important crystal structures: perovskite structuren.ethz.ch/~nielssi/download/4. semester/ac...

Important crystal structures: Perovskite structure

1 5/29/2013 L.Viciu| ACII| Perovkite structure

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


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


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

5/29/2013

L.Viciu| ACII| Perovkite structure 0, 1, ½

0, 1, ½

0, 1

0, 1

½

Elements found in the perovskite structure


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 magnetoresistance


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


rA + rO=2(rB + rO)

8

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

5/29/2013 L.Viciu| ACII| Perovkite structure

SrTiO3

5/29/2013 L.Viciu| ACII| Perovkite structure 9

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


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


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


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

Aligned dipole orientation = ferroelectric 13 5/29/2013 L.Viciu| ACII| Perovkite structure

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


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)

15

rSr2+=1.13Å rBa

2+=1.35Å


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


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


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


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

Parallel or Ferromagnetic 19 5/29/2013 L.Viciu| ACII| Perovkite structure

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

20

Bi2O2 (fluorite like layer)

AO -Rock salt layers



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:


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


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.


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


1-2-3 Superconductors YBa2Cu3O7-x ( x < 0.1): Tc = 93K

26

2 out of 6 O-Positions of the Perovskites are unoccupied

Perovskit 3 unit cells (A=Ba, A‘=Y, B=Cu)


YBa2Cu3O7-x (x 0.07 optimum for highest Tc)

Ba

Y

Ba

O(1)

O(2)

CuO2

planes O(3)

O(4)

CuO chains


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


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)


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

30

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


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


important crystal structures: perovskite structuren.ethz.ch/~nielssi/download/4. semester/ac...

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