Important crystal structures: Perovskite structure 5/29/2013 L.Viciu| ACII| Perovkite structure 1 A. Structures derived from cubic close packed 1. NaCl- rock salt 2. CaF2 – fluorite/Na2O- antifluorite 3. diamond 4. ZnS- blende B. Structures derived from hexagonal close packed 1. NiAs – nickel arsenide 2. ZnS – wurtzite 3. CdI2 – cadmium iodide 4. CdCl2 – cadmium chloride C. Non close packed structures 1. CsCl – cesium chloride 2. MoS2 - molybdenite D. Metal oxide structures 1. TiO2- rutile 2. ReO3 – rhenium trioxide 3. CaTiO3 – perovskite 4. MgAlO4 - Spinel 5/29/2013 L.Viciu| ACII| Perovkite structure 2 Perovskites: ABO3 http://en.wikipedia.org/wiki/File:Perovskite_mineral.jpg CaTiO3 CaTiO3 mineral was discovered in the Ural mountains (Rusia) in 1839 and is named after Russian mineralogist L.A. Perovski (1792–1856) 5/29/2013 L.Viciu| ACII| Perovkite structure 3 Perovskite: SrTiO3 Ti at (0, 0, 0); Corner shared TiO6 Oh Face shared SrO12 cuboctahedra 1 1 1 Sr at ( /2, /2, /2) 3O at (½, 0, 0),(0, ½, 0) and (0, 0, ½ ) Ti-O-Ti linear arrangement ABO3 0, 1, ½ • A: 12-coordinate by O (cuboctahedral) • B: 6-coordinate by O (octahedral) ½ 0, 1 (A fills the vacant centered cubic site in ReO ) 0, 1, ½ 3 0, 1 L.Viciu| ACII| Perovkite structure 4 5/29/2013 Elements found in the perovskite structure ABO3 - two compositional variables, A and B 5/29/2013 5 L.Viciu| ACII| Perovkite structure Perovskite - an Inorganic Chameleon • CaTiO3 - dielectric • NaxWO3 - mixed conductor; electrochromic • BaTiO3 - ferroelectric • SrCeO - H - protonic • Pb(Mg Nb )O - relaxor 3 1/3 2/3 3 conductor ferroelectric • RECoO3-x - mixed conductor • Pb(Zr1-xTix)O3 - piezoelectric • (Li La )TiO - lithium ion • (Ba La )TiO - semiconductor 0.5-3x 0.5+x 3 1-x x 3 conductor • (Y Ba )CuO - 1/3 2/3 3-x • LaMnO - Giant magneto- superconductor 3-x resistance 5/29/2013 L.Viciu| ACII| Perovkite structure 6 Close Packed?? • Not traditional close packing - mixed cation (A) and anion SrTiO3 AO3 (SrO3) c.c.p. layers West book ideal Perovskite: the cubic cell axis (a) can be related to the ionic radii 2r r a 2r r A O ; r + r =2(r + r ) B O 2 A O B O Examples: NaNbO3 , BaTiO3 , CaZrO3 , YAlO3 , KMgF3 Many undergo small distortions due to size effects and electronic configuration of the B ion 5/29/2013 L.Viciu| ACII| Perovkite structure 7 Size effects in perovskites (ABO3) r r t A O "tolerance factor" 2rB rO 0.8 < t < 1.0 perovskite structure; t > 1, B ion requires a smaller site; t < 0.8, the distorted perovskite structure is no longer stable and A ion needs a smaller site 0.8 0.89 1.0 t orthorhombic cubic hexagonal (SrTiO3) (GdFeO3) (BaNiO3) GdFeO (t=0.81) SrTiO3 3 BaNiO3 (t=1.13) 8 5/29/2013 L.Viciu| ACII| Perovkite structure 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 NaTaO3; 2 : 4 SrTiO3 3 : 3 LaMnO3 The structure can withstand considerable departures from ideal stoichiometry: 2- ex: O deficiency: La0.5Sr0.5TiO2.5 (50% oxygen deficient LaTiO3 ) CaFeO2.5 (the product of CaO and Fe2O3 in air) A deficiency: La1/3TaO3; La1/3NbO3; 5/29/2013 L.Viciu| ACII| Perovkite structure 9 d0 transition metals in perovskite structure n+ M 2- O O1 LUMO or Nb Conduction Band (CB) O2 O3 HOMO or Out of center distortion Valence Band (VB) 0 Schematic electronic structure of an undistorted d MO6 • 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), 2297–2306 5/29/2013 L.Viciu| ACII| Perovkite structure 10 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 1 i.e. for a d ion - ReO3 is cubic 1. Octahedrally coordinated high valent d0 cations (i.e. Ti4+, Nb5+, W6+, Mo6+). BaTiO3, KNbO3 (favored as the HOMO-LUMO splitting decreases - covalency of the M-O bonds increases) 2. Cations containing filled valence s shells (Sn2+, Sb3+, Pb2+, Bi3+) Red PbO, SnO, Bi4Ti3O12, Ba3Bi2TeO9 (2nd order JT distortion leads to development of a stereoactive electron-lone pair) 11 5/29/2013 L.Viciu| ACII| Perovkite structure BaTiO3 (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) the tetragonal distortion leads to an off-centre displacement of Ti4+ and the dipoles are pointing along c c axis cubic tetragonal tetragonal BaTiO3 is ferroelectric 5/29/2013 L.Viciu| ACII| Perovkite structure 12 Polarization due to out of center displacement of d0 ions O1 Ti in (b) - Nb0.1 – 0.2Å Ti in (a) - O2 (a) Ti position in cubic Displacement by 5-10% Ti-O bond (b) Ti displacement O3 Oh coordiantion 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 5/29/2013 Aligned dipole orientationL.Viciu| ACII| = Perovkite ferroelectric structure 13 Properties of d0 transition metals perovskites BaTiO3-first piezoelectric material discovered SrTiO3 : Insulator, normal dielectric BaTiO3 : Ferroelectric (Tc ~ 130°C) PbTiO3 : Ferroelectric (Tc ~ 490°C) KNbO3 : Ferroelectric (Tc ~ x) KTaO3 : Insulator, normal dielectric 5/29/2013 L.Viciu| ACII| Perovkite structure 14 SrTiO3 vs. BaTiO3 2+ 2+ r =1.35Å rSr =1.13Å Ba Square pyramidal coordination (TiO5) Ba2+ ion stretches the octahedra (d(Ti- Sr2+ ion is a good fit (d(Ti-O)=1.949Å), O)2 Å) this lowers the energy of LUMO (SrTiO3 is close to a ferroelectric instability) nd 2 order Jahn-Teller distortion 5/29/2013 L.Viciu| ACII| Perovkite structure 15 KNbO3 vs. KTaO3 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 t2g LUMO no Jahn-Teller distortion in KTaO3 5/29/2013 L.Viciu| ACII| Perovkite structure 16 Applications of ferroelectrics For practical applications, the ferroelectric transition should be close to room temperature BaTiO3-used as capacitor (storing electric charge) with large capacitance The most important piezoelectric is PZT (PbZrO3 + PbTiO3)- used for sensors, capacitors, actuators and ferroelectric RAM chips PZT = Pb[ZrxTi1-x]O3 best for x0.5 5/29/2013 L.Viciu| ACII| Perovkite structure 17 3dn transition metals in perovskites Compound Electrical Property Magnetic Property 0 SrTiO3 (d ) Insulating Diamagnetic 1 SrVO3 (d ) Metallic Pauli paramagnetism 2 SrCrO3 (d ) Metallic Pauli paramagnetism 3 CaMnO3 (d ) Semiconductor Antiferromagnetic 3 LaMnO3-(d ) Colossal magnetoresistance Antiferromagnetic 4 SrFeO3 (d ) Metallic 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 5/29/2013 L.Viciu| ACII| Perovkite structure 18 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 eg d-orbital (M) p-orbital (X) d-orbital (M) t2g Mn3+ (d4) Mn4+ (d3) Mn3+ (d4) O2- Mn4+ (d3) Antiparallel or Antiferromagnetic 3+ 4 Mn4+ (d3) O2- Mn (d ) 5/29/2013 L.Viciu| ACII| Perovkite structure Parallel or Ferromagnetic 19 Layered perovskites Dion-Jacobson, Ruddlesden-Popper, Aurivillius, A[A’ B O ] A [A’ B O ] (Bi O )[A M O ] n-1 nRbLaNb3n+1 2 O7 2 n-1 n 3n+1 2 2 n-1 n 3n+1 (AO)(ABO3)n NbO6 La NbO 6 AO - Bi2O2 Rb Rock (fluorite salt NbO like 6 layers La layer) NbO6 suitable systems for investigation the two-dimensional physical properties 5/29/2013 L.Viciu| ACII| Perovkite structure 20 Bi4Ti3O12=(Bi2O2)Bi2Ti3O10 Bi3TiNbO7=(Bi2O2)BiTiNbO7 n=3 n=2 Bi2O2 (fluorite like layer) 5/29/2013 L.Viciu| ACII| Perovkite structure 21 Ruddlesden-Popper (R.P.) phases of Ruthenium: (AO)n+1(RuO2)n: 1. Ca3Ru2O7 (n=2): Mott – Hubbard insulator 2. CaRuO3 (n=): paramagnet (becomes ferromagnetic upon chemical doping) 3. SrRuO3 (n=): ferromagnetic 4. Sr3Ru2O7 (n=2): metamagnet 5. Sr2RuO4 (n=1): superconducting at 1 K Sr2RuO4 5/29/2013 L.Viciu| ACII| Perovkite structure 22 La2CuO4 It may be viewed as if constructed from an …ABAB... arrangement of Perovskite cells Also known as an intergrowth structures A B A Sheets of elongated CuO6 Oh sharing only corners 23 The transparent atoms are missing 5/29/2013 L.Viciu| ACII| Perovkite structure Doped La2-xSrxCuO4 {La2-xSrxCuO4 } was the first (1986) High-Tc Superconducting Oxide (Tc ~ 40 K) for which Bednorz & Müller were awarded a Nobel Prize The first of the ‘‘High Tc superconductors’’ discovered, La1.85Sr0.15CuO4, has the same basic crystal structure as Sr2RuO4, with some subtle but important differences due to the difference in d orbital occupancy. 5/29/2013 L.Viciu| ACII| Perovkite structure 24 Perovskite