The full phase diagram of (Ga1− x In x )2O3 is obtained theoretically. The phases competing for the ground state are monoclinic β (low x), hexagonal (x ∼ 0.5), and bixbyite (large x). Three disconnected mixing regions interlace with two distinct phase-separation regions, and at x ∼ 0.5, the coexistence of hexagonal and β alloys with phase-separated binary components is expected. We also explore the permanent polarization of the phases, but none of them are polar. On the other hand, we find that ε-Ga2O3, which was stabilized in recent experiments, is pyroelectric with a large polarization and piezoelectric coupling, and could be used to produce high-density electron gases at interfaces.

We study the electronic transport coefficients and the thermoelectric figure of merit ZT in n-doped Mg 3 Sb 2 based on density-functional electronic structure and Bloch-Boltzmann transport theory with an energy-and temperature-dependent relaxation time. Both the lattice and electronic thermal conductivities affect the final ZT significantly, hence we include the lattice thermal conductivity calculated ab initio. Where applicable, our results are in good agreement with existing experiments, thanks to the treatment of lattice thermal conductivity and the improved description of electronic scattering. ZT increases monotonically in our T range (300 to 700 K), reaching a value of 1.6 at 700 K; it peaks as a function of doping at about 3×10 19 cm −3 . At this doping, ZT>1 for T>500 K.

Using first-principles calculations, we show that the maximum reachable concentration x in the (Ga1−xInx)2O3 alloy in the low-x regime (i.e., the In solubility in β-Ga2O3) is around 10%. We then calculate the band alignment at the (100) interface between β-Ga2O3 and (Ga1−xInx)2O3 at 12%, the nearest computationally treatable concentration. The alignment is strongly strain-dependent: it is type-B staggered when the alloy is epitaxial on Ga2O3 and type-A straddling in a free-standing superlattice. Our results suggest a limited range of applicability of low-In-content GaInO alloys.

FIG. 3. κ in poly-Mg3Sb2 (diamonds, same as Figure 1) and Mg3Sb1.8Bi0.2 (see text).

We predict high thermoelectric efficiency in the layered perovskite La 2 Ti 2 O 7 , based on calculations (mostly ab-initio) of the electronic structure, transport coefficients, and thermal conductivity in a wide temperature range. The figure of merit ZT computed with a temperature-dependent relaxation time increases monotonically from just above 1 at room temperature to over 2.5 at 1200 K, at an optimal carrier density of around 10 20 cm −3 . The Seebeck thermopower coefficient is between 200 and 300 µV/K at optimal doping, but can reach nearly 1 mV/K at low doping. Much of the potential of this material is due to its lattice thermal conductivity of order 1 W/(K m); using a model based on ab initio anharmonic calculations, we interpret this low value as due to effective phonon confinement within the layered-structure blocks.