We investigated the electronic and thermoelectric properties of half-Heusler alloys NiTZ (T = Sc and Ti; Z = P, As, Sn, and Sb) having an 18 valence electron count. Calculations were performed by means of density functional theory and the Boltzmann transport equation with constant relaxation time approximation, validated by NiTiSn. The chosen half-Heuslers were found to be indirect bandgap semiconductors, and the lattice thermal conductivity was comparable with the state-of-the-art thermoelectric materials. The estimated power factor for NiScP, NiScAs, and NiScSb revealed that their thermoelectric performance can be enhanced by an appropriate doping rate. The value of ZT found for NiScP, NiScAs, and NiScSb is 0.46, 0.35, and 0.29, respectively, at 1200 K.
Aliovalent doping has been recently shown to remarkably improve energy resolution in some halide scintillators. Based on first-principles calculations we report on the formation of DX-like centers in a well-known scintillator material, Tl-doped NaI (NaI:Tl), when codoped with Ca or Ba. Our calculations indicate a net binding energy favoring formation of the defect complex (TlNa−+CaNa+) involving a new cation-cation bond, instead of the isolated substitutional defects. The pair has properties of a deep DX-like acceptor complex. Doping with the aliovalent anion impurity Te is also found to induce deep centers, which can act as effective electron or hole traps. The hole trapped as TeI0 involves large lattice relaxation of the Te and an adjacent iodine, consistent with extrinsic self-trapping of the hole. Thus, in contrast to the positive effect achieved by aliovalent co-doping of the rare-earth tri-halides LaBr3:Ce and CeBr3:Ca as reported recently, co-doping with donor-like cations Ca, Ba, or the acceptor-like anion Te in monovalent NaI:Tl is found to inhibit scintillation response.
Hyperferroelectricity is an interesting phenomenon. Hexagonal ABC-type semiconductor LiZnAs was discovered to be hyperferroelectric (HyFE) [Garrity, Rabe, and Vanderbilt, Phys. Rev. Lett. 112, 127601 (2014)]. ZnO is a technologically important semiconductor and possesses a wurtzite crystal structure similar to LiZnAs. It raises an intriguing question whether ZnO is HyFE. Here we use various approaches to address this question of importance, by determining the electric equation of state, the free energy of ZnO under an open-circuit boundary condition (OCBC), as well as the vibration properties of LO phonon. We find (i) The D ∼ λ curve of ZnO, where D is electric displacement and λ = P/0.844 is a parameter directly proportional to polarization P , exhibits one and only one root at λ=0; (ii) Under OCBC, the free energy of ZnO does not produce a minimum at structural phase of nonzero polarization; (iii) The longitudinal optic (LO) phonon with computed frequency ωLO=255 cm −1 in centrosymmetric ZnO is not soft and does not have an imaginary frequency. These results corroborate the others and consistently lead to the conclusion that, although ZnO is interestingly on the edge of becoming a HyFE, it is not yet a HyFE. We further provide a physical origin explaining why ZnO is not HyFE, and reveal a possibility that may turn ZnO into a HyFE.
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