text: Perovskites may be structured in layers, with the  structure separated by thin sheets of intrusive material. Different forms of intrusions, based on the chemical makeup of the intrusion, are defined as:
  * Aurivillius phase: the intruding layer is composed of a [] ion, occurring every n  layers, leading to an overall chemical formula of []-. Their oxide ion-conducting properties were first discovered in the 1970s by Takahashi et al., and they have been used for this purpose ever since.
  * Dion−Jacobson phase: the intruding layer is composed of an alkali metal (M) every n  layers, giving the overall formula as 
  * Ruddlesden-Popper phase: the simplest of the phases, the intruding layer occurs between every one (n = 1) or multiple (n > 1) layers of the  lattice. Ruddlesden−Popper phases have a similar relationship to perovskites in terms of atomic radii of elements with A typically being large (such as La or Sr) with the B ion being much smaller typically a transition metal (such as Mn, Co or Ni). Recently, hybrid organic-inorganic layered perovskites have been developed, where the structure is constituted of one or more layers of MX-octahedra, where M is a +2 metal (such as Pb or Sn) and X and halide ion (such as ), separated by layers of organic cations (such as butylammonium- or phenylethylammonium-cation).