DEPENDENCE OF KCL - LI+ AND KBR - LI+ OFF-CENTER BEHAVIOR ON LATTICE PARAMETER

Minimum-energy configurations for impurity displacements along the 111 crystal axis in KBr:Li+ and KCl:Li+ systems were calculated over a range of lattice-parameter (LP) values: 3.200 to 3.315 for KBr:Li+, and 3.070 to 3.180 for KCl:Li+. The nearest-neighbor Born-Mayer model previously applied by the authors was utilized. The minimum-energy curves and configurations were found to be rather sensitive to the lattice parameter. Thus, in contrast to the room-temperature LP (3.293 results for KBr:Li+, where the 111 off-center well was 0.015-eV deep, the depth for the liquid-helium LP (3.26) was only 0.002 eV. The latter depth is comparable to the zero-point energy for Li+ vibration, indicating that the Li+ ion should not be localized at an off-center site at liquid-helium temperatures. This agrees with current experimental evidence at this temperature, which indicates that Li+ is on center in KBr; it also supports the low-barrier interpretation due to Bowen, Gomez, and Krumhansl for this system. Estimates are made of the dependence of the 111 motional frequencies on lattice parameter and show that the frequencies are quite sensitive, in agreement with results due to Clayman, Nolt, and Sievers. For KCl:Li+, the off-center configuration is found to persist for the liquid-helium-temperature KCl lattice parameter (3.117). An applied hydrostatic pressure of 7 kbar is found to be sufficient to restore an on-center configuration. The electric field gradient at the Li7 nucleus was calculated over the entire range of KCl lattice parameter considered. © 1969 The American Physical Society.