β-K₂V₃P₄O₁₇: A vanadium(IV) pyrophosphate with a layer structure

The high-temperature β-polymorph of the compound of formula K₂V₃P₄O₁₇ has been synthesized and its structure established by single-crystal X-ray diffraction method. Blue-green plate crystal of β-K₂V₃P₄O₁₇ crystallizes in the monoclinic space group P2₁ c (14) with a = 9.298(3), b = 4.879(2), c = 17.998(9) Å, β = 114.98(3)°, V = 740.1 ų, Z = 2, R = 0.055, and R_w = 0.050 for 586 unique reflections. The title compound consists of layers of vanadium(IV) phosphorus oxide with the potassium atoms between the layers. Each layer is built up from corner-sharing VO₅ square pyramids and P₂O₇ groups. Both differential thermal analysis and powder X-ray diffraction indicate that the phase transition from the low temperature α-polymorph to the high temperature β-polymorph occurs at ca. 915°C, and that the β-polymorph melts with decomposition at ca. 950°C. The phase transition is reconstructive and involves considerable bond scission and reformation between the two phases.