Synthesis and crystal structures of the new ternary nitrides Sr,CrN, andBa,CrN,Marten G. Barker," Michael J. Begley," Peter P. Edwards: Duncan H. Gregory " and Susan E. Smitha Department of Chemistry, The I,hitiersit.y of Nottingharn, Nottingham NG7 2RD, UKSchool of Chemistry, University of Birml'ngham, Edgbaston, Birmingham B1.5 2TT, UKTwo new ternary nitrides, Sr,CrN, and Ba,CrN,, were synthesised from the appropriate alkaline-earth-metalnitride and chromium metal or the chromium nitride, Cr,N, at high temperatures in sealed stainless-steelcapsules. Structure determination from single-crystal X-ray data showed them to be isostructural in thehexagonal space group P6,/m, Z = 2, with lattice constants a = 7.724(2), c = 5.249(1) A for Sr,CrN, anda = 8.201(2), c = 5.497(1) A for Ba,CrN,.The structures contain trigonal-planar [CrN,I6- anions and eitherSr2 + or Ba2 + cations.Whilst a very large number of ternary oxides have beenprepared containing alkaline-earth and transition elements it isonly in the past few years that the corresponding ternary nitridesystems have begun to be investigated in detail. Even now awide range of often novel structure types have been shown toexist in ternary nitrides. The valences and co-ordinationsexhibited by transition metals in these materials can be unusualoften leading to interesting physical properties. ' 7 ' Examples ofsuch nitrides are the A,MN, (313) (A = Ca, Sr or Ba; M = V,Cr, Mn or Fe)3-7 and A6MN, (615) (A = Ca; M = Ga, Feor Mn)8*9 series of materials.A particular feature of thesecompounds is the presence of a discrete [MN,I6- anionpossessing trigonal-planar co-ordination of M to nitrogen. Inthe strontium and barium compounds of manganese and iron,namely Sr,MnN,, Ba,MnN,7 and Ba,FeN,,6 the [MN,I6-anion has D,, symmetry, whilst in the corresponding calciumcompounds of vanadium, chromium and manganese, Ca3VN3,4Ca,CrN,3 and Ca3MnN3,' these anions have Czo symmetry. Inthe case of the chromium nitride this change in symmetry wasattributed to a Jahn-Teller instability for the low-spin Cr3+ion., Interestingly, the Ca,MN, compounds appear to containtriangular units of D,, symmetry * q 9 although magnetic measure-ments suggest there may be a local distortion of the anions.'In this paper we report the first preparation andcharacterisation of two ternary nitrides, Sr,CrN, andBa,CrN,, which both contain the [CrN,I6- anion having D,,symmetry as opposed to the C,, symmetry preyiously seen inCa,CrN,.3 We also outline the effects likely to govern thecrystal structure and transition-metal co-ordination in A,MN,materials in general and how these factors are likely to relate totheir magnetic properties.With the crystal data now emerging,we can begin the task of establishing coherent structuralcorrelations for these new materials.ExperimentalStarting materialsStrontium nitride was prepared by the reaction of strontiumdissolved in liquid sodium with gaseous nitrogen at 570 "C.Previously cleaned strontium (2-3 g) was added to an excess ofmolten sodium contained in a stainless-steel crucible at 250 "C.This operation was carried out in an argon-filled, evacuable,glove-box specially designed for the handling of liquid alkalimetals (ca.5 ppm 02, c 5 ppm water). On cooling, the cruciblewas sealed inside a stainless-steel vessel and heated to 570 "C for42 h under a positive nitrogen pressure (ca. 2 atm, x 2 x lo5Pa). During this time the nitrogen pressure was monitoredcontinuously by a pressure transducer and further nitrogenadded where necessary. When no further nitrogen was taken upby the alloy the vessel was cooled and then heated undervacuum at 350°C for 24 h to remove the excess of sodium.Importantly, liquid sodium does not react with nitrogen andhence serves as an inert solvent for the strontium which reactsreadily in this form with the nitrogen to give the binary nitridewhich precipitates from solution as a purple-black crystallinesolid. This method of preparation was found to give a productfree from alkaline-earth-metal oxide, an impurity which cansometimes be seen in nitride prepared by direct combinationof the elements.X-Ray powder diffraction patterns of theresulting nitride showed it to be Sr,N.'* The chromium metalpowder (99.9573, chromium nitride, Cr,N (99%), and bariumnitride, Ba3N2, used in this study were obtained commercially(Alfa).Synthesis of Sr,CrN, and Ba,CrN,Single crystals of both compounds were prepared by thereaction of the binary nitrides with chromium metal powder(99.95%) in sealed stainless-steel crucibles at 1050°C for aperiod of 4 d, after which the reaction mixture was cooled underargon at a rate not exceeding 20 "C h-l.The latter conditionswere essential for the formation of single crystals. Polycrystallinepowder samples were obtained using a similar heating cyclestarting with the binary nitride and chromium nitride (Cr2N,99%) powders. The high reactivity of the alkaline-earth-metalnitrides and the ternary nitrides towards water and oxygennecessitated that all manipulations of reaction mixtures andproducts be carried out in an argon-filled glove-box. Thereagents were mixed in the appropriate molar ratios to yieldthe correct nominal nitrogen stoichiometry and compressedinto pellets with a hand-press before being placed into stainless-steel crucibles which were then welded closed in an argonatmosphere.These crucibles were heated in a tube furnaceunder flowing argon to minimise external aerial oxidation at thehigh temperatures employed. The crucibles were cooled, cutopen in the argon-filled glove-box and the reaction productsremoved. Single crystals of each nitride were green, needle-likeand up to 0.5 mm in length. The corresponding polycrystallineproducts of each reaction were dark green powders.Initial characterisation of polycrystalline products by powderX-ray diffraction [using a Philips X'PERT (1 992) system]required the design and construction of a sample holder whichprotected the sample from the atmosphere (Fig. 1). ThisJ. Chem.SOC., Dalton Trans,, 1996, Pages 1-5 comprised an aluminium cylinder sectioned around part of itscircumference to allow the passage of X-rays to the sample. Theopen area was sealed with Mylar film which is transparent toX-rays but is relatively impermeable to oxygen and moisture.The front of the cylinder was machined to take a removablecover and an '0'-ring seal which allowed a standard samplemount to be loaded into the cylinder in the glove-box. The backface of the cylinder contained a plate which served precisely tolocate the sample in its correct position relative to the centreof the diffractometer, also as a means of mounting the cylinderonto the diffractometer. Using this sample chamber, powderpatterns of very air-sensitive materials can be recordedroutinely without any extraneous diffraction peaks from theMylar film.X-Ray diffraction patterns of both Sr,CrN, and Ba,CrN,showed them to be isostructural and to index in the hexagonalspace group P63/'m.For all the preparations carried out theproduct was found (by Rietveld refinement using the PC-RIETVELD PLUS program l 1 1 3 ) to contain a small amountof alkaline-earth-metal oxide (ca. 10 wt%) and chromiummetal (ca. 2 wt%). The presence of the oxide is probably dueto the high reactivity of the binary nitrides towards oxygenparticularly when finely ground, and the impurities thereforearise from the oxidation of unreacted starting materials.Structure determinationsThe unit-cell constants and space-group assignment wereconfirmed by oscillation and Weissenberg photographs takenof crystals sealed under argon in 0.2 mm diameter glasscapillaries.It was found necessary carefully to degas thecapillaries by heating for several hours under vacuum (ca.1 x lop5 Torr, ca. 1.33 x lo-, Pa) prior to loading them in theglove-box. Green crystals of dimensions 0.28 x 0.02 x 0.03(Sr,CrN,) and 0.19 x 0.02 x 0.04 mm (Ba,CrN,), sealed asabove, were used in the structure determinations. Data werecollected at 298 K on a Hilger and Watts Y290 four-circlediffractometer using graphite-monochromated Mo-Ka radi-ation (h 0.710 69 A). The structures were solved by heavy-atommethods in the space group P63/m (no. 176). The unit-celldimensions were refined using 25 independent reflections in therange 15 < 28 < 25".The structure determinations and full-matrix least-squares refinements were carried out using theCRYSTALS program l4 and absorption corrections were madeusing the DIFABS routine.15 The structures were solved using456 and 460 reflections respectively for Sr,CrN, and Ba,CrN,with F, > 30(F0). The function minimised in the least-squaresrefinements was Cw((F,I - IFc[), with w = o-,. Anisotropicrefinements of all atoms converged to R = 0.0192, R' = 0.0189for Sr,CrN, and R = 0.0347, R' = 0.0395 for Ba,CrN,. TheFig. 2 Structure of Sr,CrN, viewed as a projection on the ab plane.The unit cell is indicated by dashed lines. Small spheres represent Cr,medium spheres Sr and large spheres Ndata collection parameters are summarised in Table I andatomic positions are given in Table 2.Complete atomic coordinates, thermal parameters and bondlengths and angles have been deposited at the CambridgeCrystallographic Data Centre.See Instructions for Authors,J. Chem. SOC., Dalton Trans., 1996, Issue 1.Results and DiscussionThe unit cells of both compounds contain two planar triangular[MN3I6 - anions and six alkaline-earth-metal cations. Thecompounds are isostructural with Ba,FeN,6 consisting Ofeither Sr2+ or Ba2+ cations and discrete [MN,I6- units ofD,, symmetry in contrast to the situation in Ca,CrN3, inwhich the [MN,l6- units have C,, symmetry (Fig. 2). Inthe unit cell neighbouring [MN,I6- units are displaced byin z and the orientation of the triangle is reversed.Thestacking of the layers in these nitrides thus follows the se-quence [ e e e ABAB e e ] along the [OOl] direction (Fig. 3).A selection of bond lengths and angles for both ternarynitrides is given in Table 3.The three equivalent metal-nitrogen bond lengths withineach [MN,l6- unit are almost identical, being 1.728(3) 8, inSr,CrN, and 1.732(8) 8, in Ba,CrN,. These values are slightlyreduced as compared to the two shorter Cr-N distancesobserved earlier in the [MN,I6- unit in Ca,CrN, [1.766(7) A]and more significantly lower than the mean Cr-N distance[1.799(7) A] in the calcium material., The distances in thestrontium and barium 3 13 materials are also smaller than thosereported in the lithium ternary nitride Li15Cr,N, [1.760(8) and1.780(8) A] and the oxide nitride Lil,Cr,N80 [1.749(5) and1.767(6) A].l 6 The recently reported barium-chromium phase,Ba5CrN5,' with Cr" in tetrahedral co-ordination to N, alsohas two Cr-N distances [ 1.753(4) and 1.758(5) A] slightly longerthan those seen in the barium 313 composition. All of thesebond distances suggest that the metal-nitrogen bonds in thesecompounds are multiple since Cr-N single bonds areconsidered to lie in the range 2.00-2.25 A' in molecular solidsFig. 1used for air-sensitive samplesSchematic representation of powder X-ray diffraction holder Fig. 3the c axis. The unit cell is indicated by dashed lines. Key as in Fig. 2Structure of Sr,CrN, illustrating the packing of the layers along2 J. Chem. SOC., Dalton Trans., 1996, Pages 1-Table 1 Diffraction data for Sr,CrN, and Ba,CrN,Sr,CrN,a, c/A 7.724(2), 5.249( 1 )u p 271.2(2)DJg cm-, 4.368(2)p/cm-' 302.44 1 1Measured reflections 82 1Observed reflections 456Independent reflections 192F( 000) 318Largest difference peak/e k3 0.823R,' RId 1.92, 1.89Ba,CrN,8.201(2), 5.497(1)320.2( 2)5.247( 3)197.24221053460243426I .323.47, 3.95a Details in common: hexagonal, space group P63im; Z = 2; 28,,, 55";03-28 scans; octants collected khkl; parameters 15.' F, > 30(F0).W.,I - l ~ c l ) / ~ ( l ~ O l ) . (W"F,I - 1~,1)21/~(~1~,12)}~, K' = 0(FO)F2.Table 2 Positional parameters for Sr,CrN, and Ba,CrN,Atom (site)Sr/Ba (6h)xYCr (2d)xYN (6h)YYSr,CrN,0.354 55(6)0.268 93(7)0.250.666 670.333 330.750.434 7( 5 )0.3 15 8(5)0.75Ba,CrN,0.358 91(8)0.273 89(7)0.250.666 670.333 330.750.447 7(12)0.317 l(11)0.75Table 3 Selected bond lengths and angles in Sr,CrN, and Ba,CrN,Ca3VN30Ca3CrN3Ca3MnN31.762.5 2.6 2.7 2.8 2.9A-NIAFig.4 Plot of transition metal-nitrogen bond length, M-N, againstalkaline-earth-metal-nitrogen bond length, A-N, demonstrating thetwo structural regions of the 31 3 nitrides [orthorhombic, Cmcm, with[MN,I6 units of C,,, symmetry (circles) and hexagonal, P6,/m, with[MN,I6 units of D,, symmetry (triangles)]alkaline-earth-metal-nitrogen bond distances compare wellwith those observed in the previously reported strontium andbarium 3 13 materials. For example, Sr-N distances of between2.649( 13) and 2.797(9) 8, are reported in Sr3MnN3 and Ba-Ndistances of between 2.835(12) and 3.209(9) are reported inBa3MnN3.7 The Sr-N distances in Sr3CrN3 are somewhatlonger than those observed in the binary nitride, Sr2N, wherethe Sr-N distance is 2.61 18(3) 8, ( x 6 ) l o as compared to themean distance in Sr3CrN, of 2.739(1) A.This is commonlythe case for alkaline-earth-metal-nitrogen distances in ternaryand binary nitrides. A comparison of the Ba-N distance in theternary and binary nitrides cannot be made at this point sincethe crystal structure of Ba3N, is as yet unknown. Sr-NjA N-Sr-N/" Sr-N-Sr/" Sr-N-Cr/" Mean alkaline-earth-metal-nitrogen and transition metal-nitrogen bond lengths for 3 13 and 61 5 materials are shown in 2.6793(7) x 2 156.8(1) 156.8( 1) 99.85( 7)95.79(6) x 2 84.25(7) x 2 88.1(1) Table 4 together with the resulting M-N : A-N ratio for each 2.699( 3)2.778( 3) 83.19(7) x 2 96.81(7) x 2 85.6(1)2.858(3) 92.16(6) x 2 81.24(7) x 2 170.9(2) composition. Table 4 is divided into three clear regions for the66.2( 1) 173.8(1 1 3 13 nitrides: calcium nitrides with the smallest A-N distances,I 39.07(11)154.70(8)Cr-N N-Cr-N1.728(3) x 3 120 x 3Ba-NIA N-Ba-N/O2.820(2) x 2 154.1(3)2.866( 8) 97.4(1) x 22.905( 8) 84.1(2) x 23.160(7) 91.0(1) x 262.6(3)140.2( 2)157.2(2)Cr-NjA N-Cr-N/"1.732(8) x 3 120 x 3ioo.9ii j85.30(8)Ba-N-Ba/" Ba-N-Cr/"154.1(3) 101.7(1) x 284.6(2) x 2 89.3(3)95.9(2) x 2 88.0(3)79.4(1) x 2 170.8(4)177.4(3)99.8(2)82.8( 2)whilst the Cr-N triple bond is 1.57 , 4 1 9 (note also Cr-N distancesof 1.929 and 1.936 8, in Cr2N 2o and 2.070 A in CrN 21).Thetriangular units in the strontium and barium 313 compoundsare separated along z by a layer of three cations which gives alarge metal-metal separation of 5.250 8, in the strontiumcompound and 5.475 A in the barium compound. Both of thesedistances are considerably longer than the equivalent shortestCr Cr distance of 4.726 A observed in Ca3CrN3.3Strontium and barium are five-co-ordinate to nitrogen in therespective compounds with the alkaline-earth-metal cation in adistorted trigonal-bipyramidal geometry. Conversely nitrogenis at the centre of a distorted octahedron with alkaline-earth-metal cations at the axial and three of the four equatorialpositions and chromium at the remaining position.Thethe largest M-N distances and the smallest M-N : A-N ratio( ~ 0 . 7 ) ; strontium nitrides with medium A-N distances, smallerM-N distances and a ratio of ~ 0 . 6 3 ; barium nitrides withlarger A-N distances, similar smaller M-N distances to thoseof the strontium materials and similar ratios to those of thestrontium materials ( ~ 0 . 6 ) . It is worth noting that the M-Nbond lengths do not effectively change with M across the periodfor any given A (this may also be true for the 615 materialswhere M is a transition metal). Furthermore, the M-Ndistances are unchanged in all the hexagonal 3 13 materials evenwhen Sr is replaced by the larger Ba.By reducing the size of thealkaline-earth-metal cation sufficiently (for example from Sr toCa) the hexagonal structure undergoes a monoclinic distortionto give the lower-symmetry structure. The two 31 3 structuretypes, orthorhombic and hexagonal, can, therefore, betentatively defined in terms of the M-N : A-N ratios (Fig. 4).The tendency of the 313 materials to form one or other ofthe two structural types would appear to have implicationsregarding the symmetry of the [MN3I6- unit. It was suggestedthat the C,, symmetry exhibited by the [CrN3I6- unit inCa,CrN3 was due to a Jahn-Teller distortion of the low-spin( S = :) d3 Cr3+ ion in a trigonal-planar (D3,,) en~ironment.~The d2 V3+ ion, however, also formed a triangle with C,,symmetry and was shown to be diamagneti~.~ Later studiesshowed Mn3+ to form a similar [MN3I6- unit.5 Jahn-Tellerdistortion would clearly depend on the spin states of thetransition-metal ions and calculations have shown that themost stable configurations for the [MN3I6 - anions arise whenM is low spin.22 The crystallographic data herein have shownthat the equivalent strontium-barium-chromium 3 13 phases donot show any evidence of Jahn-Teller distortion at ambientJ.Chem. Soc., Dalton Trans., 1996, Pages 1-5 Table 4 Average bond lengths and A-N : M-N ratios in 31 3 and 61 5 materialsNit rideCa,VN,Ca,CrN,Ca,MnN,Sr,CrN,Sr,MnN,Ba,CrN,Ba,MnN,Ba, FeN,Ca6GaN5Ca,MnN,Ca6FeN5Av. A-NIA2.5642.535( 5)2.529( 2)2.739( 1)2.723(8)2.9 1 4( 5)2.893(9)2.887( 12)2.56( 1)2.569(2)2.56(2)Av. M-N/A1.819(6)1.799( 7)1.796(4)I .728(3)1.741( 13)I .732(8)1.737( 12)1.730( 12)1.95 l(28)1.757(4)1.770( 15)M-N : A-N0.709(2)0.7 1 O(3)0.7 1 O(2)0.631( 1)0.639(7)0.594(4)0.600( 6)0.599(7)0.76( 1)0.683(3)0.69( 1)Ref.435This work7This work76898Table 5 Site valences derived from bond-valence calculations for 313 and 615 phasesSite valenceNitrideCa,CrN,Ca,MnN,Sr,CrN,Sr,MnN,Ba,CrN,Ba,MnN,Ba, FeN,Ca3VN3Ca6GaNsCa,MnN,Ca6FeNsA1.64, 1.652.16(2), 2.20(2)1.83( l), 1.779(8)1.29(2)I .33(3)1.58(6)1.61(3)1.6(4)1.84(7)1.69( 1)1.68(4)M3.3 5( 6)3.47(7)3.67(5)4.17(3)4.2( 1)4.1(1)4.3( 1)4.2(1)2.2(2)4.1(1)4.1(4)N3.09, 2.643.13(5), 2.75(2)2.92(3), 2.83(4)2.68(2)2.75(8)2.96( 5)3.05(8)3.1(1)2.85(3), 2.5(1)2.94(1), 2.78(3)2.87(2), 2.7( 1)Ref.435This work7This work76898temperature. These nitrides form identical [MN3-J6 - unitsto those seen in the other strontium and barium materials,Ba,FeN,,6 Sr,MnN, and Ba,MnN,.7 It would appear thatthe symmetry of the trigonal-planar unit is therefore moredependent on the size of the alkaline-earth-metal cation(and hence the crystal symmetry) than on the d-electronconfiguration of the transition-metal ion.Bond-valence calculations 23 have been performed for both ofthe A,CrN, compounds using the bond-length parametersderived by Brese and O’Keeffe 24*2 for materials containinganions other than oxygen, fluorine and chlorine.The bond-valence parameters, R,,, taken for Ba-N, Sr-N and Cr-N were2.47, 2.23 and 1.85 A respectively. Calculations yielded sitevalences of 1.29(2) and 4.17(3) for Sr and Cr in Sr,CrN,and 1.58(6) and 4.1(1) for Ba and Cr in Ba3CrN,. Similarcalculations have been performed for all previously reported313 and 615 compositions and these are shown in Table 5.The site valences calculated for the strontium- and barium-chromium 313 phases are in close agreement with other 313phases containing barium or strontium. All the 313 materialshave consistently low alkaline-earth-metal site valences andconsistently high transition-metal site valences. The valuesdeviate significantly from the expected oxidation states in theA,MN, hexagonal materials (A = Sr or Ba).The uniformlyhigh transition-metal oxidation states are undoubtedly areflection of multiple M-N bonds, covalent in nature. Thevalues may also reflect some degree of compression in the M-Nsublattice. The A site valences range from x 1.3 to x 2.2 but areclosest to the expected valence of 2 in the calcium compoundsand nearer 1.5 in those of Sr and Ba. This may suggest an‘overstretching’ of the A-N sublattice or a lower formaloxidation state for A as suggested for Sr2N where bond-valencecalculations led to a similarly low value. lo Transition-metalvalences are highest in the hexagonal 313 materials and lieinvariably closer to 4 than 3 whereas the opposite is the case forthe orthorombic calcium compounds.No magnetic measure-ments have to date been performed on any of the hexagonalnitrides of Sr or Ba. It will be interesting to see what spin statesare inferred for the transition-metal ions in these compounds. Itis worth noting that the M site valences in Ca6FeN5 andCa,MnN,, both containing D,, [MN3I6- units, are alsoanomalously high. From the anion geometry in Sr,CrN, andBa,CrN, we would expect these materials to be paramagnetic.The compound Ca,CrN,, purported to contain low-spin Cr3+,exhibits an intriguing magnetic behaviour with temperaturesuggesting antiferromagnetic interaction^;^ Ca,VN,, however,is diamagnetic as might be e~pected.~ We are currently engagedin an extensive survey of the magnetic properties of 3 13 and 6 15materials and hope to relate these properties more fully to thecrystal chemistry of these nitrides.AcknowledgementsWe acknowledge the EPSRC and British Telecommunicationsplc for postdoctoral fellowships (for D.H. G. and S. E. S.respectively) and for their financial support of this work.References123456789101112F. J. DiSalvo, Science, 1990, 247, 649.N. E. Brese and M. O’Keeffe, Struct. Bonding (Berlin), 1992, 79,307.D. A. Vennos, M. E. Badding and F. J. DiSalvo, Inorg. Chem., 1990.29,4059.D. A. Vennos and F. J. DiSalvo, J. Solid State Chem., 1992, 98,318.A. Tennstedt, C. Rohr and R. Kniep, Z. Naturforsch., Teil B, 1993,48, 1831.P. Hohn, R. Kniep and A.Rabenau, 2. Kristallogr., 1991, 196,153.A. Tennstedt, C. Rohr and R. Kniep, 2. Naturforsch., Teil B, 1993,48,794.G. Cordier, P. Hohn, R. Kniep and A. Rabenau, 2. Anorg. Allg.Chem., 1990,591,58.D. H. Gregory, M. G. Barker, P. P. Edwards and D. J. Siddons,Inorg. Chem., 1995,34, 5195.N. E. Brese and M. O’Keeffe, J. Solid State Chem., 1990,87, 134.H. M. Rietveld, J. Appl. Crystallogr., 1969, 2,65.D. B. Wiles and R. A. Young, J. Appl. Crystallogr., 1981, 14, 149.4 J. Chem. SOC., Dalton Trans., 1996, Pages 1-13 C. J. Howard and R. J. Hill, Australian Atomic Energy CommissionReport No. MI 12, 1986.14 D. J. Watkin, J. R. Carruthers and D. W. Butteridge, CrystalsUsers Guide, Chemical Crystallography Laboratory, Universityof Oxford, 1993.15 N. Walker and D. Stewart, Acta Crystallogr., Sect. A, 1983,39, 158.16 A. Gudat, S. Haag, R. Kniep and A. Rabenau, 2. Naturforsch., Teil17 A. Tennstedt, R. Kniep, M. Hiiber and W. Haase, 2. Anorg. Allg.18 See, for example, R. G. Ball, B. W. Hames, P. Legzdins and19 J. T. Groves, T. Takahashi, and W. M. Butler, Inorg. Chem., 1983,B, 1989,45, 11 1.Chem., 1995,621, 511.J. Trotter, Inorg. Chem., 1980, 19, 3626.22. 884.20 S. J. Kim, T. Marquart and H. F. Franzen, J. Less-Common Met.,21 R. Blix, Z. Phys. Chem. (Munich), 1929,3,229.22 K. A. Yee and T. Hughbanks, Inorg. Chem., 1992,31, 1921.23 See, for example, I. D. Brown and D. Altermatt, Acta Crystallogr.,24 N. E. Brese and M. O’Keeffe, J. Am. Chem. SOC., 1991,113,3226.25 N. E. Brese and M. O’Keeffe, Acta Crystallogr., Sect. B, 1991,47,1990, 158,9.Sect. B, 1985,41, 244.192.Received 28th June 1995; Paper 5/04166JJ. Chem. SOC., Dalton Trans., 1996, Pages 1-5