Thermal and Electronic Transport Properties of the Half-Heusler Phase ScNiSb

Comprehensive investigation of electronic structure, phonon spectrum and thermoelectric performance of LuMSb (M = Ni, Pd, Pt) half Heusler compounds from first principles

We studied the structural, electronic, phonon spectrum and thermoelectric properties of ternary LuMSb (M = Ni, Pd, Pt) half Heusler compounds by using first principles method. The electronic properties are calculated via energy band structure and density of states by using GGA + U approximation. The calculations reveal that the replacement of Ni with Pd and Pt, energy gap decreases and LuNiSb, LuPdSb are found to have narrow indirect band gaps and exhibit semiconducting nature, while LuPtSb is found to be a gapless semiconductor. Phonon band structure calculations give only positive values of phonon frequency indicating the dynamically stability of these compounds. The thermoelectric properties have been computed using semi-classical Boltzmann transport theory. We found high Seebeck coefficient (S) and high power factor (PF) for LuNiSb and LuPdSb compounds in the whole temperature range. The ZT values of LuNiSb and LuPdSb are high in general and reach a maximum of 0.67 and 0.69 at 450 K, respectively, whereas 0.39 is the maximum ZT value for LuPtSb at the same temperature. These findings propose LuNiSb and LuPdSb compounds as promising materials for thermoelectric applications at room temperature.

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, [Formula: see text] = 20, 250, 500, 750, and 1000 [Formula: see text]C. Within pressing temperatures 20-750 [Formula: see text]C the samples maintained HH structure, however, we observed intrinsic phase separation. The material divided into three phases: stoichiometric TmNiSb, nickel-deficient phase TmNi[Formula: see text]Sb, and thulium-rich phase Tm(NiSb)[Formula: see text]. For TmNiSb sample sintered at 1000 [Formula: see text]C, we report structural transition to LiGaGe-type structure (P[Formula: see text]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 ([Formula: see text] 260 meV), while the average hole effective masses in range 0.8-2.5[Formula: see text]. TmNiSb sample pressed at 750 [Formula: see text]C achieved the biggest power factor among the series, 13 [Formula: see text]WK[Formula: see text]cm[Formula: see text], which proves that the intrinsic phase separation is not detrimental for the electronic transport.

Half-Heusler-like compounds with wide continuous compositions and tunable p- to n-type semiconducting thermoelectrics

Half-Heusler and full-Heusler compounds were considered as independent phases with a natural composition gap. Here we report the discovery of TiRu_1+xSb (x = 0.15 ~ 1.0) solid solution with wide homogeneity range and tunable p- to n-type semiconducting thermoelectrics, which bridges the composition gap between half- and full-Heusler phases. At the high-Ru end, strange glass-like thermal transport behavior with unusually low lattice thermal conductivity (~1.65 Wm⁻¹K⁻¹ at 340 K) is observed for TiRu_1.8Sb, being the lowest among reported half-Heusler phases. In the composition range of 0.15 < x < 0.50, TiRu_1+xSb shows abnormal semiconducting behaviors because tunning Ru composition results in band structure change and carrier-type variation simultaneously, which seemingly correlates with the localized d electrons. This work reveals the possibility of designing fascinating half-Heusler-like materials by manipulating the tetrahedral site occupancy, and also demonstrates the potential of tuning crystal and electronic structures simultaneously to realize intriguing physical properties.

Half-and full-Heusler compounds are considered as independent phases with a natural composition gap. Here the authors report the discovery of half-Heusler-like TiRu_1+xSb with wide continuous compositions falling in the gap region and tunable p-to n-type semiconducting thermoelectrics.

Quasi-harmonic approximation of lattice dynamics and thermodynamic properties of half Heusler ScXSb (x= Ni, Pd, Pt) from first principles

We report the lattice dynamics and thermodynamic properties of ScXSb (X= Ni, Pd, Pt) half Heusler compounds. Calculations for the structural and electronic properties were performed based on the Perdew-Burke-Ernzerhof-generalized gradient approximation density functional theory. We used the linear response density functional perturbation theory for computations addressing the elastic property, lattice dynamics, and thermodynamic properties. Results for the equilibrium lattice parameter are in reasonable agreement with reports in the existing literature. The electronic band structure behavior and bandgap agree with relevant results in research. The elastic parameters and phonon dispersions establish the mechanical stability of the compounds. Deductions from the tone of the mode-Grüneisen parameter, specific heat capacity at constant pressure and volume, and thermal expansion favor ScNiSb as the most promising material for thermal conductivity at high temperatures in the three compounds studied. The three compounds obey the Dulong-Petit law at high temperatures. Results for the Dulong-Petit limit is consistent with expectations for solid materials at high temperatures.