Objectives

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Objectives

• By the end of this section you should
• be able to identify and draw the perovskite
  structure
• understand how the perovskite structure can
  become polarisable
• know the basic properties of barium titanate
• know the basic properties of YBa2Cu3O7
• understand how properties are modified by
  appropriate substitutions

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Perovskite - an Inorganic Chameleon

• ABX3 - three compositional variables, A, B and X

• CaTiO3 - dielectric
• BaTiO3 - ferroelectric
• Pb(Mg1/3Nb2/3)O3 - relaxor ferroelectric
• Pb(Zr1-xTix)O3 - piezoelectric
• (Ba1-xLax)TiO3 - semiconductor
• (Y1/3Ba2/3)CuO3-x - superconductor

• NaxWO3 - mixed conductor electrochromic
• SrCeO3 - H - protonic conductor
• RECoO3-x - mixed conductor
• (Li0.5-3xLa0.5x)TiO3 - lithium ion conductor
• LaMnO3-x - Giant magneto- resistance

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Perovskite Structure
In SrTiO3, Ti-O a/2 1.955 Å Sr-O a?2/2
2.765 Å
ABO3 e.g. KNbO3 SrTiO3 LaMnO3
SrTiO3 cubic, a 3.91 Å
CN of A12, CN of B6
OR
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Close Packed??

• Not traditional close packing - mixed cation/anion

AX3 ccp layers. B in 1/4 of octahedral sites
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In SrTiO3, Ti-O 1.95 Å a typical bond
length for Ti-O stable as a cubic structure
larger
In BaTiO3, Ti-O is stretched, gt 2.0 Å Too
long for a stable structure. Ti displaces off its
central position towards one oxygen ? square
pyramidal coordination
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This creates a net dipole moment
Displacement by 5-10 Ti-O bond length
Random dipole orientations
paraelectric
Aligned dipole orientations
ferroelectric
Under an applied electric field, dipole
orientations can be reversed, i.e. the structure
is polarisable Dipoles tend to be frozen in at
room temperature as increase temperature,
thermal vibrations increase the polarisability
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Define the permittivity or dielectric constant of
a material by
H2O is a polar liquid ? 80 Typical ionic
solids ? 10 Air ? 1 BaTiO3 -
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Below 120C, BaTiO3 is ferroelectric with aligned
dipoles. Residual dipole disorder gives
?200-1000 At 127C, tetragonal ? cubic phase
transition. Dipoles randomise and ? increases to
5,000-10,000
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For capacitor applications, need to increase
capacitance energy stored/mass or volume by
increasing Q and thus increasing ?? How to do
this? BaTiO3 is very good at 120C but want high
?? at room temperature!
1) Partial substitution of Ba by a smaller M2
ion - Sr2 unit cell volume decreases and the
phase transition temperature decreases
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2) Disrupt dipoles by modifying B-site ions 3
Ti4 ? Mg2 2 Nb5 Ti-O 1.96 Å Nb-O 2.02Å,
Mg-O 2.12Å NbO6 octahedra may be polar MgO6
octahedra are not.
Pb(Mg1/3Nb2/3)O3 PMN a relaxor ferroelectric
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Superconductors
YBa2Cu3O7-? Perovskite? (YBa2) Cu3 O9-x
Oxygen Deficient Triple Perovskite Crosses mark
absent oxygens
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Properties

• YBa2Cu3O7
• superconductor
• - resistance lost completely at temperature Tc

YBa2Cu3O6 semiconductor - oxygen lost from base
of unit cell
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Properties

• YBa2Cu3O7
• superconductor
• - perfect diamagnet (excludes a magnetic field)

Magnetic levitation

• http//www.youtube.com/watch?vhksy_4Zmh80
• http//www.realboring.com/floating-magnet/
  Alert!

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Properties
YBa2Cu3O7-? As ? increases 1) Tc decreases 2)
symmetry changes from orthorhombic to
tetragonal (oxygen atoms rearrange in base)
O orthorhombic, T tetragonal
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Changing Properties?
Can substitute many elements into YBa2Cu3O7
structure Y ? lanthanides - no change in Tc Y
? other elements - decrease in Tc Ba ? Sr, Ca -
decrease in Tc Cu ? transition metals - decrease
in Tc Cu ? Au - very slight increase? Ba ? La -
very slight increase? Generally detrimental!
Skakle, .Mat. Sci. Eng R Reports, 23 1-40
(1998)
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Summary

• The perovskite structure, typified by the formula
  ABX3, is a highly adaptable structure
• In BaTiO3, Ti is displaced from its site to
  create a dipole - alignment of this dipole leads
  to interesting electrical properties
• BaTiO3 undergoes a paraelectric-ferroelectric
  transition at 120ºC. This may be modified by
  chemical substitutions.
• The superconductor YBa2Cu3O7-? (YBCO) can be
  described as an oxygen deficient perovskite
• The properties of YBCO change with the crystal
  structure.