Lattice-Dynamical Study of the 110°K Phase Transition in SrTiO3

The lattice-dynamical characteristic of the 110°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 $T-T_0$ 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°K are consistent with the structure given by Unoki and Sakudo in its space group $\frac{I4\mathrm{}}{mcm\left({D_{4h}}^{18}\right)}$ as well as in the oxygen parameter. This structure is shown to be a logical consequence of the condensed soft mode.