|
A perovskite 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. 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 the silicate perovskite bridgmanite.〔〔(Bridgemanite on Mindat.org )〕 ==Structure== 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. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「perovskite structure」の詳細全文を読む スポンサード リンク
|