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

Phosphates of Ca 8 Zn Ln (PO 4 ) 7 ( Ln 3+ = La – Nd, Sm – Lu) with β -Ca 3 (PO 4 ) 2 -type structure were prepared by high-temperature solid-state technique on air. All compounds of Ca 8 Zn Ln (PO 4 ) 7 are centrosymmetric and have R 3 ¯ c space group , which has been proved by several methods such as second-harmonic generation, dielectric spectroscopy, and Rietveld analysis. The influence of Ln 3+ on the structure, luminescent properties and temperature of phase transitions, which corresponds with “tetrad” effect, has been investigated. The bandgap width and location o f 4 f Ln 3+ and Ln 2+ levels relatively to the energy bands were determined using the data of luminescent spectroscopy. • Space group of Ca 8 Zn Ln (PO 4 ) 7 compounds as centrosymmetric R 3 ¯ c and Zn 2+ ions occupy only M5 position according to SXRD refinement . • The distribution of Ln 3+ between M 1 and М 3 positions depends on the ionic radius of Ln 3+ . • Temperatures of first-order reversible phase transitions correspond with the “tetrad” effect. • The energies of excitons creation and bandgap were determined as E ex = 7.94 eV and E g = 8.57 eV, respectively. • A scheme, which determines the location of Ln 3+ and Ln 2+ 4 f and 5 d levels relatively to the energy bands was developed.