J O U R N A L O F C H E M I S T R Y Materials Feature Article New directions in tin sulfide materials chemistry Tong Jiang and GeoVrey A. Ozin* Materials Chemistry Research Group, Lash Miller Chemical Laboratories, University of Toronto, Toronto, Ontario, Canada, M5S 3H6 Tin sulfide-based materials can exist in many forms, ranging from discrete molecular species, to 1D chains, 2D dense and porous sheets and 3D open frameworks.The local coordination geometry around a tin center may vary from trigonal pyramidal, to tetrahedral, trigonal bipyramidal and octahedral, and around sulfur from terminal, v-shaped to trigonal pyramidal. The oxidation state may take +2 and+4 for tin and -2, -1, 0 for sulfur. The tin sulfide chemistry is further enriched by the catenation ability of sulfur.In addition, other elements (metal and non-metal) can be incorporated into the tin sulfide structures to yield ternary and quaternary materials. More importantly, using the recent developed ‘soft chemistry’ synthetic approach, various novel porous tin (poly)sulfide materials have emerged that display interesting optical, electrical and adsorption properties. Representative tin sulfide materials will be presented and discussed in this review to demonstrate the development of tin sulfide chemistry in the last three decades.monomeric8 (SnS4)4-, dimeric9,10 (Sn2S6)4- and (Sn2S7)6- 1.0 Introduction species. In (SnS4)4-, regular Td symmetry exists around the Attributed to the versatile coordinating characteristic of tin tin(IV) site, while in (Sn2S6)4- and (Sn2S7)6-, a distorted and sulfur, tin sulfide-based solid state materials exhibit a rich tetrahedral geometry is present.The versatile bonding characstructural chemistry.1 As a main group 14 element on the fifth teristic of tin sulfide-based structures is evident in the two row of the Periodic Table, a tin atom can take a coordination dimeric tin(IV) thiostannate anions (Sn2S6)4- and (Sn2S7)6-. number of 2 to 9, and as a result, the environment around tin Both anions are constructed from two SnS4 tetrahedra, through embraces many geometrical arrangements. The most common corner-sharing in (Sn2S7)6- and edge-sharing in (Sn2S6)4-. ones are trigonal pyramidal for divalent tin, and tetrahedral, The preparation and structure of tetrameric (Sn4S10)4- have trigonal bipyramidal and octahedral for tetravalent tin.2 Sulfur, been reported by Krebs.1b The tetrameric anion consists of a main group 16 element on the third row of the Periodic four corner-sharing SnS4 tetrahedral units with an adamantane Table, displays an equally diverse coordination chemistry as geometry cluster.The stability of these tin(IV) thiostannates in tin.In addition sulfur chemistry is adorned by the catenation aqueous solution is believed to be pH dependent and generally ability of sulfur. The catenation length can be as short as in a lower pH favors higher oligomers or polymers.1b For example, S2 and as long as in a polymeric sulfur chain which may both Na4SnS4 and Na4Sn2S6 can be prepared by dissolving contain more than 200 000 sulfur atoms.3 One may think of freshly precipitated SnS2 in an aqueous solution of Na2S; numerous ways to link tin and sulfur and/or polysulfur together however, the pH for the crystallization of Na4SnS4 is around with diVerent coordination geometries around the tin and 11, while that for Na4Sn2S6 is around 9.sulfur sites. In the past three decades, various synthetic One can imagine that if a terminal sulfur on each tin site in approaches, including solid state reactions,4 hydrothermal or the dimeric (Sn2S7)6- anion coordinates to another (Sn2S7)6- organothermal5 synthesis, and molten salt (flux) methods,6 and this process continues indefinitely, a polymeric tin(IV) have been employed and developed in the search for new tin thiostannate chain would be created.Indeed, this type of tin(IV) sulfide and/or polysulfide-based solid state materials, especially thiostannate, (SnS3)2- polymer chain has been crystallized those with open-framework structures. As a result, many new from a reaction mixture of KOH, H2S and SnS2 in aqueous discrete molecular and 1D chain, 2D sheet and 3D frameworks solution, the single crystal structure of which is shown in have emerged and some of them present interesting optical Fig. 2.11 In (SnS3)2- a monomeric SnS4 building unit shares and electrical properties for device applications.7 It has proven two corners with two neighbouring units. The charge-balancing possible to incorporate other elements into the tin sulfide- and K+ cations and occluded water molecules reside between the polysulfide-based structures, to form ternary or quaternary polymeric (SnS3)2- chains.An extended hydrogen-bonding structures. As tin has stable positive oxidation states of two network is produced through the involvement of terminal and four, mixed-valence tin(II,IV ) structures are also known. In sulfur and occluded water. what follows, a brief summary of each structure type will be Tin sesquisulfide, Sn2S3, is one of the tin sulfide minerals, presented to exemplify the achievements of tin sulfide and and named ottemannite after its discovery by the German polysulfide materials chemistry over the last thirty years, and mineralogist J.Ottemann.12 In the laboratory, acicular crystals to demonstrate the structural diversity of these materials. The of this material can be prepared by heating a stoichiometric large family of organotin sulfide compounds, although importmixture of elemental Sn and S powders at 720 °C in a sealed ant and fascinating in their own right, tend to be molecular, quartz tube.13 Sn2S3 is the best known mixed-valence tin sulfide low melting, volatile and soluble in organic solvents and on compound and has an aesthetic ribbon structure as shown in this basis they are considered distinct from the solid state tin Fig. 3. The tin(IV) sites in the ribbon are octahedrally coordi- sulfides and are therefore excluded from this survey. nated with SnMS distances of 2.459–2.604 A ° . The tin(II) sites on the edge of the ribbon have a typical trigonal pyramidal geometry with two SnMS distances of 2.645 and 2.765 A ° . At 2.0 Tin sulfide molecules and chains least three ‘polytypes’ of Sn2S3, i.e. a-Sn2±xS3, b-Sn2±xS3 and c-Sn2S3, have been identified.Moh has examined the In discrete molecular tin(IV) thiostannate anions, tin centers often display a tetrahedral geometry as shown in Fig. 1 for phase diagram of tin sulfide systems and related minerals, and J. Mater. Chem., 1998, 8(5), 1099–1108 1099Fig. 2 An illustration of the zig-zag chain structure of K 2 SnS 3 ·2H 2 O11 with the K+ and water molecules omitted for clarity Fig. 3 A representation of the ribbon structure of Sn 2 S 3 ,13 the trigonal-pyramidal tin(ii) locating at the edge and the octahedral tin(iv) at the center of the ribbon reported that the non-stoichiometric a-Sn 2±xS 3 and b-Sn 2±xS 3 phases exist above 700 °C, while the c-Sn 2 S 3 phase is stable below 675 °C.12 However, there are incidents for which the Sn 2 S 3 phases do not fall into the right region of Moh’s phase diagram.13 Isostructural PbIISnIVS 3 and SnIIGeIVS 3 have also been synthesized under similar conditions. 3.0 Tin sulfide layers In general, tin sulfides favor a two-dimensional structure. Quite a few layered structure types have been reported, with various combinations of tin and sulfur local coordination geometries.The topology of tin sulfide sheets may feature a close-packing or an open structure which is decorated by regular arrays of Fig. 4 A representation of the layered structure of SnS with one pores with a variety of geometric shapes and sizes circumdistorted SnS6 octahedron highlighted (note that the sixth sulfur apex scribed by the tin and sulfur components.The sheet itself may on the neighboring tin sulfide layer is not shown)15 be flat or undulated. A summary of representative 2D tin sulfide structures is given in Table 1. octahedral geometry. As the octahedra are so distorted the tin 3.1 Tin monosulfide atom is actually displaced toward one of the faces of the octahedra as shown in Fig. 4. This leads to three short SnMS Tin monosulfide, also called herzenbergite, was first reported by the German mineralogist R. Herzenberg.14 Single crystals bonds of ca. 2.7 A° and three long SnMS bonds of almost 3.4 A° . There are two SnS layers in one unit cell and one of the long of this material can be prepared by reacting stoichiometric Sn and S elements over a temperature range of 600–750 °C.14 At distance sulfurs actually resides on the neighboring SnS layer.This weak SnMS interaction binds the two tin sulfide layers room temperature, tin monosulfide adopts the GeS structure. The corrugated tin sulfide double layers are displayed in Fig. 4. together to form a double-layer structure. Tin monosulfide undergoes a complicated thermal expansion from room tem- Each tin atom is coordinated by six sulfurs in a highly distorted Table 1 A summary of representative 2D tin sulfide structure types coordination number chemical character of tin synthetic formula Sn S sulfide layer technique SnS 6 3 dense, undulated high temperature SnS2 6 3 dense, flat CVT Rb2Sn3S7·2H2O 4, 6 2, 3 porous, flat hydrothermal Cs4Sn5S12·2H2O 5, 6 2, 3 porous, flat hydrothermal SnS-1 (Cat2Sn3S7) 5 2, 3 porous, flat hydrothermal SnS-3 (Cat2Sn4S9) 4, 5 2, 3 porous, undulated hydrothermal 1100 J.Mater. Chem., 1998, 8(5), 1099–1108perature to ca. 605 °C at which temperature it transforms to 3.3 Rb2Sn3S7·2H2O the TlI structure type. This solid state phase transition is Single crystals of Rb2Sn3S7·2H2O have been prepared by accompanied by a continuous movement of Sn and S along hydrothermal treatment of SnS2 and Rb2CO3 in a H2S satuthe entire [1 0 0] direction in the unit cell.An intralayer optical rated aqueous solution at 190 °C.22 As displayed in Fig. 6, it Ag phonon softening mechanism has been proposed to explain contains layered thiostannate (Sn3S7)2- anions with octahedral this polymorphic phase transition.Non-stoichiometric Sn1±xS SnS6 and tetrahedral SnS4 units. The (Sn3S7)2- anions are phases have also been reported with a wide variety of unit cell partially constituted by chains of the SnS6-based dense strucdimensions. 12 ture as found in bulk tin(IV) disulfide, (Fig. 5), plus Sn2S6-based open structures. One way to view these (Sn3S7)2- sheets is 3.2 Tin disulfide and polytypism that the original close-packed SnS2 layer is dissected after every two rows of octahedral SnS6 units and then sewed Tin disulfide is probably the first known tin sulfide material, the laboratory synthesis of which can be traced back some together by Sn2S6 chains, Fig. 6, to create an open 2D structure. The tetrahedral tin and double bridge (m-S)2 sulfur linkages (in two hundred years.15 Crystals of SnS2 are typically prepared through the chemical vapor transport technique (CVT) over a green), i.e.the Sn(m-S)2Sn units of the Sn2S6 chains are found to be disordered with an overall occupancy of 0.5 in the single temperature range of 600–800 °C, with I2 as a transport agent.16 Tin disulfide adopts the PbI2 layered structure with a hexag- crystal structure.The disorder of the Sn2S6 chains illustrates the versatile bonding character of tin sulfide-based structures. onal unit cell, in which tin atoms are located in the octahedral sites between two hexagonally close-packed sulfur slabs to One can imagine that the three-coordinated sulfur from the Sn2S6 chains in Fig. 6 are sometimes two-coordinated, due to form a sandwich structure, as shown in Fig. 5. The SnS2 layer can be viewed as composed of all-edge-sharing octahedral the 0.5 partial occupancy of the Sn(m-S)2Sn unit. However, these diVerent coordination environments are happily sharing SnS6 building units with the sulfurs exhibiting three-coordination and local trigonal-pyramidal symmetry. The SnS2 layers one unit cell position. The 8-atom-membered rings shown in Fig. 6 can also randomly assume many diVerent sizes. The are then stacked on top of one other along the crystallographic c-axis and held together by weak van der Waals forces. There rigid structure of SnS2 is now attached to flexible and open Sn2S6 structure units. The physical properties, especially small are many ways to stack these layers together to create various polytypes.More than 70 polytype structures of SnS2 have been molecule adsorption character, of this material, should be very interesting. established through a complicated single crystal structure analysis procedure. All of the polytypes have the same hexagonal close-packed structure within the layer, and therefore an 3.4 Cs4Sn5S12·2H2O identical unit cell parameter a of 3.647 A° ; however, they exhibit When Rb2CO3 was substituted by Cs2CO3 in the above a diVerent parameter c orthogonal to the layer which is an reaction mixture for Rb2Sn3S7·2H2O, a new tin(IV) sulfide integral number of the interlamellar spacing 5.899 A ° .17 Some material, Cs4Sn5S12·2H2O, was crystallized at 130 °C.23 It has polytypes may have a giant unit cell with a very large c a 2D porous layer structure, consisting of octahedral and parameter, as they contain many tin sulfide layers in one unit trigonal bipyramidal tin sites, Fig. 7. The basic building unit cell. For example, the polytype 258 R1 contains as many as in bulk SnS2 can also be found in this structure. In SnS2, three 258 tin sulfide sheets in one unit cell, with a c parameter of all-edge-sharing octahedral SnS6 units create a Sn3S4 broken- 761 A ° !18 It should be mentioned that one crystallite of SnS2 cube building block.Therefore, the layered structure of SnS2 often contains domains of diVerent polytypes, and the struccan be envisioned as being built from these edge-sharing Sn3S4 tural details of one polytype have to be extracted from a broken cubes. In (Sn5S12)4-, two of the primary Sn3S4 broken crystal of mixed phases.Therefore, the determination of an cubes are linked together by sharing one octahedral tin to SnS2 polytype structure has never been a trivial task.19,20 yield double Sn3S4 secondary broken-cube building blocks. Recently, Palosz proposed that the polytypes of SnS2 orig- They are further connected through double bridge Sn(m-S)2Sn inate from the partial occupancy of the tin and sulfur sites in sulfur bonds to create the 20-atom-membered rings found in the SnS2 crystal.21 Through measuring the density of SnS2 the 2D porous layers of (Sn5S12)4-.It is interesting to note polytype crystals, he found that the overall occupancy of tin that a larger pore has been created when the size of the chargeand sulfur in some polytypes can be as low as 80%.He balancing cation is increased from Rb+ to Cs+. The other attributed the diVerent stacking sequences of tin sulfide layers point worth mentioning is that by reducing the reaction to the occupancy deficiency level of the tin sulfide layers. In temperature below 200 °C, the open-layered structures of view of the many stoichiometries of tin and sulfur in stable tin sulfide materials, like SnS, Sn2S3, Sn4 S5 and SnS2, Polasz suggested that it is the versatile bonding nature of tin and sulfur that is responsible for the many stoichiometric, nonstoichiometric and lattice site-deficient structures of tin sulfides, and the occurrence of polytypes.Fig. 6 A representation of the 2D structure of Rb2Sn3S7·2H2O, the Rb+ and water molecules are omitted for clarity.The tetrahedral tin and attached double bridge Sn(m-S)2Sn sulfurs (in green) are disordered Fig. 5 A representation of the dense-packed layered structure of SnS2 16 with a partial occupancy of 0.5.22 J. Mater. Chem., 1998, 8(5), 1099–1108 1101Fig. 9 A representation of the 2D open structure of (Pr4nN)2Sn4S9, the Prn4N+ cations are omitted for clarity.The 32-membered elliptical rings are parallel to the (1 0 1) plane.28 Fig. 7 A representation of the 2D open structure of Cs4Sn5S12·2H2O, while those in SnS-3 display both distorted trigonal-bipyrami- the caesium cations and water are omitted for clarity. The 20- dal and tetrahedral symmetries. The diversity of tin sulfide membered elliptical rings are parallel to the (0 0 1) plane.23 bonding patterns is evident in these materials.Note that the tin thiostannate layer, (Sn3S7)2-, in SnS-1 has an identical Rb2Sn3S7·2H2O and Cs4Sn5S12·2H2O are created, in contrast composition to that of Rb2Sn3S7·2H2O, Fig. 6, although the to the dense layered structure of SnS2, which is crystallized at macroscopic structures of these two materials are completely temperatures above 600 °C.This exemplifies the ‘soft chemistry diVerent. approach’ to new, open framework solid state materials. Various SnS-1 and SnS-3 materials, with diVerent space groups, layer spacings and stacking sequences, have been 3.5 SnS-1 and SnS-3 obtained in the presence of a range of template cations. For example, tert-butylammonium,24 Me3HN+,25 Me4N+,26 The above two open-framework tin(IV) thiostannate materials NH4+/Et4N+,27 Et4N+,27 DABCOH+27 and quinuclidin- are built entirely of inorganic components with alkali metal ium24 have all resulted in the SnS-1 structure; while Prn4N+ cations, Rb+ and Cs+, as counter ions.It has proven feasible and Bun4N+ resulted in SnS-3.28 A comparison of some of the to employ organic cations as charge balancing moieties for the SnS-n materials is given in Tables 2 and 3.It is evident that assembly of new open-framework structures.5a For example, in the assembly of the SnS-n materials, the organic cations 2D porous materials, R-SnS-1 [(Cat+)2Sn3S7] and R-SnS-3 appear to have a particular kind of templating function. The [(Cat+)2Sn4S9], have been prepared in the presence of a large 24-atom rings present in the SnS-1 structure are templated by variety of tetraalkylammonium and amine molecules under smaller cations, while the 32-atom rings present in the SnS-3 hydrothermal reaction conditions, where R represents the structure by larger cations.The interlamellar spacing is about occluded organic cations. The basic building blocks of these 8.5–9 A ° in the SnS-1 structure, and about 14 A ° in the SnS-3 two materials are the same Sn3S4 broken cube as found in structure.The pore size and interlamellar spacing among the Cs4Sn5S12·2H2O, but, with diVerent connectivity. In SnS-1, the various SnS-n families of materials also display certain corre- Sn3S4 broken cubes are joined together by double bridge Sn(mspondence with the size of the template cations as summarized S)2Sn sulfur bonds to form hexagonally shaped 24-atom rings, in Tables 2 and 3.Interestingly, as described above, the Cs+ Fig. 8, and in SnS-3 by double bridge Sn(m-S)2Sn sulfur bonds cation with a radius of 1.81 A ° has templated the open layered as well as tetrahedral SnS4 spacer units to form elliptically structure of Cs4Sn5S12, in which 20-atom rings are present in shaped 32-atom rings, Fig. 9. The tin atoms in SnS-1 materials the tin(IV) sulfide layer with an interlamellar spacing of 7.166 A ° . feature a single kind of distorted trigonal-bipyramidal site, However, in the presence of cyclooctasulfur molecules, Cs- SnS-1 (Cs2Sn3S7·1/2S8) has been synthesized.29 Cs-SnS-1 is isostructural with (DABCOH)2Sn3S7·H2O. The Cs+ cations are located between the tin sulfide gaps to give an interlamellar distance of 8.151 A ° .The 24-atom rings are occupied by cyclooctasulfur rings which cannot be removed without harming the integrity of the tin sulfide framework. It appears that the cyclooctasulfur molecule functions as a co-templating agent with the Cs+ cation, and the Cs+ alone is not large enough to direct the formation of the SnS-1 structure.The cations in the open-framework tin chalcogenides appear to have a chargebalancing, space-filling and structure-directing role, possibly analogous to the templating properties of cations in the synthesis of zeolites and molecular sieves. It is interesting to mention that the remarkable structural diVerences among the R-SnS-1 and R-SnS-3 families of materials indicate that the bonding between tin and sulfur, and thus the constituent [Sn3S7]2- and [Sn4S9]2- tin(IV) sulfide layers, is extremely flexible.As summarized in Tables 2 and 3, Fig. 8 A representation of the 2D open structure of the shape and size of the 24-atom rings in the SnS-1 and the (Me4N)2Sn3S7·H2O, the Me4N+ cations and H2O molecules are omit- 32-atom rings in the SnS-3 structures, as well as the layer ted for clarity.The 24-membered hexagonal rings are parallel to the stacking sequence and interlamellar spacings, can be modified (1 0 1) plane.26 1102 J. Mater. Chem., 1998, 8(5), 1099–1108Table 2 A summary of the dimensions of the 24-atom rings and the interlayer spacing and layer stacking sequences of various SnS-1 materials space pore dimensionsa interlamellar stacking R-SnS-1 material group /A ° ×A ° ×A ° spacing/A ° sequence Cs2Sn3S7·0.5S8 C2/c 10.79×10.79×10.47 8.151 AB (Me4N)2Sn3S7·H2O P21/n 11.69×11.03×9.69 8.511 AA (DABCOH)2Sn3S7·H2O C2/c 10.98×10.98×9.95 8.594 AB (NH4)0.5(Et4N)1.5Sn3S7 P3121 10.94×10.94×10.88 9.011 ABC (Et4N)2Sn3S7 P21/n 11.58×10.84×10.42 8.915 AA aDimensions are defined from sulfur center to sulfur center as illustrated in Fig. 8. Table 3 A comparison of TPA-SnS-3 and TBA-SnS-3 structures brought additional flexibility to the tin sulfide-based layered structures. TPA-SnS-3 TBA-SnS-3 3.6 Tin sulfide–organic composite mesophases formula (Prn4N)2Sn4S9 (Bun4N)2Sn4S9 space group P21/n Pbcn Thermotropic tin(IV) sulfide–organic composite semiconductor pore dimensionsa/A° 20.65×9.32 19.8×11.3 materials have been synthesized recently in the presence of interlamellar dimension/A ° 14.05 14.2 long-chain amine molecules.27,32a The as-synthesized crystalline layer stacking sequence AA AB form of Meso-SnS-1 is established to have a structure that is aDimensions defined as sulfur center to sulfur center as illustrated based upon well registered yet poorly ordered porous tin(IV) in Fig. 9. sulfide layers with a layer spacing of 50 A ° , between which are sandwiched well organized hexadecylamine bilayers, Fig. 10. considerably in response to the change of size and shape of This material displays interesting thermal transitions upon the organic templates. This phenomenon may explain the heating. On warming this material to around 45 °C the hexaderelatively smaller structural variety so far observed for micro- cylamine bilayer first becomes disordered while the porous porous tin(IV) sulfide-based materials in comparison with the tin(IV) sulfide sheets remain registered.This is followed, around myriad of structure types for the microporous oxide-based 85 °C, by a transition where both the alkylamine bilayer and relatives like zeolites and aluminophosphates.In order to the porous tin(IV) sulfide lamellae become liquid crystalline at accommodate the size/shape changes of templates, the micro- which point the intralayer but not the interlayer registry is porous layers of the tin(IV) sulfide materials undergo elastic lost. The liquid crystal organic–inorganic composite phase has deformation to alter the void spaces within and between the either a nematic or a smectic C structure.Electrically, the layers, rather than forming a completely new microporous room temperature ordered phase of Meso-SnS-1 has a conducstructure type. This is to be contrasted with 3D microporous tivity of 5.3×10-8 V-1 cm-1 which increases by more than structures, in which there exists a greater restriction on the 1000 times on transforming to the LC phase where it behaves size and shape of the void space, either a cavity or channel, as a semiconducting metallogen. The conductivity of Mesowhich can barely be deformed without an accompanying SnS-1 cycles reversibly with temperature and displays disconticomplete reorganization of the 3D framework.However, it nuities that are coincident with the crystal–semiliquid crystal should be mentioned that the 3D open frameworks of RHO and semiliquid crystal–liquid crystal thermal transitions.Mesoand ZSM-5 zeolite materials have been found to display SnS-1 can readily form electrically conducting thin films which interesting framework flexibility similar to that described for are able to reversibly adsorb molecules like H2O and CO2.the 2D SnS-n materials.30 It has also been shown that a similar These properties bode well for the use of this new class of structural flexibility of the R-SnS-n structures can be induced by pressure27 and adsorbed guest molecules with a concomitant, drastic electrical property change.27,31 In addition, attributed to the 2D open structure nature of the SnS-n materials, they are found to display unique adsorption behaviour towards guest molecules.For example, they may behave like a microporous material and/or an intercalation host. In other words, the adsorption of guest molecules in the SnS-n materials is controlled by both the size/shape and the properties of guest molecules. The unique structural flexibilty and adsorption properties of these materials make them of considerable interest as potential chemoselective sensory element for molecular recognition devices.7,31 It is interesting to note that the framework flexibility of the SnS-n materials is related to the well known polytype phenomenon occurring in bulk SnS2.They all originate from the flexible bonding nature of tin(IV) and sulfur(-II) centers. However, the constituent tin sulfide layers in diVerent SnS2 polytypes are identical and all of the polytypes have the same 2D unit cell parameters and structure within the tin sulfide sheet.The only diVerence among them is the stacking sequence Fig. 10 Illustration of the structures of the as-synthesized crystalline of the constituent layers and the cell parameter along the porous layer form of mesomorphic tin(IV) sulfide (top) and its semi- stacking direction.In this regard, diVerent SnS-1 or SnS-3 liquid and liquid crystalline phases. On warming the material the structures have diVerent 3D cell parameters and space groups. alkylamine bilayer first becomes disordered around 45 °C while the Furthermore, the tin sulfide constituent layer in each particular porous tin(IV) sulfide sheets remain more-or-less registered (middle), structure has unique SnMS bond angles and distances, and and then around 85 °C, both the alkylamine bilayer and the porous therefore a distinct pore size and shape.It appears that the tin(IV) sulfide lamellae become liquid crystalline at which point the intralayer but not the interlayer registry is lost (bottom).32 regular microscopic perforations of the tin sulfide layer have J.Mater. Chem., 1998, 8(5), 1099–1108 1103inorganic–organic semiconducting LCs for electro-optical displays and chemical sensing applications. The advantages of thermotropic Meso-SnS-1 over the R-SnS-1 and R-SnS-3 materials are that it is a better electrical conductor, and most importantly, thin films can be readily fabricated for device applications by simply warming the sample to its liquid crystalline state.Thin films of the material are expected to provide more sensitive and faster response to small and large molecule analytes. 4 3D Tin sulfide frameworks In contrast to the 2D tin sulfide-based structures, 3D tin(IV) thiostannates are relatively rare, so far only two structure types have been reported.A summary of their structural characteristics is given in Table 4. K2Sn2S5 was prepared by heating a mixture of Sn, K2S, and S in an evacuated, sealed Pyrex tube Fig. 12 An illustration of the channel structure of Na4Sn3S8, viewed down the b axis34 at 320 °C.33 The 3D framework of (Sn2S5)2- is presented in Fig. 11. It is built entirely from trigonal-bipyramidal SnS5. The SnS5 building units form through edge-sharing, zig-zag chains running along the a axis as highlighted in Fig. 11. They are bridged by sulfide ligands to yield 12-membered channels running down the b axis, with charge-compensating K+ cations snugly fitting inside. As shown in Fig. 12, the structure of Na4Sn3S8 34 is closely related to that of K2Sn2S5. The only diVerence is that in Na4Sn3S8, the zig-zag chains are linked together by tetrahedral SnS4 spacer units, leaving two terminal sulfurs located in the channels (Fig. 12). The charge-balancing Na+ cations reside inside the channels. Although both materials appear to have an open channel structure, the chargebalancing cations fit tightly inside the channels, and they are thus actually quite dense. Tl2Sn2S5, isostructural with K2Sn2S5, has been crystallized by heating a stoichiometric mixture of Fig. 13 Structures of [Sn(S4)3]2- and [Sn(S4)2S6]2- 38 Tl, Sn and S powders at 350 °C.35 In view of the similar diameters of K+ (1.52 A ° ) and Tl+ (1.54 A ° ), but the smaller size 5 Tin(IV) polythiostannates of Na+ (1.16 A ° ),36 the formation of the Na4Sn3S8 structure, with two large terminal sulfurs located inside of the channel, Discrete and polymeric tin(IV) polythiostannates have been suggests that the tin sulfide-based all-inorganic frameworks characterized with varied polysulfide ligand chain lengths and favors a dense-packed, rather than an open structure.geometric tin sites, see Table 5. The best known discrete tin(IV) polythiostannate anions are [Sn(S4)3]2- and [Sn(S4)2S6]2-, both having an octahedral tin(IV) site.The former contains three bidentate tetrasulfide ligands, while the latter has two Table 4 A summary of 3D tin(IV) thiostannates bidentate tetrasulfide and one bidentate hexasulfide ligand, Fig. 13. Interestingly, these two anions often co-crystallize in coordination a single unit cell to form a disordered structure. For example, number chemical synthetic in (Et4N)2[Sn(S4)3]0.4[Sn(S4)2S6]0.6, the [Sn(S4)3]2- and formula Sn S character of structure technique [Sn(S4)2S6]2- moieties are found to be located at the same crystallographic site with a partial occupancy of 0.4 and 0.6, K2Sn2S5 5 2 12-membered channel solid state respectively;37 while in [DABCOH]2[Sn(S4)3]0.5[Sn(S4)2 Na4Sn3S8 4, 5 1, 2 14-membered channel solid state S6]0.5, they each have a partial occupancy of 0.5.27,38, In contrast to the discrete [Sn(S4)3]2- and [Sn(S4)2S6]2- anions, b-Rb2Sn2S8 features an extended 2D framework as displayed in Fig. 14.33 It contains octahedral and tetrahedral tin(IV) sites. The basic building units can be viewed as facesharing double semi-broken cubes that are linked via. double bridge Sn(m-S)2Sn sulfur bonds to create parallel chains along the a axis direction.These chains are cross-linked by tetrasul- fide S4 ligands to yield the elaborate 2D structure of b- K2Sn2S8. The charge-compensating cation, Rb+ resides between the tin(IV) polysulfide sheets. The linkage between the Table 5 A summary of representative tin(IV) polysulfide structures chemical structure tin sulfide polysulfide formula dimensionality polyhedra ligands [Sn(S4)3]2- monomer SnS6 S4 [Sn(S4)2S6]2- monomer SnS6 S6 Rb2Sn2S8 2D framework SnS6, SnS4 S4 Fig. 11 An illustration of the channel structure of K2Sn2S5, viewed Cs2Sn2S6 2D framework SnS5 S2 down the b axis33 1104 J. Mater. Chem., 1998, 8(5), 1099–1108Fig. 14 A representation of the 2D open structure of K2Sn2S8, the K+ Fig. 16 An illustration of the layered structure of the rhombohedral cations are omitted for clarity (note the tetrapolysulfide ligand)33 Sn2P2S6.Note that it is similar to the berndtite SnS2 structure shown in Fig. 5, but with one half of the octahedral tin sites replaced by P2.42 which the element A is covalently bonded to Sn and/or S. Most ternary tin sulfides are metal tin sulfides, and were prepared through a conventional direct reaction of stoichiometric SnSx (x=1, 2) with MySz, or elements Sn, M, and S in an evacuated ampoule at elevated temperatures.A large collection of metals has been studied, including Na, K, Ba, In, Tl, Ge, Pb, Sb, Mn, Cu, and Eu.39 As the applied reaction temperature is normally high, up to 1000 °C, most of the produced materials feature a dense packed 2- or 3D structure. For example, CuSn3.75S8 has been synthesized by reacting elements Cu, Sn, and S at 1100 °C.It has a defect spinel structure, with SnIV occupying 15/16 of the octahedral and CuI residing in 1/2 of the tetrahedral sites.40 Na2SnS3 was prepared by melting a stoichiometric mixture of SnS2 and Na2S at 750 °C, followed by slow cooling to room temperature at 16 °C h-1.41 It has a NaCl type structure with tin and sodium distributed over the cation positions.Recently, non-metal elements have also been sucessfully introduced into tin sulfide-based frameworks. For example, crystals of Sn2P2S6 have been prepared from stoichiometric Fig. 15 A representation of the 2D open structure of Cs2Sn2S6, the Sn, P, and S elements sealed in a quartz tube and heated at Cs+ cations are omitted for clarity (note the disulfide ligand)33 high temperatures.42 Three polymorphic Sn2P2S6 phases have been reported, two of which, monoclinic(I) and rhombohedral, display a 2D structure similar to bulk berndtite SnS2, where chains through the S4 ligands is rather flexible.In this context, it has been found that a polytype of b-Rb2Sn2S8, namely a- 50% of the octahedral metal sites are, however, replaced by P2 (Fig. 16). By contrast, the monoclinic(II) phase consists of Rb2Sn2S8, can be crystallized from an identical reaction mixture to that used to prepare b-Rb2Sn2S8 but at a slightly lower discrete P2S64- anions that are linked together via weak SMSn interactions.43 In all of these phases, the oxidation state of tin temperature.33 In comparison with monoclinic a-Rb2Sn2S8, the high-temperature product has its layers slightly shifted atoms is tin(II ).Recently, SnP2S6 was synthesized under similar reaction condition as Sn2P2S6, but with a reduced tin content along the [1 0 2] crystallographic axis to form a more symmetric, i.e. orthorhombic, lattice. of Sn5P5S of 15256 in the reaction mixture.44 SnP2S6 has an ordered defect structure of the Fe2P2S6 structure type.The tin Cs2Sn2S6 also features a 2D framework as shown in Fig. 15.33 However, in this case, it contains one unique trigonal bipyrami- centers are in the +4 oxidation state, half of the metal sites are thus vacant and this results in a 2D open structure (Fig. 17). dal tin(IV) site which shares two common edges to form polymeric chains along the c axis.These chains are cross- Both SnP2S6 and Sn2P2S6 are found to display interesting non-linear optical properties.45 linked by disulfide S2 ligands to form arrays of 14-membered distorted rectangular pores parallel to the [1 0 0] plane. It is Solid solutions of SnS2-xSex (0x2) and (Me4N)2Sn3S7-xSex (0x7) families have been synthe- clear that tin polysulfides have a diverse bonding character and many more new structures are expected to emerge with sized.16,46,47 All members of the SnS2-xSex series crystallize in the Cd(OH)2 type structure, as shown in Fig. 5, to yield an diVerent polysulfide ligands and coordination geometries around the tin centers.isostructural family of solid solutions, i.e. sulfur and selenium are randomly distributed over the chalcogenide sites in the lattice.16,46 The intercalation chemistry of the SnS2-xSex series 6 Ternary tin sulfides has been investigated by O’Hare and co-workers.They have successfully included cobaltocene [Co(g-Cp)2] into the van der All the aforementioned structures contain only tin–sulfide covalent bonds, and the metal and organic cations in the Waals’ gaps between the tin chalcogenide layers, to form a series of SnS2-xSex{Co(g-Cp)2}0.33±0.02 materials.Through X- structure are ionically bonded to the tin(IV) thio- and polythiostannate anions to compensate the charge. However, a large ray and neutron diVraction and 2H solid state NMR studies, it was found that the cobaltocene is ordered in the tin number of ternary tin sulfide SnxSyAz systems (A represents any elements other than tin or sulfur) have been studied in chalcogenide gap, with the C5 axis of the g-Cp ring parallel to J.Mater. Chem., 1998, 8(5), 1099–1108 1105Fig. 17 An illustration of the 2D open structure of SnP2S6. Note that this structure is related to the Sn2P2S6 structure shown in Fig. 15, but half of the octahedral tin sites are vacant.44 Fig. 19 A representation of the open-framework structure of [Sn5S9O2][HN(CH3)3]2, for clarity showing only one of the interpenetrated diamond-type tin sulfide networks50 [Sn20S37O8]10-, each (Sn10O4S20)8- is connected to three neighboring (Sn10O4S20)8- clusters through corner-sharing of the sulfur apex to form a 2D open-framework structure,49 while in [Sn5O2S9][HN(CH3)3]2, each (Sn10O4S20)8- being linked to four neighboring (Sn10O4S20)8- clusters to form two interwoven diamond-type structures Fig. 19.50 It is interesting to mention that the (Sn4Se10O)6- tetrameric-unit has recently been synthesized in this laboratory, a reduced size version of Fig. 18 Structure of the supercluster (Sn10O4S20)8- 48 the supertetrahedral (Sn10O4S20)8- cluster.51 In (Sn4Se10O)6-, the tin selenide cluster has an adamantane structure and the oxygen is located in the central tetrahedral void and coordi- the layers.In the intercalated materials, SnIV in the host layers nated to the four tin apexes to make them five-coordinated. is partially reduced to SnII as seen by Mo�ssbauer spectroscopy. The (Sn4Se10O)6- clusters are linked together through the four The conductivities of both the host and the host–guest intercaterminal seleniums to distorted tetrahedral SnIV centres to lated series were studied through variable temperature single form (Sn5Se10O)2-, a 3D open-framework diamond-type struc- crystal measurements.The intercalated materials display ture where the charge balance is maintained by two (CH3)4N+ reduced resistivity in comparison with the hosts. Remarkably, cations occupying the void spaces (Fig. 20). the high selenium content members, i.e. SnS2-xSex{Co(g- Cp)2}0.33±0.02 (1.85x2), exhibit superconductivity at temperatures below 6 K. In comparison, the (Me4N)2Sn3S7-xSex (0x7) family has a 2D open structure.47a It displays a similar framework architecture to that of the SnS-1 material displayed in Fig. 8. The (Me4N)2Sn3S7-xSex series is found to crystallize in an orthorhombic space group, P212121. The optical absorption edge of the (Me4N)2Sn3S7-xSex series is found to display a monotonic red shift with increasing Se content as found in the isostructural SnS2-xSex family. The results of a detailed recent study47b show that the distribution of the chalcogenides in (Me4N)2Sn3S7-xSex is random (solidsolution, Vegard law) at the length scale of the unit cell but site-selective at the level of the trigonal bipyrimidal buildingblocks.In addition to the mixed sulfur–selenium based materials, tin oxy-sulfides have also been reported. For example, the ‘supertetrahedron’ (Sn10O4S20)8- has been crystallized as a discrete anion in Na8Sn10O4S20·32H2O and Cs8Sn10O4- S20·13H2O compounds.48 The (Sn10O4S20)8- cluster is built of ten corner-sharing SnS4 tetrahedrons, Fig. 18. Four oxygen atoms are located in the supertetrahedral voids and are coordinated to six of the ten tin atoms to form SnS4O2 distorted octahedra. Recently, these supertetrahedral clusters have been linked together through sulfur bridging bonds to Fig. 20 An illustration of the 3D open framework structure of (Me4N)2(Sn5Se10O). The Me4N+ cations are omitted for clarity.51 give rise to 2D and 3D open-framework structures.In 1106 J. Mater. Chem., 1998, 8(5), 1099–1108sandwiched by two Sn metal layers in which one-half of the Sn sites are missing to form parallel grooves. The Rb+ cations are located inside the grooves. K2Au2SnS4 and K2Au2Sn2S6 were obtained by heating Sn and Au powder at 350 °C in a K2S5 and K2S9 flux, respectively.52 As shown in Fig. 23(A), the 1D structure of the former is constituted by tetrahedral SnS4 and linear AuS2 building blocks in a ratio of 152. The paralell zig-zag [Au2SnS4]2- chains are separated by K+ cations. K2Au2Sn2S6 is constructed from dimeric Sn2S6 units that are linked together by linear Au atoms to form fully extended infinite chains, Fig. 23(B). Note that although these two materials have similar chemical formulae to those of the Cu analogs discussed above, they present completely diVerent structures due to the diVerent preferred coordination geometries of CuI and AuI. The former favors a tetrahedral coordination environment while the latter favours a linear geometry. Fig21 A representaion of the 2D open structure of Rb2Cu2SnS4. Rb+ cations residing between the (Cu2SnS4)2- sheets are omitted.52 7 Quatenary tin sulfides Quaternary Rb2Cu2SnS4 and Rb2Cu2Sn2S6 have been synthesized by heating Sn and Cu powder in a Rb2S5 flux at 400 °C.52 In both materials, tetrahedral SnS4 and CuS4 units are the basic building blocks.The edge sharing of SnS4 and CuS4 creates a 2D open structure of Rb2Cu2SnS4 with 8-atom rings (Fig. 21). The rubidium cations are located between the anionic layers of [Cu2SnS4]2- to balance the charge. The corner sharing of SnS4 and CuS4 results in the 2D open struuture of A2Cu2Sn2S6 (Fig. 22). It can be viewed as a derivative of a 3D zinc blende adamantane-type structure material of formula Cu2SnS3. However, the replacement of half of the Cu atoms in Cu2SnS3 by alkali-metal atoms reduces the dimensionality of the framework structure from three to two due to the interruption of covalent bonding through the structure.Each [Cu2Sn2S6]2- slab contains three metal layers, i.e. a Cu layer Fig. 23 An illustration of the chain structures of (A) K2Au2SnS4 and Fig. 22 A representaion of the 2D open structure of Rb2Cu2Sn2S6 (B) K2Au2Sn2S6.The K+ cations residing between the chains are omitted for clarity.52 viewed parallel to the layers52 J. Mater. Chem., 1998, 8(5), 1099–1108 110717 B. Palosz, W. Steurer and H. Schulz, Acta Crystallogr., Sect. B, 8 Conclusion 1990, 46, 449. 18 B. Palosz, W. Palosz and S. Gierlotka, Acta Crystallogr., Sect. B, Representative tin sulfide-based materials that have appeared 1986, 42, 653.in the recent materials chemistry literature, ranging from 19 A. R. Verma and P. Krishna, Polymorphism and Polytypism in molecular species to 1D chain, 2D dense and porous sheets Crystals, John Wiley & Sons, New York, 1969. and 3D open frameworks, have been reviewed. In particular, 20 H. Jagodzinski, Neues Jahrb. Mineral. Abh., 1954, 3, 49. ‘soft chemistry approaches’ such as hydro- and solvothermal 21 B.Palosz, W. Steurer and H. Schulz, Acta Crystallogr., Sect. B., 1990, 46, 449. synthesis, and the molten-salt flux technique, have generated 22 W. S. Sheldrick and B. Schaaf, Z. Anorg. Allg. Chem., 1994, 620, a large collection of novel microporous tin (poly)sulfide mate- 1041. rials and the first example of a mesoporous tin sulfide, which 23 W.S. Sheldrick, Z. Anorg. Allg. Chem., 1988, 562, 23. present interesting optical, electrical, adsorption and liquid 24 C. Bowes, PhD Thesis University of Toronto, 1996. crystal properties. In the 1D, 2D and 3D extended tin (poly)sul- 25 T. Tan, Y. Ko and J. B. Parise, Acta Crystallogr., Sect. C, 1995, fides, molecular tin sulfide species, [SnS4]4-, [Sn2S6]4- and 51, 398. 26 J. B. Parise, Y. Ko, J. Rijssenbeek, D. M. Nellis K. Tan and [Sn4S10]4- are found to serve as structure building blocks. S. Koch, J. Chem. Soc., Chem. Commun., 1994, 527. This suggests that by employing modular-assembly synthetic 27 (a) T. Jiang, PhD Thesis University of Toronto, 1997; (b) T. Jiang, approaches, in combination with templating agents, many A. J. Lough, G.A. Ozin, R. L. Bedard and R. W. Broach, J. Mater. more novel binary, ternary, quaternary tin sulfide structures Chem., 1998, 8, 721; T. Jiang, A. Lough and G. A. Ozin, Adv. are expected to emerge with desired properties and functions, Mater., 1998, 10, 42. for the development of a range of new sensing, separation and 28 (a) T. Jiang, A. J. Lough, G. A. Ozin, D. Young and R. L. Bedard, Chem.Mater., 1995, 7, 245; (b) Y. Ko, K. Tan, D. M. Nellis, S. Koch catalysis applications. and J. B. Parise, J. Solid State Chem., 1995, 114, 506. 29 G. A. Marking and M. G. Kanatzidis, Chem. Mater. 1995, 7, 1915. The financial assistance of the Natural Sciences and 30 (a) D. R. Corbin, L. Abrams, G. A. Johns, M. M. Eddy, Engineering Research Council of Canada (NSERC) is deeply W.T. A. Harrison, G. D. Stucky and D. E. Cox, J. Am. Chem. Soc., appreciated. T.J. expresses her gratitude to University of 1990, 112, 4821; (b) C. A. Fyfe, H. Strobl, G. Kokotailo, Toronto for an Open Fellowship held through the course of G. J. Kennedy and G. E. Barlow, J. Am. Chem. Soc., 1988, 110, her PhD study. 3373. 31 (a) H. Ahari, C. L. Bowes, T. Jiang, A. Lough, G.A. Ozin, R. L. Bedard, S. Petrov and D. Young, Adv. Mater., 1995, 7, 375; References (b) G. A. Ozin, Supramol. Chem., 1995, 6, 125. 32 T. Jiang and G. A. Ozin, J. Mater. Chem., 1997, 7, 2213. 1 (a) N. N. Greenwood and A. Earnshaw, The Chemistry of the 33 J. Liao, C. Varotsis and M. G. Kanatzidis, Inorg. Chem., 1993, Elements, Pergamon Press, New York, 1984, ch. 10 and 15; (b) 32, 2453.B. Krebs, Z. Anorg. Allg. Chem., 1983, 22, 113; (c) D. J. Vaughan 34 J.-C. Jumas, E. Philippot and M. Maurd, J. Solid State Chem., and J. R. Craig, Mineral Chemistry of Metal Sulfides, Cambridge 1975, 14, 152. University Press, Cambridge, 1978. 35 G. Eulenberger, Z. Naturforsch., Terl B, 1981, 36, 687. 2 P. G. Harrison, Chemistry of Tin, Blackie, Glasgow, 1989. 36 (a) F. A.Cotton, G. Wilkinson and P. L. Gaus, Basic Inorganic 3 N. N. Greenwood and A. Earnshaw, A. The Chemistry of the Chemistry, John Wiley & Sons, New York, 1987; (b) J. E. Huheey, Elements, Pergamon Press, New York, 1984, p. 780. Inorganic Chemistry: Principles of Structure and Reactivity, Harper 4 B. Krebs and W. Schiwy, Z. Anorg. Allg. Chem. 1973, 398, 63. & Row, New York, 1983, p. 258. 5 (a) R. L. Bedard, L. D. Vail, S. T. Wilson and E. M. Flanigen, US 37 A. Mu� ller, J. Schimanski, M. Ro� mer, H. Bo� gge, W. Baumann, Patent, 4 880 761, 1989; R. L. Bedard, L. D. Vail, S. T. Wilson and W. Eltzner, E. Krickemeyer and U. Billerbeck, Chimia, 1985, 39, 25. E. M. Flanigen, US Patent, 4 933 068, 1990; R. L. Bedard, 38 T. Jiang, G. A. Ozin and R. L. Bedard, Adv.Mater., 1994, 6, 860; S. T. Wilson, L. D. Vail, J. M. Bennett and E. M. Flanigen, in T. Jiang, G. A. Ozin, A. Lough and R. L. Bedard, J. Mater. Chem., Zeolites: Facts, Figures, Future, ed. P. A. Jacobs and R. A. van in press. Santen, Stud. Surf. Sci. Catal., Elsevier Science Publishers B. V., 39 J. Olivier-Fourcade, J. C. Jumas, M. Ribes, E. Philippot and Amsterdam, 1989, vol. 49, part A, p. 375; (b) W. S. Sheldrick and M. Maurin, J. Solid State Chem., 1978, 23, 155. B. Schaaf, Z. Naturforsch. Teil B, 1994, 49, 655; (c) W. S. Sheldrick 40 S. Jaulmes, M. Julien-Pouzol, J. Rivet, J. C. Jumas and M. Maurin, and M. Wachhold, Angew. Chem., Int. Ed. Engl., 1997, 36, 206. Acta Crystallogr., Sect. B, 1982, 39, 51. 6 (a) M. G. Kanatzidis, Chem. Mater., 1990, 2, 353; (b) S.Dhingra 41 W. Mark, O. Lindqvist, J. C. Jumas and E. Philippot, Acta and M. G. Kanatzidis, Science, 1992, 258, 1769; (c) Crystallogr., Sect. B, 1974, 30, 2620. M. G. Kanatzidis and C. Sutorik, Prog. Inorg. Chem., 1995, 43, 151. 42 V. W. Klingen, R. Ott and H. Hahn, Z. Anorg. Allg. Chem., 1973, 7 R. L. Bedard, G. A. Ozin, H. Ahari, C. L. Bowes, T. Jiang and 396, 271. D. Young, US Patent, 5 594 263, 1997. 43 V. W. Klingen, G. Eulenberger and H. Hahn, Z. Anorg. Allg. 8 W. Z. Schiwy, S. Pohl and B. Krebs, Z. Anorg. Allg. Chem., 1973, Chem., 1973, 401, 97; H. P. B. Rimmington and A. A. Balchin, 402, 77. Phys. Status Solidi, 1971, 7, K47. 9 (a) B. Krebs, S. Pohl and W. Schiwy, Z. Anorg. Allg. Chem. 1972, 44 Z. Wang, R. D. Willett, R. A. Laitinen and D. A. Cleary, Chem. 393, 241; (b) B. Krebs, S. Pohl, S. and W. Schiwy, Angew. Chem., Mater., 1995, 7, 856. Int. Ed. Engl., 1970, 9, 897. 45 D. A. Cleary, R. D. Willett, F. Ghebremichael and M. Kuzyk, Solid 10 B. Krebs and W. Schiwy, Z. Anorg. Allg. Chem., 1973, 63, 398. State Commun., 1993, 88, 39. 11 W. Z. Schiwy, C. Blutau, D. Ga�thje and B. Krebs, Z. Anorg. Allg. 46 D. O’Hare, Chem. Rev., 1992, 121 and references therein. Chem., 1975, 412, 1. 47 (a) H. Ahari, G. A. Ozin, R. L. Bedard, S. Petrov and D. Young, 12 G. H. Moh, Neus Jahrb. Mineral. Abh., 1969, 111, 227. Adv. Chem., 1995, 7, 370; (b)H. Ahari,O� . Dag, A. G. Ozin, S. Petrov 13 R. Kniep, D. Mootz, U. Severin and H. Wunderlich, Acta and R. L. Bedard, J. Phys. Chem., in press. Crystallogr., Sect. B, 1982, 38, 2022; D. Mootz and H. Puhl, Acta 48 W. Schiwy and B. Krebs, Z. Angew. Chem., 1975, 8ystallogr., Sect. B, 1967, 23, 471. 49 J. B. Parise, Y. Ko, K. Tan, D. M. Nellis and S. Koch, J. Solid State 14 R. Herzenberg, Rev. Mineral., 1932, 4, 33. Chem., 1995, 117, 219. 15 (a) H. Wiedemeier and F. J. Csillag, Z. Kristallogr., 1979, 149, 17; 50 J. B. Parise and Y. Ko, Chem. Mater., 1994, 6, 718. (b) H. G. von Schnering and H. Wiedemeier, Z. Kristallogr., 1981, 51 H. Ahari, A. Lough and G. Ozin, unpublished work. 156, 143. 52 J. H. Liao and M. G. Kanatzidis, Chem. Mater., 1993, 5, 1561. 15 P. Woulfe, Philos. Trans. R. Soc. London, 1771, 61, 114. 16 H. P. B. Rimmington and A. A. Balchin, J. Cryst. Growth., 1972, 15, 51. Paper 7/09054D; Received 17th December, 1997 1108 J. Mater. Chem., 1998, 8(5), 1099–1108