A Structural Study of the Incommensurate-to-Ferroelectric Phase Transition in K₂SeO₄

Abstract

The crystal structure of K₂SeO₄ in the ferroelectric phase was analyzed and the structural change at the transition from incommensurate to ferroelectric phase was elucidated. The phase transition is characterized by a commensuration of structural modulation as well as an additional homogeneous structural change both in rotation of SeO₄ radicals around the y-axis and in translation of K ions along the z-axis. The spontaneous polarization in the ferroelectric phase is estimated on the basis of these structural changes and the contribution of the latter additional part is presumed to attain 60% or so. The symmetry coordinate components of Σ₂ soft mode were determined and the feature of atomic displacement responsible to this transition is discussed.