Local phase transitions from off-to on-center defects under hydrostatic pressure, studied with optical and dielectric techniques

The shallow multi-well potential of several low symmetry off-center point-defects in alkali-halides can be tuned by hydrostatic pressure into a centro-symmetric single-well potential. This has been monitored in KCl: Li+, RbCl: Ag, RbBr: Ag+ and NaBr: Cu+ by the real and imaginary dielectric response of the off-center dipoles and partly by the forbidden d → s uv transitions (which get partially allowed in the odd parity off-center potential). Both properties display a rather abrupt disappearance of the off-center dipole moment at a critical pressure, characteristic for each defect system. The dielectric data show the corresponding decrease of the orientational barrier, leading eventually to tuning from classical into tunneling behavior, and for KCl: Li+ to a pronounced increase of the tunneling splitting.