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The paper presents the Infrared and Raman spectra of the powdered [C 3 N 2 H 5 + ] 2 [I − ∙ I 3 − ] crystal at the temperature intervals of 11–270 K, covering two low-temperature phase transitions: discontinuous at 182/188 K (cooling/heating) and continuous at 254 K. The research shows that the vibrational states of the pyrazolium cations change significantly during discontinuous phase transition (III→II), while the continuous nature of successive structural transformation is more subtle and displays an insignificant change in the temperature coefficient of numerous vibrations during the II → I PT at 254 K. The spectacular changes at Raman spectra above 188 K confirm a huge rebuilding of inorganic network from [I − ∙ I 3 − ] to [I 4 2 − ]. Additionally, a complete geometry optimization was carried out in the solid state in order to obtain minimum structures and bonding properties. The theoretical results correspond well with the experimental data. Moreover, the infrared spectrum in harmonic approximation was calculated, and a comparative vibrational analysis was performed. CRYSTAL09 vibrational results appear to be in a good agreement with the experimental ones. [ABSTRACT FROM AUTHOR]

Abstract: The Raman spectrum (7–3400cm−1) measured for a metastable β-phase of benzophenone is reported for the first time. The Raman spectra measured during the structural phase transition between the metastable β-phase and the stable α-phase (in the temperature region 243–293K) are reported as well. For comparison, the Raman spectrum of the stable α-phase is presented. The structure of the benzophenone isolated molecule and its Raman spectrum calculated by the density functional method (B3LYP/6-31+G(d)) using gaussian 03 software are discussed in relation to the measured Raman spectra of the α- and β-phases. [Copyright &y& Elsevier]

Abstract: [(CH3)3PH]3[Sb2Cl9] (abbreviated as TMPCA) was found to undergo numerous reversible structural phase transitions studied by dilatometric and dielectric methods: I↔II at 375/372 (heating/cooling), II↔III at 272/272, III↔IV at 223/216 and IV↔V at 212/206K. Debay-like dispersion of the electric permittivity in the kilohertz frequency region over the phases I and II is disclosed. The molecular parameters of the relaxation process assigned to the motion of the trimethylpyridinium cations are evaluated. The pyroelectric measurements of TMPCA confirmed the polar/pyroelectric nature of the lowest temperature phase (V). Proton spin-lattice relaxation time of polycrystalline TMPCA was studied in the temperature range 80–429K. A dynamic inequivalence of two trimethylphosphonium cations performing complex reorientational motion (C3—of methyl groups, —of the cation around the P—H bond and the isotropic motion of the whole cation) was detected. A ferroelastic domain structure was observed in all phases below 375K. The infrared and Raman spectra for TMPCA were measured in the region of the internal vibrations of the trimethylphosphonium cations and [Sb2Cl9]3− anions in the wide temperature region (Raman 12–404K, IR 300–413K). The possible mechanisms of the phase transitions are discussed on the basis of the presented results. [Copyright &y& Elsevier]

Abstract: Infrared spectra of powdered 4-aminopyridinium tetrachloroantimonate(III) [4-NH2C5H4NH][SbCl4] (abbreviation: 4-APCA) in the region of the internal vibrations of the 4-aminopyridinium cation (3600 and 400cm−1) and in the temperature interval 10–300K covering three structural phase transitions are presented and discussed. The results confirm the order–disorder mechanism of the phase transition at 245K. The paraelectric–ferroelectric phase transition at 271K is hardly visible in the infrared spectra, which is in agreement with its “displacive” nature of the transition suggested by the single crystal X-ray and ac calorimetric studies. [Copyright &y& Elsevier]

Abstract: [n-C4H9NH3]2[BiBr5] exhibits five solid–solid structural phase transitions; at 309 (1st order), 291 (1st), 240 (1st), 219 (close to 2nd) and at 166K (1st). The phase transitions at 291 and 240K are accompanied by distinct dielectric anomalies. The dielectric relaxation process was found to appear over the phases (III) and (I). The infrared spectra for [n-C4H9NH3]2[BiBr5] were measured in the region of the internal vibration of the n-butylammonium cation (4000–400cm−1) between 12 and 320K. The temperature changes of wavenumbers and of a width for many of the internal modes of the n-butylammonium clearly indicate the contribution of the organic sublattice to the phase transition mechanism. [Copyright &y& Elsevier]

Abstract: Infrared spectra (3500–500cm−1) of polycrystalline (C5H5NH)5Bi2Br11 samples were investigated within the temperature range 27–456K. The assignments of the observed bands in the spectra measured at 27, 310 and 456K are proposed. A temperature dependence of the wavenumbers and full width at half maximum (FWHM) of the bands arising from some internal vibrations of pyridinium cations are analysed in order to explain the role of cations in the mechanism of the phase transition at 118 (paraelectric–ferroelectric) and 403K. It was found that numerous bands arising from the internal modes of the cations exhibit the splitting in the vicinity of both phase transitions, that indicates a distinct changes in the motional state of the pyridinium moieties. [Copyright &y& Elsevier]

Abstract: Internal vibration modes of the and crystals have been studied through their low temperature paraelastic–ferroelastic phase transitions at 151 and 205 K, respectively, using infrared spectroscopy. The studies show that the vibrational states of tetramethyphosphonium cations change markedly through the phase transitions. This is in line with the earlier suggested order–disorder mechanism of the low temperature transition. [Copyright &y& Elsevier]

Differential scanning calorimetry and linear thermal expansion studies reveal seven phase transitions in [n-C4H9NH3]2[SbBr5] crystal. The phase transitions at 261, 280 and 292 K are accompanied by distinct dielectric anomalies. The dielectric relaxation processes were disclosed over the low temperature phases (VI, VII) and over the phase (III). The infrared spectra of powdered sample of [n-C4H9NH3]2[SbBr5] in Nujol were measured in the region of the internal vibration of the n-butylammonium cation (4000–400 cm-1) in the temperature range between 26 and 320 K. The temperature changes wavenumbers and width of most of the modes of the n-butylammonium cations were analysed. [Copyright &y& Elsevier]

Infrared spectra of [(CH3)2NH2]3[Bi2Cl9] and [(CH3)2NH2]3[As2Cl9] are presented and discussed in the region of the internal vibrations of methylammonium cation (3600 and 600 cm−1) and at temperatures close to the structural phase transitions. A detail analysis has been performed for the bands arising from the stretching and deformation vibrations of the (CH3)2NH2+ groups. The results confirm the order–disorder type of the transitions related to the change of the dimethylammonium cations dynamics. FT-IR and FT-Raman spectra in the region 500–80 cm−1 at room temperature are also presented. [Copyright &y& Elsevier]

Low frequency Raman spectroscopic studies (5–400 cm−1) were carried out for single crystal samples of (C5H5NH)6Bi4Cl18 within the temperature range 37–285 K to look for possible spectral changes resulting from the phase transitions at 122 and 154 K. A detailed analysis of frequencies, band widths and intensities of the bands was made for the (a(ba)c*) and (a(bb)c*) spectra through the phase transitions to understand their nature. It was shown that the transitions are not accompanied by any mode softening, thus indicating their order–disorder nature. However, the ordering of the pyridinium cations close to 122 and 154 K exerts a strong influence on the (Bi4Cl186−) anion. [Copyright &y& Elsevier]

Abstract: [4-NH2C5H4NH][C4H3O4][C4H4O4] (abbreviated as APM) and its N,O-deuterated analogue have been synthesized. The single-crystal X-ray diffraction studies for N,O-deuterated analogue are reported. The Kurtz and Perry powder SHG technique confirms the second order nonlinear optical properties of APM. The structural phase transition of APM occurring at 312K has been examined by infrared spectroscopy. The infrared temperature studies on the polycrystalline APM showed that the modes arising from N H⋯O and O H⋯O intermolecular hydrogen bonds vibrations appear to be very sensitive to the structural phase transition. The molecular mechanism of the phase transition is explained in terms of significant changes in the hydrogen bond configuration. [Copyright &y& Elsevier]

Abstract: Calculations of the molecular structure and normal vibration frequencies have been performed using ADF (Amsterdam Density Functional) program package for initial structural data of 1-aminopyridinium iodide, (1-NH2C5H5N)+I−, low temperature phase (110K). Cambridge Structural Database survey has been carried out in order to explore hydrogen bond patterns of 1-aminopyridinium ring and to analyze the strength of intermolecular N–H⋯I− interactions. Infrared spectra of powdered (1-NH2C5H5N)+I− in a frequency range 4000–400cm−1 in a wide temperature region (from 300 to 413K), covering ferroelastic-paraelastic phase transition at 384/383K (heating/cooling), are presented and analyzed. The mechanism of the phase transition is discussed. [Copyright &y& Elsevier]

Abstract: [4-NH2C5H4NH][C4H3O4][C4H4O4] (APM) complex with rare stoichiometric ratio 1:2 has been synthesized and characterized by a single-crystal X-ray diffraction and powder X-ray diffraction in a wide temperature range (from 100K up to 338K), calorimetry and dielectric spectroscopy. The APM undergoes structural phase transition at 307/312K (cooling/heating) of first order type classified as an order–disorder. The crystal structure of APM below 307K consists of ordered 4-aminopyridinium cations, hydrogen maleate anions (HM) and neutral maleic acid molecules (MM) that interact via hydrogen bonds. The room temperature phase II and the high temperature phase one I is polar, space group P21, which is confirmed by the pyroelectric effect. The I → II phase transition is accompanied by a distinct electric permittivity anomaly (ε max ≈55 at 150kHz) that is characteristic of ferroelectric crystals. The infrared spectra of polycrystalline APM are presented and compared with other closely related analogs. [Copyright &y& Elsevier]

Abstract: Infrared spectra of the powdered (C3N2H5)5Bi2Cl11, (C3N2H5)5Bi2Br11and (C3N2H5)5Sb2Br11 crystals in the region of internal vibrations of the imidazolium cations (3600 and 400cm−1) at the temperature intervals of 10–300K, covering paraelectric–ferroelectric phase transitions, are presented and discussed in this paper. The research shows that the vibrational states of the imidazolium cations change markedly during the paraelectric–ferroelectric phase transition. The continuous nature of these transitions is well reflected in the infrared spectra, which is consistent with the previous X-ray and dielectric findings. [Copyright &y& Elsevier]

Abstract: has been synthesized and characterized by the X-ray (at 293 and 110K), calorimetric, dilatometric and dielectric measurements. At room temperature it crystallizes in the monoclinic space group, C2/m. A crystal structure consists of disordered imidazolium cations and ordered discrete tetramers of . This compound reveals a rich polymorphism in a solid state. It undergoes three solid–solid phase transitions: from phase I to II at 426/423K (heating–cooling), at 227K and at 219.5/219K. A clear dielectric relaxation process is found in the room temperature phase II. Infrared studies of the polycrystalline showed that the , (ring) and modes of the imidazolium cations appeared to be very sensitive to the phase transition. 1H NMR measurements confirmed a key role of the imidazolium cations in the phase transitions mechanisms at low temperatures. [Copyright &y& Elsevier]

Abstract: (C5H5NH)3BiCl6 crystallizes at room temperature in the triclinic space group P . The crystal is built up of the separated octahedral anions and pyridinium cations. Differential scanning calorimetry (DSC) and dilatometric measurements disclose structural phase transition of first-order type at 265/302K (cooling/heating). The dielectric investigations reveal a step-wise change of the electric permittivity at Tc characteristic of the crystals with the high-temperature plastic-like phase. Temperature-dependent infrared spectra for polycrystalline samples have been taken between 20 and 306K to clarify the cation dynamics contribution to the mechanism of the phase transition. The modes assigned to the deformation vibration of the NH+ group in the pyridinium ring appear to be the most sensitive to the phase transition. The possible molecular motions of the C5H5NH+ cations are studied by the proton magnetic resonance (1H NMR). [Copyright &y& Elsevier]

Abstract: The di(n-pentylammonium) pentabromoantimonate(III) compound has been synthesized and studied by means of a single-crystal X-ray diffraction, differential scanning calorimetry, thermal expansion, dielectric and IR techniques. Two solid–solid phase transitions of first order: at 416/388K and 225/224K (heating/cooling) have been revealed. The crystal structure of [n-C5H11NH3]2[SbBr5] has been solved at 298K, Pna21 (phase II) and at 86K P212121 (phase III). The crystal structure is composed of the anions which form an infinite chain and four independent n-pentylammonium cations. The dielectric studies have been made in the frequency range 500Hz–1MHz between 160 and 430K. The dielectric relaxation process, taking place over the low temperature phase III, has a polydispersive character. The infrared measurements carried out between 11 and 300K indicate a significant role of the organic cations in the mechanism of the phase transition from the intermediate to the low temperature at 225K. [Copyright &y& Elsevier]

Abstract: The crystal structure of [C3H5N2]6Bi4Cl18 has been determined at 293K by single crystal X-ray diffraction. The compound crystallizes in the monoclinic space group, C2/m. The structure is composed of the discrete anions and three kinds of the imidazolium cations. The cations appeared to be disordered. The infrared spectra of [C3H5N2]6Bi4Cl18 were measured in the region of the internal vibration of the imidazolium cations (4000–400cm−1) between 11 and 300K. The temperature changes of wavenumbers of the δ(ring), δ(C–H) and ω(C–H) modes in the vicinity of the 195 and 221K clearly indicate the contribution of the imidazolium cations motion to the phase transition mechanism taking place in the [C3H5N2]6Bi4Cl18 crystal. [Copyright &y& Elsevier]

Abstract: The structure of [C3N2H5]4[Bi2Br10]·2H2O, (PBB) was determined by single crystal X-ray diffraction at 100K. It crystallizes in the monoclinic space group C2/m, with a =12.992(4)Å, b =16.326(5)Å, c =8.255(3)Å, β =108.56°(3), V =1659.9(9)Å3 and Z =2. The structure consists of discrete binuclear [Bi2Br10]4− anions, ordered pyrazolium cations and water molecules. The crystal packing is governed by strong N–H⋯O and weak O–H⋯Br hydrogen bonds. A sequence of structural phase transitions in PBB was established on the basis of differential scanning calorimetry and dilatometric studies. Two reversible first-order phase transitions were found: (I→II) at 381/371K (on heating/cooling) and (II→III) at 348/338K. Dielectric response near both phase transitions is characteristic of crystals with the “plastic-like” phases. Over the phase III a low frequency dielectric relaxator is disclosed. The possible molecular motions in the PBB compound are characterized by the 1H NMR studies. The infrared spectra of polycrystalline compound in the temperature range 300–380K are reported for the region 4000–400cm−1. The observed spectral changes through the structural phase transition III→II are attributed to an onset of motion both of the pyrazolium cations and water molecules. [Copyright &y& Elsevier]

A sequence of structural phase transitions in [(CH3)2NH2]3[Bi2Cl9] (DMACB) is established on the basis of differential scanning calorimetry (DSC) and dilatometric studies. Four phase transitions are found: at 367/369, 340/341, 323/325 and 285/292 K (on cooling/heating). The crystal structure of DMACB is determined at 350 K. It crystallizes in monoclinic space group P21/n: a=8.062(2), b=21.810(4), c=14.072(3) A˚, β=92.63(3)°, Z=4, R1=0.0575, wR2=0.1486. The crystal is built of the double chain anions (“pleated ribbon structure”) and the dimethylammonium cations. Dielectric studies in the frequency range 75 kHz–900 MHz indicate relatively fast reorientation of the dimethylammonium cations over the I, II, III and IV phases. Infrared spectra are recorded in the temperature range 40–300 K and analyzed in region assigned to the symmetric and asymmetric NC2 stretching vibrations. Optical observations show the existence of the ferroelastic domain structure over all phases below 367 K. The possible mechanisms of phase transitions are discussed on the basis of presented results. [Copyright &y& Elsevier]

Infrared spectra of bis(pirrolidinium)-chloride-hexachloroantimonate(V) {(C4H8NH2+)2SbCl6−·Cl−} in the region of the internal vibrations of the pyrrolidinium cations (3500–400 cm−1) around the phase transition at Tc=134 K are presented and discussed. A detailed analysis has been performed for the bands assigned to the stretching and deformation vibrations of the C4N ring as well as the NH2+ and CH2 groups of the cation. It has been suggested that the mechanism of the phase transition is connected with a change in the dynamical state of the pyrrolidinium cation. [Copyright &y& Elsevier]

• The guanidinium derivatives, [C(NH 2) 3 ] 3 Bi 2 I 9 (GBI) and [C(NH 2) 3 ] 3 Sb 2 I 9 (GSI), as the promising compounds for a possible application in photovoltaics. • Optical, dielectric and UV–Vis results of studies on GBI and GSI. • Ferroelastic properties of [C(NH 2) 3 ] 3 Bi 2 I 9 (GBI) and [C(NH 2) 3 ] 3 Sb 2 I 9 (GSI). The guanidinium derivatives [C(NH 2) 3 ] 3 Bi 2 I 9 (GBI) and [C(NH 2) 3 ] 3 Sb 2 I 9 (GSI) are promising compounds from the viewpoint of their electrooptical and semiconducting properties. In this paper we present the optical, dielectric, UV–Vis, Raman and 1H NMR (spin-lattice relaxation time, T 1 , and second moment, M 2) results of studies on GBI and GSI. The temperature and frequency dependent ac conductivity of GBI and GSI has been analyzed in terms of the Jonscher relationship. The parameters for the ac conductivity have been estimated. The UV–Vis spectra have been analyzed for GBI and GSI and the respective calculations regarding the band gap have been carried out. GBI and GSI reveal the semiconducting properties in their high temperature phases (σ ac in the range 10–5 – 10–7 S/m). 1H NMR studies indicate the key role of the guanidium cation dynamics in the phase transitions mechanisms. [ABSTRACT FROM AUTHOR]