Study of the system Ca3(AsO4)2–CaSO4 by solid-state reaction

Abstract

Study of the system Ca₃(AsO₄)₂–CaSO₄ has led to the preparation of a series of solid solutions whose general formula is [(Ca_{21 –x}□_{3 +x})(ASO₄)_{14 – 2x}(SO₄)_2x□₂] with 0 ⩽x⩽ 1. These solid solutions are isostructural with tricalcium orthoarsenate Ca₃(AsO₄)₂. The limit of solubility of sulfate ions in tricalcium orthoarsenate corresponds to the ionic ratio R= SO₄^2–/AsO₄^3–= 0.167 (corresponding to x= 1 in the general formula). The limit compound, calcium arsenosulfate Ca₂₀(AsO₄)₁₂(SO₄)₂, is characterized by its infrared absorption spectra, crystallographic unit-cell parameter in the rhombohedral system a_r= 10.78(2)Å and density [ρ= 3.469(5) g cm ^–3]. A mechanism of SO₄^2– ion integration in the lattice of tricalcium orthoarsenate is proposed.