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Compared to their three-dimensional analogues, the layered perovskites A2BX4 (A = organic cation; X = Br, I, Cl) display higher anisotropy, elasticity and sensitivity to external stimuli. In this study, we employ a combination of diffraction, calorimetric, dielectric, vibrational and optical measurements to uncover the rich phase diversity and atypical pressure-induced transformations of a layered perovskite analogue, methylhydrazinium tetraiodoplumbate (CH3NH2NH2Pbl4, MHy2PbI4). At ambient pressure, MHy2PbI4 exists in four paraelectric phases I–IV, depending on temperature. The combined structural and spectroscopic measurements revealed that phase II (space group Pmcn) is exceptionally responsive to pressure stimuli and transforms to phase V (Pmmn) at 70 MPa; further transitions of MHy2PbI4 at 1.15 and 3.00 GPa led to the polar phases VI (Pmn21) and VII (P21). Phase VI strikingly reassembles the ambient-pressure ferroelectric phase of MHy2PbBr4. The transition from phase V to VI reduces the cation disorder and deforms the PbI6 octahedra which leads to a two-fold increase of the free exciton (FX) and the suppression of bound-exciton (BX1 and BX2) luminescence band intensities as well as to red shifts of absorption (dE/dp = −58(2) meV GPa−1) and emission signals (dE/dp = −63(2) meV GPa−1). Above 3.0 GPa, a new phase VII displays negative area compressibility with βa = −22.4(8) and βb = −6.2(6) TPa−1 along the polyanionic sheets. The structure of phase VII revealed the underlying mechanism involving pressure-triggered penetration of MHy+ cations into the voids between the highly deformed octahedra. Overall, the construction of the p–T phase diagram for MHy2PbI4 demonstrates the high structural elasticity of this hybrid perovskite, its pressure-induced polar phases isostructural with ferroelectric analog MHy2PbBr4 and unique mechanical responses to external stimuli. [ABSTRACT FROM AUTHOR]

Symmetry transformations in crystallography are traditionally represented as equations and matrices, which can be suitable both for orthonormal and crystal reference systems. Quaternion representations, easily constructed for any orientations of symmetry operations, owing to the vector structure based on the direction of the rotation axes or of the normal vectors to the mirror plane, are known to be advantageous for optimizing numerical computing. However, quaternions are described in Cartesian coordinates only. Here, we present the quaternion representations of crystallographic point-group symmetry operations for the crystallographic reference coordinates in triclinic, monoclinic, orthorhombic, tetragonal, cubic and trigonal (in rhombohedral setting) systems. For these systems, all symmetry operations have been listed and their applications exemplified. Owing to their concise form, quaternions can be used as the symbols of symmetry operations, which contain information about both the orientation and the rotation angle. The shortcomings of quaternions, including different actions for rotations and improper symmetry operations, as well as inadequate representation of the point symmetry in the hexagonal setting, have been discussed. [ABSTRACT FROM AUTHOR]

This study revisits a (001)-oriented layered lead chloride templated by 1,2,4-triazolium, Tz2PbCl4, which recently has been an object of intense research but still suffers from gaps in characterization. Indeed, the divergent reports on the crystal structures of Tz2PbCl4 at various temperatures, devoid of independent verification of chiral phases through second harmonic generation (SHG), have led to an unresolved debate regarding the existence of a low-temperature phase transition (PT) and the noncentrosymmetric nature of the low-temperature phase. Now, by combining differential scanning calorimetry, single-crystal X-ray diffraction, dielectric, as well as linear and nonlinear optical spectroscopies on Tz2PbCl4, we reveal a sequence of reversible PTs at T1 = 361 K (phase I–II), T2 = 339 K (phase II–III), and T3 = 280 K (phase III–IV). No SHG activity could be registered for any of the four crystal phases, as checked by wide-temperature range SHG screening, supporting their centrosymmetry. The dipole relaxation processes indicate a decrease in activation energy with increasing temperature, from 0.60, 0.38, to 0.24 eV observed for phase IV (space group P21/c), phase III (Pnma), and phase II (Cmcm), respectively. This change is interpreted as a result of the diminishing strength of H-bonds as the system transforms from phase IV to III and subsequently to II. The weaker H-bonds facilitate the reorientation of Tz+ cations in the presence of an external electric field. The photoluminescence spectra of Tz2PbCl4 reveal an intriguing interplay of narrow and broadband emission, linked respectively to free excitons and excitons trapped on defects. Notably, as the temperature decreases from 300 K to 16 K, both the emission bands exhibit distinctive blue and red shifts, indicative of increased in-plane octahedral distortion. This dynamic behaviour transforms the photoluminescence of Tz2PbCl4 from greenish-blue at 300 K to yellowish-green at 13 K, enriching our understanding of 2D lead halide perovskites and highlighting the optoelectronic potential of Tz2PbCl4. [ABSTRACT FROM AUTHOR]

Cooling [Fe(bbtr)3](BF4)2 (bbtr=1,4‐di(1,2,3‐triazol‐1‐yl)butane) triggers very slow spin crossover below 80 K (T1/2↓=76 K). The spin crossover (SCO) is accompanied by a hysteresis loop (T1/2↑=89 K). In contrast to isostructural perchlorate analogue [Fe(bbtr)3](ClO4)2 in which spin crossover during cooling is preceded by phase transition at TPT=126 K in tetrafluoroborate phase transition does not occur to the beginning of spin crossover (80 K). Studies of mixed crystals [Fe(bbtr)3](BF4)2(1‐x)(ClO4)2x (0.5≤x≤0.9) showed that a phase transition precedes spin crossover, however, for x≅0.46 intersection of T1/2(x) and TPT(x) dependencies takes place. The application of pressure of 1 GPa shifts the spin crossover in [Fe(bbtr)3](BF4)2 to a temperature above 270 K. High‐pressure studies of neat tetrafluoroborate and perchlorate, as well as mixed crystals [Fe(bbtr)3](BF4)2(1‐x)(ClO4)2x (0.1≤x≤0.9), revealed that at 295 K P1/2 value changes linearly with x indicating similar mechanism of spin crossover under elevated pressure in all systems under investigation. Variable pressure single crystal X‐ray diffraction studies confirmed that in contrast to thermally induced spin crossover undergoing differently in tetrafluoroborate and perchlorate an application of high pressure removes this differentiation leading to a similar mechanism depending at first on start spin crossover and then P‐3→P‐1 phase transition occurs. In this report we have shown that 2D coordination polymer [Fe(bbtr)3](BF4)2 (bbtr=1,4‐di(1,2,3‐triazol‐1‐yl)butane) treated to date as spin crossover silent shows thermally induced spin crossover phenomenon. Spin crossover in tetrafluoroborate is extremely slow. Determination of the spin crossover curve required carrying measurement in the settle mode‐cooling from 85 to 70 K took about 600 h (average velocity of change of temperature ca. 0.0004 K/min). [ABSTRACT FROM AUTHOR]

The common property of all materials is their elasticity, directly related to the chemical components and interactions in their structure. However, the progress in constructing better devices is irreversibly tied to the understanding and designing of static or dynamic structural responses of strained materials. Such structural-property relationships have been explained for polymeric frameworks involving flexible 1,6-hexanediamine (HDA) linkers, with discrete HDA conformers. In the structure of ambient-pressure polymer Cd(HDA)2(NO3)2 the HDA linkers are conformationally disordered. This tetragonal phase α is stable down to 190 K, when the HDA linkers order in different conformations, which triggers a ferroelastic transition to the triclinic phase δ; the flash-cooling of phase α overcools it to 100 K. High pressure induces a ferroelastic transition to triclinic phase β at 1.10 GPa, followed by an isostructural transition to phase γ at 2.00 GPa. All four phases α–δ differ in the configurations of HDA conformers and Cd-coordination involving nitrate linkers. The unusual convex-shaped monotonic compression of Cd(HDA)2(NO3)2 phases α, β and γ has been explained by the mechanism of buckling-sticks: the increasing strain and mounting energy accumulated in the buckling HDA linkers is released by their conversions to shorter conformers. An analogous conformational transition takes place in Cd2(HDA)3(NO3)4 at 1.50 GPa. In another polymer Cu(HDA)2(MeCN)2·2BF4 the conformational changes induce its very high compressibility and partial amorphization above 1.20 GPa, caused by the non-coordinated conversions of the HDA conformers. [ABSTRACT FROM AUTHOR]

Due to its high electron affinity and electron mobility in a wide absorption range of the visible solar spectrum, [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) is often used as an efficient acceptor in organic photovoltaics. In turn, imines are additives to the active layer of organic solar cells, mainly due to the free electron pair of the imine nitrogen atom and the presence of various chemical groups affecting the polarity and conformations of molecules. However, the attainable efficiency is not as high as expected. Therefore, we have systematically investigated two imines and their mixtures with PC71BM by spectroscopic (the high pressure UV–Vis and frequency domain dielectric), thermoelectric, and mechanical methods for organic, flexible photovoltaics. Both the imines, (N,NʹE,N,NʹE)-N,Nʹ-([2,2ʹ:5ʹ,2ʺ-terthiophene]-5,5ʺ-diylbis(methanylylidene))bis(benzo[d]thiazol-2-imine) (SC3) and (6E)-N-((5-(5-(5-((E)-(4-(4-(4-fluorophenyl)thiazol-2-yl)phenylimino)methyl)thiophen-2-yl)thiophen-2-yl)thiophen-2-yl)methylene)-4-(4-(4-fluorophenyl)thiazol-2-yl)benzenamine (SC13), have the same core composed of three thiophene rings but different terminal chains of the molecules. In the imine SC3, the imine bond is followed by benzothiazole rings on both sides of the core, while in SC13, a thiazole ring separates two benzene rings, the terminal one F-substituted. The difference in molecular structure affects the electric properties of the neat imine and its mixed layers. An addition of PC71BM to the imines improves their electric conductivity. The mechanical studies focused on the stress at break and elongation showed superior behaviour compared to fullerene derivative. High pressure systematically reduces the band gap energy, Eg, from 1.68 eV at 0.16 GPa to 1.51 eV at 2.69 GPa for PC71BM, from 1.77 eV at 0.1 MPa to 1.53 eV at 4.15 GPa for SC3, and from 1.99 eV at 0.11 GPa to 1.8 eV at 3.10 GPa for SC13, as determined by the UV–Vis absorbance measurements in a diamond-anvil cell. These Eg reductions reflect the compressed intermolecular interactions that can be used to monitor the structural stability of these compounds. Based on the dielectric studies it was found that the relaxation processes registered for both imines are probably the grain boundary relaxation. Two processes also appear in the systems with PC71BM, but none of them is the one characteristic of imines. The high-frequency process has a dipole character while the low-frequency one is probably the grain boundary relaxation of these systems. The mechanism of quasi-DC conduction in various temperature ranges in the studied systems was also determined. [ABSTRACT FROM AUTHOR]

Phenyl-substituted perylene diimide (PTCDI-Ph) derivatives are prospective organic, electronic and optoelectronic materials. In this study, the effects of modified intermolecular interactions and molecular conformations on the energy gap and the optical properties of PTCDI-Ph were explored at high pressure and various temperatures by calorimetry, X-ray diffraction, UV-vis absorption, and photoluminescence spectroscopy. Below 498 K, PTCDI-Ph transforms from phase I to phase II, which is stable down to 93 K at least. Above 3.0 GPa, phase II transforms into phase III. The compressed intermolecular distance between perylene cores linearly narrows the band gap, due to the intensified π–π stacking. The negative linear compressibility of the crystal along [100] coincides with the negative thermal expansion in this direction. Owing to the strong pressure and temperature dependence of the luminescence spectrum, the PTCDI-Ph derivatives constitute a promising class of highly stimuli-responsive elastic organic materials with broad potential for tailoring their optoelectronic properties through physical and chemical modifications. [ABSTRACT FROM AUTHOR]

The aggregation of molecules is usually associated with a specific type of interaction, which can be altered by thermodynamic conditions. Under normal conditions, the crystal structure of selenourea, SeC(NH2)2, phase α is trigonal, space group P31, Z = 27. Its large number of independent molecules (Zα′ = 9) can be associated with the formation of an NH...N hydrogen bond substituting one of 36 independent NH...Se hydrogen bonds, which prevail among intermolecular interactions. Phase α approximates the trigonal structure with a threefold smaller unit cell (Z = 9), which in turn approximates another still threefold smaller unit cell (Z = 3). The temperature‐induced transformations of selenourea have been characterized by calorimetry and by performing 21 single‐crystal X‐ray diffraction structural determinations as a function of temperature. At 381.0 K, phase α undergoes a first‐order displacive transition to phase γ, with space group P3121 and Z reduced to 9, when the NH...N bond is broken and an NH...Se bond is formed in its place. Previously, an analogous competition was observed between NH...N and NH...O hydrogen bonds in high‐pressure phase III of urea. The lattice vectors along the (001) plane in low‐ and high‐temperature phases of selenourea are related by a similarity rule, while the lattice dimensions along direction c are not affected. This similarity rule also applies to the structures of phase γ and hypothetical phase δ (Z = 3). The thermally controlled transition between enantiomorphic phases of selenourea contrasts with its high‐pressure transition at 0.21 GPa to a centrosymmetric phase β, where both the NH...Se and NH...N bonds are present. The compression and heating reduce the number of independent molecules from Z′ = 9 in phase α, to Z′ = 2 in phase β and to Z′ = 1.5 in phase γ. [ABSTRACT FROM AUTHOR]