Determination of the off-center parameters in NaBr:Cu+by pressure effects of the absorption bands

The temperature dependence of the oscillator strength and the peak position of the parity-forbidden absorption bands of the ${\mathrm{Cu}}^{+}$ ion in NaBr and NaCl crystals under various hydrostatic pressures up to 14.8 kbar are measured in detail. These results reveal the state of the ${\mathrm{Cu}}^{+}$ ion in a shallow off-center system NaBr:${\mathrm{Cu}}^{+}$; the distance of the ${\mathrm{Cu}}^{+}$ ion from the lattice site (off-center distance) is estimated to be 0.$5_5$ Å and the depth of the valley of the adiabatic potential (off-center valley) to be 25±3 meV. The distance between the ${\mathrm{Cu}}^{+}$ ion and ${\mathrm{Br}}^{\mathrm{-}}$ ion suggests that the 〈110〉 or the 〈111〉 directions are preferable for an off-center displacement. The plausible off-center potential for the ${\mathrm{Cu}}^{+}$ ion in NaBr is represented in a summary of these quantitative results. The adiabatic potential for the ${\mathrm{Cu}}^{+}$ ion in NaBr is changed by pressure from a shallow off-center type to a harmonic on-center type passing through an anharmonic on-center type.