Structural Phase Transitions of a Layered Organic–Inorganic Hybrid Compound: Tetra(cyclopentylammonium) Decachlorotricadmate(II), [C₅H₉NH₃]₄Cd₃Cl₁₀

A layered organic–inorganic hybrid compound, tetra­(cyclopentylammonium) decachlorotricadmate­(II) (<b>1</b>), in which the two-dimensional [Cd₃Cl₁₀]^4–_<i>n</i> networks built up from three face-sharing CdCl₆ octahedra are separated by cyclopentylammonium cation bilayers, has been discovered as a new phase transition material. It undergoes two successive structural phase transitions, at 197.3 and 321.6 K, which were confirmed by differential scanning calorimetry measurements, variable-temperature structural analyses, and dielectric measurements. The crystal structures of <b>1</b> determined at 93, 298, and 343 K are solved in <i>P</i>2₁2₁2₁, <i>Pbca</i>, and <i>Cmca</i>, respectively. A precise analysis of the structural differences between these three structures reveals that the origin of the phase transition at 197.3 K is ascribed to the order–disorder transition of the cyclopentylammonium cations, while the phase transition at 321.6 K originates from the distortion of the two-dimensional [Cd₃Cl₁₀]^4–_<i>n</i> network