We report the results of a comprehensive study on the structural, electronic, and optical properties of Ga 2 O 3 in its ambient, monoclinic ͑␤͒ and high-pressure, hexagonal ͑␣͒ phases in the framework of all-electron density functional theory. In both phases, the conduction band minimum is at the zone center while the valance band maximum is rather flat in the k space. The calculated electron effective mass m e * / m 0 comes out to be 0.342 and 0.276 for ␤-Ga 2 O 3 and ␣-Ga 2 O 3 , respectively. The dynamic dielectric function, reflectance, and energy-loss function for both phases are reported for a wide energy range of 0-50 eV. The subtle differences in electronic and optical properties can be attributed to the higher symmetry, coordination number of Ga atoms, and packing density in ␣-Ga 2 O 3 relative to that in ␤-Ga 2 O 3 .

We report on the structural and electronic transport properties of BiCuSeO based compounds, that have recently been reported as promising thermoelectric materials with figure of merit ZT > 0.8 at 923 K, and share the same crystal structure as the high-Tc iron based 1111 oxypnictides. We show that the substitution of Bi 3+ by Sr 2+ induces a strong decrease of the electrical resistivity up to the solubility limit reached for x = 0.35, which originates from the strong increase of the carriers concentration. Two anomalies in the resistivity curves have been observed, one for the undoped compound near 260 K and the other for the doped samples at very low temperature. However, structural and magnetic measurements do not provide indications of structural or magnetic phase transition or superconductivity as it had been previously suggested in BiCu 1−x OS. We show that the thermoelectric properties of Bi 1−x Sr x CuSeO materials can be well understood through the analysis of the electronic band structure and the density of states close to the Fermi level and we provide possible directions toward the enhancement of the thermoelectric figure of merit of these materials.

Fe-doping enables n-type BiCuSeO and improves the performances of p-type BiCuSeO.

Since 2010, we have evidenced the very promising thermoelectric properties of layered oxychalcogenides, with parent compound BiCuSeO, which could be used in thermoelectric conversion systems in the 300-600 • C temperature range. These materials, that were first studied in the early 2000s for their optoelectronic properties, exhibit thermoelectric figure of merit values around 1.4 at 650 • C, which makes them the best lead-or tellurium-free p-type thermoelectric materials ever reported to date. In this paper, we will review the chemical, structural and physical properties of this family of materials, with an emphasis on the links between crystal structure, electronic structure and functional properties.

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