Anomalies in the properties of Hf(S 2− xTe x) 1− y and Hf(Se 2− xTe x) 1− y near the metal-insulator transition

Abstract

The solid solutions Hf(S 2− x Te x ) 1− y and Hf(Se 2− x Te x ) 1− y were prepared to examine changes in the electronic structure in a narrow composition range near the metal-insulator transition. Powder X-ray diffraction analysis and resistivity measurements are presented. The x 3 dependence of the hexagonal c parameter for the sulfur solutions is due to large packing mismatches in the layers. In the selenium solutions, anomalous behavior is observed in the composition dependence of the a and c hexagonal lattice parameters in the range 0.05 < x < 0.10, y ∼ 0. Between these concentration limits, both lattice parameters show positive deviations from smooth behavior and the diffraction linewidths broaden. The temperature dependence of the resistivity suggests that the Hf(S 2− x Te x ) 1− y solutions have an energy of activation for conduction at room temperature for 0 < x < 7 8 ; the sample of composition x = 5 8 has a metal-insulator transition as a function of temperature below room temperature. The band gap goes to zero with composition for Hf(Se 2− x Te x ) 1− y in the range x = 0.080 to x = 0.095. Nonstoichiometry, phase separation, and changes from covalent to metallic bonding explain the structural and electronic changes observed in the seleno-telluride system near the metal-insulator transition.