Symmetry Inhomogeneity of Ca<sub>9–<i>x</i></sub>Zn<sub><i>x</i></sub>Eu(PO<sub>4</sub>)<sub>7</sub> Phosphor Determined by Second-Harmonic Generation and Dielectric and Photoluminescence Spectroscopy
Dina V. Deyneko, Sergey M. Aksenov, Ivan V. Nikiforov, S. Yu. Stefanovich, Bogdan I. Lazoryak
Abstract
A series of Ca9–xZnxEu(PO4)7 phosphates (0 ≤ x ≤ 1) with a β-Ca3(PO4)2-type structure was successfully synthesized and studied. Three areas of compositions, I (0 ≤ x ≤ 0.5), II (0.5 < x ≤ 0.8), and III (0.8 < x ≤ 1), were distinguished. In these regions the unit cell parameters and the full width at half-maximum (FWHM) of reflexes in the X-ray diffraction patterns change monotonously. The fracture in these dependences was observed on the frontiers between regions I/II and II/III. The limits of these areas were determined by second-harmonic generation (SHG), dielectric, and photoluminescence spectroscopy methods. It was shown that in Ca9–xZnxEu(PO4)7 the Eu3+ cations occupy low-symmetry sites. With an increase in of Zn2+ concentration, the structure symmetry changes from noncentrosymmetric (space group (SG) R3c) to centrosymmetric (SG R3̅c). In this case, the distortion of europium polyhedra in the structure increases and, as a result, the luminescence intensity rises. It was found that in β-Ca3(PO4)2-type solid solutions by replacing Ca2+ with Zn2+ the luminescence intensity can be finely tuned. Measured quantum yields (QY) reach 65%. It can be concluded that centrosymmetric phases show higher luminescence intensity. This method of property management may be common for β-Ca3(PO4)2-type structure phosphors, since the host matrix is very sensitive to changes in composition.