Static and Dynamic Paraelectric Behavior of〈110〉Off-CenterAg+Ions in RbCl and RbBr

Abstract

Controversial results and considerable uncertainty have existed so far on the question of possible off-center behavior and paraelectric properties of ${\mathrm{Ag}}^{+}$ ions in RbCl and RbBr. In a comprehensive study, the static and dynamic paraelectric properties of both systems at low temperature are investigated, using the electrodichroism of the ${\mathrm{Ag}}^{+}$ uv absorption. The static electro-optical measurements, performed for three field orientations at the $A$, $D_1$, and $D_2$ band, reveal for both systems paraelectric behavior from $〈110〉$-oriented dipoles with dipole moment values of 0.78 and $0.95e$ Å for RbCl: ${\mathrm{Ag}}^{+}$ and RbBr: ${\mathrm{Ag}}^{+}$, respectively. Measurements on the time-dependent electrodichroism after rapid changes of the electric field show the existence of two relaxation processes with rates different by several orders of magnitude. The observed relative amount of both processes depends strongly on the applied-field direction. This complex relaxation behavior can be quantitatively accounted for by a $〈110〉$ dipole model which is based on predominance of 90° reorientation over 60° reorientation. This behavior, which cannot be understood for a dipole in an octahedral crystal field, is attributed to the dressing of the dipole by a strong $E_g$ lattice distortion, which allows easy 90° rotation within a {100} plane, but inhibits the change of this plane by 60° dipole reorientation. The observed temperature and field dependence of the relaxation rates indicate reorientation by tunneling processes at $T<\mathrm{}5\mathrm{}°$K, and classical thermally activated reorientation at higher temperature. Several of the earlier discrepancies can be explained by this $〈110〉$ dipole and two-relaxation-time model.