The extraordinary thermal stability of [NH₂CHNH₂]₃Bi₂I₉ (FA₃Bi₂I₉) crystals, up to around 600 K, has been enhanced by using the evaporating method. DSC measurements have revealed reversible structural phase transitions: at 203 K (Phase I → Phase II), 173 K (II → III), 134.7 K (III → IV), 131.4 K (IV → V), and 120 K (V → VI). The crystal structures of FA₃Bi₂I₉ adopt the polar space groups P6₃mc at 220 K (Phase I) and Cmc2₁ at 190 K (Phase II). The pyroelectric current (I_pyr) measured in the temperature range covering all the phases of FA₃Bi₂I₉ confirmed their polar nature. The reversibility of the spontaneous polarization in each phase has been confirmed by the observed hysteresis loops. All the phase transitions are dielectrically active. The dielectric response (ε*(ω,T) close to 134.7 and 131.4 K) is characteristic of ferroelectrics with a critical slowing-down process. Optical bandgaps estimated from the UV-vis measurements and calculated using the DFT method are equal to 1.85 and 1.89 eV, respectively. On the basis of the structural, dielectric, and spectroscopic results, the molecular mechanisms of the phase transitions have been proposed.

Highly stable ferroelectrics with reversible high-temperature phase transitions and switchable nonlinear optical behaviour are much coveted targets for emerging optoelectronic applications. Here, we demonstrate a cyano-bridged perovskite [(CH₃)₃NOH]₂[KCo(CN)₆] (TMAO-Co), a new analogue of the multi-axial ferroelectric [(CH₃)₃NOH]₂[KFe(CN)₆] (TMAO-Fe) with improved thermal stability and enhanced second-order nonlinear optical response. Indeed, for TMAO-Co the Curie temperature (T_c) is shifted to a higher value of ca. 416 K (improvement by ca. 10 K versusTMAO-Fe); the separation between T_c and the decomposition threshold is 46 K. TMAO-Co is a biaxial ferroelectric as revealed by P(E) hysteresis loop measurements along the a and c crystallographic directions with spontaneous polarization values of 0.9 and 0.63 μC cm⁻² at 293 K, respectively. The SHG response of TMAO-Co is two times higher than that of TMAO-Fe. The improved stability of TMAO-Co to thermal and optical loads allowed for demonstration of bistable switching of nonlinear optical response between SHG-on and SHG-off states by temperature sweeping. Structurally, TMAO-Co reproduces the unusual characteristics of TMAO-Fe, i.e. the first-order phase transition between (polar) monoclinic to (nonpolar) cubic phases involving bond switching and is assisted by the pronounced increase of disorder of the TMAO cations above T_c. Combined temperature-resolved Raman and infrared spectroscopic measurements were employed to track the symmetry increase above T_c, which is primarily associated with changes in hydrogen-bonding. Consistent with the bond-switching character of the phase transitions, a pronounced shift to higher wavenumbers is observed for the O–H stretching modes. The DFT calculations demonstrate that the system's polarization along the a-axis mostly comes from the rotation of the [(CH₃)₃NOH]₂ cluster, while the atomic displacement of the framework contributes largely to that along the c axis.

We report the synthesis and characterization of iron-bearing sodium borosilicate glasses with an Fe₂O₃content lower than 10 mol%.

Herein, a multi-phase ferroelectric and ferroelastic pyrrolidinium-based organic–inorganic hybrid (C ₄ H ₈ NH ₂ ) ₂ [SbCl ₅ ] is presented.

The preparation of materials featuring more than one ferroelectric phase represents a promising strategy for controlling electrical properties arising from spontaneous polarization, since it offers an added advantage of temperature-dependent toggling between two different ferroelectric states. Here, we report on the discovery of a unique ferroelectric–ferroelectric transition in diisopropylammonium tetrabromocadmate (DPAC, (C₆H₁₆N)₂[CdBr₄]) with a T_c value of 244 K, which is continuous in nature. Both phases crystallize in the same polar orthorhombic space group, Iab2. The temperature-resolved second-harmonic-generation (SHG) measurements using 800 nm femtosecond laser pulses attest to the polar structure of DPAC on either side of the phase transition (PT). The dc conductivity parameters were estimated in both solid phases. The anionic substructure is in the form of [CdBr₄]^2– discrete complexes (0D), while in the voids of the structure, the diisopropylammonium cations are embedded. The ferroelectric properties of phases I and II have been confirmed by the reversible pyroelectric effect as well as by P–E loop investigations. On the basis of the dielectric responses, the molecular mechanism of the PT at 244 K has been postulated to be of mixed type with an indication of its displacive nature.

A novel photoluminescent–ferroelectric crystal of (C ₄ H ₁₀ N) ₂ [Cd ₂ I ₆ ] ( PCdI ) has been obtained and characterized.