Perovskite

Perovskite is a relatively common mineral of the mineral class of " oxides and hydroxides " with the chemical composition of CaTiO3. Chemically, it is a calcium titanium oxide or calcium titanate, ie, a compound from the group of titanates.

The perovskite structure is an important structure type for industrially important compounds, such as ferroelectrics, the term perovskite structure refers here but on a cubic crystal structure, but is not present in the eponymous perovskite. Due to the small ionic radius of Ca2 ions in the crystal structure of the actual CaTiO3 perovskite is distorted, whereby the orthorhombic crystal system in the lower symmetrical crystallized. The crystals of the perovskite thus have a pseudo-cubic shape, they easily form distorted, usually metallic -looking pseudo- cube with a black to reddish-brown color.

Etymology and history

The first description of perovskite comes from the German mineralogist Gustav Rose (1798-1873) from the year 1839. He discovered the unknown mineral in the Druze a rock sample from Achmatowsk near Zlatoust ( Ural ), which he made ​​from the top Bergmeister chamberlain St. Petersburg had received. Rose described the crystal form, certain hardness ( 5.5 on the Mohs hardness scale ), and the density of the mineral and conducted numerous chemical investigations, which he could still valid today constituents calcium and titanium (IV ) oxide known exactly. He named the new mineral perovskite after the Russian politicians and mineralogist Lev Alekseyevich Perovsky ( 1792-1856 ). Achmatowsk today is the type locality of the perovskite.

Since the founding of the International Mineralogical Association perovskite is the internationally recognized mineral name for naturally occurring calcium titanium CaTiO3.

Classification

In the now outdated but still in use 8th edition of the mineral classification by Strunz the perovskite belonged to the mineral class of " oxides and hydroxides " and then to the Department of " oxides with the molar ratio of metal: oxygen = 2: 3", where he was named the " perovskite - series" with the system number. Formed IV/C.10 and the other members Barioperowskit, Isolueshit, Latrappit, loparite - (Ce), Lueshit, Macedonit, Natroniobit and Tausonit.

The 9th edition used since 2001, valid and of the International Mineralogical Association (IMA ) of the Strunz'schen Mineral classification assigns the perovskite also in the class of " oxides and hydroxides ," there, however, in the department of " oxides with the molar ratio of metal: oxygen = 2: 3, 3: 5 and comparable " one. This department is also further divided according to the size of the cations involved, so that the mineral is found according to its composition in the subsection "With large and medium-sized cations " where there along with Lueshit the " perovskite Lueshit group " with the system no. Forms 4.CC.30 and the other members Barioperowskit, Lakargiit, Latrappit and Natroniobit.

The mainly common in English-speaking classification of minerals according to Dana assigns the perovskite into the class of " oxides and hydroxides " and there in the department of " oxides ". Here it is also named the " perovskite - group " with the system no. To find 04:03:03 and the other members Latrappit, loparite - (Ce), Lueshit, Tausonit, Isolueshit, Barioperowskit and Lakargiit within the sub-division of "simple oxides with a cation charge of 3 ( A2O3 ) ".

Modifications and varieties

Perovskite can in addition to calcium and titanium contain traces up to larger amounts of other metals. Instead of calcium, alkali metals, rare earth metals and rare iron be present, often on the titanium positions is also niobium and tantalum and zirconium subordinate. Varieties with a very high content of rare earth metals (especially cerium ) are referred to as Knopit, very niobreiche perovskites as Dysanalyt, with a combination of both as loparite. Taking into account the elements contained commonly, the chemical composition of the perovskite may also be expressed more generally as (Ca, Na, Fe 2 , Ce, Sr) (Ti, Nb) O3.

Education and Locations

Perovskite is formed by crystallization from titanium- rich magmas and is a frequent component of silica- poor ( mafic ) rocks such as syenite, carbonatite or kimberlite, but it also occurs in carbonate-rich metamorphic rocks such as the skarn. Also in carbonate chondrites ( stony meteorites ) perovskite could be detected. Artificially perovskite can be produced by the reaction of calcium oxide CaO with TiO2:

Among the accompanying minerals ( paragenesis ) include Fe3O4 nepheline (K, Na) AlSiO4, titanite CaTiSiO5, ilmenite and magnetite FeTiO3.

Worldwide, there are numerous finding of perovskite, in addition to the type locality Achmatowsk and other places in the Urals, these include Kola (Russia) peninsula, the Eifel and the Emperor chair ( Germany ), Zermatt ( Switzerland ) and the Val di Susa and Val Malenco ( Italy).

Morphology

Perovskite crystallizes most cube -shaped, the dice are slightly distorted due to the orthorhombic symmetry. Less common are octahedral or monocrystals kuboktaederförmige. The octahedral faces are, as well as partly Rhombendodekaederflächen, at least hinted present in many cube-shaped crystals. The possible crystal forms are shown below.

Octahedron

Cuboctahedron

Crystal structure

Perovskite crystallizes in the orthorhombic crystal system, space group Pbnm ( Raumgruppen-Nr. 62) with the lattice parameters a = 5.39 Å, b = 5.45 Å and c = 7.65 Å and four formula units per unit cell.

Ideally perovskite would crystallize in the cubic crystal system in the space group Pm3m ( Raumgruppen-Nr. 221) with a lattice parameter of the unit cell of about 3.80 Å. Because of this to small ionic radius of Ca2 cations, the crystal structure of CaTiO3 is still distorted, which explains the lower orthorhombic symmetry. The term perovskite structure as an important type of structure, refers generally to the cubic crystal structure in which the eponymous prototype itself therefore does not crystallize.

The analogous compounds of the larger homologues of calcium, strontium and barium contrast crystallize in the undistorted cubic structure: strontium titanate SrTiO3, also known as the mineral Tausonit, with a lattice parameter of a = 3.90 Å; Barium titanate BaTiO3, since 2007, also known as mineral ( Barioperowskit ), with a = 4.01 Å and only one formula unit per unit cell. The degree of distortion of a compound in the ABO 3 perovskite structure may also be estimated based on the tolerance factor t Goldschmidt, which is defined as follows:

Where rA is the radius of the A cation, rB is the radius of the B cation and rO is the radius of the anion (usually oxygen). The perovskite structure exists in the range 0.89 < t < 1.02, where t = 1 corresponds to the strontium titanate in the undistorted cubic structure.

The crystal structure of perovskite can be described in two different ways. The titanium atoms are each surrounded by six oxygen atoms in the form of octahedra. This [ TiO6 ] octahedra form a three-dimensional network via common corners, which can be described using the Niggli notation as follows:

In the gaps of this network are the calcium atoms that have a coordination sphere of twelve oxygen atoms in the form of a cuboctahedron as coordination polyhedra. Alternatively, the structure can be described as a cubic close packing of spheres that is established jointly by calcium and oxygen. Every fourth of the octahedral sphere packing is occupied by titanium and indeed the one that is surrounded only by oxygen atoms. Since as many as octahedral packing particles are present in a close packing of spheres, there will again be the sum formula CaTiO3.

In a cubic perovskite - type crystallize a number of other compounds, including ferroelectrics industrially important as the above mentioned barium titanate ( BaTiO3 ), although other oxides such as CaZrO3 or CaSnO3, and fluorides, and nitrides such as KNiF3 compositions, and KMnF3 ThTaN3.

Of the perovskite structure, other structures can be derived. The perovskite there is only one type of A- atoms. If you replace half of all A atoms systematically (ie, every second ) to an atom A ', we obtain the elpasolite structure ( AA'BO6 ). Their unit cell corresponds to eight times of the perovskite.

Removed by twelve oxygen atoms surrounding the central atoms (in the case of the perovskite so that calcium atoms ) out of the structure, is created another common type of structure, the rhenium ( VI) oxide structure that can be described as a defect structure variant of the cubic perovskite structure.

Deposits in the Earth's mantle

A major constituent of the lower mantle ( 660-2900 km depth ) is also called perovskite. It is to iron - and magnesium - containing silicate rock in the composition ( Mg, Fe) SiO 3.

Use of materials having a perovskite structure

Perovskites such as barium titanate were applied as ferroelectrics and dielectrics, for example, as in ceramic capacitors. With perovskites the later Nobel laureates Johannes Georg Bednorz and Karl Alexander Müller succeeded in 1986, the breakthrough in the new ceramic high-temperature superconductors. These were lanthanum -barium-copper oxides.

Perovskites are also discussed as a base material cheaper solar cells, after in 2012 several technological breakthroughs have been achieved (eg Henry Snaith, University of Oxford). The research group led by Michael Grätzel thus achieved on small prototypes under laboratory conditions, efficiencies of 15%. The problem is the still high levels of toxic lead.