Paraelastic properties and reorientation behavior of〈110〉off-centerAg+defects in RbCl and RbBr

The stress-induced alignment of substitutional ${\mathrm{Ag}}^{+}$ defects in RbCl and RbBr was studied at low temperatures, using the uv absorption ($A$, $D_1$, and $D_2$ bands) of the ${\mathrm{Ag}}^{+}$ defect. The elasto-optical behavior, measured under stress $S$ of $〈100〉$, $〈111〉$, and $〈110〉$ symmetry, yielded results which are in quantitative agreement with a Curie-law paraelastic behavior of $〈110〉$ oriented permanent elastic dipoles. With one particular exception (which will be discussed), the three stress symmetries yield internally consistent and very precise data which test and confirm the elastic dipole model with an unprecedented accuracy. For both RbCl and RbBr systems, the $E_g$ part of the elastic-dipole model (= level splitting under $S_{100}$) is found considerably larger than the $T_{2g}$ part (or $S_{111}$ splitting effect). These static elastic dipole properties can be used to understand the peculiar dynamic behavior of the ${\mathrm{Ag}}^{+}$ defect: The high preference of reorientation by tunneling between next-nearest-neighbor (90°) instead of nearest-neighbor (60°) dipole states, can be explained by the very different way the $T_{2g}$ and $E_g$ elastic-dipole distortions are "dressing" and hindering these two motions. Elasto-optical measurements of the relaxation behavior as a function of stress of different symmetry yield results which agree with the predictions for one-phonon or thermally activated relaxation processes involved in the dipole reorientation.