Perovskite Structures
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The name “perovskite” originally refers to a type of minerals with chemical formula CaTiO3, named after Russian mineralogist L. A. Perovski. In general, any crystals with the same structure as CaTiO3 are categorized into the perovskite structure.

Most perovskite crystals are oxides with the general formula ABO3, where A and B represent two different cations and O represents oxygen elements. The A-site cations are larger than B-site cations. Perovskite oxides (ABO3) can be described as the framework of corner-shared BO6 octahedra with 12-coordinated A cations. The unit cell of an ideal perovskite structure is a cube, where the A-site cations locate at the corners of the cube, the B-site cation sits in the body center and oxygen sits in the face centers as shown in Figure 3533a. The structure is a corner-linked oxygen octahedral network. The octahedra are linked in a regular cubic array, resulting in high-symmetry cubic m-3m prototype structure. The 6-fold-coordinated B-site in the center of the oxygen octahedron is occupied by a small highly charged cation with a valence state of 3+, 4+, 5+ or 6+, and the larger 12-fold coordinated A-site between octahedra is occupied by a larger cation with a valence state of 1+, 2+, or 3+.

The main advantage of the perovskite structures is the large flexibility in tailoring the chemical composition and lattice parameter(s) of the system by substituting the different cations that present on both the A and B sites without changing the overall structure completely.

Schematic illustration of ideal perovskite structure

Figure 3533a. Schematic illustration of ideal perovskite structure.

However, most perovskite structures do not have such ideal cubic symmetry and are normally distorted, even for the prototype perovskite, CaTiO3. For instance, the properties of the cations on the A-sites and on the B-sites often induce common distortions such as cation displacements within the octahedra and tilting of the octahedra. In general, the degree of distortion in ABO3 perovskites can be determined by [1],

           Goldschmidt tolerance factor ----------------------------- [3533]

where,
          RA, RB and RO -- The ionic radii of the A-site cation, B-site cation and oxygen anion, respectively.
          t -- Goldschmidt tolerance factor.

In an ideal cubic perovskite crystalline structure the A-site and B-site cations optimize their equilibrium bond distances to the oxygen elements without inducing any distortion of the unit cell at t = 1. When 0.75 < t < 1.05 (that almost all the perovskites have), a distorted perovskite structure can normally be stabilized. Note that most of the cubic perovskites have t values in between 0.8 and 0.9.

All the ferroelectric materials today are based on corner-linked oxygen octahedral structures. The simplest configuration is the well-known perovskite structure. In EELS, perovskite type ferroelectric and high-k dielectric materials, such as BaTiO3 and SrTiO3, normally show only one interband plasmon peak [2–5].

For perovskite cell structures, octahedral oxygen (O) bonding around the atom (Mn for LaMnO3 as shown in Figure 3533b) in B-site generates the crystal field around the B-site atom. The valence states of the atoms (e.g. Mn) are fixed by charge neutrality. In the case of LaMnO3, if the parent compound is doped with Sr2+ for La3+, then Mn3+ (d3) is replaced by Mn4+ in the lattice.

Octahedral oxygen bonding around the Mn atom in LaMnO3

Figure 3533b. Octahedral oxygen bonding around the Mn atom in LaMnO3.

Table 3533a. Atomistic positions in cubic perovskites.

Site Location Co-ordinates
A cation (2a) (0, 0, 0)
B cation (2a) (1/2 , 1/2 , 1/2)
O anion (6b) (1/2 , 1/2 , 0) (1/2 , 0, 1/2) (0, 1/2 , 1/2 )

Table 3533b. Atomistic positions in orthorhombic perovskites.

Site Location Co-ordinates
A cation (4c) ±[(u, v, 1/4)(1/2-u, v+1/2, 1/4)]
B cation (4b) (1/2, 0, 0) (1/2, 1/2, 0) (0, 0, 1/2) (0, 1/2, 1/2)
O(1) anion (4c) ±[(m, n, 1/4) (1/2-m, n+1/2, 1/4)]
O(2) anion (8d) ± [(x, y, z) (1/2-x, y+1/2, 1/2-z) (-x, -y, z+1/2) (x+1/2, 1/2-y, -z)]
* u, v, m, n are dependent on the particular structure under consideration.

Table 3533c. Atomic positions for rhombohedral perovskites.

Site Location Co-ordinates
A cation (6a) (0, 0, 1/4)
B cation (6b) (0, 0, 0)
O anion (18e) (x, 0, 1/4)
* The co-ordinates are based on hexagonal axes.

Table 3533d. Atomic positions for hexagonal perovskites.

Site Location Co-ordinates
A cation 2a (0, 0, z)
A cation 4b (1/3 , 2/3 , z)
B cation 6c (x, 0, z)
O(1) anion 6c (x, 0, z)
O(2) anion 6c (x, 0, z)
O(3) anion 2a (0, 0, z)
O(4) anion 4b (1/3 , 2/3 , z)

Table 3533e. Examples of perovskite crystals.

Substance

A-site B-site Mobile ions Space Group Crystal structure Lattice constant (nm) Angles Ionic conductivity/Scm-1 (at °C) Remarks
Ag3SI     Ag+         1x10-2 at 25 No-oxide anti-perovskite-type
Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF) Ba & Sr Co & Fe             Good catalytic activity above 600 °C
BaTiO3           0.401      
BiFeO3       R3CH Rhombohedral a = 0.55775; b = 0.55775; c = 1.38616 90, 90, 120    
CaCu3Ti4O12 Ca2+ & Cu2+               High permittivity: good for capacitors
CaSnO3           0.392      
CaTiO3           0.384      
CaZrO3           0.402      
CeAlO3       P4/mmm   a = 0.37669; c = 0.37967      
CeCrO3       Pm-3m   0.389      
CeFeO3         Rhombohedral 0.39 90, 90, 90    
CeGaO3           0.3879      
CeVO3       Pbnm   a = 0.5514; b = 0.5557; c = 0.7808      
CrBiO3         Tetragonal a = 0.777; c = 0.808 90, 90, 90    
CsCdBr3           0.533      
CsHgBr3           0.577      
CsHgCl3           0.544      
CsIO3           0.466      
CsPbCl3     Cl-         1.2x10-3 at 500 No-oxide perovskite
CsPbBr3     Br-         8x10-4 at 500 No-oxide perovskite
DyAlO3       Pbnm   a = 0.521; b = 0.531; c=0.74      
DyFeO3       Pbnm   a = 0.5302; b = 0.5598; c = 0.7623      
DyMnO3       Pbnm   a = 0.5842; b = 0.7378; c = 0.528      
ErFeO3       Pbnm   a = 0.5263; b = 0.5582; c = 0.7591      
EuAlO3       Pbnm   a = 0.5271; b = 0.5292; c = 0.7458      
EuFeO3       Pbnm   a = 0.5372; b = 0.5606; c = 0.7685      
GdAlO3         Rhombohedral a = 1.056; b = 1.056; c = 1.289 90, 90.6, 90    
GdAlO3       Pbnm   a = 0.5247; b = 0.5304; c = 0.7447      
GdCoO3           a = 0.3732; b = 0.3807; c = 0.3676      
GdCrO3       Pbnm   a = 0.5312; b 0.5515; c = 0.76 15      
GdFeO3       Pbnm   a = 0.5349; b = 0.5611; c = 0.7669      
GdMnO3           0.382      
GdScO3       Pbnm   a = 0.5742; b = 0.7926; c = 0.5482      
HoFeO3       Pbnm   a = 0.5278; b = 0.5591; c = 0.7602      
KIO3           0.441      
KMgF3           0.397      
KNiF3           0.401      
KZnF3           0.405      
LaAlO3           0.378      
LaAlO3       R-3c Rhombohedral a = 0.5.3647; c = 1.31114 60.1, 90, 90    
LaCoO3       R-3CR Rhombohedral a = 0.53416; b = 0.53416; c = 0.53416 60.99, 60.99, 60.99    
LaCrO3       Pbnm   a = 0.5479; b = 0.77562; c = 0.55161      
LaFeO3       Pbnm   a = 0.55647; b = 0.78551; c = 0.5556      
LaGaO3           0.388      
LaGaO3       Pbnm   a = 0.55245; b = 0.54922; c = 0.7774      
La0.51Li0.34TiO2.94
La & Li Ti Li+         1.4x10-3 at 27 A-site deficient
LaMnO3       Pbnm   a = 0.55367; b = 0.57473; c = 0.76929      
LaNiO3       R-3CH Rhombohedral a = 0.54573; b = 0.54573; c = 1.31601 90, 90, 120    
LaRhO3       Pbnm   a = 0.5524; b = 0.5679; c = 0.79      

La0.9Sr0.1Ga0.8Mg0.2O2.85

La & Sr Ga & Mg O2-         1.5x10-1 at 800 Doped single perovskite oxide
LaTiO3       Pbnm   a = 0.56301; b = 0.55844; c = 0.7901      
LaVO3       Pbnm   a = 0.55518; b = 0.7848; c = 0.5554      
LuFeO3       Pbnm   a = 0.5213; b = 0.5547; c = 0.7565      
NdAlO3       R-3c   a = 0.53223; b = 0.53223; c = 1.29292 90, 90, 120    
NdCoO3       Pbnm   a = 0.53312; b = 0.75482; c = 0.53461      
NdCrO3       Pbnm   a = 0.54798; b = 0.76918; c = 0.54221      
NdFeO3       Pbnm   a = 0.5587; b = 0.7761; c = 0.54505      
NdGaO3       Pbnm   a = 0.54276; b = 0.54979; c = 0.77078      
NdMnO3       Pbnm   a = 0.57119; b = 0.7589; c = 0.54119      
NdScO3       Pbnm   a = 0.5555; b = 0.5744; c = 0.7972      
NdVO3       Pbnm   a = 0.5461; b = 0.558; c = 0.7762      
PrAlO3       R-3c Rhombohedral a = 0.53337; b = 0.53337; c = 1.29766 90, 90, 120    
PrCoO3       Pm-3m   0.378      
PrCrO3       Pbnm   a = 0.5444; b = 0.5484; c = 0.771      
PrFeO3       Pbnm   a = 0.5482; b = 0.5578; c = 0.7786      
PrGaO3       Pbnm   a = 0.54526; b 0.54947; c = 0.7121      
PrMnO3       Pbnm   a = 0.5450; b = 0.5786; c = 0.7589      
PrMnO3           0.382      
PrVO3           a = 0.548; b = 0.559; c = 0.776      
PuAlO3         Rhombohedral 0.533 56.07, 90, 90    
PuCrO3           a = 0.546; b = 0.551; c = 0.776      
PuMnO3       Pbnm   a = 0.54; b = 0.5786; c = 0.7589      
PuVO3       Pbnm   a = 0.548; b = 0.561; c = 0.778      
RbIO3           0.452      
SmCoO3       Pm-3m   0.375      
ScAlO3       Pbnm   a = 0.4937; b = 0.52321; c = 0.72045      
SmAlO3       Pbnm   a = 0.52912; b = 0.52904; c = 0.7474      
SmCrO3       Pm-3m   0.386      
SmFeO3       Pbnm   a = 0.54; b = 0.5597; c = 0.7711      
SmVO3           0.389      
SmVO3           a = 0.54; b = 0.5591; c = 0.768      
SrCe0.95Yb0.05O3-α Sr Ce & Yb H+         1x10-2 at 900 Doped single perovskite oxide + hydrogen
SrCoO3-δ Sr Co             Used as MIEC cathodes in IT-SOFC
SrCo0.8Fe0.2O3-δ (SCF) Sr Co & Fe             Good catalytic activity above 600 °C
SrCo1−xNbxO3−δ Sr Co & Nb5+              
SrTiO3           0.391      
SrZrO3           0.410      
TbFeO3       Pbnm   a = 0.53268; b = 0.55978; c = 0.76406      
YAlO3           0.368      
YAlO3       Pbnm   a = 0.51377; b = 0.52736; c = 0.73085      
YbFeO3       Pbnm   a = 0.5233; b = 0.5557; c = 0.757      
YCrO3       Pbnm   a = 0.5247; b = 0.5518; c = 0.754      
YFeO3       Pbnm   a = 0.52819; b = 0.55957; c = 0.76046      
YScO3       Pbnm   a = 0.5431; b 0.5712; c = 0.7894      

In Table 3533a, some dopants are introduced into the perovskite structures for different purposes. For instance, Sb, Mo or Sn are introduced [15-17] at the Co positions in SrCoO3-δ in order to avoid the formation of the unwanted 2H-hexagonal structure. [6-8]

Table 3533f. Other characteristics of perovskite structures.

Characteristics
Space group Pm-3m (221)

 

 

 

 

 

 

 

 

 

[1] V. M. Goldschmidt, T. Barth, G. Lunde, and W. Zachariasen, Skrifter Norske Videnskaps-Akad. Oslo, Mat.-Nat. Kl. 2, 117 (1926).
[2] K.S. Katti, M. Qian, F. Dogan, M. Sarikaya, J. Am. Ceram. Soc. 85 (2002) 2236–2243.
[3] K. van Benthem, C. Elsasser, R.H. French, J. Appl. Phys. 90 (2001) 6156–6164.
[4] S. Schamm, G. Zanchi, Ultramicroscopy 88 (2001) 211–217.
[5] J. Zhang, A. Visinoiu, F. Heyroth, F. Syrowatka, M. Alexe, D. Hesse, H.S. Leipner, Phys. Rev. B 71 (2005) 064108.
[6] Aguadero, A.; Pérez-Coll, D.; Alonso, J.A.; Skinner, S.J.; Kilner, J. A new family of Mo-doped SrCoO3−δ perovskites for application in reversible solid state electrochemical cells. Chem Mater 2012, 24, 2655-2663.
[7] Wang, S.F.; Hsu, Y.F.; Yeh, C.T.; Huang, C.C.; Lu, H.C. Characteristics of SrCo1−xSnxO3−δ cathode materials for use in solid oxide fuel cells. Solid State Ionics 2012, 227, 10-16.
[8] Aguadero, A.; Alonso, J.A.; Perez-Coll, D.; De la Calle, C.; Fernández-Díaz, M.T.; Goodenough, J.B. SrCo0.95Sb0.05O3-δ as cathode material for high power density solid oxide fuel cells. Chem. Mater 2010, 22, 789–798.

 

 

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