Design and discovery of a novel half-Heusler transparent hole conductor made of all-metallic heavy elements.

Transparent conductors combine two generally contradictory physical properties, but there are numerous applications where both functionalities are crucial. Previous searches focused on doping wide-gap metal oxides. Focusing instead on the family of 18 valence electron ternary ABX compounds that consist of elements A, B and X in 1:1:1 stoichiometry, we search theoretically for electronic structures that simultaneously lead to optical transparency while accommodating intrinsic defect structures that produce uncompensated free holes. This leads to the prediction of a stable, never before synthesized TaIrGe compound made of all-metal heavy atom compound. Laboratory synthesis then found it to be stable in the predicted crystal structure and p-type transparent conductor with a strong optical absorption peak at 3.36 eV and remarkably high hole mobility of 2,730 cm(2) V(-1) s(-1) at room temperature. This methodology opens the way to future searches of transparent conductors in unexpected chemical groups.

First-principles study on thermodynamic properties and phase transitions in TiS(2).

Structural and vibrational properties of TiS(2) with the CdI(2) structure have been studied to high pressures from density functional calculations with the local density approximation (LDA). The calculated axial compressibility of the CdI(2)-type phase agrees well with experimental data and is typical of layered transition-metal dichalcogenides. The obtained phonon dispersions show a good correspondence with available experiments. A phonon anomaly is revealed at 0 GPa, but is much reduced at 20 GPa. The thermodynamic properties of this phase were also calculated at high pressures and high temperatures using the quasi-harmonic approximation. Our LDA study on the pressure-induced phase transition sequence predicts that the CdI(2)-type TiS(2), the phase stable at ambient conditions, should transform to the cotunnite phase at 15.1 GPa, then to a tetragonal phase (I4/mmm) at 45.0 GPa. The tetragonal phase remains stable to at least 500 GPa. The existence of the tetragonal phase at high pressures is consistent with our previous findings in NiS(2) (Yu and Ross 2010 J. Phys.: Condens. Matter 22 235401). The cotunnite phase, although only stable in a narrow pressure range between 15.1 and 45.0 GPa, displays the formation of a compact S network between 100 and 200 GPa, which is evidenced by a kink in the variation of unit cell lengths with pressure. The electron density analysis in cotunnite shows that valence electrons are delocalized from Ti atoms and concentrated near the S network.

Prediction of high-pressure polymorphism in NiS2 at megabar pressures.

A sequence of pressure-induced phase transitions of vaesite, NiS(2), with pyrite structure, has been established from static LDA calculations. A dozen AX(2) candidate structures have been studied at high pressures including cotunnite (α-PbCl(2)), which is commonly observed in other AX(2) compounds at high pressures. At 150 GPa, vaesite transforms to a tetragonal phase (P4(2)/n) rather than cotunnite. This tetragonal structure is characterized by layers of Ni atoms in eight-fold coordination with S atoms rather than the nine-fold coordination observed in cotunnite. With further compression to about 7.5 Mbar, the tetragonal phase transforms into a hexagonal AlB(2)-type structure (P6/mmm) which is characterized by planar hexagonal layers of S intercalated by Ni atoms where each Ni atom is 12-fold coordinated by S atoms. Calculated band structures and valence electron density maps show S-S and Ni-S bonded interactions for NiS(2) under these extremely compressed conditions. The tetragonal phase may have geophysical implications if present in the Earth's core.