Structural chemistry of binary and ternary hafnium-rich tellurides

Abstract

While ternary group V tellurides have received much attention recently, the chemistry of the group IV transition metal tellurides, especially that of hafnium, has remained virtually unexplored. Brewer's ideas concerning strong, intermetallic interactions have served as a synthetic guide in the preparation of several structurally unique compounds. When possible the electronic transport properties have been measured. Extended Hiickel calculations serve to rationalize the observed structural features and properties. A family of isostructural ternary tellurides of the composition Hf8MTe6 (M = Mn, Fe, Co, Ni, Ru) have been synthesized. Single crystal structures of the Mn and Fe containing materials have been determined. These materials crystallize in the orthorhombic space group Pmmn (No. 59) and the structure possesses tricapped trigonal-prismatic Hf clusters centered by M. The Te atoms are 4, 5, and 6 coordinate with respect to Hf and serve to sheath the metal framework to produce large Te-lined cavities within the structure. Receptivity measurements between 77 and 250K show Hf8MnTe6 and Hf8FeTe6 to be metallic as predicted by band structure calculations. The same calculations also indicate that Hf-M bonding is of overriding importance in stabilizing this structure type. The synthesis of the new ternary hafnium tellurides Hf5MTe3 (M = Fe, Co) is reported. The single crystal structure of Hf5FeTe3 has been determined. Hf5FeTe3 crystallizes in the orthorhombic space group Pnma (No. 62). M-centered, tricapped, trigonal prisms of hafnium constitute the basic structural unit of this structure type as well. The more metal-rich composition of Hf5MTe3 is achieved through edge sharing of clusters within such chains to form double chains. Tellurium coordination is nearly identical in the Hf5MTe3 and Hf8MTe6 structures producing strikingly similar channels that run parallel to the b axis. Four-probe resistivity measurements show metallic behavior for Hf5FeTe3, in accord with electronic structure calculations. Again, these calculations also show that Hf-M bonding is of primary importance in stabilizing these metal-metal bonded structures. The Hf-rich region of the Hf-Te binary system was also investigated in the course of the studies on ternary systems. Three new phases have been synthesized. Hf2.64Zro.36Te4 (Nb3Te4-type) was synthesized at IOOO'C in a eutectic LiCI/RbCl flux. Hf5Te4 and lif4.35Zro.65Te4 (Ti5Te4-type) were prepared via vapor phase transport reaction at 1000'C. Like Hf3-xZrxTe4, this structure type can incorporate Zr. At the composition Hf3Te2, a remarkable layered material is formed related to bcc bulk metal. Essentially no Zr is incorporated in this material. The structures of each of these materials has been determined by single crystal x-ray diffraction. The composition and synthetic requirements for a fourth phase which crystallizes in the WC structure remain unclear.