Dielectric relaxation and conductivity in barium salt of 12-tungstophosphoric acid hydrate

Barium salt of 12-tungstophosphoric acid (WPA) hydrate BaHPW12O40.7H(2)O (BaHWPA-7H20) has been investigated both in the microwave (X-band) region and in the lower frequency (5 Hz-500 kHz) region. This work is a continuation of our previous investigations on the influence of monovalent and bivalent cations on the dielectric properties of WPA hydrate salts. The real and imaginary parts of the permittivity (epsilon, epsilon) as a function of frequency and temperature, were determined in both frequency regions. The observed relaxations were attributed to specific relaxation processes. In the lower frequency range, the relaxation time tau=1.0 x 10(-6) s was assigned to barium ion jumps. In the microwave region, the dielectric relaxation time tau =1.7 x 10(-11) s was attributed to polyatomic ions (H3O+) and/or H2O molecules reorientations. A phase transition was observed at 330 K. The low value of the activation energy for reorientation in the microwave frequency region is discussed. The d.c. conductivity and its temperature dependence were determined. Measurements were made in the temperature interval from 286 to 353 K. In the upper part of this temperature interval a PTCR effect was observed. (C) 2002 Published by Elsevier Science B.V