CRYSTALLINE INSOLUBLE ACID SALTS OF TETRAVALENT METALS .28. SYNTHESIS OF CRYSTALLINE TI(HPO4)2.2H2O BY THE HF PROCEDURE AND SOME COMMENTS ON ITS FORMATION AND STRUCTURE

Crystalline Ti(HPO4)2·2H2O has been obtained by slow decomposition of titanium fluoro-complexes in the presence of phosphoric acid. From the formation of intercalation compounds, X-ray powder patterns and density measurements, as well as from comparison with several acid salts of tetravalent metals, the following conclusions were derived: 1. (1) The crystal structure of Ti(HPO4)2·2H2O is built up of polymeric macro-anions, Tin(PO4)2n2n-, having negative fixed charges neutralized by protons (which can be easily exchanged by other cations), while the water is placed between the layers of these macro-anions. Polar organic molecules can also be intercalated between the macro-anions. 2. (2) The crystal structure of Ti(HPO4)2·2H2O in analogous to that of γ-Zr(HPO4)2·2H2O. 3. (3) The structural differences between the α and γ-phases are not due, as previously reported in the literature, to differences in the inter-layer structural arrangement of the same type of macro-anions but to a different structure of the macro-anions itself. From the conditions of the formation of the γ-Ti(HPO4)2·2H2O and γ-Zr(HPO4)2·2H2O it is concluded that the formation of the γ-phases is favoured by a high mole ratio [H2XO4-]/[M(IV)] in the solution. Some suggestion on the nomenclature for insoluble acid salts are proposed. © 1979.