Crystal growth of “defective” half-Heusler Nb0.83CoSb

Abstract

‘Defective’ half Heuslers are a new class of defect stabilized Heusler alloys. They have a cubic structure analogous to their XYZ half-Heusler parent phase but with as large as 20% vacancies at the X-site. The presence of a copious amount of vacancies has several interesting outcomes including a significantly suppressed thermal conductivity which makes them potential thermoelectric materials for high temperature applications. They also exhibit vacancy ordering. The nature of this ordering is as yet an unresolved issue. Having high-quality single-crystals of these materials can be useful for performing advanced spectroscopy experiments to throw light on this issue. In this manuscript we report the crystal growth of NbCoSb using the chemical vapor transport technique. By using iodine as a transporting agent and a temperature profile described in the manuscript, mm-sized high-quality crystals of NbCoSb are obtained. The grown crystals were characterized x-ray diffraction (powder and Laue) and electron microscopy (FESEM and HRTEM) techniques. In HRTEM, diffuse bands due to short-range vacancy ordering, and weak spots due to superstructure formation were found to coexist with the fundamental diffraction spots due to the overall hH cubic symmetry. The differential scanning calorimetry revealed a reversible phase transition near 1200 °C which is likely a vacancy order–disorder transition. The electrical resistance shows a linearly decreasing behavior upon cooling.