REFINED TREATMENT OF THE MODEL OF LINEARLY COUPLED ANHARMONIC-OSCILLATORS AND ITS APPLICATION TO THE TEMPERATURE-DEPENDENCE OF THE ZONE-CENTER SOFT-MODE FREQUENCIES OF KTAO3 AND SRTIO3

The temperature behavior of the zone-center soft modes of KTaO3 and SrTiO3 is remeasured betwen 5 and 300 K by the technique of hyper-Raman spectroscopy, the experimental errors of soft-mode frequency and damping constant being reduced to below ±1 cm-1. The results are taken as a stimulus to a refined treatment of the prototype dynamical model of a ferroelectric crystal, i.e., a lattice of anharmonic oscillators randomly driven by a heat bath and coupled to each other by a dipolarlike intercell interaction. While the intercell coupling is described as previously in terms of a molecular-field approximation, the pseudoharmonic frequency of the anharmonic oscillator at each lattice site is calculated as exactly as possible within the framework of the statistical-linearization approach. The effective classical potential method is used to test a variety of local anharmonic-oscillator potentials. In the case of KTaO3 excellent fits to the experimental values of the soft-mode frequency Ω0(T) are obtained on the basis of only three parameters characterizing globally the harmonic limit, the anharmonicity, and the dipolar interaction, respectively. In the case of SrTiO3 some uncertainty arises from the unknown dependence of these parameters on the octahedral rotation in the antiferrodistortive phase below 105 K. In both materials Ω0(T) turns out to be too smooth for deducing details of the local anharmonic-oscillator potential like its anisotropy and the ratio of sextic and quartic anharmonicity parameters. On the other hand, local potentials of the multiple-well type with quartic anharmonicities and negative harmonic terms can definitely be excluded. © 1995 The American Physical Society.