New Mixed Calcium Phosphate-Vanadates Ca9Sm(PO4)7(1–x)(VO4)7x: Symmetry Inhomogeneity and Its Effect on Photoluminescent Properties

New mixed calcium oxo-salts with the general formula Ca9Sm(PO4)7(1–x)(VO4)7x and the β-Ca3(PO4)2 structure are prepared. It is studied how ferroelectric, optical, and luminescent properties of these compounds are affected by the substitution of $\text{PO}_{4}^{3-}$ by $\text{VO}_{4}^{3-}$ . Successful introduction of $\text{VO}_{4}^{3-}$ groups into the structure is confirmed by Raman and IR spectroscopy. The formation of a continuous series of solid solutions is established, and the presence of regions with different space groups is demonstrated by the second-harmonic generation method. It is shown that a centrocymmetric region with the $R\bar{3}c$ space group is formed for 0.2 ≤ x ≤ 0.4 in Ca9Sm(PO4)7(1–x)(VO4)7x. It is established that the photoluminescence intensity depends on the changes in the anionic part of the structure: for x = 0.2, the integrated intensity increases by 10% with respect to the Ca9Sm(PO4)7 phosphate and by 110% with respect to the Ca9Sm(VO4)7 vanadate. The absorption edge of the diffuse reflectance spectra shifts towards the visible region with increasing vanadate content. The bandgap decreases from 4.5 eV for Ca9Sm(PO4)7 to 2.8 eV for Ca9Sm(VO4)7. Due to the strong luminescence of Sm3+ ions in the red visible spectrum, the studied series can be considered as color correcting luminescent materials for LED lighting.