Organic-inorganic layer compounds: physical properties and chemical reactions

Abstract
In contrast with intercalation compounds, which can exist both with and without organic molecules between the planes of inorganic material, ‘molecular composite’ compounds have organic groups covalently or ionically bound to inorganic layers. In such crystals the aim is to combine magnetic or optical properties characteristic of the inorganic solid state, like magnetism and luminescence, with properties found in the organic solid state like mesomorphism or polymerization. This contribution surveys experiments from our laboratory on the structural, physical and chemical properties of one series of ‘molecular composites’, the layer perovskite halide salts (RNH 3 ) 2 MX 4 (R = organic group; M = Cr, Mn, Cd; X = Cl, Br). When R is an n -alkyl group these compounds undergo structural phase transitions triggered by variations in the hydrogen-bonding of the −NH 3 to the inorganic layer. Optical microscopy is used to follow the phase transitions and map ferroelastic domains. Compounds with M = Cr are ferromagnets with strongly temperature-dependent visible absorption spectra, whose intensity correlates with the degree of magnetic order. In Mn and Cd salts more elaborate functional groups can be inserted in the organic sidechains with the aim of making topochemical polymerizations in the solid state.