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Properties of Solids
Published in W. M. Haynes, David R. Lide, Thomas J. Bruno, CRC Handbook of Chemistry and Physics, 2016
W. M. Haynes, David R. Lide, Thomas J. Bruno
The crystal structures of the allotropic forms of the elements are presented in terms of the Pearson symbol, the Strukturbericht designation, and the prototype of the structure. The temperatures of the phase transformations are listed in degrees Celsius and the pressures are in GPa. A consistent nomenclature is used, whereby all allotropes are labeled by Greek letters. The lattice parameters of the units cells are given in nanometers (nm) and are considered to be accurate to 2 in the last reported digit.
Electronic, mechanical, vibrational and optical properties of TaIrX (X = Ge and sn): a DFT approach
Published in Molecular Physics, 2022
The structural parameters computed are presented in Table 1. The half-Heusler alloys crystallise in the usual face-centred cubic type of MgAgAs structure with space group F-43 m (no. 216) and Strukturbericht Designation C1b. The unit cell of the HH alloy contains atoms occupying the Wyckoff positions as X(X = Ge and Sn) in 4a (0,0,0), Ta in 4b(1/2,1/2,1/2) and Ir in 4c(1/4,1/4,1/4). Figure 1 presents the crystal structure of TaIrX (X = Ge and Sn). The lattice constant of TaIrGe is lesser than that of TaIrSn because the atomic mass of Ge is lesser than that of Sn. The bulk modulus of TaIrSn is more significant than that of TaIrGe. This implies that TaIrSn has a higher tendency to resist volume deformation than TaIrGe. Results from the present work agree with those of other works in the literature.
Stability of X-IV-IV half Heusler semiconductor alloys: a DFT study
Published in Molecular Physics, 2021
Half- Heusler X-IV-IV semiconductor generally crystallizes in a non-centrosymmetric structure that belongs to the space group F-43 m and with strukturbericht designation (C1b). The Z and Y atoms occupy the Wyckoff positions, 4a (0, 0, 0) and 4b (1/2, 1/2, 1/2), in a rock-salt like a sublattice, and the Z and X atoms occupy the 4a and 4c (1/4, 1/4, 1/4) sublattice forming the covalent zinc-blende structure in the three possible configurations in a half heusler compound. The half Heusler semiconductors can either have valence electrons of 8, 18 or 28 in line with the Slater-Pauling rule [25]. The half Heusler 18 valence electron of X-IV-IV is of the focal point here. Other combination options for 18 valence electrons include; XI-I-VI, XIII-V, XI-III-IV, I-XII-V, II-XII-IV, III-XII-III, X-II-VI, and X-III-V [26]. Some 18-valence electron semiconducting half Heusler alloys have been predicted from first principles and experiments for technological applications [27]. Such alloys include experimental simulation of TiCoSb and TiNiSn, as reported by Birkel et al. [28]. The optoelectronic and thermoelectric properties NiHfSn and NiZrSn were also reported by Hamioud et al. [29]; they reported the compounds to be mechanically and thermodynamically stable. Also, Ta-based half-Heusler TaXY (X=Ru, Rh; X=Sb, Bi, Sn and Pb) was investigated by Hoat using the full-potential linearized augmented plane-wave (FP-LAPW) method within the density functional theory (DFT) and the classical Boltzmann transport theory for thermoelectric applications. He reported a figure of merit (ZT) of 0.99 [30]. The results obtained by Ouardi et al. [31,32] in the synthesis of the substitutional series of NiTi1−xMxSn (where M = Sc, V and 0 < x ≤ 0.2) and NiTi0.3−xScxZr0.35Hf0.35Sn (where 0 < x ≤ 0.05) concerning their electronic structure and transport properties shows the possibility of creating n- and p-type Thermoelectric materials with significantly high power factors and ZTs within a single Heusler compound. More recently, Khandy et al. explored the thermoelectric properties and stability of HH PtZrSn and PtHfSn semiconductor alloys and predicted both alloys to be stable with ZTs of 0.24 and 0.57 respectively [33]. In another work, using the PBE-GGA in WIEN2k, Khandy and Chai reported ZTs of 0.91/0.81 at an optimal strain of 10% at 800 K/room temperature for ZrRhSb HH semiconductor [34].