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ferrimagnet TN = 585 C (strong coupling between Oh and Td sites)
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Magnetism in spinels
- Fe2O3 inverse defect spinel, used in disk storage
~5 m film deposited on plastic tape
Fe(III)[Fe1.67(III)□0.33]O4
Td Oh
medium-hard ferrimagnet 1 Fe(III) Td d5 N=5
1.67 Fe(III) Oh d5 N=5
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ReO3
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Perovskites (CaTiO3)
Simple perovskites have an ABX3 stoichiometry. The A cation and X anions, taken together, comprise a close-packed array, with B cations filling 1/4 of the octahedral sites.
An ordered AA’BX3 perovskite
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Perovskites
ABX3 CN A = 12 B = 6 X = 2
common for oxides and fluorides (ex NaFeF3)
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Ruddlesden-Popper phases
K2NiF4
Sr3Fe2O7
Ca4Mn3O10
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YBa2Cu3O7
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Tl2Ba3Ca2Cu3O10
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Ferroelectrics
Ideal perovskite structure has cubic symmetry (centrosymmetric)
But structures are often distorted to be non-centrosymmetric
These can be ferroelectric
In BaTiO3 , the Ti cation is a little smaller than the Oh site (Ti-O ~ 1.95Å), and is displaced ~0.1Å off site center towards an oxide ligand, forming a dipole
Above Tc (=120 C) the dipoles are randomly oriented, and structure is cubic (paraelectic)
Below Tc - all dipoles orient along the same direction (ferroelectric)
Note: ferroelectricity is named by analogy to ferromagnetism, but it is not common for Fe-containing materials
Also: antiferroelectric ferrielectric
one difference – dipole ordering is tied to structural change
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BaTiO3
Dielectric constant vs temp
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Ferro/piezoelectricsCaTiO3 is not ferroelectric, the smaller Ca2+ ion reduces Oh site and Ti4+ is not small enough to displace off center
BaxSr1x TiO3 (BST) is ferroelectric with a lower Tc, so the max in ε’ occurs at a lower temp. It’s used in dynamic RAM (DRAM) capacitor elements
ε’
Ex: water 80
TiO2, MgTiO3 10–100
BST ferroelectrics 4000-8000
piezoelectrics – crystals polarize under applied mechanical stress and vice versa (applied E across crystal generates lattice strain)
crystals must be noncentrosymmetric
P = d P = polarization, σ = mechanical stress
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Piezoelectrics
Piezoelectrics: ex: quartz crystal, BaTiO3
PbZrxTi1xO3 (PZT) actuators, x~0.5 highest d
positioning - apply E induce
Qz transducers (pressure measurement)
use from sensed pressure to produce E signal
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Two-zone transport
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MX2
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Layered structuresMO2 and MS2 structures and intercalation
Two basic structure types with different cation coordnation geometries
1. CdI2 structure, cations in Oh sites, filling alternate layers
To observe CDW typical tunnelling parameters of 2-3 nA and 10-20 mV gap voltage were observed. The atomic lattice can be seen simul- taneously when the current is increased to higher values (30 - 40 nA).
TaS2 (and TaSe2) exhibit an electronic phase transition from a normal into a condensed state which is called the Charge Density Wave (CDW) state. The transition is caused by an electron-phonon coupling. STM images of TaS2 show a triangular atomic lattice (a0=0.33 nm) with a superimposed CDW lattice of about 3.5 a0. The CDW lattice is rotated 11° with respect to the atomic lattice.