Abstract
The phonon dispersion curves of the transverse acoustic (TA) and the lowest transverse optic (TO) branches in the (001) direction of two (KTN) crystals with niobium concentrations of 0.8 and 1.2 at.% have been measured by inelastic neutron scattering at various temperatures. A pronounced temperature dependence has been observed for both the lowest TO phonon at the centre of the Brillouin zone and the TA phonons in the (001) direction for reduced wave vectors larger than 0.04. The temperature dependence of the TA phonons at large wavevectors shows a significant minimum. From the observation of this minimum it is conjectured that both crystals undergo phase transitions. The measured data have been analysed in the framework of a shell model with an anisotropic, nonlinear core - shell interaction. It is shown that the anomalous behaviour of the TA branch is due to an exchange of vibrational character between the phonons of the lowest TO branch and those of the TA branch. An estimate of the size of dipolar clusters around the niobium impurities is derived from the analysis of the dispersion of the lowest TO modes in the long-wavelength range.
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