Syntheses and structures of Ta2(WO2)0.87H0.26(PO4)4 and Ta2(MoO2)(PO4)4

Tantalum hydrogen phosphate, β-TaH(PO4)2, has a three-dimensional structure that is stable to remarkably high temperature (∼600 °C) presumably due to the presence of strong hydrogen bonds. Impedance measurements indicate a low conductivity, 2.0 × 10−6 S/cm at 200 °C in 5% H2. In further studies aimed at enhancing the conductivity by aliovalent doping, we have investigated systematically the synthesis of compounds in the TaH(PO4)2–W2P2O11 system at 380 °C. As a result, a new phase, Ta2(WO2)0.87H0.26(PO4)4, was identified and subsequently the molybdenum analog Ta2(MoO2)(PO4)4 was also prepared. The structures were determined by single crystal X-ray diffraction techniques. The structures of Ta2(WO2)0.87H0.26(PO4)4 and Ta2(MoO2)(PO4)4 can be formally derived from the structure of β-TaH(PO4)2 by the replacement of two P–OH protons with an MO22+ (M = Mo and W) group together with a change in the orientation of some phosphate tetrahedra.