Collective effects in a random-site electric dipole system: KTaO3: Li

Abstract

Li, substituting for K in KTa${\mathrm{O}}_3$, creates a local electric dipole, due to its off-center position with respect to the cubic site. We have studied such crystals with different amounts of Li (and in a few cases also doped with Nb, substituting for Ta) by nuclear magnetic resonance, dielectric relaxation, pyroelectricity, ultrasound, and birefringence methods. Birefringence and dielectric susceptibility results show that collective effects between the Li dipoles occur below a fairly well-defined concentration-dependent temperature of the order of 50 K, but nuclear magnetic and dielectric relaxation indicate the absence of criticality at the onset of these effects. These collective effects are related to those arising in spin-glasses. We discuss the data in the light of theoretical models and computer simulations of systems of randomly interacting moments, which predict an apparent condensation into a system of metastable clusters without long-range order.