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FIR Reflectivity, Pyroelectric Effect and Dielectric Constant in 8% Brominated BCCD
Author(s) -
Vieira L. G.,
Almeida A.,
Ribeiro J. L.,
Chaves M. R.,
Klöpperpierper A.,
Albers J.
Publication year - 1997
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 - 10.1002/1521-3951(199712)204:2<863::aid-pssb863>3.0.co;2-9
Subject(s) - pyroelectricity , ferroelectricity , dielectric , raman spectroscopy , phase transition , hydrostatic pressure , condensed matter physics , materials science , phase (matter) , phase diagram , analytical chemistry (journal) , chemistry , optics , thermodynamics , physics , optoelectronics , organic chemistry , chromatography
Betaine calcium chloride dihydrate is known to exhibit a rich number of commensurate and incommensurate phases ( k = δ ( T ) c *) between a room temperature paraelectric phase (Pnma) and a lower temperature ferroelectric phase (Pn2 1 a). Partial bromination of this compound induces important changes in the phase diagram of the material. This work reports recent results of FIR reflectivity, dielectric dispersion and pyroelectric effect measured on 8% brominated BCCD. In agreement with previous Raman results reported by Schaack and coworkers, the FIR reflectivity ( E ∥ b ) spectra (20 cm —1 < ω < 300 cm —1 show that the sequence of commensurate and incommensurate phases observed in the pure compound is shifted towards lower temperatures and that the structure remains modulated at lower temperatures (modulation wavenumber δ ≈ 1/4 at T = 20 K). The temperature and frequency dependence of the dielectric constants ε a and ε b , as well as the behaviour of the pyroelectric coefficient λ b observed along a sequence of thermal cycles with and without a bias field, indicate a progressive deviation from thermal equilibrium and a subsequent freezing of the structure at low temperatures. The experimental results are discussed within the framework of a phenomenological model based on a Landau‐type free energy expansion, previously reported to account for the phase transition sequence observed in the pure crystal under a positive hydrostatic pressure.