Phase relations in the K2W2O7–K2WO4–KPO3–Bi2O3 system and structure of K6.5Bi2.5W4P6O34

The phase relations in the cross-section of the K2W2O7–K2WO4–KPO3 containing 15 mol% Bi2O3 were undertaken using flux method. Crystallization fields of K6.5Bi2.5W4P6O34, K2Bi(PO4)(WO4), Bi2WO6, KBi(WO4)2 and their cocrystallization areas were identified. Novel phase K6.5Bi2.5W4P6O34 was characterized by single-crystal X-ray diffraction: sp. gr. P−1, a=9.4170(5), b=9.7166(4), c=17.6050(7) Å, α=90.052(5)°, β=103.880(5)° and γ=90.125(5)°. It has a layered structure, which contains {K7Bi5W8P12O68}∞ layers stacked parallel to ab plane and sheets composed by potassium atoms separating these layers. Sandwich-like {K7Bi5W8P12O68}∞ layers are assembled from [W2P2O13]∞ and [BiPO4]∞ building units, and are penetrated by tunnels with K/Bi atoms inside. FTIR-spectra of K2Bi(PO4)(WO4) and K6.5Bi2.5W4P6O34 were discussed on the basis of factor group theory.