Phase-transition-induced dipolar relaxation, a new method for the investigation of phase transformations in dielectric materials, applied toK2SnCl6

A new method for the investigation of lattice dynamics at structural phase transformations is presented, TIDR (transition-induced dipolar relaxation). It relies on the fact that the relative positions of molecules and likewise of dipolar units within a crystal are determined by the minima in lattice potential. As a consequence of phase transformations, the lattice potential may change, thus changing the orientation of the dipoles and leading to polarization or depolarization currents. If intrinsic dipolar molecules are not present in the sample, a small concentration of dipolar defects which can act as indicators can easily be introduced via a mild neutron irradiation and subsequent recoil processes of the activated atoms. This technique has been applied to ${\mathrm{K}}_2$Sn${\mathrm{Cl}}_6$, which exhibits two structural phase transitions at $T_1=262\mathrm{}$ K and $T_2=253\mathrm{}$ K. ppm-amounts of Sn${\mathrm{Cl}}_5^{-}$ dipoles were created by neutron-capture-induced ligand recoil. From the measurements a libration of the Sn${\mathrm{Cl}}_5^{-}$ units about two cubic main axes by (6 ± 2)° below $T_2$ could be deduced. This result is in good agreement with recent neutron scattering data on the tilting of the intact Sn${\mathrm{Cl}}_6^{2-}$ octahedra. On the other hand, the data supply indications of 90° jumps of the Sn${\mathrm{Cl}}_5^{-}$ dipoles at $T>~T_1$.