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New Type of Ferroelectric Soft Mode in Gadolinium Molybdate
Author(s) -
Petzelt J.,
Dvořák V.
Publication year - 1971
Publication title -
physica status solidi (b)
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.51
H-Index - 109
eISSN - 1521-3951
pISSN - 0370-1972
DOI - 10.1002/pssb.2220460141
Subject(s) - phonon , soft modes , condensed matter physics , raman spectroscopy , ferroelectricity , brillouin zone , phase transition , raman scattering , infrared , dielectric , materials science , far infrared , phase (matter) , degenerate energy levels , instability , optics , physics , quantum mechanics , optoelectronics
A group analysis is made of external lattice vibrations in Gd 2 (MoO 4 ) 3 for k = (1, 1, 0) in the high‐symmetry D 2d 3phase and for the Γ‐point in the low‐symmetry C 2v 8phase. It is shown that the phase transition is induced by an instability of one double degenerate T 1 ‐phonon at the M‐point of the Brillouin zone which changes into two A 1 ‐phonons, infrared and Raman active below the phase transition. The far‐infrared reflectivity and transmission measurements show that these critical modes are extremely weak and lie at 45 and 56 cm −1 at −150 °C. When rising the temperature they become indistinct and probably disappear above the phase transition. This behaviour together with known dielectric and Raman scattering experiments is discussed in terms of the recent theory of the soft‐mode behaviour in improper ferroelectrics.