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

Abstract
The crystal structure of K 2 SeO 4 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 SeO 4 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 Σ 2 soft mode were determined and the feature of atomic displacement responsible to this transition is discussed.

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