Perovskite (structure)

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Structure of a perovskite with a chemical formula ABX3. The red spheres are X atoms (usually oxygens), the blue spheres are B-atoms (a smaller metal cation, such as Ti4+), and the green spheres are the A-atoms (a larger metal cation, such as Ca2+). Pictured is the undistorted cubic structure; the symmetry is lowered to orthorhombic, tetragonal or trigonal in many perovskites.[1]
A Perovskite mineral (calcium titanate) from Kusa, Russia. Taken at the Harvard Museum of Natural History.

A perovskite structure is any material with the same type of crystal structure as calcium titanium oxide (CaTiO3), known as the perovskite structure, or XIIA2+VIB4+X2−3 with the oxygen in the face centers.[2] Perovskites take their name from the mineral, which was first discovered in the Ural mountains of Russia by Gustav Rose in 1839 and is named after Russian mineralogist L. A. Perovski (1792–1856). The general chemical formula for perovskite compounds is ABX3, where 'A' and 'B' are two cations of very different sizes, and X is an anion that bonds to both. The 'A' atoms are larger than the 'B' atoms. The ideal cubic-symmetry structure has the B cation in 6-fold coordination, surrounded by an octahedron of anions, and the A cation in 12-fold cuboctahedral coordination. The relative ion size requirements for stability of the cubic structure are quite stringent, so slight buckling and distortion can produce several lower-symmetry distorted versions, in which the coordination numbers of A cations, B cations or both are reduced.

Natural compounds with this structure are perovskite, loparite, and silicate perovskite.[2]

Structure[edit]

The perovskite structure is adopted by many oxides that have the chemical formula ABO3.

In the idealized cubic unit cell of such a compound, type 'A' atom sits at cube corner positions (0, 0, 0), type 'B' atom sits at body centre position (1/2, 1/2, 1/2) and oxygen atoms sit at face centred positions (1/2, 1/2, 0). (The diagram shows edges for an equivalent unit cell with A in body centre, B at the corners, and O in mid-edge).

The relative ion size requirements for stability of the cubic structure are quite stringent, so slight buckling and distortion can produce several lower-symmetry distorted versions, in which the coordination numbers of A cations, B cations or both are reduced. Tilting of the BO6 octahedra reduces the coordination of an undersized A cation from 12 to as low as 8. Conversely, off-centering of an undersized B cation within its octahedron allows it to attain a stable bonding pattern. The resulting electric dipole is responsible for the property of ferroelectricity and shown by perovskites such as BaTiO3 that distort in this fashion.

The orthorhombic and tetragonal phases are most common non-cubic variants.

Complex perovskite structures contain two different B-site cations. This results in the possibility of ordered and disordered variants.

Common occurrence[edit]

As pressure increases, the O2− ions compress so olivine transforms to the spinel structure, which at still higher pressure converts to a perovskite structure and a coexisting periclase structure.[2]

At the high pressure conditions of the Earth's lower mantle, the pyroxene enstatite, MgSiO3, transforms into a denser perovskite-structured polymorph; this phase may be the most common mineral in the Earth.[3] This phase has the orthorhombically distorted perovskite structure (GdFeO3-type structure) that is stable at pressures from ~24 GPa to ~110 GPa. However, it cannot be transported from depths of several hundred km to the Earth's surface without transforming back into less dense materials. At higher pressures, MgSiO3 perovskite transforms to post-perovskite.

Although the most common perovskite compounds contain oxygen, there are a few perovskite compounds that form without oxygen. Fluoride perovskites such as NaMgF3 are well known. A large family of metallic perovskite compounds can be represented by RT3M (R: rare-earth or other relatively large ion, T: transition metal ion and M: light metalloids). The metalloids occupy the octahedrally coordinated "B" sites in these compounds. RPd3B, RRh3B and CeRu3C are examples. MgCNi3 is a metallic perovskite compound and has received lot of attention because of its superconducting properties. An even more exotic type of perovskite is represented by the mixed oxide-aurides of Cs and Rb, such as Cs3AuO, which contain large alkali cations in the traditional "anion" sites, bonded to O2− and Au anions.

Material properties[edit]

Perovskite materials exhibit many interesting and intriguing properties from both the theoretical and the application point of view. Colossal magnetoresistance, ferroelectricity, superconductivity, charge ordering, spin dependent transport, high thermopower and the interplay of structural, magnetic and transport properties are commonly observed features in this family. These compounds are used as sensors and catalyst electrodes in certain types of fuel cells[4] and are candidates for memory devices and spintronics applications.[5]

Many superconducting ceramic materials (the high temperature superconductors) have perovskite-like structures, often with 3 or more metals including copper, and some oxygen positions left vacant. One prime example is yttrium barium copper oxide which can be insulating or superconducting depending on the oxygen content.

Chemical engineers are considering a cobalt-based perovskite material as a replacement for platinum in catalytic converters in diesel vehicles.[6]

Applications[edit]

Physical properties of interest to materials science among perovskites include superconductivity, magnetoresistance, ionic conductivity, and a multitude of dielectric properties, which are of great importance in microelectronics and telecommunication. Because of the flexibility of bond angles inherent in the perovskite structure there are many different types of distortions which can occur from the ideal structure. These include tilting of the octahedra, displacements of the cations out of the centers of their coordination polyhedra, and distortions of the octahedra driven by electronic factors (Jahn-Teller distortions).[7]

Photovoltaics[edit]

Main article: Perovskite solar cell

Synthetic perovskites have been identified as possible inexpensive base materials for high-efficiency commercial photovoltaics[8] – they showed a conversion efficiency of up to 15%[9] and can be manufactured using the same thin-film manufacturing techniques as that used for thin film silicon solar cells.[10] A group of methylammonium tin and lead halides is of interest for use in dye-sensitized solar cells.[11][12] In 2014, efficiencies have reached up to 17.9% certified power conversion efficiency.

Another approach uses organic-inorganic perovskite-structured semiconductors, the most common of which is the triiodide (CH
3
NH
3
PbI
3
). They exhibit high charge carrier mobility and charge carrier lifetime that allow light-generated electrons and holes to move far enough to be extracted as current, instead of losing their energy as heat within the cell. CH
3
NH
3
PbI
3
effective diffusion lengths are some 100 nm for both electrons and holes.[13]

These perovskites are deposited by low-temperature solution methods (typically spin-coating). Other low-temperature (below 100°C) solution-processed films tend to have considerably smaller diffusion lengths. Stranks et al. described nanostructured cells using a combination of methylammonium iodide and lead iodide with a small amount of chloride substitution and demonstrated one amorphous thin-film solar cell with an 11.4 % conversion efficiency, and another that reached 15.4 % using vacuum evaporation. The film thickness of about 500 to 600 nm implies that the electron and hole diffusion lengths were at least of this order. They measured values of the diffusion length exceeding 1 µm for the mixed perovskite, an order of magnitude greater than the 100 nm for the pure iodide. They also showed that carrier lifetimes in the mixed perovskite are longer than in the pure iodide.[13]

For CH
3
NH
3
PbI
3
, open-circuit voltage (VOC) typically approaches 1 V, while for CH
3
NH
3
PbI(I,Cl)
3
, VOC > 1.1 V has been reported. Because the band gaps (Eg) of both are 1.55 eV, VOC-to-Eg ratios are higher than usually observed for similar third-generation cells. With higher band-gap perovskites, VOC up to 1.3 V has been demonstrated.[13]

The technique offers the potential of low cost because of the low temperature solution methods and the absence of rare elements. Cell durability is currently insufficient for commercial use.[13]

Planar heterojunction perovskite solar cells can be manufactured in simplified device architectures (without complex nanostructures) using only vapor deposition. This technique produces 15% solar-to-electrical power conversion as measured under simulated full sunlight. [14]

Lasers[edit]

Also in 2014 researchers demonstrated that perovskite can generate laser light. Methylammonium lead iodide perovskite (CH3NH3PbI3-xClx) cells fashioned into optically pumped vertical-cavity surface-emitting lasers (VCSELs) convert visible pump light to near-IR laser light with a 70% efficiency.[15][16]

Hydrolysis[edit]

In Sep. 2014, researchers of EPFL in Lausanne, Switzerland reported achieving water photolysis at 12.3% efficiency in a highly efficient and low-cost water-splitting cell using perovskite photovoltaics.[17][18]

See also[edit]

Examples of perovskites[edit]

References[edit]

  1. ^ A. Navrotsky (1998). "Energetics and Crystal Chemical Systematics among Ilmenite, Lithium Niobate, and Perovskite Structures". Chem. Mater. 10 (10): 2787. doi:10.1021/cm9801901. 
  2. ^ a b c Wenk, Hans-Rudolf; Bulakh, Andrei (2004). Minerals: Their Constitution and Origin. New York, NY: Cambridge University Press. ISBN 978-0-521-52958-7. 
  3. ^ John Lloyd; John Mitchinson. "What's the commonest material in the world". QI: The Book of General Ignorance. Faber & Faber. ISBN 0-571-23368-6. 
  4. ^ Kulkarni, A; FT Ciacchi; S Giddey; C Munnings; SPS Badwal; JA Kimpton; D Fini (2012). "Mixed ionic electronic conducting perovskite anode for direct carbon fuel cells". International Journal of Hydrogen Energy 37 (24): 19092–19102. doi:10.1016/j.ijhydene.2012.09.141. 
  5. ^ J. M. D. Coey, M. Viret; S. von Molnar (1999). "Mixed-valence manganites". Advances in Physics 48 (2): 167–293. Bibcode:1999AdPhy..48..167C. doi:10.1080/000187399243455. 
  6. ^ Alexandra Witze (2010). "Building a cheaper catalyst". Science News Web Edition. 
  7. ^ Lufaso, Michael W.; Woodward, Patrick M. (2004). "Jahn–Teller distortions, cation ordering and octahedral tilting in perovskites". Acta Crystallographica Section B Structural Science 60: 10. doi:10.1107/S0108768103026661. 
  8. ^ Bullis, Kevin (8 August 2013). "A Material That Could Make Solar Power “Dirt Cheap”". MIT Technology Review. Retrieved 8 August 2013. 
  9. ^ Cartwright, Jon (11 September 2013). "A Flat-Out Major Advance for an Emerging Solar Cell Technology". Science (journal). 
  10. ^ Liu, Mingzhen; Johnston, Michael B.; Snaith, Henry J. (2013). "Efficient planar heterojunction perovskite solar cells by vapour deposition". Nature 501 (7467): 395–8. Bibcode:2013Natur.501..395L. doi:10.1038/nature12509. PMID 24025775. 
  11. ^ B.V. Lotsch (2014). "New Light on an Old Story: Perovskites Go Solar". Angewadte Chemie Int. Ed. 53 (3): 635–637. doi:10.1002/anie.201309368. 
  12. ^ R. Service (2013). "Turning Up the Light". Science 342: 794–797. doi:10.1126/science.342.6160.794. 
  13. ^ a b c d Hodes, G. (2013). "Perovskite-Based Solar Cells". Science 342 (6156): 317–318. doi:10.1126/science.1245473. PMID 24136955.  edit
  14. ^ Liu, M.; Johnston, M. B.; Snaith, H. J. (2013). "Efficient planar heterojunction perovskite solar cells by vapour deposition". Nature 501 (7467): 395. doi:10.1038/nature12509.  edit
  15. ^ High-efficiency perovskite photovoltaic material also lases
  16. ^ "Study: Perovskite solar cells can double as lasers". Rdmag.com. 2014-03-28. Retrieved 2014-08-24. 
  17. ^ Jingshan Luo et.al (26 September 2014). "Water photolysis at 12.3% efficiency via perovskite photovoltaics and Earth-abundant catalysts". Science 345 (6204): 1593–1596. doi:10.1126/science.1258307. 
  18. ^ "Harvesting hydrogen fuel from the Sun using Earth-abundant materials". Phys.org. Sep 25, 2014. Retrieved 26 September 2014. 

Further reading[edit]

  • Tejuca, Luis G (1993). Properties and applications of perovskite-type oxides. New York: Dekker. p. 382. ISBN 0-8247-8786-2. 
  • Mitchell, Roger H (2002). Perovskites modern and ancient. Thunder Bay, Ontario: Almaz Press. p. 318. ISBN 0-9689411-0-9. 

External links[edit]