The transformation of Gd 2 (MoO 4 ) 3 from the prototypic phase to the ferroelectric-ferroelastic phase should be caused by instability of two degenerate modes of lattice vibration each of which has a wave length equal to 2 a in both the tetragonal a and b directions. Upon this recognition and a few assumptions, a comprehensive investigation is made into the temperature dependences of electrical, mechanical and optical properties of Gd 2 (MoO 4 ) 3 . Of the deduced results those which have experimental data available for comparison are found to agree qualitatively well with the experimental results. The order of the phase transformation is affirmed to be first.
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Kêitsirô Aizu (1971) studied this question.
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