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

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

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 A=12, CN of B=6 OR

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

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

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

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

Below 120°C, BaTiO3 is ferroelectric with aligned dipoles. Residual dipole disorder gives ´~200-1000 At ~127°C, tetragonal  cubic phase transition. Dipoles randomise and ´ increases to ~5,000-10,000

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

a relaxor ferroelectric 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

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

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

Properties YBa2Cu3O7 superconductor - perfect diamagnet (excludes a magnetic field) Magnetic levitation http://www.youtube.com/watch?v=hksy_4Zmh80 http://www.realboring.com/floating-magnet/ Alert!

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

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)

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.