Ammonium-exchanged phase of γ-titanium phosphate

Abstract

The monoammonium salt of γ-titanium phosphate has been prepared by hydrothermal treatment of π-Ti2O(PO4)2·2H2O in the presence of urea and phosphoric acid, and its crystal structure was obtained by Rietveld analysis using powder X-ray diffraction data. γ-Ti(PO4)(NH4HPO4) crystallizes in the monoclinic space group P21/m with a = 5.0725(3) A, b = 6.3101(3) A, c = 11.2435(5) A, β = 97.980(3)° (Z = 2). The structure consists of 2D titanium phosphate layers in the ab-plane. The titanium atoms and one of the phosphate groups are located nearly in the ab-plane of the layer. All the oxygen atoms of this phosphate group are involved in titanium coordination sphere. The other phosphate group located in the layers edges links two neighboring titanium atoms in the a-direction through two of its oxygen atoms. The remaining two oxygens are pointed toward the interlayer space being involved in hydrogen bond interactions with the ammonium ions. Each ammonium ion is shared by four oxygens belonging to four different phosphate hydroxyl groups. γ-Ti(PO4)(NH4HPO4) is stable until 453 K, while above this temperature, it transforms to γ’-Ti(PO4)(NH4HPO4) high temperature polymorph stable until 573 K. Thermal decomposition of this material leads to cubic TiP2O7 structure, with previous formation of two intermediate pseudo-layered compounds: Ti(PO4)(NH4HP2O7)0.5 and Ti(PO4)(H2P2O7)0.5. The activation energy of thermal decomposition has been calculated as a function of the extent of conversion applying the Kissinger–Akahira–Sunose (KAS) isoconversional method to the thermogravimetric data.