Lattice-Dynamical Study of the 110°K Phase Transition in SrTiO3
The lattice-dynamical characteristic of the 110\ifmmode^\circ\else\textdegree\fi{}K transition in SrTi${\mathrm{O}}_{3}$ has been elucidated by inelastic neutron scattering measurements. The transition is caused by a ${\mathrm{T}}_{25}$ soft-phonon mode at the [111] zone boundary, confirming the model recently proposed by Fleury, Scott, and Worlock. The square of the frequency of this soft mode is proportional to $T\ensuremath{-}{T}_{0}$ above the transition temperature ${T}_{0}$. At the transition, this zone boundary becomes a superlattice point, enlarging the unit cell. The results of elastic intensity measurements at the superlattice points at 78\ifmmode^\circ\else\textdegree\fi{}K are consistent with the structure given by Unoki and Sakudo in its space group $\frac{I4}{mcm({{D}_{4h}}^{18})}$ as well as in the oxygen parameter. This structure is shown to be a logical consequence of the condensed soft mode.