Electrical properties of the fast-ion conductor lanthanum fluoride

Audio-frequency complex-impedance measurements have been performed on pure and alkaline-earth-doped lanthanum trifluoride at zero pressure over the temperature range 5.5-310 K and at pressures up to 0.3 GPa in the vicinity of room temperature. Studies have been made both parallel and perpendicular to the optical axis. The real part of the dielectric constant is found to be given by epsilonsub(parallel) approx. = 12.6 and epsilon sub(perpendicular) approx.14.4 and both components increase slightly with temperature. Also, for the calcium-doped material, four relaxations have been observed. The activation enthalpy for the principal relaxation is close to that for the motion of free vacancies and thus is attributable to the reorientation of a bound vacancy. In addition, the anisotropy ratio for the relaxation is found to be A sub(perpendicular)/A sub(parallel) approx. = 1.8, which is opposite to the anisotropy observed for DC conductivity. An explanation of the anisotropy ratio is discussed. The remaining relaxations are tentatively attributed to other jumps of bound vacancies. Finally, the pressure-dependent conductivity has been measured in the association region, yielding an activation volume of about 2.4 cm3 mol-1. The value parallel to the optical axis is slightly larger than that perpendicular to the optical axis. In addition, the thermal expansion coefficient of the activation volume is found to be negative. The value of 2.4 cm3 mol-1 is interpreted to be the activation volume for the motion of vacancies. (author)