Transition metal chalcogenides: new views on an old topic

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 metal-rich 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 K4Ti2P6S25 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.