Perovskite-like materials exhibit desirable photophysical and electric properties that make them suitable for a remarkable breadth of applications in electronics and physics. In this contribution, we report on the multiphase ferroelectric and ferroelastic phenomena in a pyrrolidinium-based hybrid metal–organic material: (C4H8NH2)3[Sb2Cl9]. The title compound is the first pyrrolidinium derivative within the halobismuthates­(III) and haloantimonates­(III) families that is featured by the ferroelectric property. From a structural point of view, the crystal structure is built of [Sb2Cl9]3– ∞ perovskite-like layers, interdigitated by layers of pyrrolidinium cations. The rich solid-state dynamics of pyrrolidinium cations endowed (C4H8NH2)3[Sb2Cl9] with a complex sequence of temperature-dependent phase transitions. Remarkably, polar properties have been found to occur in all six phases, including room-temperature Phase I. Insights from variable-temperature single-crystal X-ray diffraction, dielectric spectroscopy, and T1 spin–lattice relaxation measurements revealed the general mechanism of most phase transitions, as related to the progressive ordering of nonequivalent pyrrolidinium cations. Noncentrosymmetry is probed by room-temperature second harmonic generation (SHG), while the ferroelectric property was evidenced through P(E) and dielectric measurements. The experimental values of spontaneous polarization were justified and analyzed in the context of theoretical values derived from quantum-chemical calculations. Optical measurements show that the integrity of the sample survives all of the phase transitions, despite sometimes significant deformations of the unit cell. The changes of symmetry associated with structural phase transitions are accompanied by an intriguing evolution of the ferroelastic domain structure with temperature.

Novel room-temperature ferroelectric crystals with complex sequences of phase transitions.

We report combined density functional studies and thermodynamic considerations on Ti-related and native defects in lithium borohydride and sodium alanate. Ti atoms introduced into the bulk of LiBH(4) are thermodynamically unfavorable for all their oxidation states, while high oxidation states of Ti(n+) cations may become thermodynamically stable in the bulk of NaAlH(4) at certain thermodynamic conditions. Neutral hydrogen vacancies and interstitials or cation vacancies are less stable than their charged counterparts in both compounds. In sodium alanate, the formation of native defects leads to changes of the coordination number of aluminum, while in lithium borohydride BH(4) groups change their mutual orientation but B-H bonds remain intact. The electronic band alignment in LiBH(4) and NaAlH(4) is different.

Two novel guanidinium iodoantimonate(III) and iodobismuthate(III) crystals, [C(NH2)3]3[Sb2I9] and [C(NH2)3]3[Bi2I9], have been synthesized and their structures have been determined by means of single-crystal x-ray diffraction studies at three temperatures (293, 348 and 362 K). Both compounds appeared to be isomorphous in corresponding phases. The crystal structure of the title compounds is composed of discrete M2I93− (M = Sb, Bi) anions and C(NH2)3+ guanidinium cations. A non-equivalence of two guanidinium cations has been found. Both guanidinium analogs exhibit a rich sequence of phase transitions. In Gu3Sb2I9, three solid–solid structural phase transformations of the first order type are detected at 119/121, 341/344 and 355/362 K (on cooling/heating) by the DSC and dilatometric techniques. Gu3Bi2I9 displays four first order phase transitions: 179/185, 202/215, 287/291 and 358/368 K. The low temperature phases appear to have ferroic (ferroelastic) properties. The prototypic paraelastic phase for both compounds belongs to hexagonal symmetry (space group P63/mmc). The dielectric response has been measured in a wide frequency region (100 Hz–1 MHz), but no dielectric dispersion has been detected. Possible mechanisms of the phase transitions in Gu3M2I9 (M = Sb, Bi) are discussed on the basis of the presented results.

Diisobutylammonium bromide is found to be a unique improper ferroelastic in which the elastic degrees of freedom seem to play the essential role, giving rise to a domain pattern resembling that of martensitic phase transitions. A weak canted ferroelectricity turns out switchable by an electric field.

Using optical second harmonic generation (SHG) we observe, for the first time, a structural change in the energetic material (dry-aminated production grade) TATB (1,3,5-triamino-2,4,6-trinitrobenzene) upon heating. This structure change is verified by X-ray powder diffraction (XPD). The kinetics of this structure change are measured and are found to closely match commonly accepted thermal decomposition parameters for the first step, analogous to observations made for the β to δ phase transition in HMX (octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine). Consequently, these solid structure transitions appear to be coupled to the chemical decomposition mechanism or it may be the first physical process occurring. We apply this technique as a diagnostic to an ignition experiment. This experiment demonstrates that the structure change can be rapid (it is thermally activated) and that SHG can be used to indicate not only the solid structure change (large increase in SHG intensity) but also the onset to significant decomposition (subsequent drop in SHG intensity). The ability to selectively change the efficiency of the SHG, in a single piece of explosive, is also demonstrated and may have novel nonlinear optical applications. This application of SHG to TATB constitutes a fundamentally new probe in the dynamics of TATB decomposition, ignition, and combustion and may have application to optical integrated devices for frequency conversion and information encoding.

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