Intrinsic defects and their influence on optical properties of ALa9(GeO4)6O2 (A = Li, Na, K, Rb, Cs) oxyapatites prepared by spray pyrolysis

Abstract

Abstract New germanate ALa9(GeO4)6O2 (A = Li, Na, K, Rb, Cs) oxyapatites, showing promise as host lattices for phosphors with emission in the visible and infrared spectral ranges, have been obtained for the first time by ultrasonic spray pyrolysis. The discussion is focused on native defectiveness and optical properties of these compounds, depending on the nature of alkaline ions. Spray pyrolysis method was consciously used to obtain the compounds with intrinsic defects as confirmed by electron paramagnetic resonance. Reviewing available EPR data, the registered EPR response was assigned to the paramagnetic centers related to germanium tetrahedra embracing the oxygen vacancies. According to quantum-chemical calculations the oxygen-deficient centers in ALa9(GeO4)6O2 are presumably stabilized by antisite defects in the cation sublattice. Diffuse reflectance and pulse cathodoluminescence spectra of the compounds were investigated in detail, establishing a distinct dependence of optical properties on the type of alkaline ion. The observed optical absorption maxima at 275 and 370 nm were attributed to the F-like centers. The inverse dependence of luminescence intensity on the degree of defectiveness was interpreted on the assumption that these centers were involved in non-radiative energy relaxation.