Crystal structure, dielectric and optical properties of β-Ca3(PO4)2-type phosphates Ca9-xZnxLa(PO4)7:Ho3+

Abstract

A series of new phosphates Ca9-xZnxLa(PO4)7:Ho3+ with the β-Ca3(PO4)2-type structure was synthesized by the solid state route. An intense near infra-red (NIR) emission according to intraconfigural 4f-4f transitions of Ho3+ ions 5I7 – 5I8 (~2 μm) and 5I6 – 5I8 (~1.156 μm) was observed. The obtained phases were studied by a combination of methods including synchrotron powder X-ray diffraction, dielectric spectroscopy, second harmonic generation, differential scanning calorimetry, luminescence spectroscopy. The structure of Ca8ZnLa(PO4)7 was refined by the Rietveld method in centrosymmetric space group R3‾c. The Ca2+ → Zn2+ substitution in the M5 site leads to a transformation from polar R3c space group (x = 0 – 0.5) to centrosymmetric R3‾c space group (x = 0.6–1) and to the increased integral intensity of luminescence with maxima at x = 1. It was concluded that the crystal site engineering in the Ho3+-containing β-Ca3(PO4)2-type hosts offers a promising way to obtain new NIR phosphors for use in the creation of biocompatible bone tissue fillers.