Dielectric Response of Alkaline Earth Chromates containing Cr3+ and Cr5+ compounds
Date Issued
2012
Date
2012
Author(s)
Li, Huai-Min
Abstract
Abstract
The synthesis, characterization and dielectric properties of polycrystalline alkaline earth chromates Ca2+Cr23+O42-, Ca22+Cr23+O52-, and A32+Cr25+O82- (A = Ca, Sr, and Ba) compounds were investigated. Powder X-ray diffraction patterns reveal that all the samples are single phase without any impurity. The dielectric properties of these compounds were studied in the frequency range from 20 Hz to 1 MHz between 20 K and 320 K.
The Cr3+ valence contains CaCr2O4 sample exhibits a high dielectric permittivity (ε'' ~ 104) at room temperature with frequency dispersion. In addition, ε''(T) also shows
maxima at ~ 40 K. The corresponding tanδ (T) curves exhibit two relaxations at T < 40
K and ~ 260 K. The peak temperature TP of tanδ shifts to a higher temperature as the
frequency increases in both relaxations. The overall behavior of high temperature ε'' and
tanδ is similar to that observed in giant dielectric constant materials. The observed giant
dielectric response was explained in terms of internal (grain boundary) barrier layer
capacitance (IBLC) effect and maxima at ~ 40 K is related to the development of
long-range antiferromagnetic order. Moreover, calcium rich Cr3+ valence Ca2Cr2O5
sample shows very small value of ε'' compare to the value of CaCr2O4.
The ε''(T) curves of A3Cr2O8 (A = Ca, Sr, and Ba) samples reveal that the value
of real dielectric permittivity decreases with increasing A-site ionic radius. The tanδ(T)
curves of these compounds exhibit dielectric relaxation at high temperature. These peak
temperatures also found to be decreased with increasing A-site ionic radius. The peak
temperatures TP of tanδ shifts to a higher temperature as the frequency increases in all
samples. The structural, orbital or spin fluctuations are likely origin for the observed
results of Cr5+ valence compounds.
Subjects
Dielectric constant
dissipation factor
AC conductivity
Antiferromagnetic order
Spin-dimer
Jahn - Teller Effect
Type
thesis
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