1. Perovskites: crystal structure, important compounds and properties

2. Very stable structure, large number of compounds, variety of properties, many practical applications. Key role of the BO 6 octahedra in ferromagnetism and ferroelectricity. Extensive formation of solid solutions  material optimization by composition control and phase transition engineering. A 2+ B 4+ O 2- Ideal cubic perovskite structure (ABO 3 )

3. Oxygen Octaedra Octaedral site B cations : Nb, Ta, Ti, Zr, Fe, Mn, …. Dodecaedral site A cations : K, Na, Ca, Sr, Ba, Pb, Bi, Y, La, … 3 Cubic perovskite SrTiO 3

4. A 2+ B 4+ O 2- Distorted perovskite structure (CaTiO 3 ) Ideal cubic perovskite structure (SrTiO 3 ) The perovskite structure is stable when 0.89  t  1.06 (taking r O = 0.14 nm). Ideal cubic structure only observed at room temperature when t is close to 1. If A ions are small, t <1, and tetragonal, orthorhombic and rhombohedral deformations of structure due to rotation and tilting of the BO 6 octahedra are observed. Other types of deformations are induced by the appearance of spontaneous polarization in ferroelectric perovskites. Because of its flexibility, the perovskite structure can accommodate ions with different size and charge (A I B V O3, A II B IV O 3, A III B III O 3 ) and the same site can be shared by ions with different charge (Ba(Mg 1/3 Nb 2/3 )O 3, Pb(Mn 1/3 Nb 2/3 )O 3 ). The perovskite structure provides the building blocks for the assembly of other important crystal structures: Roddlesden- Popper phases (Ca n+1 Ti n O 3n+1 ), Aurivillius compounds ((Bi 2 O 2 ) 2+ (Bi m-1 Ti m O 3m+1 ) 2- ) and oxygen-deficient brownmillerite compounds (CaAlFeO 5 ) Goldschmidt’s tolerance factor

5. A BO 6 Orthorhombic perovskite ABO 3

6. Reference compound PropertiesExisting and potential applications Notes BaTiO 3 Ferroelectricity, piezoelectricity, high dielectric constantMultilayer ceramic capacitors (MLCCs), PTCR resistors, embedded capacitance Most widely used dielectric ceramic T C = 125°C (Ba,Sr)TiO 3 Non-linear dielectric propertiesTunable microwave devicesUsed in the paraelectric state Pb(Zr,Ti)O 3 Ferroelectricity, piezoelectricityPiezoelectric transducers and actuators, ferroelectric memories (FERAMs) PZT: most successful piezoelectric material Bi 4 Ti 3 O 12 Ferroelectric with high Curie temperatureHigh-temperature actuators, FeRAMsAurivillius compound T C = 675°C (K 0.5 Na 0.5 )NbO 3, Na 0.5 Bi 0.5 TiO 3 Ferroelectricity, piezoelectricityLead-free piezoceramicsPerformances not yet comparable to PZT but rapid progress (Pb,La)(Ti,Zr)O 3 Transparent ferroelectricOptoelectronic devicesFirst transparent ferroelectric ceramic BiFeO 3 Magnetoelectric coupling, high Curie temperatureMagnetic field detectors, memories Most investigated multiferroic compound. T C = 850°C PbMg 1/3 Nb 2/3 O 3 Relaxor ferroelectricCapacitors, actuatorsHigh permittivity, large electrostrictive coefficients, frequency-dependent properties SrRuO 3 FerromagnetismElectrode material for epitaxial ferroelectric thin films (La, A)MnO 3 A = Ca, Sr, Ba Ferromagnetism, giant magnetoresistance, spin- polarized electrons Magnetic field sensors, spin electronic devices SrTiO 3 Incipient ferroelectricity, thermoelectric power, metallic electronic conduction when n-doped, mixed conduction when p-doped, photocatalyst Alternative gate dielectric material, barrier layer capacitors, substrate for epitaxial growth, photoassisted water splitting Multifunctional material LaGaO 3 BaIn 2 O 5 Oxyde-ion conductionElectrolyte in solid oxide fuel cells (SOFCs)BaIn 2 O 5 is an oxygen deficient perovskite with brownmillerite structure. BaCeO 3, BaZrO 3 Proton conductionElectrolyte in protonic solid oxide fuel cells (P- SOFCs) High protonic conduction at 500-700°C (La,Sr)BO 3 (B = Mn, Fe, Co) Mixed conduction, catalystCathode material in SOFCs, oxygen separation membranes, membrane reactors, controlled oxidation of hydrocarbons, Used for SOFC cathodes LaAlO 3 YAlO 3 Host materials for rare-earth luminescent ions,Lasers Substrates for epitaxial film deposition