Perovskite (structure)

Structure

The perovskite structure is adopted by many oxides that have the chemical formula ABO ₃ .

In the idealized cubic unit cell of such a compound, type ‘B’ atom sits at cube corner positions (0, 0, 0), type ‘A’ atom sits at body center 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 center, B at the corners, and O in mid-edge).

The invention relates to the stability of the outer space, the effects of which can be reduced, and the presence of several lower-symmetry distorted versions, in which the coordination numbers of A cations, B cations or both are reduced. Tilting of the BO ₆ octahedra reduces the co-ordination of an undersize. A cation of 12 to as low as 8. Conversely, an off-centering of an undersize The resulting electric dipole is responsible for the property of ferroelectricity and shown by perovskites such as BaTiO ₃ 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

The most common mineral in the earth is bridgmanite, a magnesium-rich silicate which adopts the perovskite structure at high pressure. As pressure increases, the SiO ₄ ^4- tetrahedral units in the dominant silica-bearing minerals become unstable compared with SiO ₆ ^8- octahedral units. The most abundant material is a perovskite-structured mineral with the formula (Mg, Fe) SiO ₃ , with the second most abundant material likely to rocksalt-structured (Mg, Fe) O oxide , periclase . ^[2]

At the high pressure conditions of the Earth’s lower mantle , the pyroxene enstatite , MgSiO ₃ , transforms into a perovskite-structured polymorphic denser ; This phase may be the most common mineral in the Earth. ^[4] This phase has the orthorhombically distorted perovskite structure (GdFeO ₃ -type structure) that is stable at pressures from ~ 24 GPa to ~ 110 GPa. However, it can not be transported from depths of several hundred kilometers to the Earth’s surface without transforming back into less dense materials. At higher pressures, MgSiO ₃ 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 NaMgF ₃ are known. A large family of metallic perovskite compounds can be represented by RT ₃ M (R: rare-earth or other relatively large ion, T: metal ion transition and M: light metalloids). The metalloids occupy the octahedrally coordinated “B” sites in these compounds. RPd ₃ B, RRh ₃ B and CeRu ₃ These are examples. MgCNi ₃ is a metallic perovskite compound and has received a 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 Cs₃ AuO, which contains large alkali cations in the traditional “anion” sites, bonded to O ^2- and Au ^– anions.

Material properties

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. These compounds are used as catalysts and catalysts in certain types of fuel cells ^[5] and are candidates for devices and spintronicsapplications. ^[6]

Many superconducting ceramic materials (the high temperature superconductors ) have perovskite-like structures, often with 3 vacancies. One prime example is yttrium barium copper oxide which may 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. ^[7]

Applications

Physical properties of interest to materials science Among perovskites include superconductivity , magnetoresistance , Ionic conductivity , and a multitude of dielectric properties, qui are of great importance in microelectronics and telecommunication . Because of the flexibility of bonds 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 polyhedra coordination, and distortions of the octahedra driven by electronic factors ( Jahn-Teller distortions ).^[8]

Photovoltaics

Synthetic perovskites have been identified as possible inexpensive base materials for high-efficiency commercial photovoltaics ^[10] [11] – they showed a conversion efficiency of up to 15% ^[11] [12] and can be manufactured using the same thin-film manufacturing as used for thin film silicon solar cells. ^[13] Methylammonium tin halides and methylammonium lead halides are of interest for use in dye-sensitized solar cells . ^[14] [15] In 2016, power conversion efficiency reached 21%. ^{[ quote needed ]}In July 2016, a team of researchers led by Dr. Alexander Weber-Bargioni showed that perovskite PV cells could reach a theoretical peak efficiency of 31%. ^[16]

Among the methylammonium halides, the most common is methylammonium lead triiodide ( CH
3 NH
3 PbI
3 ). It has a high load carrier mobility and load carrier lifetime that allow light-generated electrons and holes to move far away from the current world. CH
3 NH
3 PbI
3 effective diffusion are some 100 nm for both electrons and holes. ^[17]

Methylammonium halides are deposited by low-temperature solution methods (typically spin-coating ). Other low-temperature (below 100 ° C) solution-processed films tend to have smaller distribution smaller diffusion lengths. Stranks et al. Described nanostructured cells using a mixed halide lead methylammonium (CH ₃ NH ₃ PbI _3-x Cl _x ) 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 100 nm for the pure iodide. They are shown to be in the mixed perovskite are longer than in pure iodide. ^[17]

For CH
3 NH
3 PbI
3 , open-circuit voltage (V _OC ) typically approaches 1 V, while for CH
3 NH
3 PbI (I, Cl)
3 with low Cl content, V _OC > 1.1 V has been reported. Because the band gaps (E _g ) of Both are 1.55 eV, V _OC -to-E _g ratios are Higher Than usually Observed for similar third-generation solar cells. With greater bandgap perovskites, V _OC up to 1.3 V has been demonstrated. ^[17]

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. ^[17]

Planar heterojunction perovskite solar cells can be manufactured using 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. ^[18]

Lasers

Also in 2008 the researchers showed that perovskite can generate laser light. LaAlO ₃ doped with neodymium gave laser emission at 1080 nm. ^[19] In 2014 it was shown that mixed methylammonium lead halide (CH ₃ NH ₃ PbI _3-x Cl _x ) cells optically pumped into vertical-cavity surface-emitting lasers (VCSELs) convert visible light to near-IR laser light pump with a 70% efficiency. ^[20] [21]

Light Emitting Diodes

Due to their high photoluminescence quantum efficiencies , perovskites may be good candidates for use in light-emitting diodes (LEDs). ^[22] However, the propensity for radiative recombination has largely been observed at liquid nitrogen temperatures.

Photoelectrolysis

In September 2014, researchers of EPFL in Lausanne, Switzerland reported effective water electrolysis at 12.3% efficiency in a highly efficient and low-cost water-splitting cell using perovskite photovoltaics. ^[23] [24]

See also

• Diamond anvil
• Post-perovskite
• spinel
• Goldschmidt tolerance factor
• Ruddlesden-Popper phase

Examples of perovskites

Simple

• Strontium titanate
• Calcium titanate
• Lead titanate
• Bismuth ferrite
• Lanthanum ytterbium oxide
• Silicate perovskite
• Lanthanum manganite
• yttrium aluminum perovskite (YAP) ^[25]

Solid solutions

• Lanthanum strontium manganite
• LSAT (lanthanum aluminate – strontium aluminum tantalate)
• Lead scandium tantalate
• Lead zirconate titanate

References