Cation Distribution Studies in Double Orthophosphates with Whitlockite Structure

Abstract

The structure of β-Ca₃ (PO₄)₂ is known to be similar to that of whitlockite (R3c, Z =21). The cations are distributed in five different positions, the position Ca(4) being only half occupied. The whitlockite network is stable while the filling of the Ca(4) position changes from 0 to 1. The presence of vacancies allows to provide heterovalent substitutions 3Ca²⁺ = 2M³⁺ + and Ca²⁺ + 0 = 2Me⁺ with limits of compositions Ca₉M (PO₄)₇ and Ca_loMe(PO₄)₇ respectively. The possible cation size can be changed from 0.55 Å (Fe³⁺) to 1.51 Å (K⁺). In order to check these suggestions we have synthesized Ca₉M(PO₄)₇ (M = rare earth element, Y, Bi, Fe, Al, In, Sc) compounds, The double phosphates were studied by luminescence, infrared spectroscopy and X-ray diffraction. All these compounds are isostructural to β-Ca₃(PO₄)₂. The cell parameters gradually decrease for M = La-Ho, Y and remain constant for M = Er-Lu. This divergence can be attributed to the change of the coordination number of the rare earth element. A considerable decrease of parameters is observed for the compounds of small-size trivalent elements, their IR spectra show more bands. By luminescence it was found that rare earth elements and Y, Bi occupy Ca(1), Ca(2), Ca(3) positions. The study of Ca_9.18 Fe_0.88 (PO₄)₇ structure shows that Fe³⁺ occupies octa-hedral Ca(5) site. The considerable decrease of lattice parameters and changes in IR spectra of compounds with M = Fe, Al, In, Sc result from the occupation of Ca(5) Site with trivalent cations and from redistribution of charges in the cation sublattice of the β-Ca₃(PO₄)₂ type structure.