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

A series of new phosphates Ca9-xZnxLa(PO4)(7):Ho3+ with the beta-Ca-3(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 I-5(7) - I-5(8) (similar to 2 mu m) and I-5(6) - I-5(8) (similar to 1.156 mu m) 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 R (3) over barc. The Ca2+ -> Zn2+ substitution in the M5 site leads to a transformation from polar R3c space group (x = 0 - 0.5) to centrosymmetric R (3) over barc 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 beta-Ca-3(PO4)(2)-type hosts offers a promising way to obtain new NIR phosphors for use in the creation of biocompatible bone tissue fillers.