Phase transitions, structure and ion conductivity of zirconium hydrogen phosphates, H1±XZr2−XMX(PO4)3·H2O (M = Nb, Y)

Structure transformations and proton conductivity of hydrogen zirconium phosphates with the NASICON structure, H{sub 1{+-}X}Zr{sub 2-X}M{sub X}(PO{sub 4}){sub 3}.H{sub 2}O (X = 0, 0.02 and 0.1, M = Nb, Y), were studied by X-ray powder diffraction, calorimetry, IR- and impedance spectroscopy. Substitution of zirconium by niobium leads to decrease of the lattice parameters, while yttrium doping leads to their increase. H{sub 0.9}Zr{sub 1.9}Nb{sub 0.1}(PO{sub 4}){sub 3} structure was determined at 493 and 733 K. This phase crystallizes in rhombohedral space group R3-bar c with lattice parameters a = 8.8564(5) A, c = 22.700(1) A at 493 K and a = 8.8470(2) A, c = 22.7141(9) A at 733 K. The a parameter and lattice volume were found to decrease with temperature increasing. Structure transformations upon heating are caused mainly by the decrease of the M1 site and C cavities. Ion conductivity of obtained materials was found to increase in humid atmosphere. Activation energies of conductivity were calculated. Rhombohedral-triclinic phase transition found by X-ray powder diffraction was proved by calorimetry data. According to XRD and IR spectroscopy data hydrogen bond in HZr{sub 2}(PO{sub 4}){sub 3} was found to be weaker than in hydrated material.