Presentation is loading. Please wait.

Presentation is loading. Please wait.

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.

Similar presentations


Presentation on theme: "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."— Presentation transcript:

1

2 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 YBa 2 Cu 3 O 7 understand how properties are modified by appropriate substitutions

3 Perovskite - an Inorganic Chameleon ABX 3 - three compositional variables, A, B and X CaTiO 3 - dielectric BaTiO 3 - ferroelectric Pb(Mg 1/3 Nb 2/3 )O 3 - relaxor ferroelectric Pb(Zr 1-x Ti x )O 3 - piezoelectric (Ba 1-x La x )TiO 3 - semiconductor (Y 1/3 Ba 2/3 )CuO 3-x - superconductor Na x WO 3 - mixed conductor; electrochromic SrCeO 3 - H - protonic conductor RECoO 3-x - mixed conductor (Li 0.5-3x La 0.5+x )TiO 3 - lithium ion conductor LaMnO 3-x - Giant magneto- resistance

4 Perovskite Structure ABO 3 e.g. KNbO 3 SrTiO 3 LaMnO 3 SrTiO 3 cubic, a = 3.91 Å In SrTiO 3, Ti-O = a/2 = 1.955 Å Sr-O = a 2/2 = 2.765 Å CN of A=12, CN of B=6 OR

5

6 Close Packed?? Not traditional close packing - mixed cation/anion AX 3 ccp layers. B in 1/4 of octahedral sites

7 In SrTiO 3, Ti-O ~ 1.95 Å a typical bond length for Ti-O; stable as a cubic structure In BaTiO 3, 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 larger

8 This creates a net dipole moment : Displacement by 5-10% Ti-O bond length Random dipole orientationsparaelectric Aligned dipole orientationsferroelectric 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

9 Define the permittivity or dielectric constant of a material by: H 2 O is a polar liquid; ´ ~ 80 Typical ionic solids; ´ ~ 10 Air; ´ ~ 1 BaTiO 3 :-

10 Below 120°C, BaTiO 3 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

11 For capacitor applications, need to increase capacitance [energy stored/mass or volume] by increasing Q and thus increasing How to do this? BaTiO 3 is very good at 120°C but want high at room temperature! 1) Partial substitution of Ba by a smaller M 2+ ion - Sr 2+ ; unit cell volume decreases and the phase transition temperature decreases

12 2) Disrupt dipoles by modifying B-site ions 3 Ti 4+ Mg 2+ + 2 Nb 5+ Ti-O ~ 1.96 Å;Nb-O ~2.02Å, Mg-O ~ 2.12Å NbO 6 octahedra may be polar; MgO 6 octahedra are not. Pb(Mg 1/3 Nb 2/3 )O 3 PMN a relaxor ferroelectric

13 Superconductors YBa 2 Cu 3 O 7- Perovskite? (YBa 2 ) Cu 3 O 9-x Oxygen Deficient Triple Perovskite Crosses mark absent oxygens

14 Properties YBa 2 Cu 3 O 7 superconductor - resistance lost completely at temperature T c YBa 2 Cu 3 O 6 semiconductor - oxygen lost from base of unit cell

15 Properties YBa 2 Cu 3 O 7 superconductor - perfect diamagnet (excludes a magnetic field) Magnetic levitation http://www.youtube.com/watch?v=hksy_4Zmh80 http://www.realboring.com/floating-magnet/ Alert!http://www.realboring.com/floating-magnet/

16

17 Properties YBa 2 Cu 3 O 7- As increases: 1) T c decreases 2) symmetry changes from orthorhombic to tetragonal (oxygen atoms rearrange in base) O = orthorhombic, T = tetragonal

18 Changing Properties? Can substitute many elements into YBa 2 Cu 3 O 7 structure: Y lanthanides - no change in T c Y other elements - decrease in T c Ba Sr, Ca - decrease in T c Cu transition metals - decrease in T c Cu Au - very slight increase? Ba La - very slight increase? Generally detrimental! Skakle,. Mat. Sci. Eng: R: Reports, 23 1-40 (1998)

19 Summary The perovskite structure, typified by the formula ABX 3, is a highly adaptable structure In BaTiO 3, Ti is displaced from its site to create a dipole - alignment of this dipole leads to interesting electrical properties BaTiO 3 undergoes a paraelectric-ferroelectric transition at 120ºC. This may be modified by chemical substitutions. The superconductor YBa 2 Cu 3 O 7- (YBCO) can be described as an oxygen deficient perovskite The properties of YBCO change with the crystal structure.


Download ppt "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."

Similar presentations


Ads by Google