Site occupancy, luminescence and dielectric properties of β-Ca3(PO4)2-type Ca8ZnLn(PO4)7 host materials

Phosphates of Ca8ZnLn(PO4)7 (Ln3+ = La – Nd, Sm – Lu) with β-Ca3(PO4)2-type structure were prepared by high-temperature solid-state technique on air. All compounds of Ca8ZnLn(PO4)7 are centrosymmetric and have R3¯c space group, which has been proved by several methods such as second-harmonic generation, dielectric spectroscopy, and Rietveld analysis. The influence of Ln3+ on the structure, luminescent properties and temperature of phase transitions, which corresponds with “tetrad” effect, has been investigated. The bandgap width and location of 4f Ln3+ and Ln2+ levels relatively to the energy bands were determined using the data of luminescent spectroscopy. •Space group of Ca8ZnLn(PO4)7 compounds as centrosymmetric R3¯c and Zn2+ ions occupy only M5 position according to SXRD refinement.•The distribution of Ln3+ between M1 and М3 positions depends on the ionic radius of Ln3+.•Temperatures of first-order reversible phase transitions correspond with the “tetrad” effect.•The energies of excitons creation and bandgap were determined as Eex = 7.94 eV and Eg = 8.57 eV, respectively.•A scheme, which determines the location of Ln3+ and Ln2+ 4 f and 5d levels relatively to the energy bands was developed.