1. 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

2. 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
(Li0.5-3xLa0.5+x)TiO3 - lithium ion conductor
LaMnO3-x - Giant magneto- resistance

3. Perovskite Structure In SrTiO3, Ti-O = a/2 = 1.955 Å
Sr-O = a2/2 = Å
ABO3 e.g. KNbO3
SrTiO3
LaMnO3
SrTiO3 cubic, a = 3.91 Å
CN of A=12, CN of B=6 OR

5. Close Packed?? Not traditional close packing - mixed cation/anion
AX3 ccp layers.
B in 1/4 of octahedral sites

6. 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, > 2.0 Å Too long for a stable structure.
Ti displaces off its central position towards one oxygen
 square pyramidal coordination

7. 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

8. Define the permittivity or dielectric constant of a material by:
H2O is a polar liquid; ´ ~ 80
Typical ionic solids; ´ ~ 10
Air; ´ ~ 1
BaTiO3 :-

9. Below 120°C, BaTiO3 is ferroelectric with aligned dipoles.
Residual dipole disorder gives ´~
At ~127°C, tetragonal  cubic phase transition.
Dipoles randomise and ´ increases to ~5,000-10,000

10. 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 120°C 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

11. a relaxor ferroelectric
2) Disrupt dipoles by modifying B-site ions
3 Ti4+  Mg 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

12. Superconductors YBa2Cu3O7- Perovskite? (YBa2) Cu3 O9-x
Oxygen Deficient
Triple Perovskite
Crosses mark absent oxygens

13. Properties YBa2Cu3O7 superconductor
- resistance lost completely at temperature Tc
YBa2Cu3O6
semiconductor
- oxygen lost from base of unit cell

14. Properties YBa2Cu3O7 superconductor
- perfect diamagnet (excludes a magnetic field)
Magnetic levitation
Alert!

16. Properties YBa2Cu3O7- As  increases: 1) Tc decreases
2) symmetry changes from orthorhombic to tetragonal
(oxygen atoms rearrange in base)
O = orthorhombic, T = tetragonal

17. 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, (1998)

18. 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.