Читать книгу Solid State Chemistry and its Applications - Anthony R. West - Страница 65

So far, we have considered A sites to be occupied, fully or partially, by large cations such as K⁺, Ba²⁺ and La³⁺. In a family of hybrid organic–inorganic perovskites, the A sites are large enough to contain ammonium, NH₄ ⁺ and small organic derivative cations such as methylammonium, MA (CH₃NH₃)⁺, tetramethylammonium, TMA [(CH₃)₄N]⁺ and formamidinium, FA (NH₂=CHNH)⁺. The inorganic BX₃ framework of corner‐sharing octahedra typically consists of the divalent cations of Ge, Sn or Pb with the larger halogens Cl, Br and I. These hybrid perovskites display a range of structural variants, including (i) tilted structures based on the BX₃ octahedra and (ii) ordered arrangements, based on the non‐spherical A cations, which may undergo phase transitions to disordered cubic structures with increasing temperature. In recent years, there has been great interest in these hybrid perovskites for high efficiency, solar cell applications.

A very extensive family of hybrid layered or intergrowth perovskites has been reported which contain large organic cations that cannot fit within the A‐site cages. Instead the structures consist of BX₃ layers in various orientations: [100], [110] or [111], separated by layers of organic cations. The inorganic BX₃ component may be either a single layer or a multilayer block, similar to those found in the [100] orientation in the Ruddlesden–Popper family of perovskite‐rock salt intergrowth structures, Section 1.17.14. The polar, NH₃ ⁺ end of the organic component is bonded ionically to the anions of the inorganic layer and usually, the organic component consists of double layers in a head‐to‐head arrangement for bonding to the inorganic layers to either side. Given the many possible compositions for the organic component, which usually consists of chains of different lengths and with various side groups attached, there is great scope for generating novel hybrid layered perovskites. This provides one way to modify the properties that are associated primarily with the inorganic component of the structure.