Hydrothermal synthesis and crystal structures of Sr2V(PO4)2(H2PO4) and Sr2Fe(PO4)2(H2PO4)

Abstract

Two new phosphates, Sr₂M(PO₄)₂(H₂PO₄)(M = V^III or Fe^III), have been synthesised hydrothermally at 230 °C and characterized by single-crystal X-ray diffraction and thermal analysis. Crystal data: Sr₂V(PO₄)₂(H₂PO₄), monoclinic, space group C2/c, a= 11.078(3), b= 11.742(3), c= 8.951(3)Å, β= 125.35(2)°, Z= 4 and R= 0.0356; Sr₂Fe(PO₄)₂(H₂PO₄), as above except a= 11.082(2), b= 11.737(2), c= 8.961(2)Å, β= 125.45(1)° and R= 0.0273. The two compounds are isostructural. The structure consists of _∞¹[M(PO₄)_4/2(H₂PO₄)_2/2] units along the [101] direction, held together by hydrogen bonding and O–Sr–O bonds. The interaction between the structural unit and the interstitial species has been evaluated using the valence-matching principle to account for the presence of an interstitial H₂O in the structure of the related compound Ca₂V(PO₄)(HPO₄)₂·H₂O. Dehydration of the compounds at 750 and 650 °C yields Sr₂V(PO₄)(P₂O₇) and Sr₂Fe(PO₄)(P₂O₇), respectively.