Electronic structure and optical properties of ALa9-xEux(GeO4)6O2 (A = Li, Na, K, Rb, Cs, La1/3; x=0, 0.07)

The new single-phase oxyapatites ALa(9)(GeO4)(6)O-2 and ALa(8.93)Eu(0.07)(GeO4)(6)O-2 (A = Li, Rb, space group P6(3)/m) are prepared using the ethylenediaminetetraacetic acid (EDTA) assisted process and the solid state method. Detailed characterization of the fine features of electronic structure within the families of oxyapatites ALa(9)(GeO4)(6)O-2 and ALa(8.93)Eu(0.07)(GeO4)(6)O-2 (A = Li, Na, K, Rb) has been performed using photoluminescence measurements and density functional theory (DFT) calculations. The trivalent europium ions may occupy two crystallographically nonequivalent sites (4f) and (6h) with the C-3 and C-s symmetry, respectively, and can be distinguished using the photoluminescence excitation on and the photoluminescence spectra from potassium and rubidium germanates, in contrast to those for lithium and sodium compounds. All the compounds should be semiconductors with wide direct band gaps of about 3.8 eV for alkali metal derivatives and about 3.2 eV for the vacancy-stabilized La-9.33(GeO4)(6)O-2 composition. The thermodynamic stability, the lattice parameters and the band gap value of CsLa9(GeO4)(6)O-2 not yet synthesized as a single phase have been qualified using DFT calculations. (C) 2017 Elsevier B.V. All rights reserved.