Abstract
The dielectric constant of NaNH 4 SeO 4 ·2H 2 O along the c axis under stress-free and clamped conditions, e 33 f and e 33 c , were experimentally determined, and its elastic stiffnesses under constant electric field and constant electric polarization conditions, c 66 E and c 66 P , and other elastic stiffnesses were also determined experimentally. It was clarified that both e 33 c and 1/ c 66 P ( s 66 P ) follow the Curie-Weiss law, but the phase transition in NaNH 4 SeO 4 ·2H 2 O is caused by predominant dielectric instability more than elastic instability. It was predicted basing on the Landau theory that one of three elastic shear stiffnesses, c 66 E , shows discontinuous behaviour at the Curie temperature T c , but the other two, c 44 E and c 55 E , show continuous behaviours at T c , if the phase transition is of first order. The prediction was confirmed to be the case by the experimental results.
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