Phase transitions, structure and ion conductivity of zirconium hydrogen phosphates, H_(1 ± X)Zr_(2-X)M_X(PO_4)_3·H_2O (M = Nb, Y)

摘要

Structure transformations and proton conductivity of hydrogen zirconium phosphates with the NASICON structure, H_(1 ± X)Zr_(2-X)M_X(PO_4)_3·H_2O (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_(0.9)Zr_(1.9)Nb_(0.1)(PO_4)_3 structure was determined at 493 and 733 K.This phase crystallizes in rhombohedral space group R3c 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-tridinic phase transition found by X-ray powder diffraction was proved by calorimetry data. According to XRD and IR spectroscopy data hydrogen bond in HZr_2(PO_4)_3 was found to be weaker than in hydrated material.