Phase formation in the systems TiOCl2-H3PO4-MF(HF)-H2O (M = K, Rb, Cs)

From solutions containing 2–17 wt % TiO₂ at the molar ratios M/Ti = 1–4, F/Ti = 2–4, and PO ₄ ^3? /Ti = 0.5–10 under mild conditions, fluoro- and oxo(hydroxo) fluorophosphate titanates were isolated: crystalline M₂TiF₆ (M = K, Rb, Cs) and K₂Ti₂O_2.5F₂PO₄ · 2H₂O, and amorphous K₃Ti₄O(OH)F₇(PO₄)₃ · 5H₂O, Cs₂Ti₃O₂F₇PO₄ · 6H₂O, and CsTi₃O₃F₄PO₄ · 3H₂O. In a mixture with M₂TiF₆ and KCl, phosphate-ion-containing crystalline phases of unidentified composition were detected. The phases were studied by elemental, crystal-optical, X-ray powder diffraction, thermal, IR spectroscopic, and electron microscopic analyses. Annealing fluorophosphate titanates gives a mixture of MTiOPO₄ and TiO₂. All the mentioned alkali metal fluorophosphates contain the tetrahedral ion PO ₄ ^3? and titanium polyhedra with bonds Ti-F and Ti-O; some of them also contain bridging oxygen connecting titanium atoms: Ti-O-Ti; i.e., these substances are polymeric.