La 4 Ga 2 Se 6 O 3 : A Rare-Earth Oxyselenide Built from One-Dimensional Strips.

The oxyselenide La 4 Ga 2 Se 6 O 3 was obtained by reaction in NaCl flux. Its monoclinic crystal structure (space group C 2 / c , a = 21.2832(13) Å, b = 11.6272(7) Å, c = 6.0006(4) Å, β = 106.3430(10)°, Z = 4), which is a new type, consists of strips of edge-sharing OLa 4 tetrahedra and zigzag chains of corner-sharing GaSe 4 tetrahedra. The separation into distinct structural blocks with mostly ionic vs covalent bonding character was supported by analysis of the electron localization function and crystal orbital bond index. An experimental band gap of 1.8 eV was extracted from optical diffuse reflectance spectra. First-principles calculations suggest that the thermoelectric power factor of this compound would be enhanced by n-doping.