Important crystal structures: Perovskite structuren.ethz.ch/~nielssi/download/4. Semester/AC...
Transcript of Important crystal structures: Perovskite structuren.ethz.ch/~nielssi/download/4. Semester/AC...
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
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Perovskites: ABO3
http://en.wikipedia.org/wiki/File:Perovskite_mineral.jpg
CaTiO3 mineral was discovered in the Ural mountains (Rusia) in 1839 and is named after Russian mineralogist L.A. Perovski (1792–1856)
CaTiO3
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Perovskite: SrTiO3
ABO3
• A: 12-coordinate by O (cuboctahedral)
• B: 6-coordinate by O (octahedral)
(A fills the vacant centered cubic site in ReO3)
Ti at (0, 0, 0);
Sr at (1/2, 1/2, 1/2)
3O at (½, 0, 0),(0, ½, 0) and (0, 0, ½ )
Ti-O-Ti linear arrangement
Face shared SrO12 cuboctahedra Corner shared TiO6 Oh
4
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L.Viciu| ACII| Perovkite structure 0, 1, ½
0, 1, ½
0, 1
0, 1
½
Elements found in the perovskite structure
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ABO3 - two compositional variables, A and B
Perovskite - an Inorganic Chameleon
• 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
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Close Packed?? • Not traditional close packing - mixed cation (A) and anion
SrTiO3
AO3 (SrO3) c.c.p. layers West book
Examples: NaNbO3 , BaTiO3 , CaZrO3 , YAlO3 , KMgF3
Many undergo small distortions due to size effects and electronic configuration of the B ion
;2
22 OA
OB
rrrra
ideal Perovskite: the cubic cell axis (a) can be related to the ionic radii
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rA + rO=2(rB + rO)
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Size effects in perovskites (ABO3)
""2
factortolerancerr
rrt
OB
OA
GdFeO3 (t=0.81) BaNiO3 (t=1.13)
cubic (SrTiO3)
hexagonal (BaNiO3)
0.8 0.89 1.0
t orthorhombic (GdFeO3)
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
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SrTiO3
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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: ex: O2- 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;
perovskite structure: great stability
d0 transition metals in perovskite structure
Schematic electronic structure of an undistorted d0 MO6
O3
O1
O2
Nb
Bhuvanesh, N. S. P. and Gopalakrishnan, J.; J. Mater. Chem., 1997, 7(12), 2297–2306
• 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
HOMO or Valence Band (VB)
LUMO or Conduction Band (CB)
Out of center distortion
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Mn+ O2-
Jahn-Teller of the second order
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)
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 d1 ion - ReO3 is cubic
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the tetragonal distortion leads to an off-centre displacement of Ti4+ and the dipoles are pointing along c axis
BaTiO3 (1) At temp. >120ᵒC : cubic perovskite structure (a=4.018Å) (2) At temp.< 120ᵒC : tetragonal structure (a=3.997Å, c=4.031 Å)
(1) (2)
c
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tetragonal BaTiO3 is ferroelectric cubic tetragonal
Views on the [100] direction = a axis
Displacement by 5-10% Ti-O bond length creates a net dipole moment
(a) Ti position in cubic Oh coordiantion
(b) Ti displacement
-
- 0.1 – 0.2Å
O3
O1
O2
Nb
Ti in (a)
Ti in (b)
The ordering of the displaced ions in the perovskite structure depends on:
1. The valence requirements of anions
2. Cation-cation repulsions
Polarization due to out of center displacement of d0 ions
An applied electric field can reverse the dipole orientations the structure is polarisable
Random dipole orientations = paraelectric
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SrTiO3 : Insulator, normal dielectric BaTiO3 : Ferroelectric (Tc ~ 130°C) PbTiO3 : Ferroelectric (Tc ~ 490°C) KNbO3 : Ferroelectric (Tc ~ x) KTaO3 : Insulator, normal dielectric
Properties of d0 transition metals perovskites
BaTiO3-first piezoelectric material discovered
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SrTiO3 vs. BaTiO3
Sr2+ ion is a good fit (d(Ti-O)=1.949Å), (SrTiO3 is close to a ferroelectric instability)
Ba2+ ion stretches the octahedra (d(Ti-O)2 Å) this lowers the energy of LUMO 2nd order Jahn-Teller distortion
Square pyramidal coordination (TiO5)
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rSr2+=1.13Å rBa
2+=1.35Å
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KNbO3 vs. KTaO3
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
Ferroelectric Normal dielectric
Similar bonds and behavior like in BaTiO3
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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
Applications of ferroelectrics
PZT = Pb[ZrxTi1-x]O3 best for x0.5
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3dn transition metals in perovskites Compound Electrical Property Magnetic Property
SrTiO3 (d0)
SrVO3 (d1)
SrCrO3 (d2)
CaMnO3 (d3)
LaMnO3-(d3)
SrFeO3 (d4)
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
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Pauli paramagnetism is the paramagnetism induced by the excited conduction electrons
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
(1) Superexchange
d-orbital (M) d-orbital (M) p-orbital (X)
Antiparallel or Antiferromagnetic
(2) Double exchange
Mn3+ (d4) Mn4+ (d3)
t2g
eg
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Mn3+ (d4) Mn4+ (d3) O2-
Mn3+ (d4) Mn4+ (d3) O2-
Layered perovskites
La
NbO6
NbO6
NbO6
La
Rb
NbO6
RbLaNb O 2 7
Ruddlesden-Popper, A2[A’n-1BnO3n+1] (AO)(ABO3)n
Dion-Jacobson, A[A’n-1BnO3n+1]
Aurivillius, (Bi2O2)[An-1MnO3n+1]
suitable systems for investigation the two-dimensional physical properties
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Bi2O2 (fluorite like layer)
AO -Rock salt layers
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Bi2O2 (fluorite like layer)
Bi4Ti3O12=(Bi2O2)Bi2Ti3O10 Bi3TiNbO7=(Bi2O2)BiTiNbO7
n=2 n=3
Sr2RuO4
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
Ruddlesden-Popper (R.P.) phases of Ruthenium: (AO)n+1(RuO2)n:
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It may be viewed as if constructed from an …ABAB... arrangement of Perovskite cells
Also known as an intergrowth structures
La2CuO4
Sheets of elongated CuO6 Oh sharing only corners
A
B
A
The transparent atoms are missing 23
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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.
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Perovskite –type superconductors: YBa2Cu3O7-x
2 out of 6 O-Positions in
the structure are
unoccupied
Cu-Atom coordination:
1/3 square-planar
2/3 square-pyramidal
(superconducts over 77 K (Boiling point of N2)
Perovskit CaTiO3
Triple unit cell
YBa2Cu3O7-x 25
Y
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1-2-3 Superconductors YBa2Cu3O7-x ( x < 0.1): Tc = 93K
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2 out of 6 O-Positions of the Perovskites are unoccupied
Perovskit 3 unit cells (A=Ba, A‘=Y, B=Cu)
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YBa2Cu3O7-x (x 0.07 optimum for highest Tc)
Ba
Y
Ba
O(1)
O(2)
CuO2
planes O(3)
O(4)
CuO chains
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YBa2Cu3O7-
icorthorhomb2,400
tetragonal OC
O (1) site almost missing CuO2 planes are the SC layers
= 0.08 Tc=93K > 0.56 not superconductor (tetragonal structure)
YBa2Cu3O7-x: intergrowth structure
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Layers stacked in the sequence: Cu(1)O–BaO–Cu(2)O2–Y–Cu(2)O2–BaO–Cu(1)O
Cu(1)O
BaO
Cu(2)O2
Cu(2)O2
Y
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)O2]-2 layers
1212 CuBa2YCu2O7 (YBa2Cu3O7)
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0201 (La1-xSrx)2CuO4
1212 HgBa2CaCu2O6
1212 CuBa2YCu2O7 (Usually written YBa2Cu3O7)
1223 TlBa2Ca2Cu3O9
2201 Bi2Sr2CuO6
2234 Tl2Ba2Ca3Cu4O12
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.
Naming Scheme of the cuprates 1223 TlBa2Ca2Cu3O9
Annu. Rev. Mater. Sci. 1997. 27:35–67
Changing Properties? Can substitute many elements into YBa2Cu3O7 structure:
Y lanthanides - no change in Tc
Ba Sr, Ca - decrease in Tc
Cu transition metals - decrease in Tc
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
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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
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Generally detrimental!
Y other elements - decrease in Tc
Ba La - very slight increase?
Cu Au - very slight increase?
Composition Physical Property Possible or present application
CaTiO3 Dielectric Microwave applications
BaTiO3 Ferroelectric Non volatile computer memories
PbZr1-xTixO3
(Pb,La)(Zr,Ti)O3
Piezoelectric
Optical
Sensors
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
conductor
Gas diffusion membranes
BaInO2.5 Ionic conductor Electrolyte in solid oxide fuel cells
AMnO3-x Giant magneto resistance Read heads in hard disks
YAlO3, KNbO3 Optical Laser
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