Transition metal chalcogenides: new views on an old topic

Journal of

A~OY5 AND COM~D~DS ELSEVIER

Journal of Alloys and Compounds 219 (1995) 73-82

Transition metal chalcogenides: new views on an old topic Wolfgang Tremel *, Holger Kleinke, Volkmar Derstroff, Christian Reisner lnstitut far Anorganische Chemie und Ana?ytische Chemie, Johannes Gutenberg-Universitiit Maim, Becherweg 24, D-55128 Maim, Germany

Abstract

In this contribution we discuss a group of layered tellurides with modular structures (tinker toy phases) and possible low temperature routes for the synthesis of chalcogenides with porous structures. Layer compounds of general composition [(M2Te2)(ATe2)](MTe2)n and related phases have been synthesized as main group counterparts of layered metal-rich early transition metal tellurides. Special features of these compounds are (i) the unusual square planar Te coordination of the main group atoms Ga, Si, and Ge and (ii) their modular structure based on four building blocks. Similarly to layered metalrich early transition metal tellurides these phases are electronically stabilized by extensive bonding between early transition metals and main group "heteroatoms". Size effects are important for the structural stability of the tinker toy phases; attempts to substitute A = Ga by the group homologues B or In lead to the formation of alternate phases such as Ta4BTe8 containing metal clusters with interstitial atoms or intercalate phases such as InxNb3Te4. Reactions in thiophosphate fluxes and under solvothermal conditions have been explored in order to synthesize materials with microporous structures. The formation of (poly)thiophosphates such as K2MP2S7 (M = V, Cr) and I~TizP6S25 shows that reactions in molten thiophosphates are controlled by the redox equilibria and the basicity of the flux. The reaction conditions prevent the use of templates which are needed for the formation of compounds with porous structures. Reaction of telluroarsenates with Cr(CO)6 under solvothermal condition, however, results in the formation of a unique polytelluride [Cr(en)3][Te6] with a microporous structure. Solvated Cr(en)33÷ cations serve as templates in the synthesis of this material. Keywords: Early transition metal chalcogenides; Low-dimensional compounds; Cluster compounds; Flux reactions; Thiophosphates; Solvothermal reactions; Microporous structures

1. Introduction

Transition metal chalcogenides have attracted wide interest during recent decades primarily because of their interesting structural chemistry, unusual electronic properties and rich intercalation chemistry, which put this class of compounds at the interface of chemistry, mineralogy, solid state physics and materials science [1]. Pioneering investigations on superconductivity and charge density waves have been performed on this class of compounds [1,2]. The concept of tailoring materials properties by intercalating guest species in the van der Waals gap of layered chalcogenides has triggered numerous investigations [3]. Extending the idea of intercalating transition metals into layered sulphides and selenides to the corresponding tellurides gave a n u m b e r of surprises. Firstly, transition metal derivatives of ditellurides do not form intercalates M~'MQ2 (M' = 3 d transition metal, M = Nb, * Corresponding author.

Elsevier Science S.A.

SSDI 0925-8388(94)05064-3

Ta (1)) as known for the sulphides and selenides [3], but metal-rich layer compounds (2) such as M M ' T e / ( M ' = F e , Co, Ni; M = N b , Ta) are obtained instead [4]. Although the reason for the formation of these compounds is not fully understood it is clear that their electronic stability is based on strong bonding interactions between early and late transition metals. The reaction of main group metals such as Sn, Pb ~3r Bi with early transition metal dichalcogenides yields the "misfit" layer compounds (AQ)I +x(MQ2), (A = Sn, Pb,

x@ x @ x @ x

X

X

X

X

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I4:. Tremel et al. / Journal of Alloys and Compounds 219 (1995) 73-82

II II

Ln; n = 1-3) [5], whose structures contain NaCl-type AQ slabs alternating with (MQ2)n sandwiches (3). We considered whether, by analogy with the metal-rich layer compounds mentioned above, new niobium and tantalum compounds could be synthesized whose structures are stabilized by extensive bonding between early transition metals and main group elements. A special impetus for our investigations emanated from studies of centred zirconium and lanthanide halide clusters by Corbett and co-workers [6].

Fig+ 1. View of a single layer of M2ATe+ down [001]: o, A atoms; ©, M atoms; Q , Te atoms.

2. Layer compounds with modular structures [(M2Te2)(ATe2)](MTe2). (A=Ga, Si, Ge; 0 ~ n ~ l ) and related phases

2.1. Linear variants of the tinker toy phases [(M2 Te2) (A Te2)](MTe2)n All materials of this family can be obtained from the elements by conventional high temperature reactions (800 °C< T< 1000 °C) in evacuated sealed silica tubes with halogens as transport reagents. All compounds crystallize in layered structures based on an (AA)(BB) or (AA)(BB)(CC) stacking of Te atoms as found in the MoS2 structure. The metal atoms fill trigonal prismatic voids in an ordered fashion, whereas the main group atoms are situated in square planar Te coordination at the common faces of two adjacent trigonal prismatic sites. In the parent structure of this series with composition M2ATe 4 (M = Nb; A = Ga, Si; n = 0) [7], which is shown in Fig. 1, the two metal atoms form

a metal-metal bond (dM-M= 2.9 A) across the rectangular faces of two adjacent prisms (4). The unusual square planar Te coordination of the main group element is stabilized by bonding interactions to the neighbouring transition metal atoms. It is important to note that each A atom is surrounded by four metal neighbours in order to achieve a maximum stabilization of its unusual square planar Te environment.

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A comparison of the mean A - M separations (2.852 /~ for Nb2GaTe4 and 2.794 ~ for Nb2SiTe4) and the M - M distances (2.933(11) ~ for Nb2GaTe4 and 2.889(8) /~ for Nb28iTe4) with the difference of 0.27/~ in the covalent-ionic radii of Ga and Si and the mean Ga-Te and Si-Te distances (0.12/~) indicates that the M-M and A - M and A T e interactions are equally affected by the different valence electron concentration (VEC). In addition, the Te-Te distances of 3.392 /~ for Nb2GaTe 4 and 3.428/~ for N b 2 S i T e 4 a r e found across the shared edges of the M containing prisms perpendicular to the M-M bond. These distances, substantially shorter than the sum of the Te van der Waals radii, are consistent with a further partial oxidation of the Te and a concomitant reduction of the Nb atoms. Finally, the decrease in the mean interlayer Te-Te distance from about 3.70/~ (NbESiTe4) to about 3.64 /~ (Nb2GaTe4) is compatible with increasing interlayer bonding and further Te oxidation in the compound with the smaller VEC. The electronic structure of Nb2SiTe4 (and NbEGaTe4) can be rationalized in a first approximation from that of the two structural motifs, an [(Si°)(Te2-)4]8- unit (5) (isoelectronic to XeF4) and the [(Nb 4+)2(Te 2-)8] 8entity 4. The frontier orbitals of both fragments are well known. 5 possesses two filled lone pair orbitals which can act as electron donors. 4 has a sequence of six molecular orbitals (tr, ~', 3, ~*, 7r*, and tr*, 6), of which, in agreement with the assumed metal-metal bond, only the tr orbital is occupied. The remaining ~and ~ type orbitals are empty and have the proper directionality for donor-acceptor interactions with the lone pairs of 5, i.e. the metal atoms are formally reduced and the main group atoms are oxidized. Since each Nb 4+ atom is accepting electrons from the Si° atoms

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Fig. 2. (a) DOS for Nb2SiTe4: - - -, total DOS; - - , Nb contribution. (b) COOP, - - , Nb-Te; - - - , Nb-Nb; - . - , Nb-Si. The Fermi level corresponding to Nb2SiTe4 and Nb2GaTe4 is marked by horizontal lines.

a reasonable assignment of oxidation states is (Nb3+)2(Si2+)(Te2-)4. This is in agreement with the relative electronegativities of Si-Ge and Nb-Ta, and the computed 3s2 (or 4s 2) configuration of the main group atoms in the M2ATe4 type compounds (however, one should be careful in pushing this formalism too far, as similar reasoning leads to a formal oxidation state of B + in Ta4BTes (vide infra) which is not realistic). This simple picture is reflected in the density of states (DOS) and the corresponding crystal orbital overlap population (COOP) diagrams for Nb2SiTe4 in Fig. 2. The 3s states of Si (outside the energy window) are filled. The Nb-Nb bonding states which are confined to a region of approximately 1 eV below the Fermi level indicate localized metal-metal bonds within the units 4. Nb-Nb bonding contributions in the energy window between - 1 5 and - 1 6 eV point to through bond coupling by the Te ligands. The Si-Nb bonding contributions are localized in the same energy interval and comparable in size with Nb-Te bonding. Most intralayer and interlayer Te-Te interactions are weakly

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antibonding for Nb2SiTe4. As a general statement, Te-Te bonding (especially interlayer interactions) increases for the more electron deficient NbzGaTe4. According to the computational and experimental results NbzSiTe4 is semiconducting and electronically saturated, whereas its Ga analogue is predicted to be metallic. Using a variant 7 of the building block 4 and considering further that a sufficient electronic stabilization of the square planar coordinated main group atoms is possible only if the maximum number of four metal neighbours is supplied, a whole series of layer compounds with modular structures of linear type and the general formula [(MzTez)(ATez)](MTe2), ( A = G a , Si, Ge; 0~
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3, 0).

abab-type

abcabc-type

Fig. 4. Schematic representation of two possible isomers of the tinker toy phases M2ATe4 and M3ATe6.

pounds [(M2Te2)(ATe2)](MTe2), generates members of an infinite adaptive series with the limiting compositions M2ATe4 and M 3 A T e 6. The characteristic features of the members of this series are that building blocks 7 with isolated metal atoms are "thinned out" when we move from one end (M3ATe6) to the other (M2ATe,). The boundary members, [(M2Te2)(ATe2)](MTe/)I/2 and [(MzTe2)(ATe2)](MTe2)I/3, as well as two other members of this series are shown in a schematic fashion in Fig. 3; other compounds have been characterized most recently [8]. Interestingly, structural isomers are possible. Alternative structures for M2ATe4and M3ATe6 are shown in Fig. 4. Although the first and second nearest neighbour environments of A and M atoms are identical, the connectivities of the building blocks 5, 4, and 7 are different. Why are the isomers in Fig. 4 not observed? The infinite parallel arrangement of motifs shown in Fig. 4 is not compatible with the presence of compressed and elongated building blocks

W. Tremel et al. / Journal of Alloys and Compounds 219 (1995) 73--82

77

5 and 4 as observed in the real structures of M2ATe4 and MaATe6.

2.2. Cyclic variants of the tinker toy phases The building block 7 is composed of two simpler fragments, an isolated metal atom in trigonal prismatic coordination (8) and an empty trigonal prismatic site (9). Considering these simple trigonal prismatic fragments as building blocks for layer compounds with modular structure, new combinations, e.g. 10 and 11, which satisfy the boundary condition of having four metal neighbours for each main group atom in square planar Te coordination, are possible. One of them, 11,

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a cyclic variant of the tinker toy phases with composition [(TaTe)(I-qTe)5(GaTe2)3(Ta2Te2)6]--- Ta13Ga3Te24, has been synthesized [9]. Again, the modular structure principle of these materials based on only four building blocks 5, 4, 8 and 9 is immediately apparent. 8 with an isolated metal atom is surrounded by three empty prisms 9. These prisms share two rectangular faces with the double prisms 4. The common Te atoms of the pairs of 4 are now bonded to four (instead of three) metal atoms; this leads to a composition with a higher metal content than in the linear variant of the modular layer phases. Fragment 5 fills the void space between the double prisms; the "coordination" of the main group atoms within 5 is completed by two metal containing blocks 4. This triangular assembly propagates in plane through two empty prisms. The bonding description given for the linear variants of the "tinker toy phases" is valid for 11 as well. The metal-metal separations within units 4 are about 2.9/~, and the shortest intralayer Te-Te distances (d-re--re= 3.459/~) are associated with the M2 pairs in 4, although Te-Te separations with the triangular faces of the isolated central 9 (d-re--r,= 3.499/~) are almost comparable, but even the Te-Te distances of fragments 4 within the sandwich layers (i.e. parallel to the stacking direction) (drc--r, = 3.651/~) are significantly shorter than the sum of the van der Waals distances. The shortest distances (d-r,--r, = 3.519 .~,) are observed along the common edge of the fragments 4. As for the linear "tinker toy phases" the longest intralayer and shortest Te-Te interlayer distances occur between Te atoms bonded to Ta atoms of isolated prisms 8. Although the same structural principles apply for the linear [(M2Te2)(ATe2)](MTe2)n and cyclic variants of the tinker toy phases such as Ta13Ga3Te24, a formal assignment of oxidation states is less straightforward.

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According to the results of band structure calculations the two different types of metal atoms are similar. A formulation [(Ta (3)+)13(Ga +)3(Te 2-)24] indicates that a "polytelluride" network is formed by gradual oxidation of the anion matrix. Similarly to Nb2GaTe4, the formal oxidation state of the main group atoms is Ga + because mainly the s states of the valence shell are filled. The conductivity properties of the modular layer compounds can be predicted by a simple "jigsaw puzzle exercise". Whereas the valence electrons of the building blocks 5 and 4 are tied up by the formation of quasilocalized M-M and M-A bonds, 8 can donate at least a portion of its d electrons to the conduction band of the solid. Therefore, all modular layer compounds containing unit 8 are metallic. Nb2SiTe4, whose structure is built up from units 5 and 4 only, is electron precise and semiconducting. The valence band of the isostructural Nb2GaTe4 contains defect electrons which leads to metallic behaviour.

,b

(a)

2.3. Size effects: formation of intercalation compounds and cluster phases From structural chemistry it is well known that size effects and radius ratios control the structure of materials to a major extent. Similarly, the size of the main group element determines the structure of compounds MnATez,. For, example, all attempts to replace gallium in Nb2GaTe4 and hypothetical Nb13Ga3Te24 by its heavier homologue indium invariably afforded In/Nb3Te4 [10], where the size of the channels in the binary host structure Nb3Te4 allows the inclusion of a variety of elements such as In, TI, or alkali metals [11~. On the contrary, replacement of gallium (roov= 1.26 A) by the isoelectronic but much smaller boron (rcov=0.88 A) leads to the formation of a unique cluster compound with an interstitial main group atom, Ta4BTe8 [12], which can be synthesized from the elements at 1050 °C. Ta4BTe8 is of special interest because it provides a link between the centred zirconium and rare earth halide clusters and the "reduced" niobates and molybdates. The structure of Ta4BTe8 (Fig. 5) contains parallel chains of B-centred trans edge-sharing Ta6 octahedra which are coordinated by Te atoms above all free edges. Thus, the fragments are related to the [MeX12] type arrangement as illustrated in a side-on view in Fig. 5(b). The chain has the composition Ta4rzTa2BTe4Tes/z=Ta4BTe8 . The individual chains are interconnected by ditelluride groups. Those Ta and Te atoms which are not shared by adjacent clusters of the chain provide the links to parallel but rotated chains Ta4BTei4Tei-~zTea2 using the symbolism of Sch~ifer and yon Schnering [13].

(b) Fig. 5. (a) The unit cell of Ta4BTe8 viewed down [001]. (b) Chain of condensed Ta6BTet2 clusters: o, B atoms; ©, Ta atoms; Q , Te atoms.

Many structural features of the [Ta6BTe12] cluster may be compared with those of related clusters among the well-known rare earth halides. The Ta-Ta interatomic distances along the joined edges (d-ra_Xa= 2.938(7)/~) are almost equal to those in elemental Ta (d-ra_-ra=2.92 /~) [14]. The octahedron is extremely elongated along the chain direction (3.564(3)/~), and the avera~ge distance between the apex and basal atoms is 3.189 A. Te-Te closed shell repulsions are probably responsible for the relative elongation of the octahedra. The distorted structure of the cluster leads to the occurrence of two different Ta-B bond distances (2.198(3) /~ and 2.309(2) /~). These Ta-B distances

I~. Tremel et al. / Journal of Alloys and Compounds 219 (1995) 73-82

are comparable with those of B-centred Zr6 clusters, e.g. in K2Zr6CI15B (dzr-B = 2.304/~) [15]. The presence of the interstitial atom is reflected in the metal-metal distances. Interstitial atoms expand the cluster: thus the average Ta-Ta distance is significantly longer than typical Ta-Ta distances of 2.9-3.0/~ in tantalum halide [M6X12]n+ clusters [13]. Remarkably, only few chalcogenide cluster compounds containing interstitial atoms are known so far, Ta2S2C [16], M4ATe4 (M =Nb, Ta; A--Ga, Si, Cr, Fe, Co, Ni) [17], Nb4Te9OI 4 [18], and Cp6Zr6S9 [19] being known examples. Chains of related construction have been found in Sc4C16Z (Z = B, N) [20] and NaM0406 [21], the principal difference being that in the structure of Sc4CI6Z and NaM0406 the chains are interconnected by isolated chlorine and oxygen atoms respectively (instead of Te22groups in the case of Ta4BTes) and sodium atoms are located in the cavities between four chains. The electronic structure of Ta4BTes may be estimated in several ways. The formal electron counting procedure leads to the formulation [(Ta2"25+)4(B3+)(Te2-)4(Te22-)2]. According to a theoretical analysis for NaMo406, the optimum number of M-M bonding electrons is 13 [22] compared with 11 for Ta4BTes. Bond order summations using Pauling's formula [14] do not lead to conclusive results. Band structure calculations at the extended Hiickel level show that as a consequence of short Te-Te interactions along the chain direction a fraction of the "anion" states is raised above the Fermi level; therefore, the electron deficiency of Ta4BTes is less than expected from a comparison with the related oxide NaM0406; conductivity studies reveal in agreement with the computational results metallic properties for the telluride. Ta4BTes provides a conceptual link between the condensed empty cluster systems encountered among the oxoniobates and oxomolybdates and the condensed rare earth and zirconium halide clusters with interstitial atoms. In the former class of compounds, matrix effects [23], i.e. the size of the metal oxide clusters, prevent interstitial atoms from being encapsulated in the cluster; the electron deficiency of the cluster must be compensated by countercations in intercluster cavities. On the contrary, the cluster halides of group 3 and group 4 transition metals can "optimize" the cluster electron count by a broad variety of intersitial atoms and additional countercations. Matrix effects do not prevent group 5 tellurides, formally isoelectronic to the oxoniobates, from balancing their electron deficit by the formation of centred dusters. The smooth encapsulation of boron may be due to thermodynamic (no stable alternate phases, boron tellurides are not stable), structural (cluster proportions are optimal for the bonding of boron interstials) or electronic (correct electron count) factors.

79

3. Low temperature routes to microporous chalcogenides The recent developments in the synthesis of materials with porous structures have evolved from zeolites with crystalline aluminosilicate frameworks to compounds where both A1 and Si have been replaced by other main group and transition elements [24]. The basic idea behind this substitutional chemistry is to combine different main group elements to produce compounds having framework structures isoelectronic to SiO2 or known aluminosilicates with the ultimate goal of tailoring the properties of zeolites for catalytic reactions. Surprisingly, a replacement of the anionic part of the lattice has hardly been tackled. Considering the numerous applications of microporous oxides in chemical technology and the importance of many chalcogenides such as ZnS or CuFeS2 in electronic devices [25], one obvious extension of zeolite chemistry is to combine the desired properties of both types of materials by replacing the anionic oxygen component in zeotype compounds by its higher congeners sulphur and selenium, i.e. to attempt the synthesis of chalcogenosilicates and phosphates with porous structures. Template reactions are possible synthetic routes to the target materials. Reactions in molten fluxes and hypercritical solvents could provide the necessary mild reaction conditions. Low melting alkali metal thiophosphate fluxes have been used very successfully in the synthesis of new quaternary transition metal thiophosphates [26]. Most reactions have been carried out in the temperature range between 350 °C and 500 °C with alkali metal sulphides, phosphorus pentasulphide, sulphur and the corresponding transition metals as starting materials. The structures of the reaction products show that, as for metal polychalcogenides, polythiophosphates [PmS~]~- act as reactants and mineralizers. K2MP2S7 (M = V, Cr) [26] is a new solid state thiophosphate. Its structure is built up from isolated K ÷ cations and one-dimensional M2P2S72- chains (Fig. 6). The chain contains dimeric [M(PS4)]2 units with octahedrally coordinated M atoms and thiophosphate groups acting as triply bridging ligands. Similar dimetal units have been found in the structure of V2P4S13 [27]. The dimeric units with magnetically coupled metal atoms are linked by P2S64- groups to one-dimensional chains, the metal-metal distances within the dimeric units being approximately 3.6 /~. The anionic chain may thus be formulated as l[(M3+)2(PS43-)2(P2S64-)] 4-. The simultaneous occurrence of PS43- and P2S64- groups in the chain compound indicates that reactions in polythiophosphate fluxes are controlled by acid-base and redox equilibria. The complexity of the redox equilibria in polythiophosphate fluxes is well illustrated by the structure of

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W. Tremel et al. / Journal of Alloys and Compounds 219 (1995) 73~2

%

Fig. 7. The 2[M2P6S2514- layer in K4Ti2P6S25: O, Ti atoms; o, p atoms; Q , S atoms. Fig. 6. The I**[MP2S712- chain in K2MP2S7 (M=V, Cr): ©, M atoms; O, P atoms; Q , S atoms.

the layer compound K4Ti2P6Sz5 (Fig. 7), which was synthesized from a K2S-P2Ss-S flux and elemental Ti. The structure contains Ti2P6S25 chains which are crosslinked by polysulphide groups to form layers. These layers are separated by the alkali metal atoms. The structure contains two types of thiophosphate groups: P2874- groups built up from two corner-sharing PS43and P2594- anions, the latter being made up from two

PS 4 groups linked by an $3 unit instead of an isolated S atom. Thus, the anion contains Ti4÷ centres as typical Ti polysulphide compounds and may be formulated as [(Ti4÷)2(P2874-)(P2594-)214-. The Ti atoms are octahedrally coordinated by o n e P2S74- and t w o P2894e n t i t i e s in a bidentate fashion. The latter cross-link the linear TiP2S7Ti chain fragments to a honeycomb-like arrangement; the length of the Sx units within the S3PSxPS 3 groups is determined by steric factors such

W. Tremel et al. I Journal of Alloys and Compounds 219 (1995) 73-82

as space filling or ring strain, but redox equilibria should be important as well. The reactive flux method proves very useful in the synthesis of new transition metal thiophosphates. A wide variety of early transition metal, lanthanide and actinide compounds with unexpected compositions and structures has been obtained [26]. Still, the use of organic templates in molten salt reactions is problematic: solvothermal reactions may be more useful here. This is illustrated by the synthesis of a novel polychalcogenide, [Cr(en)3][Te6] (en=ethylenediamine), with microporous structure [28]. This unique compound is obtained by reaction of Cr(CO)6 with an alloy of the nominal composition BaAs4Te4 in en. Under solvothermal conditions the Cr(CO)6 undergoes oxidative decarbonylation, and the chelating solvent coordinates to the hard Cr 3÷ cations. The highly charged Cr(en)33÷ species serves as a template in the formation of a Te6 3polyanion with an unprecedented microporous structure. The characteristic structural features of the T e 6 3 polyanion are puckered sheets of Te atoms as shown in Fig. 8(a). Each of these sheets is made up of Te6 fragments that are bonded to each other by Te-Te single bonds (dTe_Xe=2.763(1) ~). The Te6 fragments contain a central Te3 ring with one exocyclic Te atom bonded to each Te atom of the rin~. The Te-Te distances within the ring are at 3.138(1) A about 0.4/~ longer and the exocyclic Te-Te separation are at 2.918(1)/~ about 0.2/~ longer than a typical Te-Te single bond distance in polytellurides. The Te-Te distances between the layers (3.3.76(1)/~) are much shorter than the sum of the van der Waals radii (as indicated by the broken line in Fig. 8(b)). If only Te-Te distances less than 2.95 ~ are considered as bonding the structure of the polyanion consists of isolated Te42- groups. This description is in accord with conventional bonding considerations in polychalcogenides. However, the interactions between three Te42- anions are repulsive; the presence of numerous partially bonding distances indicates that a more intricate bonding description is needed. Alternatively, if we counted all Te-Te distances less than 3.15 /~ as bonding, a layer structure with puckered Te6 motifs is found, the ind!vidual layers being separated by solvated Cr 3÷ ions. Finally, if we count all Te-Te distances smaller than 3.4 A as bonding, a three-dimensional framework structure with pores containing solvated cations emerges (Fig. 8(c)). According to bond length-bond strength considerations and the results of band structure calculations the Te layers are weakly bonded to each other (bond order, about 0.15). The exocyclic Te atoms become hypervalent and fourelectron-three-centre bonds are formed. Thus, repulsive interactions are transformed into non-bonding interactions. The computational results show that the Te6 3-

a

(b)

81

:

©

(c)

Fig. 8. (a) View of a 2Te sheet in the structure of [Cr(en)3][Te6] down [001]. (b) Assembly of two 2**Te sheets with cations (e, en ligands omitted) in micropores. (c) Space filling picture of the [Cr(en)3l[Te6] structure with solvated cations in micropores: C), Te atoms; ©, C, N atoms; o, H atoms.

anion is electron precise and [Cr(en)3][Te6] should be semiconducting.

Acknowledgements This work was supported by the Deutsche Forschungsgemeinschaft and the Fonds der Chemischen Industrie.

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