Crystal structure of a new salt-inclusion vanadyl(IV) phosphate CaF(H ₂ O)[VO(PO ₄ )], related to sincosite

The novel phase Ca(VO)F(PO 4 )·H 2 O was prepared by hydrothermal synthesis at a temperature of 230 °C and a pressure of 100 atm. Its crystal structure was determined using single-crystal X-ray diffraction, and refined as an inversion twin against the F 2 data. The compound crystallizes in the monoclinic space group P 2 1 , with unit-cell parameters a = 6.3198(8), b = 6.3230(7), c = 6.5826(10) Å, β = 105.68(2)°, and Z = 2. The structure is based on alternating corner sharing VO 5 F octahedra and PO 4 tetrahedra to form layers of four-connected nets. The negatively charged vanadyl-phosphate [VO(PO 4 )] − ∞ layers alternate along the [001] and [010] directions with Ca salt component [CaF(H 2 O)] + that ensures electroneutrality of the compound. The water molecules participate in the coordination of Ca atoms, and serve as donors of bifurcated hydrogen bonds. The compound is a new structural representative of the topology derived from the VOPO 4 ·2H 2 O – a mixed protonic-electronic conductor. The CaF 2 salt incorporation in the interlayer space changes the electronic state of V 5+ to V 4+ and reduces the structure symmetry from tetragonal to monoclinic. The Ca(VO)F(PO 4 )·H 2 O is discussed as a new member in the sincosite family of natural and synthetic compounds, which crystallize as polymorphic modifications.