Syntheses and structures of Ta 2(WO 2) 0.87H 0.26(PO 4) 4 and Ta 2(MoO 2)(PO 4) 4

Abstract

Tantalum hydrogen phosphate, β-TaH(PO 4) 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% H 2. In further studies aimed at enhancing the conductivity by aliovalent doping, we have investigated systematically the synthesis of compounds in the TaH(PO 4) 2–W 2P 2O 11 system at 380 °C. As a result, a new phase, Ta 2(WO 2) 0.87H 0.26(PO 4) 4, was identified and subsequently the molybdenum analog Ta 2(MoO 2)(PO 4) 4 was also prepared. The structures were determined by single crystal X-ray diffraction techniques. The structures of Ta 2(WO 2) 0.87H 0.26(PO 4) 4 and Ta 2(MoO 2)(PO 4) 4 can be formally derived from the structure of β-TaH(PO 4) 2 by the replacement of two P–OH protons with an MO 2 2+ (M = Mo and W) group together with a change in the orientation of some phosphate tetrahedra.