Half-Heusler phase TmNiSb under pressure: intrinsic phase separation, thermoelectric performance and structural transition

Half-Heusler (HH) phase TmNiSb was obtained by arc-melting combined with high-pressure high-temperature sintering in conditions: p = 5.5 GPa, T_HPHT = 20, 250, 500, 750, and 1000 ^∘ C. Within pressing temperatures 20–750 ^∘ C the samples maintained HH structure, however, we observed intrinsic phase separation. The material divided into three phases: stoichiometric TmNiSb, nickel-deficient phase TmNi _1-x Sb, and thulium-rich phase Tm(NiSb) _1-y . For TmNiSb sample sintered at 1000 ^∘ C, we report structural transition to LiGaGe-type structure ( P 6_3 mc , a = 4.367(3) Å, c = 7.138(7) Å). Interpretation of the transition is supported by X-ray powder diffraction, electron back-scattered diffraction, ab-initio calculations of Gibbs energy and phonon dispersion relations. Electrical resistivity measured for HH samples with phase separation shown non-degenerate behavior. Obtained energy gaps for HH samples were narrow ( ≤ 260 meV), while the average hole effective masses in range 0.8–2.5 m_e . TmNiSb sample pressed at 750 ^∘ C achieved the biggest power factor among the series, 13 WK ^-2 cm ^-1 , which proves that the intrinsic phase separation is not detrimental for the electronic transport.