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The accepted view of the dynamics of the structural phase transition in SrTi03 (T~-100'K) and KMnF3 (T, -186'K) had been that of a soft zone-boundary (j =qR) phonon driving the transition with a frequency " that goes to zero as T-T~f rom above. Recently, Riste et al. observed in the neutron-scattering spectra of SrTi03 a temperature-dependent central component centered around (& = 0 and q =q~in addition to the phonon sidebands centered around & = ". We report on higher-resolution neutron studies of the central component and sidebands in SrTi03, as well. as the observation of a central peak in KMnF3. The central component is interpreted as arising from a low-frequency resonance in the self-energy of the soft mode and is characterized by a frequency~0 related to &"by the anharmonic coupling constant p, &0=&p The temperature behavior of the SrTi03 spectra as T decreases shows a rapid increase in the intensity of the central component as the sideband frequency (,)"decreases. Near T" the intensity of the central peak diverges and~"approaches a finite limiting value. Since (do is proportional to the total intensity, the diverging intensity implies that (, )0-0 as T-T~. It is shown that the temperature dependence of~o is stronger than that predicted from mean-field theory. The energy width of the central component is &0.02 meV and we observe a narrow temperature-dependent q width. In KMnF3, the central component is clearly seen even though the soft phonons are already overdamped at 40 K from T~. The observed spectra have a similar behavior with decreasing temperature as studied in SrTi03.

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A neutron-scattering experiment to look for spin-wave excitation-energy changes of magnetite at the transition point 119°K showed no such change. During this work we discovered, however, that the lowtemperature structure gives rise to weak extra neutron-diffraction lines that can be indexed as (h, k, l±!) in cubic indices. From the observed neutron polarization sensitivity of the intensities and the increasing structure factors with increasing sinO/X, we conclude that the extra reflections are not directly due to magnetic ordering (ferrimagnetic spirals or Fe2+-Fe H ordering on the octahedral sites). Rather they are caused by a lowering of the crystal symmetry.

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