Abstract
Structural phase changes in K 2 SeO 4 have been found at 129.5°K and 93°K by introducing free radical SeO 4 - as a probe. The structure in the temperature range between 129.5°K and 93°K is expected to be orthorhombic with the crystallographic axes coincident with those of room temperature. The unit cell may contain twelve molecules located at three different sites. A superstructure, a tripled lattice parameter along the a axis, has been found by X-ray diffraction method and an expected space group is Pnam . It is ferroelectric along the c axis below 93°K. A possible unit cell contains twelve molecules located at six different sites and is monoclinic in the space group P 2 1 , in which the superstructure is maintained. The crystal probably consists of fine twins with the common c axis and they are related with each other by mirror planes. The higher transition is of second order and the lower of first order. Conformations of radicals SeO 2 - and SeO 3 - in the crystal lattice are illustrated.
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