Zr-based quaternary half-Heusler alloy systems ZrNimX0.5Sb1.5−m (X Fe/In): Studies on phase evolution, crystal structures and electronic properties

Abstract

Half Heusler(hH) compounds have demonstrated exceptional capability in a wide range of functional applications as semiconductors. Although there are theoretical predictions about newer compounds and their thermodynamic stability, experimental validation is often missing. In this study, we report two quaternary multicomponent Zr-based hH alloy systems, namely ZrNi0.5Fe0.5Sb and ZrNiIn0.5Sb0.5, designed by combining 19 and 17 VEC (valence electron count) alloy systems. The structural features, including the crystal structures and compositions, were established using multiple techniques like X-ray diffraction, scanning and transmission electron microscopy. Both these systems crystallized in signature hH cubic structure (F4 ̅ m) having lattice parameters 0.6091 nm and 0.6104 nm, respectively. The measurement of Seebeck coefficients over a wide temperature range showed p-to n-type semiconductor transition in ZrNi0.5Fe0.5Sb at around 888 K due to bipolar conduction. Subsequently, the partial substitution of Co for Fe sites (ZnNi0.5Fe0.3Co0.2Sb) completely suppressed the bipolar conductivity, making it a n-type semiconductor and increased the absolute value of Seebeck coefficient, by an order of magnitude, to − 133μV/K. The alloy ZrNiIn0.5Sb0.5 showed n-type semiconductor behavior throughout the measurement temperature range. This study conducts an in-depth examination of the microstructural phase evolution, chemical environment of the elements forming the novel hH phase and demonstrates the tunability of electronic properties through aliovalent substitutions at various lattice sites.