An organic-inorganic hybrid compound, [NH3(CH2)5NH3]SbCl5, exhibits a switchable second harmonic generation (SHG) effect between SHG-OFF and SHG-ON states and tunable dielectric behaviors between high and low dielectric states, connected with the changes in the dynamics of 1,5-pentanediammonium cations during its centrosymmetric-to-noncentrosymmetric symmetry breaking phase transition at 365.4 K.

C 6 N 2 H 18 ][SbI 5 ] (1), a novel metal halide semiconductor with dielectric relaxation behavior, has been successfully synthesized, in which the cavities between the one-dimensional [SbI 5 ] n 2− polyanions are occupied by 2-methyl-1,5pentanediammonium (2-MPDA) cations. 1 undergoes a reversible solid-state phase transition at T C = 192.7 K and shows a step-like dielectric anomaly. Interestingly, above T C , distinct dielectric dispersion in a wide temperature range is also witnessed. Remarkably, the solid state UV−vis diffuse reflectance spectrum of 1 exhibits a slightly gentler absorption edge at about 650 nm; that is, 1 adopts an indirect band gap with 1.92 electron volts. The combined narrow band gap, strong photoconductivity effect, and excellent dielectric relaxation might shed light on the exploitation of lead-free hybrid metal halide molecular materials with promising application prospects in thermoresponsive relaxation-type dielectric materials and photovoltaic conversion devices.

A layered organic-inorganic hybrid compound, tetra(cyclopentylammonium) decachlorotricadmate(II) (1), in which the two-dimensional [Cd3Cl10](4-)n 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 1 determined at 93, 298, and 343 K are solved in P2₁2₁2₁, Pbca, and Cmca, 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-)n network.

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A new one-dimensional organic−inorganic material, 1,5-pentanediammonium pentabromoantimonate (III) (1), exhibits a centrosymmetric-to-non-centrosymmetric symmetry breaking phase transition at 366.5 K, showing a prominent second harmonic generation (SHG) response and dielectric anomalies. The differential scanning calorimetry results indicate the phase transition is a second-order one. The variable-temperature structural analyses reveal that the space group changes from Pnma at 393 K in the high-temperature phase to P2 1 2 1 2 1 at 293 K in the low-temperature phase, accompanied by the loss of a symmetry plane and inversion center. The crystal structure is composed of one-dimensional zigzag chains of corner-sharing SbBr 6 octahedra and 1,5pentanediammonium cations. The origin of the phase transition can be attributed to both the deformation of the zigzag chains and the order−disorder transition of the 1,5-pentanediammonium cations. The compound is SHG-active below the transition temperature, demonstrating its second-order nonlinear optical properties. It is also SHG-inactive above the transition temperature, which further confirms the symmetry breaking phenomenon. These findings will pave a new way to explore organic−inorganic multifunctional phase transition material.