Incommensurate structures of the CH3NH3Co(COOH)3 compound

摘要

The present article is devoted to the characterization of the structural phase transitions of the [CH3NH3][Co(COOH)3] (>1) perovskite-like metal–organic compound through variable-temperature single-crystal neutron diffraction. At room temperature, compound >1 crystallizes in the orthorhombic space group Pnma (phase >I). A decrease in temperature gives rise to a first phase transition from the space group Pnma to an incommensurate phase (phase >II) at approximately 128 K. At about 96 K, this incommensurate phase evolves into a second phase with a sharp change in the modulation vector (phase >III). At lower temperatures (ca 78 K), the crystal structure again becomes commensurate and can be described in the monoclinic space group P21 (phase >IV). Although phases >I and >IV have been reported previously [Boča et al. (2004). Acta Cryst. C>60, m631–m633; Gómez-Aguirre et al. (2016). J. Am. Chem. Soc. >138, 1122–1125; Mazzuca et al. (2018). Chem. Eur. J. >24, 388–399], phases >III and >IV corresponding to the Pnma(00γ)0s0 space group have not yet been described. These phase transitions involve not only the occurrence of small distortions in the three-dimensional anionic [Co(HCOO)3]⁻ framework, but also the reorganization of the [CH3NH3]⁺ counter-ions in the cavities of the structure, which gives rise to an alteration of the hydrogen-bonded network, modifying the electrical properties of compound >1.