perovskite

perovskite is a calcium titanium oxide mineral with the chemical formula CaTiO₃. The term has been extended to a broad class of compounds sharing the same crystal structure, which is of immense scientific and technological importance. These materials exhibit a wide array of remarkable properties, including superconductivity, magnetoresistance, and high efficiency in photovoltaic cells. The study of these compounds spans disciplines from mineralogy and solid-state chemistry to materials science and electrical engineering.

Structure and composition

The defining characteristic is the eponymous crystal structure, known as the perovskite structure. This is an ABX3 framework where 'A' and 'B' are cations of different sizes and 'X' is an anion, typically oxygen or a halogen. In the natural mineral, the 'A' site is occupied by calcium, the 'B' site by titanium, and 'X' by oxygen. The structure is a three-dimensional network of corner-sharing BX6 octahedra, with the larger 'A' cation occupying the cuboctahedral cavities. This arrangement can tolerate significant distortion, leading to variants like the orthorhombic structure of CaTiO3 and the ideal cubic form found in strontium titanate. Pioneering work on this structure was conducted by Victor Goldschmidt, who established the tolerance factors predicting its stability. The flexibility of the framework allows for extensive chemical substitution, enabling the creation of materials like lanthanum strontium manganite and methylammonium lead iodide.

Properties

Materials adopting this structure display an extraordinarily diverse range of physical properties. They can be insulators, semiconductors, metals, or even superconductors, as seen in BaPbBiO. Many exhibit strong correlated electron effects, leading to phenomena such as colossal magnetoresistance in LaCaMnO. Certain compositions are ferroelectric, with barium titanate being a classic example crucial for capacitors. Others are multiferroic, combining ferroelectric and magnetic orders. A major area of research is their exceptional performance in optoelectronics; organometal halide variants demonstrate high charge carrier mobility and strong light absorption, making them outstanding for photovoltaics. Their ionic conductivity is also exploited in devices like the solid oxide fuel cell.

Occurrence and synthesis

The natural mineral is relatively rare, occurring in mafic and ultramafic rocks such as those in the Ural Mountains and the Zermatt-Saas Zone. It is also found in skarn deposits and certain meteorites. Most technologically important materials are synthetic. Common synthesis methods include solid-state reaction, where powdered oxides like TiO2 and CaCO3 are heated at high temperatures. For thin films, techniques such as chemical vapor deposition and spin coating are prevalent, the latter being key for creating layers for solar cells. The synthesis of hybrid organic-inorganic types, pivotal for perovskite solar cell research, often involves solution processing of precursors like PbI2 and methylammonium iodide.

Applications

The technological impact of these materials is vast and growing. The most prominent application is in photovoltaics, where cells have achieved power conversion efficiencys over 25%, rivaling silicon technology. They are also used in light-emitting diodes for displays and lighting. Their ferroelectric properties are utilized in non-volatile memory, sensors, and actuators. Barium titanate and related compounds form the dielectric layer in multilayer ceramic capacitors found in virtually all electronics. As piezoelectric materials, they are used in transducers and ultrasonic imaging. Furthermore, their ionic conductivity makes them suitable as electrolytes in fuel cells and as potential membranes for oxygen separation.

History and etymology

The mineral was first discovered in the Ural Mountains of Russia by Gustav Rose in 1839 and named in honor of the Russian mineralogist Lev Perovski. The crystal structure was first described in detail for the mineral by Victor Goldschmidt and his colleagues in the 1920s. The broader significance of the structure became apparent in the mid-20th century with the study of ferroelectricity in barium titanate by B. T. Matthias and others at Bell Labs. The modern era of intense research began in 2009 when Tsutomu Miyasaka and his team first used an organometal halide variant as a light absorber in a dye-sensitized solar cell, sparking the revolution in photovoltaic research. Subsequent breakthroughs by groups like that of Henry Snaith at the University of Oxford rapidly advanced their efficiency. Category:Oxide minerals Category:Crystal structures Category:Functional materials