Synthesis, Structure, and Luminescence Properties of Anion-Substituted Germanates Ca₂La_7.2Eu_0.8(GeO₄)_6−x(PO₄)_xO_2+x/2 with an Apatite-Type Structure

The effect of the substitution of [GeO₄]⁴⁻ groups by [PO₄]³ in the anionic sublattice of apatite-structure inorganic luminophores activated with Eu³⁺ ions on their luminescence and crystal chemical properties has been studied. A number of solid solutions with a general formula of Ca₂La_7.2Eu_0.8(GeO₄)_6−x(PO₄)_xO₂ (x = 0.18, 0.3, 0.48, 0.6, 0.78) have been synthesized. Using the luminescence spectroscopy and electron paramagnetic resonance (EPR) methods, it has been shown that Eu³⁺ in the structure of the synthesized crystal phosphors undergoes reduction to Eu²⁺. For compounds with x = 0.18 and 0.48, the effect of the composition on the strength of the crystal field acting on Eu³⁺ ions has been shown. The phonon sublattice has been studied by the IR and Raman spectroscopy methods. A decrease in the integrated luminescence intensity for the selected type of substitution has been shown. The data obtained can be used to design effective luminophores for some fields of technology, such as the design of scitillation detectors, television systems, and photodiodes.