Dielectric relaxation in PbMoO4 crystals

After excitation with external electric fields E
p
(10 V/cm < E
p
< 104 V/cm) during cooling down dielectric relaxations can be observed in short circuit measurements at about 185 K. The depolarisation glow peak exhibits a complex behaviour which is effected by the direction of the external field with respect to crystal axes as well as the different origin of the samples. In addition to investigations on current-voltage characteristics both the polarisation time t
p
as well as the polarisation temperature T
p
is studied as parameter. The reorientation of dipolar defects is suggested to be responsible for the occurrence of relaxations. From the anisotropic results of orientation dependent measurements a dipolar defect is discussed to be the most probable in the scheelite structure in question. The model involves an oxygen vacancy as a mobile defect. Light irradiation during the application of polarising fields gives rise to the appearance of thermally stimulated de-polarisation peaks even below the peak temperature down to T
p
4 K. A characteristic temperature T
p
= 70 K and exciting wavelength λ
irr
= 420 nm are found to be most effective in generating the relaxation. The decrease of dipolar relaxation time due to optical excitation is explained by lowered bonding. The potential barrier of the excited state is supposed to be smaller than in the ground state.