Rhenium silicide as a new class of thermoelectric material

Abstract

The microstructure, defect structure and thermoelectric properties of binary and some ternary Re silicide have been investigated as a new class if thermoelectric material. Binary Re silicide is identified to contain many Si vacancies, which are arranged in an ordered manner in the underlying tetragonal C11_b structure so that the silicide is formulated to be ReSi_1.75 with a monoclinic unit cell and contains four differently oriented domains accompanied by the twinned microstructure. The density and arrangement of Si vacancies can be controlled by ternary alloying. When the number of valence electrons of a ternary element is smaller than that for Re, the density of Si vacancies decreases with ternary additions, whereas the density of Si vacancies increases with ternary additions when the number of valence electrons of a ternary element is larger than that for Re. For both cases, the variation of the density of Si vacancies upon ternary alloying is accompanied by the introduction of the so-called shear structure.Binary ReSi_1.75 exhibits nice thermoelectric properties as exemplified by the high value of dimensionless figure of merit (ZT) of 0.70 at 800 °C when measured along [001], although the ZT value along [100] is just moderately high. The ZT value is further increased to 0.8 with a small amount (2% substitution for Re) of Mo addition, by which an incommensurate microstructure is formed as result of extensive shear operation on the nano-scale.