The crystals of (CH₃NH₃)₃Bi₂Br₉ have been grown which at room temperature are isomorphous with Cs₃Bi₂Br₉ and (CH₃NH₃)₃Sb₂Br₉. The space group is P3m1, a = 0.807, c = 1.008 nm, V = 0.5685 nm³, Z = 1. The crystals reveal, when cooling, three phase transitions at 188 ± 0.5, 140 ± 1, and 101.5 ± 0.5 K. The high temperature phase (I) behaves as a plastic one with respect to a freedom of rotation of methylammonium cations. The two high temperature transitions I II and II III can be related, similarly to (CH₃NH₃)₃Sb₂Br₉, to a freezing of reorientation of those cations. At 101.5 K a transition to the improper ferroelectric phase IV was discovered. The features of this phase are non-typical and the nature of the transition is unknown.

A ferroelectric crystal $[4-{\mathrm{NH}}_2{\mathrm{C}}_5{\mathrm{H}}_4\mathrm{NH}]\left[{\mathrm{SbCl}}_4\right]$ has been synthesized. The x-ray diffraction studies indicate that it is made of polyanionic chains of $({\mathrm{SbCl}}_4^{-}{)}_n$ forming a tunnel-like structure and the 4-aminopyridinium cations connected via weak hydrogen bonds to the chlorine atoms. The x-ray and differential scanning calorimetry studies show that the crystal undergoes a complex sequence of phase transitions: ${P2\mathrm{}}_1/c\stackrel{240/245\mathrm{K}}{\leftrightarrow }\mathrm{}\left(\mathrm{Cc}\right)\mathrm{CcC}2/cC2/c.\mathrm{}$ The pyroelectric measurements reveal that $[4-{\mathrm{NH}}_2{\mathrm{C}}_5{\mathrm{H}}_4\mathrm{NH}]\left[{\mathrm{SbCl}}_4\right]$ becomes ferroelectric below 270.5 K with the spontaneous polarization of the order of 3.5 ${\mathrm{m}\mathrm{C}/\mathrm{m}}^2$ measured along the [102] direction. The dielectric results confirm the order-disorder nature of the paraelectric-ferroelectric phase transition. The dynamics of the 4-aminopyridinium cations plays an essential role in the ferroelectric phase transition mechanism.

A ferroelectric inorganic–organic hybrid based on in situ substituted stilbazolium cation (TAMS²⁺ = trimethylamino-N-methyl stilbazolium) has been obtained. Single-crystal X-ray structure analysis demonstrates that the NLO-phore organic component of the complex is embedded into the inorganic polymeric [Bi₂Cl₈]^{2 −} framework and shows a polar arrangement with the cooperation of the nonsymmetric inorganic component. The measurement of ferroelectric and nonlinear optical properties show typical ferroelectricity and second harmonic generation (SHG) responses as well as a significant improvement of the laser damage threshold and thermal stability of relative organic material.

A new pyridinium compound, (C5H5NH)5Bi2Br11, has been synthesized. The x-ray diffraction studies indicate that its structure is built up of four independent pyridinium cations and isolated Bi2Br115− groups. At room temperature it crystallizes in the monoclinic space group P21/n. Two phase transitions of second- and first-order type are detected by the differential scanning calorimetry measurements at 118 and 405/403 K (on heating–cooling), respectively. The corresponding anomalies in the linear thermal expansion at 118 and 405 K are observed along three crystallographic directions. The dielectric investigations reveal dielectric absorption and dispersion in the radio-frequency region. The experimental results were analyzed on the basis of dielectric formula which represents the sum of two independent relaxators. The pyroelectric measurements reveal that (C5H5NH)5Bi2Br11 becomes ferroelectric below 118 K and is characterized by the appearance of spontaneous polarization of the order of 3⋅10−3 C/m2 along the b axis.

Halogenoantimonates(iii) and halogenobismuthates(iii) are a highly versatile class of organic–inorganic hybrid materials, applicable in optoelectronics and switchable dielectric devices.