Stuctural Phase Transition in K₂SeO₄

Abstract

X-ray scattering from K₂SeO₄ crystal at temperatures near T_m=129.5 K is measured to elucidate the nature of the mechanism of the structural phase transition. On approaching T_m the super-lattice reflection at (2/3, 0, 4) decreases slowly, indicating that the transition at T_m is of the second order. The diffuse intensity near (2/3, 0, 4) increases near T_m and its distribution shows a strong anisotropy elongating along the a^*-direction. It is suggested that the transverse optical phonon polarized along the c-direction softens at q=(2π⁄3a, 0, 0) near T_m.