PREPARATION AND CHEMICAL-VAPOR TRANSPORT OF MIXED-VALENT TITANIUM(III, IV)-PHOSPHATES - WITH A NOTE ON THE CRYSTAL-STRUCTURE OF TITANIUM(IV) ORTHOPHOSPHATE TI5P4O20

The results of isothermal reactions (1000 degrees C) as well as those of chemical vapour transport experiments in the phase triangle TiO2/TiP2O7/TiPO4 reveal the existence of three mixed valent titanium(III,IV)-phosphates with approximate compositions Ti9P7O35 (5TiO(2)(.)3TiP(2)O(7)(.)TiPO(4); Ti4+:Ti3+ = 8:1), Ti18P14O69 (9TiO(2)(.) 5TiP(2)O(7)(.)4TiPO(4); Ti4+:Ti3+ = 7:2) and Ti9P8O36 (3TiO(2)(.)2TiP(2)O(7)(.)4TiPO(4); Ti4+:Ti3+ = 5:4). Black crystals of the titanium(III, IV)-phosphates can be grown using halogens with additional small amounts of reducing agents (e.g. Ti, TiPO4) as transport agents. Furthermore two chromium-titanium-phosphates, Ti8CrP7O35 and Ti14Cr4P14O69, isotypic to Ti9P7O35 and Ti18P14O69 respectively, were obtained by chemical vapour transport (transport agent: chlorine; brownish transparent crystals). The five new phosphates were characterized by their X-ray powder patterns and by Xray fluorescence analysis. The existence of Ti5P4O20 as the only anhydrous titanium(IV)-phosphate besides TiP2O7 has been confirmed. Preliminary results of its crystal structure determination are given. The unit cell dimensions indicate a close structural relationship among Ti5P4O20 and the mixed valent titanium(III, IV)-phosphates.