Two new phosphates, CaM2(PO4)2(HPO4)(M = FeIII or VIII), have been synthesised hydrothermally at 400 °C and characterized by single-crystal X-ray diffraction. Crystal data: CaFe2(PO4)2(HPO4), monoclinic, space group P21/n, a= 6.3663(6), b= 19.752(2), c= 6.5030(8)Å, β= 91.626(9)°, Z= 4 and R= 0.0240; CaV2(PO4)2(HPO4), as above except a= 6.359(2), b= 19.973(2), c= 6.4744(4)Å, β= 91.51(1)° and R= 0.0390. The two compounds are isostructural. The framework consists of corner-sharing PO4 tetrahedra, HPO4 groups and dimers of edge-sharing MO6 octahedra forming M2O10 units. The structure of CaFe2(PO4)2(HPO4) was further defined by thermogravimetric and Mössbauer studies.
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Kwang‐Hwa Lii (1994) studied this question.