Abstract
Optical observation of twin structures, X-ray examinations and the measurement of the dielectric constant of (NH 4 ) 3 H(SeO 4 ) 2 were carried out in reference to the phase transition at 29°C ( T c ). It was found that (NH 4 ) 3 H(SeO 4 ) 2 undergoes the improper ferro-elastic phase transition at T c . The crystal is determined to be monoclinic withthe space group A 2/ a below T c and trigonal with the space group R \bar3 m above T c . The crystal has spontaneous strain ( e 22 - e 11 )/2=-3.0×10 -3 and e 13 =3.3×10 -3 at room temperature. It was clarified by both observation of domains and symmetry consideration that the crystal exhibits twin structures with two types of domain walls and three kinds of domain orientations at room temperature. The behaviour of the dielectric constant observed near T c is well explained by the Landau theory.
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