On the existence of pyrophosphates of tetravalent metals having a layered structure

After an accurate investigation of the thermal behaviour (TG and DTA curves) of several samples of α-Zr(HPO₄)₂·H₂O and α-Ti(HPO₄)₂·H₂O, prepared both by the refluxing and HF procedures, γ-Zr(HPO₄)₂·2 H₂O and γ-Ti(HPO₄)₂·2 H₂O, new pyrophosphate phases of tetravalent metals have been isolated.The structure of these compounds, deduced from X-ray diffraction data, density measurements and considerations of bond length and angles, arises from the packing of layers of α and γ type, identical to those present in the starting materials and held together by POP bridges.These layered M(IV)P₂O₇ are obtained after the condensation process of the HPO₄ groups present in α and γ-M(IV)(HPO₄)₂ and are stable until the transition to the respective cubic pyrophosphates takes place.The range of stability of these phases changes on going from the α to the γ phase and in part depends on the degree of crystallinity and/or the crystal size of the starting layered exchanger. The chemical implications of α- and γ-L-M(IV)P₂O₇ are examined and discussed.