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 Nb Ti in (b) - O2 0.1 – 0.2Å Ti in (a) - O3 (a) Ti position in cubic Displacement by 5-10% Ti-O bond (b) Ti displacement 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), (SrTiO is close to a ferroelectric instability) O) 2 Å) this lowers the energy of LUMO 3 2nd 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 –type superconductors: YBa2Cu3O7-x
(superconducts over 77 K (Boiling point of N2)
2 out of 6 O-Positions in
the structure are
unoccupied
Cu-Atom coordination:
Perovskit Y 1/3 square-planar CaTiO 3 2/3 square-pyramidal
Triple unit cell
YBa2Cu3O7-x 5/29/2013 L.Viciu| ACII| Perovkite structure 25 1-2-3 Superconductors
YBa2Cu3O7-x ( x < 0.1): Tc = 93K
CuO chains O(1)
Ba O(2)
CuO2 Y planes O(3) O(4) Ba
2 out of 6 O-Positions of the YBa Cu O (x 0.07 optimum for Perovskites are unoccupied 2 3 7-x highest Tc) Perovskit 3 unit cells (A=Ba, A‘=Y, B=Cu) 26 5/29/2013 L.Viciu| ACII| Perovkite structure YBa2Cu3O7- = 0.08 Tc=93K > 0.56 not superconductor (tetragonal structure)
400C, O tetragonal 2orthorhombic
O (1) site almost missing CuO2 planes are the SC layers
5/29/2013 L.Viciu| ACII| Perovkite structure 27 YBa2Cu3O7-x: intergrowth structure Layers stacked in the sequence:
Cu(1)O–BaO–Cu(2)O2–Y–Cu(2)O2–BaO–Cu(1)O
Cu(1)O UNIQUE SEQUENCE OF LAYERS: BaO 1) Charge reservoirs layers (insulating), such Cu(2)O 2 as [Cu(1)O] Y 2) Spacing layers: such as [BaO]-2 layers Cu(2)O2 BaO 3) Separating layers: such as [Y]-1 layer 4)Superconducting layers [Cu(2)O ]-2 layers Cu(1)O 2
1212 CuBa2YCu2O7 (YBa2Cu3O7)
5/29/2013 L.Viciu| ACII| Perovkite structure 28 Naming Scheme of the cuprates
1223 TlBa Ca Cu O 2 2 3 9 I . the number of insulating layers between adjacent conducting blocks
II. the number of spacing layers between identical CuO2 blocks
III. the number of layers that separate adjacent CuO2 planes within the conducting block
IV. the number of CuO2 planes within a conducting block.
0201 (La1-xSrx)2CuO4
1212 HgBa2CaCu2O6
1212 CuBa2YCu2O7 (Usually written YBa2Cu3O7)
1223 TlBa2Ca2Cu3O9
2201 Bi2Sr2CuO6
2234 Tl2Ba2Ca3Cu4O12 Annu. Rev. Mater. Sci. 1997. 27:35–67 L.Viciu| ACII| Perovkite structure 29 5/29/2013 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 Ba La - very slight increase?
Cu transition metals - decrease in Tc Cu Au - very slight increase? Generally detrimental! Skakle, .Mat. Sci. Eng: R: Reports, 23 1-40 (1998)
It is believed that the superconductivity depends on the number of
CuO2 planes per unit cell
YBa2Cu3O7 (1212): 2 CuO2 layers Tc=93K
Bi2Sr2Ca2Cu3O10 (Bi-2223): 3 CuO2 layers Tc=110K
Tl2Ba2Ca2Cu3O10 (Tl-2223): 3 CuO2 layers Tc=125K
HgBa2Ca2Cu3O8 (Hg-1223): 3 CuO2 layers Tc=134K
30 5/29/2013 L.Viciu| ACII| Perovkite structure Composition Physical Property Possible or present application
CaTiO3 Dielectric Microwave applications
BaTiO3 Ferroelectric Non volatile computer memories
PbZr1-xTixO3 Piezoelectric Sensors (Pb,La)(Zr,Ti)O3 Optical Electro-optical modulator
Ba1-xLaxTiO3 Semiconductor Semiconductor applications
GdFeO3, LaMnO3 Magnetic Magnetic memories, ferromagnetism
Y0.33Ba0.67CuO3-x Superconductor Magnetic detectors
LnCoO3-x Mixed ionic and electronic Gas diffusion membranes conductor
BaInO2.5 Ionic conductor Electrolyte in solid oxide fuel cells
AMnO3-x Giant magneto resistance Read heads in hard disks
YAlO3, KNbO3 Optical Laser
5/29/2013 L.Viciu| ACII| Perovkite structure 31