Paraelastic Properties and Reorientation behavior of Off-Center Ag(+) Defects in RbCl and RbBr.

摘要

The stress-induced alignment of substitutional Ag defects in RBCL and RbBr was studied at low temperatures, using the uv absorption (A, D1, and D2 bands) of the Ag defect. The elasto-optical behavior, measured under stress S of , , and symmetry, yielded results which are in quantitative agreement with a Curie-law paraelastic behavior of oriented permanent elastic dipoles. With one particular exception 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 is found considerably larger than the T(2g) part (or S111 splitting effect). These static elastic dipole properties can be used to understand the peculiar dynamic behavior of the Ag defect: The high preference of reorientation by tunneling between next-nearest-neighbor (90 deg) instead of nearest-neighbor 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.