A new off‐center defect system, RbCl:Li+, studied with dielectric techniques under hydrostatic pressure

RbCl crystals with Li+ doping, spanning a concentration range of nearly three orders of magnitude, are systematically studied in their real and imaginary dielectric response under hydrostatic pressure (up to 7 kbar). A dielectric constant anomaly and loss peak around 40 K is found, which can be fitted to a Langevin‐Debye behavior of classical dipoles with an activation energy for reorientation of 510 K. For small Li+ doping (N < 1018 cm−3), this dielectric response scales with the Li+ concentration, yielding (for Lorentz corrected fields) a dipole moment of 〈p〉 = 0.52 e Å. The potential barrier for the Li+ dipole reorientation is found to be drastically reduced under hydrostatic pressure, shifting the observable relaxation to low temperatures. Though this pressure tuning process is interrupted by the NaCl → CsCl structural phase‐transition at 4.6 kbar, it can be extrapolated that for about 8 kbar (i.e., 2% compression) RbCl:Li+ should become a tunneling system similar to KCl:Li+. Consequences of the observed behavior for the theoretical understanding and host‐lattice trends of Li+ off‐center defects and for the behavior of FA(Li) centers are discussed.