Important crystal structures: Perovskite structure. 5/29/2013 L.Viciu ACII Perovkite structure

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1 Important crystal structures: Perovskite structure 1

2 A. Structures derived from cubic close packed 1. NaCl- rock salt 2. CaF 2 fluorite/na 2 O- antifluorite 3. diamond 4. ZnS- blende B. Structures derived from hexagonal close packed 1. NiAs nickel arsenide 2. ZnS wurtzite 3. CdI 2 cadmium iodide 4. CdCl 2 cadmium chloride C. Non close packed structures 1. CsCl cesium chloride 2. MoS 2 - molybdenite D. Metal oxide structures 1. TiO2- rutile 2. ReO3 rhenium trioxide 3. CaTiO3 perovskite 4. MgAlO 4 - Spinel 2

jpg CaTiO 3 CaTiO 3 mineral was discovered in the Ural

3 Perovskites: ABO 3 CaTiO 3 CaTiO 3 mineral was discovered in the Ural mountains (Rusia) in 1839 and is named after Russian mineralogist L.A. Perovski ( ) 3

1, ½ 5/29/2013 ½ 0, 1 0, 1, ½ 0, 1 ABO 3 A: 12-coordinate by O (cuboctahedral) B: 6-coordinate by

4 Perovskite: SrTiO 3 Ti at (0, 0, 0); Sr at ( 1 / 2, 1 / 2, 1 / 2 ) Corner shared TiO 6 Oh Face shared SrO 12 cuboctahedra 3O at (½, 0, 0),(0, ½, 0) and (0, 0, ½ ) Ti-O-Ti linear arrangement 0, 1, ½ 5/29/2013 ½ 0, 1 0, 1, ½ 0, 1 ABO 3 A: 12-coordinate by O (cuboctahedral) B: 6-coordinate by O (octahedral) (A fills the vacant centered cubic site in ReO 3 ) L.Viciu ACII Perovkite structure 4

5 Elements found in the perovskite structure ABO 3 - two compositional variables, A and B 5/29/2013 L.Viciu ACII Perovkite structure 5

6 Perovskite - an Inorganic Chameleon 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 magnetoresistance 6

7 Close Packed?? Not traditional close packing - mixed cation (A) and anion SrTiO 3 West book AO 3 (SrO 3 ) c.c.p. layers ideal Perovskite: the cubic cell axis (a) can be related to the ionic radii 2 ra ro a 2 rb ro ; 2 r A + r O = 2(r B + r O ) Examples: NaNbO 3, BaTiO 3, CaZrO 3, YAlO 3, KMgF 3 Many undergo small distortions due to size effects and electronic configuration of the B ion 7

8 Size effects in perovskites (ABO 3 ) 2 r r 0.8 < t < 1.0 perovskite structure; t > 1, B ion requires a smaller site; t r A B O r O " tolerance factor " t < 0.8, the distorted perovskite structure is no longer stable and A ion needs a smaller site orthorhombic (GdFeO 3 ) cubic (SrTiO 3 ) hexagonal (BaNiO 3 ) t GdFeO 3 (t=0.81) SrTiO 3 BaNiO 3 (t=1.13) 8

9 perovskite structure: great stability allowed variation in the tolerance factor (t) and the subsequent distortions with the preservation of the basic framework A and B sites are relatively insensitive to charge distributions: ex: various valence combinations for A and B cations 1 : 5 NaTaO 3 ; 2 : 4 SrTiO 3 3 : 3 LaMnO 3 The structure can withstand considerable departures from ideal stoichiometry: ex: O 2- deficiency: La 0.5 Sr 0.5 TiO 2.5 (50% oxygen deficient LaTiO 3 ) CaFeO 2.5 (the product of CaO and Fe 2 O 3 in air) A deficiency: La 1/3 TaO 3; La 1/3 NbO 3; 9

10 d 0 transition metals in perovskite structure M n+ O 2- LUMO or Conduction Band (CB) O1 Nb O2 HOMO or Valence Band (VB) O3 Out of center distortion Schematic electronic structure of an undistorted d 0 MO 6 Small gap between HOMO and LUMO allows for symmetry distortion This distortion is called Jahn-Teller effect of the second order The distortion is favored because it stabilizes the HOMO, while destabilizing the LUMO Bhuvanesh, N. S. P. and Gopalakrishnan, J.; J. Mater. Chem., 1997, 7(12),

11 Jahn-Teller of the second order The 2nd order JT distortion reduces the symmetry and widens the band gap The stabilization of HOMO disappears when electrons start filling the band i.e. for a d 1 ion - ReO 3 is cubic 1. Octahedrally coordinated high valent d 0 cations (i.e. Ti 4+, Nb 5+, W 6+, Mo 6+ ). BaTiO 3, KNbO 3 (favored as the HOMO-LUMO splitting decreases - covalency of the M-O bonds increases) 2. Cations containing filled valence s shells (Sn 2+, Sb 3+, Pb 2+, Bi 3+ ) Red PbO, SnO, Bi 4 Ti 3 O 12, Ba 3 Bi 2 TeO 9 (2nd order JT distortion leads to development of a stereoactive electron-lone pair) 11

12 BaTiO 3 (1) At temp. >120ᵒC : cubic perovskite structure (a=4.018å) (2) At temp.< 120ᵒC : tetragonal structure (a=3.997å, c=4.031 Å) Views on the [100] direction = a axis (1) (2) c the tetragonal distortion leads to an off-centre displacement of Ti 4+ and the dipoles are pointing along c axis cubic tetragonal tetragonal BaTiO 3 is ferroelectric 12

Polarization due to out of center displacement of d 0 ions (a) Ti position in cubic Oh coordiantion O1 Nb O3 (b) Ti displacement O2 Ti in (b) Ti in (a) - - 0.1 0.

13 Polarization due to out of center displacement of d 0 ions (a) Ti position in cubic Oh coordiantion O1 Nb O3 (b) Ti displacement O2 Ti in (b) Ti in (a) Å Displacement by 5-10% Ti-O bond length creates a net dipole moment The ordering of the displaced ions in the perovskite structure depends on: 1. The valence requirements of anions 2. Cation-cation repulsions An applied electric field can reverse the dipole orientations the structure is polarisable Random dipole orientations = paraelectric 13 Aligned dipole orientation = ferroelectric

14 Properties of d 0 transition metals perovskites BaTiO 3 -first piezoelectric material discovered SrTiO 3 : Insulator, normal dielectric BaTiO 3 : Ferroelectric (Tc ~ 130 C) PbTiO 3 : Ferroelectric (Tc ~ 490 C) KNbO 3 : Ferroelectric (Tc ~ x) KTaO 3 : Insulator, normal dielectric 14

15 SrTiO 3 vs. BaTiO 3 r Sr 2+ =1.13Å r Ba 2+ =1.35Å Square pyramidal coordination (TiO 5 ) Sr 2+ ion is a good fit (d(ti-o)=1.949å), (SrTiO 3 is close to a ferroelectric instability) Ba 2+ ion stretches the octahedra (d(ti- O) 2 Å) this lowers the energy of LUMO 2 nd order Jahn-Teller distortion 15

16 KNbO 3 vs. KTaO 3 Ferroelectric Normal dielectric Similar bonds and behavior like in BaTiO3 Ta 5d orbitals are more electropositive and have a larger spatial extent than Nb 4d orbitals (greater spatial overlap with O 2p), both effects raise the energy of the t 2g LUMO no Jahn-Teller distortion in KTaO 3 16

17 Applications of ferroelectrics For practical applications, the ferroelectric transition should be close to room temperature BaTiO 3 -used as capacitor (storing electric charge) with large capacitance The most important piezoelectric is PZT (PbZrO 3 + PbTiO 3 )- used for sensors, capacitors, actuators and ferroelectric RAM chips PZT = Pb[Zr x Ti 1-x ]O 3 best for x

18 3d n transition metals in perovskites Compound Electrical Property Magnetic Property SrTiO 3 (d 0 ) SrVO 3 (d 1 ) SrCrO 3 (d 2 ) CaMnO 3 (d 3 ) LaMnO 3- (d 3 ) SrFeO 3 (d 4 ) Insulating Metallic Metallic Semiconductor Colossal magnetoresistance Metallic Diamagnetic Pauli paramagnetism Pauli paramagnetism Antiferromagnetic Antiferromagnetic Spiral antiferromagnetic Unpaired electrons in the d shell leads to magnetic interactions through the oxygen p orbitals Dramatic change in resistivity in an applied magnetic field gives rise to colossal magnetoresistance Pauli paramagnetism is the paramagnetism induced by the excited conduction electrons 18

19 Magnetism in perovskites There are two interaction mechanisms : 1. superexchange that leads to antiparallel spin alignment 2. double exchange that leads to parallel spin alignment (2) Double exchange (1) Superexchange e g d-orbital (M) p-orbital (X) d-orbital (M) t 2g Mn 3+ (d 4 ) Mn 4+ (d 3 ) Antiparallel or Antiferromagnetic Mn 3+ (d 4 ) Mn 4+ (d 3 ) O 2- Mn 4+ (d 3 ) O 2- Mn 3+ (d 4 ) Parallel or Ferromagnetic 19

20 Layered perovskites Dion-Jacobson, A[A n-1 B n O 3n+1 ] RbLaNb O 2 7 Ruddlesden-Popper, A 2 [A n-1 B n O 3n+1 ] (AO)(ABO3)n Aurivillius, (Bi 2 O 2 )[A n-1 M n O 3n+1 ] NbO 6 La NbO 6 Rb NbO 6 La NbO 6 AO - Rock salt layers Bi 2 O 2 (fluorite like layer) suitable systems for investigation the two-dimensional physical properties 20

21 Bi 4 Ti 3 O 12 =(Bi 2 O 2 )Bi 2 Ti 3 O 10 Bi 3 TiNbO 7 =(Bi 2 O 2 )BiTiNbO 7 n=3 n=2 Bi 2 O 2 (fluorite like layer) 21

22 Ruddlesden-Popper (R.P.) phases of Ruthenium: (AO) n+1 (RuO 2 ) n : 1. Ca 3 Ru 2 O 7 (n=2): Mott Hubbard insulator 2. CaRuO 3 (n= ): paramagnet (becomes ferromagnetic upon chemical doping) 3. SrRuO 3 (n= ): ferromagnetic 4. Sr 3 Ru 2 O 7 (n=2): metamagnet 5. Sr 2 RuO 4 (n=1): superconducting at 1 K Sr 2 RuO 4 22

23 It may be viewed as if constructed from an ABAB... arrangement of Perovskite cells Also known as an intergrowth structures La 2 CuO 4 A B A The transparent atoms are missing Sheets of elongated CuO 6 Oh sharing only corners 23

Doped La 2-x Sr x CuO 4 {La 2-x Sr x CuO 4 } was the first (1986) High-T c Superconducting Oxide (T c ~ 40 K) for which Bednorz & Müller were awarded a Nobel Prize The first of the High Tc

24 Doped La 2-x Sr x CuO 4 {La 2-x Sr x CuO 4 } was the first (1986) High-T c Superconducting Oxide (T c ~ 40 K) for which Bednorz & Müller were awarded a Nobel Prize The first of the High Tc superconductors discovered, La 1.85 Sr 0.15 CuO 4, has the same basic crystal structure as Sr 2 RuO 4, with some subtle but important differences due to the difference in d orbital occupancy. 24

25 Perovskite type superconductors: YBa 2 Cu 3 O 7-x (superconducts over 77 K (Boiling point of N 2 ) 2 out of 6 O-Positions in the structure are unoccupied Perovskit CaTiO 3 Y Cu-Atom coordination: 1/3 square-planar 2/3 square-pyramidal Triple unit cell YBa 2 Cu 3 O 7-x 25

26 1-2-3 Superconductors YBa 2 Cu 3 O 7-x ( x < 0.1): Tc = 93K CuO chains O(1) Ba O(2) Y CuO 2 planes O(3) O(4) Ba 2 out of 6 O-Positions of the Perovskites are unoccupied Perovskit 3 unit cells (A=Ba, A =Y, B=Cu) YBa 2 Cu 3 O 7-x (x 0.07 optimum for highest Tc) 26

27 YBa 2 Cu 3 O 7- = 0.08 Tc=93K > 0.56 not superconductor (tetragonal structure) tetragonal 400 C, O 2 orthorhombic O (1) site almost missing CuO 2 planes are the SC layers 27

28 YBa 2 Cu 3 O 7-x : intergrowth structure Layers stacked in the sequence: Cu(1)O BaO Cu(2)O 2 Y Cu(2)O 2 BaO Cu(1)O Cu(1)O BaO Cu(2)O 2 Y Cu(2)O 2 BaO Cu(1)O UNIQUE SEQUENCE OF LAYERS: 1) Charge reservoirs layers (insulating), such as [Cu(1)O] 2) Spacing layers: such as [BaO]-2 layers 3) Separating layers: such as [Y]-1 layer 4)Superconducting layers [Cu(2)O 2 ]-2 layers 1212 CuBa 2 YCu 2 O 7 (YBa 2 Cu 3 O 7 ) 28

Naming Scheme of the cuprates 1223 TlBa 2 Ca 2 Cu 3 O 9 I. the number of insulating layers between adjacent conducting blocks II. the number of spacing layers between identical CuO 2 blocks III.

29 Naming Scheme of the cuprates 1223 TlBa 2 Ca 2 Cu 3 O 9 I. the number of insulating layers between adjacent conducting blocks II. the number of spacing layers between identical CuO 2 blocks III. the number of layers that separate adjacent CuO 2 planes within the conducting block IV. the number of CuO 2 planes within a conducting block (La 1-x Sr x ) 2 CuO HgBa 2 CaCu 2 O CuBa 2 YCu 2 O 7 (Usually written YBa 2 Cu 3 O 7 ) 1223 TlBa 2 Ca 2 Cu 3 O Bi 2 Sr 2 CuO Tl 2 Ba 2 Ca 3 Cu 4 O 12 Annu. Rev. Mater. Sci : /29/2013 L.Viciu ACII Perovkite structure 29

30 Changing Properties? Can substitute many elements into YBa 2 Cu 3 O 7 structure: Y lanthanides - no change in T c Ba Sr, Ca - decrease in T c Cu transition metals - decrease in T c Y other elements - decrease in T c Ba La - very slight increase? Cu Au - very slight increase? Generally detrimental! Skakle,.Mat. Sci. Eng: R: Reports, (1998) It is believed that the superconductivity depends on the number of CuO 2 planes per unit cell YBa 2 Cu 3 O 7 (1212): 2 CuO 2 layers Tc=93K Bi 2 Sr 2 Ca 2 Cu 3 O 10 (Bi-2223): 3 CuO 2 layers Tc=110K Tl 2 Ba 2 Ca 2 Cu 3 O 10 (Tl-2223): 3 CuO 2 layers Tc=125K HgBa 2 Ca 2 Cu 3 O 8 (Hg-1223): 3 CuO 2 layers Tc=134K 30

31 Composition Physical Property Possible or present application CaTiO 3 Dielectric Microwave applications BaTiO 3 Ferroelectric Non volatile computer memories PbZr 1-x Ti x O 3 (Pb,La)(Zr,Ti)O3 Piezoelectric Optical Sensors Electro-optical modulator Ba 1-x La x TiO 3 Semiconductor Semiconductor applications GdFeO 3, LaMnO 3 Magnetic Magnetic memories, ferromagnetism Y 0.33 Ba 0.67 CuO 3-x Superconductor Magnetic detectors LnCoO 3-x Mixed ionic and electronic Gas diffusion membranes conductor BaInO 2.5 Ionic conductor Electrolyte in solid oxide fuel cells AMnO 3-x Giant magneto resistance Read heads in hard disks YAlO 3, KNbO 3 Optical Laser 31

per unit cell Motif: Re at (0, 0, 0); 3O at ( 1 / 2, 0), (0, 0, 1 / 2 ) Re: 6 (octahedral coordination) O: 2 (linear coordination) ReO 6

Lattice: Primitive Cubic 1ReO 3 per unit cell Motif: Re at (0, 0, 0); 3O at ( 1 / 2, 0, 0), (0, 1 / 2, 0), (0, 0, 1 / 2 ) Re: 6 (octahedral coordination) O: 2 (linear coordination) ReO 6 octahedra share