Study of structural and physical properties of early transition metal chalcogenides and halides

Abstract

Many tantalum and niobium rich chalcogenides have been synthesized and because of their interesting anisotropic structures and properties, the physical properties of these compounds have been studied. The group V metal-rich chalcogenides were synthesized in vapor phase transport reactions. The vapor phase transport synthesis affords good single crystals for measurements of electrical resistivities. A comparison of the temperature dependent resistivities of Ta2S and Ta3S2 show that the latter compound is a considerably poorer conductor for all temperatures from 15 to 270 'K, as predicted in an earlier band structure study of these materials. The truly one-dimensional compound Ta4Te4Si shows a resistivity maximum at 160 K and below 60 K exhibits a broad metal-insulator transition. This behavior of Ta4Te4Si is phenomenologically reminiscent of NbSe3 for which the anomalies are convincingly attributed to the charge density waves. In contrast the isoelectronic compound, Y4 Br4Os shows semiconducting behavior with an activation energy (Ea) for conduction is 0. 16 eV, in accord with the calculated band structure presented. The resistivity anomaly of Ta4Te4Si is rapidly removed by alloying since cation disorder suppresses the phase transition, even if the alloying metals are isoelectronic. Alloying Ta4Te4Si with niobium damps the resistivity anomalies while the resistivity maximum of Ta4Te4Si is disappeared by inclusion of zirconium. Physical properties of other metal-rich chalcogenides, and the resistivity and susceptibility apparatus built in our laboratory are discussed.