J. MATER. CHEM., 1991, 1(1), 51-57 Single-crystal Conductivity Study of the Tin Dichalcogenides SnS2- $ex Intercalated with Cobaltocene Carl A. Formstone, Mohamedally Kurmoo, Emma T. FitzGerald, P. Anthony Cox* and Dermot O’Hare* Inorganic Chemistry Laboratory, University of Oxford, South Parks Road, Oxford OX 1 3QR, UK Single crystals of the n-type semiconducting tin dichalcogenides SnS2-,Se, (x=O, 0.3,0.5, 1.3, 1.85 and 2), which have a two-dimensional layered structure, have previously been intercalated with cobaltocene (CoCp2, Cp =q5-C5H5)to give the series of compounds SnS2 -xSex(CoCp2)o.33. Four-contact resistivity measurements have been carried out on these host and intercalate samples. The sulphur-rich intercalates (x=O.O, 0.3,0.7 and 1.3) were found to be semiconducting, whereas the selenium-rich intercalates (x= 1.85 and 2.0)were found to be metallic.These findings confirm earlier observations made in a photoelectron spectroscopy study. The resistivity of the semiconducting intercalates closely follows the functional form exp[( TolT)”4],characteristic of variable- range hopping. The values of To were in the range 3.4-3.7x lo8 K. The metallic intercalates show a transition to a superconducting state at 6.1 K for x=2.0 and 5.7 K for x= 1.8.In the diselenide case this has been confirmed by observation of a magnetic response appropriate to a type II superconductor below 6 K. This is the highest reported T, for a two-dimensional layered structure intercalated by an organometallic guest molecule. Keywords: Intercalation; Superconductivity; Cobaltocene; Variable-range hopping; Conductivity 1.Introduction Two-dimensional layered materials, such as the tin dichalcog- enides SnS,-,Se, (O<x<2), have been much studied with regard to their pronounced electronic and structural aniso- tropy.’?’ The SnS, -,Sex layered compounds crystallise in the Cd(OH),-type structure (space group Phl) to form an isostructural series of solid solution^.^ There are several other series of this kind, such as Tal-,W,S,(0~xIl), TaS, -,Se,(O I.xI2) and Zr, -,Sn,Se,(O Ix Il).4 The three- dimensional structure is built from repeatedly stacked XMX lamellae bound together by van der Waals interactions between adjacent planes of hexagonally close-packed chalcog- enide atoms (X).5 In SnX2 (X =S, Se) the metal atoms are co- ordinated in nearly octahedral sites.The layered MX, structure permits a variety of guest molecules to be inserted into the interlamellar gaps of the host materials.6 For example, the intercalation of metallic tantalum and niobium dichalcogenides by organic amines has led to the discovery of a new class of two-dimensional supercond~ctors.~This has raised questions as to the dimen- sionality of superconductivity in layered structures and the relationship between charge density waves (CDWs) and the superconducting BCS mechanisms.8 There are also examples of molecular crystals with a layered structure that exhibit superconductivity at low temperature, such as salts of bis(ethylenedithi0)-tetrathiafulvalene (BEDT-TTF).9 The importance of the process of charge transfer from the guest to the host in intercalation reactions has been recognised for some time.” In view of this, the possibility of ‘fine-tuning’ the electronic structure of the host material by intercalation has been utilised in several past studies.’ In fact, there are many examples of semiconducting layered structures being induced into a metallic state by intercalation of electron donor guest species, for example, LiTiS,,” Ko.5WS212 and Ko.5MoS2.13 Although a vast amount of effort has been expended on the characterisation of layered TX, materials (T =Ti, Mo, W, Ta, Nb; X=S, Se, Te) intercalated by hydrazine, organic amines and metallocenes,’ little attention has been paid to the intercalation of non-transition-metal dichalcogenides such as SnS, and SnSe,, especially in single-crystalline form.14y1 The synthesis of large single crystals (ca.2mmx 4 mm x 0.5 mm) of intercalated materials has often proved difficult. Given the poor kinetics of the general intercalation reaction,6 the majority of such reactions are only possible with microcrystalline host compounds. Consequently, single- crystal studies have been carried out mainly on the host16 rather than on the intercalate materials. Single crystals are particularly attractive for conductivity studies, which can give a great deal of information, especially when combined with other techniques such as solid-state photoelectron spectroscopy (PES).A prime example might be the extensive physical characterisation of single-crystalline phosphorus-doped SnS, and its cobaltocene (CoCp,) interca- late, S~S,(COC~~)~.,,, where Cp =q5-C5H5.17 Previous studies on the tin dichalcogenide hosts SnS, -,Sex have involved electrical measurements, such as the tem-perature dependence of the Seebeck coefficient or resistivity, on single-crystalline SnS2,18*19 SnSSe,” SnSo.7Sel,3,21 SnSo.lSel.921 and SnSe,.” The successful intercalation of cobaltocene (CoCp,) into the series of single crystals SnS2-,Se, (05x52) has been achieved in this laboratory.22 The widely differing properties of the host compounds coupled with their structural uniform- ity make them very interesting from an electronic viewpoint, especially in the intercalated form in which extensive electron transfer between the guest and host entities would be expected.Photoelectron spectroscopy has demonstrated a semicon-ductor-to-metal transition on moving from the sulphur-rich members to the selenium-rich members of the SnS,-,Se, (CoCp,),,,, intercalate series.” The aim of the present study was to confirm the differing electronic properties of these intercalate materials by using single-crystal four-contact a.c./ d.c. measurements in the temperature range 2-300 K. 2. Experimental 2.1 Preparation of SnS, -xSex Single Crystals The host single crystals SnSz-,Se, described in this work were all grown with the use of the iodine vapour transport method.’, The high-purity elements (>99.99%) with a 1YO molar quantity of phosphorus dopant and the transport agent Table 1 Growth conditions, appearance and c-spacing for SnS, -$ex temperature, composition T,,T,/"C growth time/h colour c-spacing/A SnS, SnS1,7Seo.3 685,645 670,630 12 48 orange red 5.928 5.953 SnS,,,,Se,,,, SnS,,,,Se,.,, SnSo.lsSe,,8, SnSe, 650,610 620,580 570,530550,510 48 100 50 72 dark red black black black 6.008 6.103 6.136 6.141 I, (5 mgcm-3) were sealed in evacuated quartz ampoules (10 cm x 1 cm).A three-zone furnace provided a stable tem- perature gradient between the reaction zone (7'') and the growth zone (T,) of the ampoule. Table 1 gives the growth conditions for some members of the series as well as the appearance24 and c-spacing of the crystals. The crystal structure was analysed using a Phillips PW 1710 powder diffractometer.The stoichiometry of the host crystals was determined using a JEOL FX 2000 analytical electron micro~cope.~~ The X-ray emission from microcrystal- line samples excited by a 200 keVt electron beam was detected and the end members of the series (SnS, and SnSe,) used as standards for the stoichiometry determination. 2.2 Synthesis and Characterisation of Intercalated Samples The synthesis and manipulation of the air-sensitive intercalate SnSz -xSex(CoCp2)o.33 single crystals were carried out under an atmosphere of dinitrogen. The acetonitrile solvent used was pre-dried over molecular sieves and then distilled over CaH,, followed by thorough degassing.The crystalline host (ca. 100 mg) was added to a solution of freshly sublimed CoCp, (ca. 150 mg) in acetonitrile (ca. 5cm3). In order to avoid damage to the brittle crystals, the reaction was carried out at ca. 65°C without stirring. After the reaction was complete, typically 5-21 days, the CoCp, solution was removed and the intercalate washed with aceto- nitrile (4 x 20 cm3) until the filtrate was colourless. The single crystals were then dried in vacua for several hours and characterised by X-ray diffraction under an N2 atmosphere within a sealed cell. The product crystals were deemed to be fully intercalated when all the host reflections had disappeared from the spectrum. Table 2 shows the lattice expansion, stoi- chiometries, appearance and reaction conditions for the intercalates produced in this study.The stoichiometry in each case was determined by elemental microanalysis. 2.3 Conductivity Experiment The air-sensitive intercalate materials were handled in a glove bag under an inert atmosphere. Under such conditions four contacts were attached to these samples, which had previously t 1 eV x1.602x J J. MATER. CHEM., 1991,VOL. 1 been cut into a parallelepiped geometry, typically 2 mm x 4mm xO.2 mm. A sample would then be sealed inside the specimen holder ensuring complete isolation from atmospheric oxygen and water. Conductivity data were, in fact, reproduc- ible over several days for a given sample sealed within the cryostat.The host crystals presented no special problems other than the necessity for gentle handling, in order that structural damage should be avoided. Colloidal Ag paint was used to attach the metallic contacts to the samples, the experimental results being identical when colloidal Au paint was used instead. This observation was taken as evidence that any reaction between the Ag paint and the chalcogen-rich crystal surfaces was unimportant. For all the samples measured, ohmic contacts were established within the chosen current range. Samples were occasionally rejected if cracks were noticed in them during routine investigation under a microscope. Resistance measurements were taken with a Hewlett-Pack- ard HP 3478A multimeter interfaced to a RML 3802 micro- computer, which managed the experiment over a wide range of temperature (2-300 K) in conjunction with an Oxford Instruments 3 120 temperature controller.The temperature changes within an Oxford Instruments cryostat were brought about as slowly as possible to allow equilibration of the temperature within the sample holder unit. The measurements were all four-contact in type, but only the SnSo.15Se,.85 and SnSe, host and intercalate crystals were studied using an a.c. method (15 Hz), which required the use of a Brookdeal 9503 'lock-in' amplifier. 3. Results and Discussion 3.1 Experimental Data for the Host Materials Fig.1 presents a plot of log,,(resistivity/n cm) versus tempera-ture/K for the entire series of host single crystals (phosphorus doped) SnS,-,Sex, where x=O.O, 0.3, 0.7, 1.3, 1.85 and 2.0.This plot is intended to give an impression of the wide variation in conductivity behaviour in the host structures as sulphur is replaced by selenium. A closer examination of the data presented in Fig. 1 suggests that the host materials can be divided into two distinct classes on the basis of their resistivity variation with temperature. For class I (x=O.O, 0.3, 0.7 and 1.3) the resistivity spans several orders of magnitude in the temperature range 100-300 K. For class I1 (x = 1.85 and 2.0) the resistivity decreases in the range 298 to ca. 150 K then increases down to 2 K. The lowest resistivity corresponds to 140 and 160 K for x= 1.85 and x=2.0, respectively. Materials in the first class (I) seem to be well described by an Arrhenius-type model for conductivity.Fig. 2 gives a logarithmic plot of resistivity us. inverse temperature for SnS,-,Se,, where x=O.O, 0.3, 0.7 and 1.3. The activation energies (E,) decrease steadily from 0.45 eV (x=O.O) to 0.09 eV (x= 1.3) across this series. The resistivity versus temperature plots for SnSz -,Sex, Table 2 Stoichiometries, reaction conditions, lattice expansion and appearance for the intercalated layered materials [host(CoCp,),] reaction conditions host temp./"C time/da ys stoichiometry (y) Ac/A colour SnS, 65 5 0.31 5.46 dark blue SnS1.,Se0.3 65 7 0.31 5.24 light blue SnS1sSe0.s 65 9 0.31 5.20 light blue SnS0.7Se1.3 65 14 0.33 5.26 black SnSo.15Se1.85 65 17 0.33 5.50 black SnSe, 65 21 0.33 5.56 black J.MATER. CHEM., 1991, VOL. 1 6*o 1 5.0 4.0 E 3.0 x c.'.-.-2> 2.0.-fn 1.0 0,-53 0 -12 1 o.loo/\ (b)/ 5 0.osol -2.0 1,0.0401-77 155 233 3 10 temperature/K Fig. 1 Plot of log (resistivity/R cm) us. temperature/K for the host single crystals, SnS, -,Sex, where 01x 2. (a) SnSe,; (b) SnSe1.85S0.15; (c) SnSe1.3S0.7 (d)SnSe0.7S1.3; (e)SnSe0.3S1.7;(f)SnS2 6-o 3 0.002 0.005 0.008 0.010 0.013 temperature- '/K -Fig. 2 Plot of log (resistivity/R cm) us. K/temperature for the host single crystals, SnS2-,Se,, where x=O.O, 0.3, 0.7 and 1.3. (a) SnSe1.3S0.7;(b)snse0.7s1.3; (c) SnSe0.3S1.7;(d)SnS2 where x= 1.85 and x=2.0, are given in Fig.3. The materials in the second class (11) can be modelled by considering them be very low activation energy semiconductors (E, z lo-, eV), so that the carrier mobility term (p) becomes important com- pared to the carrier concentration (n) at elevated tempera- tures (T>150 K). This would account for their metallic-like behaviour in the range 150-300 K. An attempt has been made elsewhere to model the resistivity us. temperature behaviour of an organic conductor, (NMP),TCNQ, according to:26 p(T)= l/(nep)=AT"exp(E,/kBT) (34 where E, is the activation energy and a is the mobility factor, relating to the type of scattering mechanism in operation. Fig. 3 presents an attempt at the best fit for the resistivity uersus temperature (100-300 K) experimental data with theor- etical data derived from eqn.(3.1) (solid line). Consequently, an estimate can be made for the activation energies (E,) and mobility factors (a) of these class I1 materials. Table 3(a) summarises the resistivity (p) at 298 K, the mobility parameter (a) and the activation energy (E,) for the host crystals SnS, -,Sex. 1 0.020 1,90 147 20 5 262 320 temperature/K Fig. 3 Plot of resistivity/R cm us. temperature/K for the host crystals SnS,-,Se,, where x= 1.85 and 2.0. (a)SnSe,; (b) SnSel,,,So,ls 3.2 Nature of the Host Material Conductivity Phosphorus doping was initially adopted as a means to produce SnS, samples of sufficient conductivity for X-ray and ultraviolet photoelectron spectroscopy (XPES and UVPES) studies.,, The phosphorus-doped SnX, samples are all n-type semiconducting on the evidence of this PES study.Electro- chemical measurements have shown P-doped SnS, electrodes to be n-type semiconducting on the basis of electrochemical impedance studies in aqueous sol~tion.~~*~* The undoped SnSo.7Sel.3, SnSo.l Se,., and SnSe, single crystals are n-type semiconducting according to previous Hall and Seebeck coefficient measurements.21 It is interesting to speculate as to the nature of the phos- phorus doping in these materials. One might imagine that P atoms are able to substitute for Sn atoms in the individual XSnX sandwich structure. This would be analogous to the situation found in the layered metal thiohypophosphate com- pounds, M2P2S629 (M=Fe, Ni, Mn, etc.), in which co-ordination sites between the sulphide layers are occupied by metal atoms (M) or P2 units.On the other hand, one might consider the possibility of P atoms occupying interlamellar positions, a situation that arises in the layered compound Po.2VS2.30 In both these cases P atoms would be acting as electron donors in accordance with the n-type semiconductiv- ity of the SnX, host materials. In support of the latter possibility, it was noticed that undoped SnS, was intercalated by cobaltocene in 1,2-dimethoxyethane (DME) solution at room temperature much less rapidly than P-doped SnS, under the same conditions. This could be explained by the presence of intersandwich P atoms disrupting the interlayer van der Waals bonding, thereby allowing cobaltocene molecules easier access to the interlamellar spaces.The experimental conductivity data presented above show a gradually decreasing activation energy in these host materials (Table 3) from SnS, (0.45 eV) to SnSe, (0.04 eV) via SnS0.,Se,., (0.09 eV). This is to be expected on the basis of Mott's impurity model for doped semic~nductors,~' which predicts that as the medium becomes more polarisable the energy required to ionise impurity electrons from the donor levels into the host conduction band will decrease. No work has been done to estimate the level of P doping throughout J. MATER. CHEM., 1991, VOL. 1 Table 3 Summary of important resistivity data on hosts and intercalates (4hosts (x) p/Qcm at 298 K E,/eV 0.0 387.6 0.45 0.3 7.67 0.37 0.7 2.87 0.28 1.3 1.21 0.09 1.85 0.114 0.05 2.0 0.054 0.04 0.0 3.90 1.54 0.3 38.6 7.48 0.7 69.2 9.77 1.3 61.7 12.5 1.85 1.1 x 2.0 1.1 x the series, but an estimate of 1015 cm-3 for the carrier density in P-doped SnS2 has been made.28 The conductivity behaviour of the so-called class I1 host materials (x= 1.85 and 2.0,Fig.1) can be interpreted in terms of the carrier mobility factors (p)in semiconductors with small activation energies (E, =lo-, eV). The model presented on the basis of eqn. (3.1) seems to work well above 100 K, but below this temperature there is some deviation possibly arising from additional conduction via impurity sites.It has previously been demonstrated that in two-dimen- sional layered systems the carrier mobility (p)is highly tem- perature dependent:, ’ P( Tl )/AT2)=(TIIT2)-“; a>1.5 (3.2) This strong temperature dependence above ca. 100 K has been related to an optical phonon scattering mechanism. This is unique to two-dimensional layered materials, since the carriers are confined to individual XMX layers with mainly short-range interactions coupling the carriers to the optical modes of the lattice. These vibrational modes involve modu- lation of the XMX sandwich thickness in layered materials. In this study the exponent (a)in the mobility temperature dependence expression in SnS2 -,Sex crystals was found to be a= 1.70 and 1.72 for x= 1.85 and 2.0, respectively.This correlates well with theoretical predications and other exper- imental data on layered systems,, suggesting that scattering of conduction electrons at T>75 K may well be related to this mechanism in these particular SnS, -xSex hosts. 3.3 Experimental Data for the Intercalate Materials As with the host materials it is convenient to divide the intercalate compounds into two distinct groups. Consider first the mainly sulphur-rich intercalate single crystals SnS2 -,Sex (CoCp,),.,,, where x=O.O, 0.3, 0.7 and 1.3. For these samples log resistivity is plotted against Tp1I4in Fig. 4 in order to demonstrate the excellent agreement between the experimental data and the Mott variable-range hopping (VRH) law.Least- U ----1.70 1.72 3.67 4.0 3.42 3.9 3.60 3.9 3.64 3.9 --5.00-4.00-9> c..-2 3.00-.-UJ2 Y 0 2 2.00-1.00-0.001 I I 1 , I I I , I I I 1 I 1 1 1 1 I I , 0.22 0.24 0.26 0.28 0.30 0.32 temperature-1’4/K”’4 Fig. 4 Plot of log (resistivity/Q cm) us. temperature-’’4/K-1/4 for the intercalates SnS, -xSex(CoCp2)o,31, where x =0.0, 0.3, 0.7 and 1.3. (a) SnS2{Co(Cp)2}; (b) SnSe0.3S1.7{Co(Cp)2}0.31;(c) SnSe0.7S1.3 {cO(cp2}0.32;(d) SnSel .~s0.~{c0(cP)2}0.31 intercalate materials a much reduced anisotropy is observed. (P 11 /PI =10).The resistivity versus temperature variations for the SnS2 -,S~,(COC~~)~~,~ compounds, where x = 1.85 and 2.0, are given in Fig.5 and 6, respectively. On cobaltocene inter- cala tion the room- temperature single-cr ystal conductivity increases significantly in both cases (see Table 3). On cooling these samples the resistivity decreases as expected for metallic samples. At 5.7 K (x = 1.85) the resistivity drops sharply (width 1.5 K)as in Fig. 5. At 6.1 K (x=2.0) a similar transition (width 0.7 K) is observed with the resistivity falling to zero as in Fig. 6. Measurements on several different samples indicate that these transitions generally take place in the range 4.8-5.7 K in the SnSo,15Sel.85 intercalates and 6.1-6.5 K in the SnSe, intercalates. Within this variety of samples were several crys- tals of undoped SnSe2 intercalated with cobaltocene.The absence of phosphorus seemed to make no difference. squares fitting of the resistivity data to the expre~sion~,-~~ P =PornTo)”2exPC(To/T)”1 (3.3) gives o=0.25f0.02. The values of p at 298 K,o,po and Tofor each composition are given in Table 3. Notice that the room-temperature resis- tivity increases for x=0.3, 0.7 and 1.3 upon intercalation, whereas for x=O.O the opposite is true. Experiments have demonstrated that there is considerable anisotropy in the host single-crystal conductivity (pll/pl =loo), whereas in the J. MATER. CHEM., 1991, VOL. 1 1.2x10-3 1 7 18, 141 9.0 x 0 77 155 233 310 temperature/K Fig. 5 Plot of resistivity/R cm us. temperature/K for the intercalate SnS0.15Se1,85(CoC~2)0.33 0 77 155 233 310 temperature/K Fig.6 Plot of resistivity/R cm us. temperature/K for the intercalate SnSe2(CoCPz)o33 The superconductivity has been confirmed in the diselenide case by magnetic susceptibility measurements, which have demonstrated the Meissner effect below 6 K appropriate to a type I1 supercond~ctor.~~After cooling to 4.2 K in zero applied magnetic field, an initial diamagnetic response was observed as in Fig. 7. At 4.2 K the magnetisation falls to a low value above 300 G; the presence of flux trapped within the sample is shown on returning to zero applied field. Below 3 K extensive hysteresis of the magnetic moment is observed. Table 3(b) summarises the data for the metallic intercalates (x= 1.85, 2) giving the resistivity at 298 K and the supercon- ducting transition temperature (T,).Finally, in order to illustrate clearly the difference in electrical properties of the intercalates at either end of the series, Fig. 8 gives the log,, resistivity uersus temperature variation for the entire intercalate single crystal series. The clear semiconductor-to-metal transition appears to occur in the stoichiometry range 1.3<x<1.85. 3.4 Intercalate Conductivity Data Previous UVPES measurements indicate that a transition from semiconducting to metallic character occurs on passing -750 -500 -250 0 250 500 7 field/G 2.00 4.00 6.00 8.00 10.00 Fig. 7 Plot of molar magnetisation us. applied magnetic field for the1.o x10-5 1,diselenide (x 2.0) intercalate at 2.5 K35= 0 77 155 233 310 ternperature/K Fig.8 Plot of loglo(resistivity/R cm) us. temperature/K for the intercalate single crystals, SnS, -,S~,(COC~~)~.~~, where 0 5x I 2. (') SnSel .8SS0.1dc0(cP)2)0.3 1; (b) SnSe,(Co(Cp)Z}0.3 1; (c) SnS2{c0(cP)2 )O.31; (d) SnSe0.3S1 .7{c0(cP)2}0,31; SnSe0.7Sl .3{c0(cP)2}0,31; (f) Snsel ,3s0.7{c0(cp)2~0.31 through the series of organometallic intercalates SnX, (COC~,),,,,.~~The present conductivity study has confirmed this observation, but at the same time several other points of interest have emerged. First, a change in conduction mechan- ism has been observed on going from the host to intercalate semiconductors (x=O.O, 0.3, 0.7 and 1.3), and, secondly, there has been an observation of superconductivity in the metallic members (x = 1.85 and 2.0) of the intercalate series.For the semiconducting SnS2 -xSe,(CoCp2),, intercalates (x=O.O, 0.3, 0.7 and 1.3) it has been established that a Mott VRH law can be successfully applied to the experimental resistivity data as in Fig. 4. Previous XPS experiments2, indicate that these intercalate materials are mixed valent in both tin and cobalt. Upon intercalation roughly 10% of the Sn" sites in the host materials are reduced to Sn" and roughly 66% of the cobaltocene molecules are oxidised (Co" to Co"'). This can be interpreted by considering each cobaltocene molecule to be an electron donor impurity sitting adjacent to a tin acceptor atom with chalcogen atoms separating them.A rigid band model, which would predict a partially filled Sn(5s, 5p) acceptor band,12 does not seem to apply in these semiconducting sulphur-rich cases. There must be strong localising influences affecting the ionised impurity electron, such as the distortion of the Sn-X bond lengths upon Sn reduction and the presence of an attractive [CoCp,] poten-+ tial. In fact, the UVPES data provided evidence for a localised band (Sn 5s2) situated just below the empty conduction band in the intercalates (x I1.3). An Anderson model,36 similar to that used to interpret the electronic structure of disordered solids, might be applicable to these intercalate materials. Evidence from crystallographic studies suggests that the cobaltocene molecules are close- packed between the SnX, layers.37 However, disorder may arise in several different ways; for example, there may be an uneven distribution of chalcogen atoms in the SnX, layers.However, recent STM investigations have revealed that the chalcogen distribution may be random in WSSe single-crystal solid solutions.38 Nevertheless, the energy of Sn 5s2 states may well be scattered over a small energy range to give an Anderson band as in amorphous semiconductors. The Fermi level (EF) may be situated above the mobility edge in these intercalate (x=O.O, 0.3, 0.7 and 1.3) systems with the Sn 5s2 states localised in the band-gap region as envisaged in the Anderson localisation model. The mixed valency observed in the intercalate XPS data2, was interpreted as involving a hopping of impurity electrons between the cobaltocene and tin sites, the XPS timescale being very fast relative to the timescale of the hopping process.A mechanism of thermally activated hopping conduction between the cobalt and tin sites seems entirely reasonable on the basis of this discussion. The experimental data give an extremely good fit to the exp[( T0/T)'/4] dependence. The hopping mechanism in 6 dimensions yields a T''('+@ expre~sion,~~so the intercalate experimental data strongly suggest an isotropic three-dimensional (T-'I4) hopping pro- cess rather than a two-dimensional (T-1/3) process. The experiments carried out to investigate the anisotropy of con- duction in these intercalate systems suggest that the current carriers are not confined to a single layer to such an extent as in the host systems.The higher resistivity of some of these intercalates relative to their host compounds (see Table 3), despite substantial electron transfer to the Sn atoms, may depend on the limiting nature of the thermally activated hopping process rather than the presence of a large energy gap. For the SnSo.15Sel.85 and SnSe, hosts there is a dramatic increase in conductivity upon intercalation; a change from a semiconducting to a metallic conduction mechanism is also observed (see Table 3). The impurity band in these intercalates may now overlap the conduction band to some extent to give metallic character. Moving through the series towards the selenium-rich materials, the increasing conduction band width, the increasing screening effect of the chalcogenide layers and the increasing degree of band filling are all factors that may contribute to this changeover. An Anderson (insulator-to- metal) transition provides a good model for the electronic changes in these intercalate systems.Within this scheme the mobility edge is assumed to move past the Fermi level, so that states at the bottom of the impurity band are still localised, whereas states at the top of the band are considered to be delocalised. A rigid band model still seems inadequate even for the metallic intercalates, as this would not explain the observation of mixed tin valency in SnSe2(CoCp,),,,, by xPs.22 The anomaly in the resistivity versus temperature behaviour of the diselenide intercalate (Fig.6) may be related to some sort of structural phase transition taking place at ca. 100 K. The periodic lattice distortions (PLDs) that occur in some low-dimensional metals are driven by strong electron-phonon coupling interactions. In metallic 2H-TaS, and 2H-TaSe2. anomalies are found to arise in the resistivity data39 consistent J. MATER. CHEM., 1991, VOL. 1 with the formation of superlattices in these low-dimensional systems. Similar anomalies have been observed in systems such as the charge-transfer salt K-(BEDT-TTF),CU(NCS),.~ In the tin dichalcogenide intercalates the superconducting transition temperature (T,)increases as sulphur is replaced by selenium. This change can be understood in terms of the enhanced degree of electron charge transfer to the empty Sn(5s, 5p) conduction band as suggested by XPS data.7 This can be related to the increasing polarisability of the medium as selenium is added, since screening of the ionised electron from the [CoCp,] attractive potential becomes more effect- + ive.This would probably lead to a greater value in N(EF)for the pure diselenide relative to the other case, which would be expected to lead to a decrease in the T, value as predicted by BCS theory. Previous XPS data2, have indicated that unoxidised cobaltocene is in electronic equilibrium with the oxidised form in the SnX, intercalates. The superconductivity does not seem to be destroyed by the presence of the paramagnetic cobaltocene molecules between the SnX, layers. This can be compared to the TaS2(CoCp2)o.25 and TaS2(CrCp2)o.25 intercalates in which T, is roughly the same (ca.4 K) despite the presence of diamagnetic [CoCp,] and paramagnetic + [CrCp,] cations between the TaS, layer^.^ Local magnetic + moments are generally thought to be responsible for the breaking of Cooper pairs in a spin-disorder scattering mechanism. 4. Conclusion For this work it was essential to prepare high-quality single crystals of the layered tin dichalcogenides SnS, -xSex interca- lated with cobaltocene. The variation of the sulphur and selenium content has produced an interesting change in electronic structure (semiconducting to metallic) through this intercalate series.The main result of this work has been the successful confirmation of this transition, which was originally observed using photoelectron spectroscopy.22 The resistivity measurements for the semiconducting intercalates obey an exp[( T,/T)]1/4 temperature dependence. This suggests that these intercalated materials may conduct via a variable-range hopping mechanism, in contrast to their respective host compounds that obey the Arrhenius law. The metallic intercalates have also proved to be extremely interest- ing, not least in the sense that the SnS,,,,Se,~,, and SnSe, host materials are themselves semiconducting. Such semicon- ductor-to-metal transitions induced by intercalation are uncommon,12 but it is very rare for the resulting metal to be superconducting (T,z6 K).Another example is the metal KxMoS, (T,z7 K), one of a family of intercalates produced by alkali-metal insertion into the semiconducting MoS2 host.13 However, the present case is important as it involves the intercalation of an organometallic species into a layered structure with paramagnetic CoCp, molecules sitting in the van der Waals spaces. Further work will concentrate on the superconducting materials. 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