Effect of Charge-balancing Species on Sm³⁺ Incorporation in Calcium Vanadinite

Abstract

Apatites are often seen as good potential candidates for the immobilization of halide-rich wastes. In particular, phosphate apatites have received much attention in recent years, however, their synthesis often produces complicated multi-phase systems, with a number of secondary phases forming [1.2]. Calcium vanadinite (Ca₅(VO₄)₃Cl) demonstrates a much simpler phase system, with only a single Ca₂V₂O₇ secondary phase which can easily be retarded by the addition of excess CaCl₂. However, when doping with SmCl₃ (as an inactive analogue for AnCl₃) the Sm forms a wakefieldite (SmVO₄) phase rather than being immobilized within the vanadinite, a result of having to form an energetically unfavourable Ca vacancy in order for the lattice to remain neutral overall. It has been postulated that charge-balancing the lattice via co-substitution of a monovalent cation will be less disfavoured and therefore help stabilise formation of a (Ca_5-2xSm_xA_x)(VO₄)₃Cl solid solution (A = monovalent cation). This has been investigated using a combined modelling and experimental approach. Static lattice calculations performed using Li⁺, Na⁺ and K⁺ as charge-balancing species have shown the energy cost to be less than half that of charge-balancing via formation of a Ca vacancy. As a result, solid state synthesis of (Ca_5-2xSm_xLi_x)(VO₄)₃Cl, (Ca_5−2xSm_xNa_x)(VO₄)₃Cl and (Ca_5-2xSm_xK_x)(VO₄)₃Cl solid solutions have been trialled, and analysis of the resulting products has shown a significant reduction in both the SmVO₄ and Ca₂V₂O₇ secondary phases across all dopant levels.