Relaxor ferroelectrics
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Transcript of Relaxor ferroelectrics
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Ref.- L.E. Cross, Ferroelectrics 76, 241 (1987).
Relaxors and Diffuse Phase Transitions
Empirical
δ – diffusiveness coefficient T
ε′
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No X-ray line splitting due to pseudo cubic structure
Line splitting below the structural phase
transition
Very weak anisotropy (pseudo-cubic)
Strong optical anisotropy (birefringence)
No structural phase transition across Tm
1st or 2nd order transition with macroscopic
symmetry change at TC
Strong frequency dispersion
No frequency dispersion
Strong deviation from C-W obeys modified C-W Law
Obeys Curie-Weiss law:
Broad/ diffuse phase transition about Tmax
Sharp narrow phase transitions
Relaxor Ferroelectric Proper Ferroelectric
€
1ε−1εm
=(T −Tm )
γ
C1
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Perovskite - chemical order on B-site - Lone pair electron at A-site
Pb(Zn1/3Nb2/3 )O3
(Sr,Ba)Nb2O6 - Tungsten Bronze (Pb,Ba)Nb2O6 Defect Order-Disorder Site Occupancy
La-dependent (Pb,La) (Zr,Ti) O3 - Defect
All relaxors have some lattice feature that breaks translational symmetry and this frustrates long-range order and dipole alignment to create local clusters of polarizations.
Crystal structure models of the A(BI1/2BII1/2)O3 type perovskite: (a) the ordered structure with a small rattling space and (b) the disordered structure with a large rattling space [open circle = BI (lower valence cation) and solid circle = BII (higher valence cation)].
When an electric field is applied to a disordered perovskite, the B ions (usually high valence ions) with a large rattling space can shift easily without distorting the oxygen framework. Larger polarization can be expected for unit magnitude of electric field; in other words, larger dielectric constants and larger Curie-Weiss constants should be typical in this case. On the other hand, in ordered perovskites with a very small rattling space, the B ions cannot move easily without distorting the octahedron. A smaller permittivity and a Curie-Weiss constant are expected.