High-Pressure Structural and Optical Studies of Pure Low-Dimensional Cesium Lead Chlorides CsPb2Cl5 and Cs4PbCl6.

We report high-pressure single-crystal X-ray diffraction, optical absorption, and photoluminescence investigations of all-inorganic perovskite-related materials CsPb2Cl5 and Cs4PbCl6. The crystal structure of CsPb2Cl5, composed of alternate layers of Cs+ cations and Pb-Cl frameworks, is stable under pressure up to at least 4.2 GPa. Because external stress is mainly absorbed by the Cs+ layers, the optical absorption edge of the crystal only slightly red-shifts with increasing pressure, which correlates well with a moderate shortening of the Pb-Cl bonds. A quite different response to pressure shows Cs4PbCl6, the crystal built of isolated PbCl64- octahedra and Cs+ cations. During the compression at around 3.4 GPa, the trigonal phase I, space group R3̅c, transforms to the orthorhombic phase II, space group Cmce, which at around 4 GPa transforms into phase III. On decompression, phase II is not restored, but phase III converts through a diffuse phase transition into another high-pressure phase IV, which is stable in a wide pressure range and transforms to the initial phase I only around atmospheric pressure. The red shift of the absorption edge and the profound modification of the absorption spectrum in phase II were ascribed to the deformation of the PbCl64- octahedra. The transition to phase III induces a blue shift of the absorption edge, while the transition to phase IV is associated with a large red shift. Photoluminescence was detected in phases I and II with the intensity quenched with increasing pressure.

Dipole-Moment Modulation in New Incommensurate Ferrocene.

Despite 70 years of research on metallocenes and their applications, there are still unresolved regions in its phase diagram of the prototypic sandwich compound, ferrocene Fe2+[C5H5]-2 (FeCp2), and its molecular 5-fold symmetry cannot be reconciled with the dielectric response of this crystal. We found a new phase I″ of ferrocene, which reveals the relationships between the molecular conformation, intermolecular interactions, and electric permittivity of this compound. Between 172.8 and 163.5 K, the conformational disorder of ferrocene molecules transforms into the incommensurate modulation. The structure of phase I″ is described in the (3+2)-dimensional superspace, where the molecular conformations, rotations and inclinations of the Cp rings, molecular tilts, and displacements of the Fe2+ cations, as well as the CH···π bonds in the crystal environment, are modulated. These geometric changes combine into the FeCp2 bending distortion, breaking the 5-fold symmetry and generating waves of molecular dipole moments with their amplitudes approaching 4 × 10-30 C·m.

Thermodynamic Stability, Structure, and Optical Properties of Perovskite-Related CsPb2Br5 Single Crystals under Pressure.

CsPb2Br5 belongs to all inorganic perovskite-related quasi-two-dimensional materials that have attracted considerable attention due to their potential for optoelectronic applications. In this study, we solve numerous controversies on the physical properties of this material. We show that optical absorption in the visible spectrum and green photoluminescence are due to microcrystallites of the three-dimensional CsPbBr3 perovskite settled on the CsPb2Br5 plates and that carefully cleaned crystal plates are devoid of these features. The high-pressure structural and spectroscopic experiments, performed on the single crystals free of CsPbBr3 impurities, evidenced that the layered tetragonal structure of CsPb2Br5 is stable at least up to 6 GPa. The absorption edge is located in the ultraviolet at around 350 nm and continuously red shifts under pressure. Moderate band gap narrowing is well correlated to the pressure-induced changes in the crystal structure. Although the compressibility of CsPb2Br5 is much higher than for CsPbBr3, the response in optical properties is weaker because the Pb-Br layers responsible for the optical absorption are much less affected by hydrostatic pressure than those built of Cs+ cations. Our study clarifies the confusing data in the literature on the optical properties and thermodynamic stability of CsPb2Br5.

Comment on "Improper molecular ferroelectrics with simultaneous ultrahigh pyroelectricity and figures of merit" by Li et al.

Li et al. (Science Advances, 29 January, p. eabe3068) claim the discovery of two improper ferroelectrics, dabcoHClO4 and dabcoHBF4 (dabco = 1,4-diazabicyclo[2.2.2]octane), and that these materials exhibit superior pyroelectric figures of merit. This information is misleading because of the fundamental methodological errors and false conclusions, not to mention that these ferroelectrics were reported more than 20 years ago. They are proper ferroelectrics, for which the spontaneous polarization is the macroscopic order parameter. We show that the useful pyroelectric coefficients of dabcoHClO4 and dabcoHBF4 are about 1000 times lower than the coefficients reported by Li et al.

Material Parameters Identification of Historic Lighthouse Based on Operational Modal Analysis.

In the present paper, the identification of the material parameters of a masonry lighthouse is discussed. A fully non-invasive method was selected, in which the material properties were determined via numerical model validation applied to the first pair of natural frequencies and their related mode shapes, determined experimentally. The exact structural model was built by means of the finite element method. To obtain experimental data for the inverse analysis, operational modal analysis was applied to the structure. Three methods were considered: peak picking (PP), eigensystem realization algorithm (ERA) and natural excitation technique with ERA (NExT-ERA). The acceleration's responses to environmental excitations, enhanced in some periods of time by sheet piling hammering or by sudden interruptions like wind stroke, were assumed within the analysis input. Different combinations of the input were considered in the PP and NExT-ERA analysis to find the most reasonable modal forms. A number of time periods of a free-decay character were considered in the ERA technique to finally calculate the averaged modal forms. Finally, the elastic modulus, Poisson's ratio and material density of brick, sandstone and granite masonry were determined. The obtained values supplement the state of the art database concerning historic building materials. In addition, the numerical model obtained in the analysis may be used in further cases of structural analysis.

Band Gap Engineering in MASnBr3 and CsSnBr3 Perovskites: Mechanistic Insights through the Application of Pressure.

Here we report on the first structural and optical high-pressure investigation of MASnBr3 (MA = [CH3NH3]+) and CsSnBr3 halide perovskites. A massive red shift of 0.4 eV for MASnBr3 and 0.2 eV for CsSnBr3 is observed within 1.3 to 1.5 GPa from absorption spectroscopy, followed by a huge blue shift of 0.3 and 0.5 eV, respectively. Synchrotron powder diffraction allowed us to correlate the upturn in the optical properties trend (onset of blue shift) with structural phase transitions from cubic to orthorhombic in MASnBr3 and from tetragonal to monoclinic for CsSnBr3. Density functional theory calculations indicate a different underlying mechanism affecting the band gap evolution with pressure, a key role of metal-halide bond lengths for CsSnBr3 and cation orientation for MASnBr3, thus showing the impact of a different A-cation on the pressure response. Finally, the investigated phases, differently from the analogous Pb-based counterparts, are robust against amorphization showing defined diffraction up to the maximum pressure used in the experiments.

Vitrification and New Phases in the Water:Pyrimidine Binary Eutectic System.

The binary diagram for pyrimidine:water mixtures has been determined by differential scanning calorimetry, in situ single-crystal, and powder X-ray diffraction experiments. The eutectic point has been located near the 1:4 n/n ratio at 234.5 K. The eutectic and nearly eutectic mixtures easily vitrify, and the vitrification could be kinetically induced for 1:3 n/n mixtures, too. Depending on the cooling rate, the 1:4 mixture freezes in the glass state, as a conglomerate of the glass and crystalline phases, or as the eutectic mixture of pyrimidine phase I and hexagonal ice Ih. When heated above 160 K, the glass phase transforms to a novel crystalline phase, tentatively identified as a pyrimidine hydrate, which in turn at ca. 200-210 K transforms into a eutectic mixture of pyrimidine phase I and hexagonal ice Ih. The pyrimidine-water binary diagram and novel crystalline and amorphous phases are relevant to the thermodynamic behavior of hydrophilic pyrimidine and its natural and synthetic derivatives in humid environments. The presently determined binary diagram can be straightforwardly applied for assessing the contents of water in highly hygroscopic pyrimidine samples.

Differences in photoinduced optical transients in perovskite absorbers for solar cells.

Methylammonium lead iodide films and powdered crystals were studied by time-resolved absorption and emission spectroscopy on the time scales from femtoseconds to nanoseconds. Strikingly different transient absorption signals were observed, changing from strong long-wavelength band-edge bleach to weak signatures of band-shift, which depended on the absorber form (films or polycrystals) and preparation method (stoichiometric or non-stoichiometric). The observed differences were correlated with the variation in absorption and emission spectra, changes in photo-induced carrier lifetimes and solar cell efficiency. These differences also pointed out that similar perovskite absorbers can provide significantly different transient responses and emphasize that special care must be taken when interpolating the obtained findings to the processes occurring in the most efficient devices.

Photovoltaic Hybrid Perovskites under Pressure.

High-pressure studies on methylammonium trihaloplumbates, of general formula [CH3NH3]+PbX3- (abbreviated MAPbX3, where X = Cl, Br, I), and its analogues shed new light on the materials for harvesting solar energy and open new perspectives for photovoltaic science and technology. However, there are considerable discrepancies between the reported structural, calorimetric, and spectroscopic results and even between the results obtained by the same technique, for example, of X-ray diffraction. The origins of these discrepancies and possible pitfalls in the diffraction and spectroscopic studies on MAPbX3 crystals have been investigated. Several new effects revealed in this study involve phase transitions of exceptionally slow kinetics and the coexistence of phases. They strongly affect photovoltaic properties and are essential for theory, predictions, and technological applications.

Mechanism of Pressure-Induced Phase Transitions, Amorphization, and Absorption-Edge Shift in Photovoltaic Methylammonium Lead Iodide.

Our single-crystal X-ray diffraction study of methylammonium lead triiodide, MAPbI3, provides the first comprehensive structural information on the tetragonal phase II in the pressure range to 0.35 GPa, on the cubic phase IV stable between 0.35 and 2.5 GPa, and on the isostructural cubic phase V observed above 2.5 GPa, which undergoes a gradual amorphization. The optical absorption study confirms that up to 0.35 GPa, the absorption edge of MAPbI3 is red-shifted, allowing an extension of spectral absorption. The transitions to phases IV and V are associated with the abrupt blue shifts of the absorption edge. The strong increase of the energy gap in phase V result in a spectacular color change of the crystal from black to red around 3.5 GPa. The optical changes have been correlated with the pressure-induced strain of the MAPbI3 inorganic framework and its frustration, triggered by methylammonium cations trapped at random orientations in the squeezed voids.

Impact of structural differences in carcinopreventive agents indole-3-carbinol and 3,3'-diindolylmethane on biological activity. An X-ray, ¹H-¹4N NQDR, ¹³C CP/MAS NMR, and periodic hybrid DFT study.

Three experimental techniques (1)H-(14)N NQDR, (13)C CP/MAS NMR and X-ray and Density Functional Theory (GGA/BLYP with PBC) and Hirshfeld surfaces were applied for the structure-activity oriented studies of two phyto-antioxidants and anticarcinogens: indole-3-carbinol, I3C, and 3,3'-diindolylmethane, DIM, (its bioactive metabolite). One set of (14)N NQR frequencies for DIM (2.310, 2.200 and 0.110 MHz at 295K) and I3C (2.315, 1.985 and 0.330 MHz at 160K) was recorded. The multiplicity of NQR lines recorded at RT revealed high symmetry (chemical and physical equivalence) of both methyl indazole rings of DIM. Carbonyl (13)C CSA tensor components were calculated from the (13)C CP/MAS solid state NMR spectrum of I3C recorded under fast and slow spinning. At room temperature the crystal structure of I3C is orthorhombic: space group Pca21, Z=4, a=5.78922(16), b=15.6434(7) and c=8.4405(2)Å. The I3C molecules are aggregated into ribbons stacked along [001]. The oxygen atomsare disorderedbetween the two sites of different occupancy factors. It implies that the crystal is built of about 70% trans and 30% gauche conformers, and apart from the weak OH⋯O hydrogen bonds (O⋯O=3.106Å) the formation of alternative O'H⋯O bonds (O'⋯O=2.785Å) is possible within the 1D ribbons. The adjacent ribbons are further stabilised by O'H⋯O bonds (O'⋯O=2.951Å). The analysis of spectra and intermolecular interactions pattern by experimental techniques was supported by solid (periodic) DFT calculations. The knowledge of the topology and competition of the interactions in crystalline state shed some light on the preferred conformations of CH2OH in I3C and steric hindrance of methyl indole rings in DIM. A comparison of the local environment in gas phase and solid permitted drawing some conclusions on the nature of the interactions required for effective processes of recognition and binding of a given anticarcinogen to the protein or nucleic acid.

Comment on the phase transition mechanism in diglycine methanesulfonate.

The transition mechanism revised: The transition in diglycine methanesulfonate at 133 K has an order-disorder character. It has been evidenced that the previously postulated by Zhou and coworkers (Chem. Asian J.- 2014, 9, 996-1000) displacive mechanism of this transition is a simple consequence of the improper analysis of the X-ray diffraction and calorimetric experimental data.

Crystal structures, phase transitions, and pressure-induced ferroelectricity in [C(NH2)3]5SO4(SO3-OC2H5)2F.

A guanidinium compound, [C(NH(2))(3)](5)SO(4)(SO(3)-OC(2)H(5))(2)F, with complex anionic sublattice has been synthesized and characterized by calorimetric and dielectric measurements at ambient and high hydrostatic pressures, as well as by single-crystal X-ray diffraction at varied temperatures. At room temperature, the crystal structure is orthorhombic, with the space group Pnma. In this phase, each of the two crystallographically nonequivalent ethoxysulfonate anions is disordered between two sites. On cooling, one of these anions starts to set in order at 228 K, where the crystal transforms in a continuous manner to the intermediate orthorhombic phase, with the space group P2(1)2(1)2(1). This transition belongs to the exceptionally rare pure gyrotropic phase transitions, the order parameter of which is described by the third-rank gyrotropic tensor. The ordering of the second ethoxysulfonate anion occurs suddenly at 187 K, inducing a first-order phase transition to the low-temperature phase of space group Pna2(1). The dissimilar response of both ethoxysulfonate anions to the temperature variation can be attributed to the different hydrogen bonding patterns they form with the cationic framework. Despite the polar symmetry, the low-temperature phase is not ferroelectric at ambient pressure, but it acquires ferroelectric features at elevated pressures above 140 MPa, as evidenced by the polarization reversal in an external electric field. The ferroelectric properties disappear on increasing pressure above 220 MPa, where the phase transition strongly modifying the crystal properties, but fully reversible, takes place. In the pressure-induced phase, a Debye-like dipolar relaxation process has been found and characterized as a function of pressure. The unusual properties of [C(NH(2))(3)](5)SO(4)(SO(3)-OC(2)H(5))(2)F under hydrostatic pressure have been summarized in the p-T phase diagram.

Simple guanidinium salts revisited: room-temperature ferroelectricity in hydrogen-bonded supramolecular structures.

Dielectric, calorimetric, and X-ray diffraction methods have been employed to characterize the crystals of guanidinium tetrafluoroborate and guanidinium perchlorate, both built of two-dimensional honeycomb hydrogen-bonded sheets. The room-temperature ferroelectricity of these isosymmetric complexes (space group R3m) has been evidenced by the polarization switching in an external electric field and pyroelectric effect. The analysis of structural data as a function of temperature showed that the high values of spontaneous polarization of about 8.5 µC cm(-2) originate mainly from the ionic displacements, while the exceptional thermally induced increase of polarization is related with the apparent weakening of the N-H···F/N-H···O hydrogen bonds at elevated temperatures. An excellent correlation between the donor-acceptor distance and the relative displacement of the ions in the crystal lattice along the polar direction has been found. The huge entropy change at the two-closely spaced high-temperature phase transitions in guanidinium perchlorate, together with the large crystal polarization, suggest a large electrocaloric effect, the property strongly desired for solid-state cooling applications.

Pressure-induced collapse of guanidinium nitrate N-H···O bonded honeycomb layers into a 3-D pattern with varied H-acceptor capacity.

Highly favoured N-H···O bonded honeycomb layers in guanidinium nitrate, C(NH(2))(3)(+)NO(3)(-), have been destabilized by a pressure of 0.6 GPa, and the novel motif of 3-dimensional N-H···O bonded aggregation in high-pressure phase IV determined for in situ grown single-crystal by X-ray diffraction. The mechanism of the transition involves the collapse of voids present in phases I, II and III. In the P/T phase diagram a large hysteresis of the phase IV boundaries is caused by the strongly reconstructive character of the transition and pressure dependent H-accepting capacity of oxygen atoms.

Bias-field and pressure effects on the one-dimensional dielectric response in N-H(+)...N hydrogen-bonded 1,4-diazabicyclo[2.2.2]octane hydrobromide crystal.

Unusual dielectric properties of 1,4-diazabicyclo[2.2.2]octane hydrobromide [C(6)H(13)N(2)](+).Br(-) (dabcoHBr) have been investigated at ambient and hydrostatic pressures and at biasing dc electric field. The crystal exhibits a huge dielectric constant along the hydrogen-bonded chains, exceeding 1500, while in the perpendicular direction it behaves as a typical nonpolar dielectric. Though the dynamics of protons in the N-H(+)...N hydrogen bonds is essential for these properties, of key importance are weak protonic correlations leading to the formation of short-range ordered regions. The complex dielectric response of dabcoHBr is due to several contributions involving dipolar fluctuation within the polar nanoregions, fluctuations of boundaries, and excitation of solitonic kinks propagating along the chains as a result of coherent proton transfers. A relatively low dc biasing electric field distinctly modifies the dielectric response, making it reminiscent of ferroelectric relaxors. Profound changes are also induced by hydrostatic pressure, which counteracts the proton correlations and the short-range polar order formation. At elevated pressures, the hexagonal structure of dabcoHBr undergoes a phase transition, associated with a loss of the unusual dielectric properties. This is due to the breaking of the N-H(+)...N hydrogen bonds, which destroys the one-dimensional topology of the polycationic chains and results in formation of the phase built of hydrogen-bonded ionic pairs. The phase diagram, illustrating the phase boundary between the high- and low-dielectric constant phases of dabcoHBr, is presented.

Giant dielectric anisotropy and relaxor ferroelectricity induced by proton transfers in NH+...N-bonded supramolecular aggregates.

A huge dielectric effect has been observed in a pure and water-soluble hydrogen-bonded organic crystal, 1,4-diazabicyclo[2.2.2]octane hydroiodide [C6H13N2]+.I(-) (dabcoHI). In this structure, the dabco cations are NH+...N bonded into linear aggregates, where the protons are disordered at two nitrogen atoms and the crystal acquires the symmetry of space group P6m2. This nonpolar crystal exhibits a barely temperature-dependent dielectric constant exceeding 1000 at ambient conditions. The dielectric response is extremely anisotropic, more than 2 orders of magnitude higher along the linear hydrogen bonded chains than in perpendicular directions. The physics underlying this effect originates from proton transfers in the NH+...N bonds, leading to disproportionation defects and formation of polar nanodomains, which, on the macroscopic scale, results in one-dimensional relaxor ferroelectricity. Such properties are unprecedented for the materials with hydrogen bonds highly polarizable due to proton disorder. The proton disordering in dabcoHI is analogous to this in H2O ice, where the hydrogen bonds remain disordered until the lowest temperature.

From nonpolar to ferroelectric crystal structure: the temperature-tuned growth of two guanidinium ethoxysulfonate polymorphs.

Guanidinium ethoxysulfonate, [C(NH2)3]+[C2H5O-SO3]-, was synthesized, and two polymorphs, both stable at normal conditions, were grown from an aqueous solution by only a slight change in the crystallization temperature. The nonpolar polymorph I is built of hydrogen-bonded bilayers, while the ferroelectric polymorph II consists of single-layers. The diversity in the crystals' architecture and properties originates from the excessive number of proton-acceptor sites. At 298 K, the structure of polymorph I is orthorhombic, space group Pbam, formed of supramolecular hydrogen-bonded sheets. Within such a sheet, the ethoxysulfonate anions assume alternately cis and trans conformations, both disordered at room temperature and at 150 K. The anisotropy of the crystal structure is mirrored by a strong anisotropy of its thermal expansion. Upon cooling at 120 K, the crystal undergoes a first-order order-disorder phase transition. The structure of polymorph II is also reinforced by the two-dimensional network of NH...O hydrogen bonds, but the supramolecular motif formed is different from that of polymorph I. The H-bonded strongly corrugated sheets are stacked, forming a densely packed single-layer structure. All the anions assume the same trans conformation. At 298 K, they are disordered between the two sites related by the mirror symmetry plane. The onset of ordering of the anions coincides with the Curie point at TC = 211 K, at which the dielectric constant exceeds 4000. The continuous paraelectric-ferroelectric phase transition is associated with the symmetry change Pnma --> Pna21. Despite the apparent order-disorder character of the transition, the transition mechanism also involves a substantial displacement of the ions and a rearrangement of the H-bonded network.

Anomalous protonic-glass evolution from ordered phase in NH...N hydrogen-bonded dabcoHBF4 ferroelectric.

Dielectric properties, spontaneous polarization, and phase transitions of the NH+...N bonded ferroelectric dabcoHBF4 (i.e., 1,4-diazabicyclo[2.2.2]octane tetrafluoroborate, [C6H13N2]+ BF4-) have been related to one-dimensional arrangement of the cations and to their conformational properties. The onset of conformational transformation lowering the symmetry of the cations, rearrangement of the anions, and proton disordering in NH+...N hydrogen bonds, linking the cations into linear chains, lead to a ferroelectric-ferroelectric phase transition at T23 = 153 K. A weak coupling between the protonic and anionic sites in dabcoHBF4 results in the formation of distinct phase-diagram regions: the high-temperature paraelectric phase with disordered protons, the intermediate ferroelectric phase with the protons ordered, and the low-temperature ferroelectric phase where the protons become disordered again. The lowest temperature phase remains ferroelectric owing to the ionic displacements, while the protons assume the glass state. In this phase the H+ transfers involve local formation of neutral, monocationic, and dicationic species. Such an anomalous formation of protonic glass state from the ordered phase depends on the subtle structural features pertaining to the proton transfers in bistable hydrogen bonds. In paraelectric phase I, between the mp and T12 = 374 K, the anions are orientationally disordered, the protons are disordered in the hydrogen bonds and the cations rotate about the [z] direction; in ferroelectric phase II below T12, the protons and cations order, the dabco cations assume a planar conformation of ethylene bridges, and the anions exhibit a residual temperature-dependent gradual ordering (two 80:20 occupied sites of the anion are still observed at 332 K); and in ferroelectric phase III below T23, the cations assume left- and right-twisted propeller conformations and the anions are ordered but the protons become disordered in the hydrogen bonds.