Soft modes and the structure of ferroelectric tetragonal KTN. II. The lattice dynamics of the cubic phase

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Abstract
For pt. I see abstr. A11594 of 1973. The thermal diffuse streaks observed in cubic KTN with X-rays a and neutrons are attributed to strong anisotropy in the transverse phonon dispersion surfaces. The frequencies of these modes fall sharply on the (100) symmetry planes, and the thermal diffuse intensity, which is inversely proportional to the square of the mode frequency, rises rapidly. Close to the zone boundary this intensity is dominated by the acoustic branch, which is flat at an energy of 7 meV for 0.3a*5 with uOI=uOII=-uTa,Nb. Near the zone centre, the temperature dependence of the streak is given by AT+BT(T-Tc)-1 where AT is the acoustic mode contribution and BT(T-Tc)-1 arises from a soft optic mode with frequency omega varies as (T-Tc)12/. The eigenvectors of this optic mode correspond with the atomic displacements at the ferroelectric transition.