Phase transition of pyridinium tetrachloroiodate(III), PyHICl4, studied by a single crystal X-ray analysis and dielectric and heat capacity measurements

Change of a local environment of a polar pyridinium ion, which is associated with the phase transition of crystalline pyridinium tetrachloroiodate(III) at T_c = 217 K, was investigated by a single crystal X-ray analysis and dielectric and heat capacity measurements. The site symmetry 2/m of the ion at T > T_c indicates an orientational disorder in the high-temperature phase (HTP). The energy difference ΔE between the stable and meta-stable orientations of the pyridinium ion at the 2/m site was estimated to be ΔE/R ? 560 K at 280 K in the HTP. Below the T_c, an antiferroelectric ordering of the ions was revealed.     

Particle size effect on the new phase transition in a tridymite compound, CsCoPO4

A new phase transition (III–IV) was found at 311 K in CsCoPO₄ by DSC measurements. The crystal structure of all the phases, I–IV, in CsCoPO₄ was studied by synchrotron-radiation X-ray powder diffraction. The diffractometry revealed that CsCoPO₄ had the same crystal structure as that of corresponding phases in CsZnPO₄. An extremely large particle size effect was found on III–IV phase transition in CsCoPO₄; the phase transition enthalpy decreases with decreasing the particle size around 0.1 mm. Such large particle size effects had been also observed in CsZnPO₄. However, no III–IV phase transition was observed in the particle smaller than 0.1 mm of CsZnPO₄, while such a critical size was not found in CsCoPO₄.     

Reversible phase transition of 2‐carboxypyridinium perchlorate–pyridinium‐2‐carboxylate (1/1)

The title salt, C₆H₆NO₂⁺·ClO₄⁻·C₆H₅NO₂, was crystallized from an aqueous solution of equimolar quantities of perchloric acid and pyridine‐2‐carboxylic acid. Differential scanning calorimetry (DSC) measurements show that the compound undergoes a reversible phase transition at about 261.7 K, with a wide heat hysteresis of 21.9 K. The lower‐temperature polymorph (denoted LT; T = 223 K) crystallizes in the space group C2/c, while the higher‐temperature polymorph (denoted RT; T = 296 K) crystallizes in the space group P2/c. The relationship between these two phases can be described as: 2a_RT = a_LT; 2b_RT = b_LT; c_RT = c_LT. The crystal structure contains an infinite zigzag hydrogen‐bonded chain network of 2‐carboxypyridinium cations. The most distinct difference between the higher (RT) and lower (LT) temperature phases is the change in dihedral angle between the planes of the carboxylic acid group and the pyridinium ring, which leads to the formation of different ten‐membered hydrogen‐bonded rings. In the RT phase, both the perchlorate anions and the hydrogen‐bonded H atom within the carboxylic acid group are disordered. The disordered H atom is located on a twofold rotation axis. In the LT phase, the asymmetric unit is composed of two 2‐carboxypyridinium cations, half an ordered perchlorate anion with ideal tetrahedral geometry and a disordered perchlorate anion. The phase transition is attributable to the order–disorder transition of half of the perchlorate anions.     

Temperature dependence of nonequivalent potential wells for pyridinium ion reorientation in pyridinium tetrachloroiodate(III), PyHICl4, studied by 1H-14N nuclear quadrupole double resonance

The reorientation of a pyridinium ion in the paraelectric and antiferroelectric phase of PyHICl(4) is investigated using (1)H-(14)N nuclear quadrupole double resonance (NQDR). The (14)N nuclear quadrupole resonance frequencies are measured. The temperature variations of the principal values of the time-averaged electric-field-gradient (EFG) tensor at the nitrogen position are used to determine the occupation probabilities of the six orientations of a pyridinium ion in both crystallographic phases. The energy difference between various orientations is determined. The molar transition entropy associated with the reorientation of the pyridinium ions is calculated and compared to the experimental value.     

Columnar bis(pyridinium) ionic liquid crystals derived from 4-hydroxypyridine: synthesis,mesomorphism and emission properties

A series of flexibly linked bis(pyridinium) salts with various counterions (Br^?, PF₆^?, BF₄^? and OTf-) was designed and prepared starting from corresponding N-alkylated 4-pyridones precursors with mesogenic 3,4,5-tris(alkyloxy)benzyl moieties (alkyl = dodecyl or tetradecyl). These salts were investigated for their liquid crystalline properties by a combination of differential scanning calorimetry, polarising optical microscopy and temperature-dependent powder X-ray diffraction (XRD). Their thermal stability was checked by thermogravimetric analysis. All bis(pyridinium) salts, except the triflate salt with shorter terminal carbon chain, display an enantiotropic liquid crystalline behaviour with a hexagonal columnar (Col_h) phase assigned on the basis of its characteristic texture and XRD studies. It was found that these luminescent bis(pyridinium) salts show weak emission in dichloromethane solutions at room temperature and a pronounced red-shifted emission in solid state. The emission properties of these bis(pyridinium) salts do not depend significantly on the nature of counterion employed.     

Low-temperature phase transitions in (C5H5NH)Ag5I6

The solid electrolyte (C₅H₅NH)Ag₅I₆ undergoes two low-temperature transitions, probably associated with the ordering of the pyridinium ions. The two phases are monoclinic and have structures closely related to that of the hexagonal phase at 240°K. The transition to the γ-phase at 230°K is second order, and that from the γ- to the δ-phase at 180°K, first order. In the γ-phase, the monoclinic a and b axes correspond to the orthohexagonal B and A axes, respectively; in the δ-phase, the correspondence is reversed. If the pyridinium ions order in the γ-phase, its space group is most probably Cc. If the pyridinium ions are ordered in the δ-phase, the N atoms must lie on twofold axes in $\text{C2}\text{c}$, but the space group Cc is also possible for this phase.     

Effect of CuO Doping on the Electrical Behavior of ZrO2

Electrical conductivity has been measured at different temperatures for ZrO2 doped with various molar ratios of CuO. The conductivity increases due to migration of vacancies, created by doping. The conductivity was found to increase with increase in temperature till 220℃ and thereafter decrease due to collapse of the fluorite framework. A second rise in conductivity around 500 ℃ was observed due to phase transition of ZrO2. X-ray powder diffraction, DTA and IR studies were carried out for confirming doping effect and phase transition in ZrO2.     

Ferroelectricity of pyridinium perchlorate, a molecular level point of view

The behavior of ferroelectric and thermodynamic parameters of pyridinium perchlorate (PyClO(4)) was simulated based on experimental NMR data and properties of the individual constituting ions. Dynamic (2)H NMR spectroscopy was used to investigate the order-disorder character of ferroelectricity in PyClO(4). Quadrupole echo and inversion recovery experiments were performed in the range from 140 to 300 K. The spectra can be simulated by a rotational jump motion of the pyridinium cations about their pseudo-C(6) axis. This confirms that the ferroelectricity of this compound below a first-order phase transition at 248 K is primarily due to the ordering of cations along a ferroelectric axis. In an intermediate phase between 248 and 233 K, the cation-anion sublattice displacement mechanism also gives a small positive contribution to ferroelectricity. In the family of the ferroelectric pyridinium salts, the paraelectric-ferroelectric phase transition temperature increases with the size and polarizability of the constituting anions, suggesting that the main interaction for ferroelectric ordering occurs via an indirect superexchange mechanism, whereas in compounds with small anions of low polarizability the direct dipole-dipole interaction dominates and leads to antiferroelectric order.     

Preparation and properties of anion exchange membranes having pyridinium or pyridinium derivatives as anion exchange groups

Anion exchange membranes with pyridinum groups and various pyridinium derivative groups were prepared from a copolymer membrane composed of chloromethylstyrene and divinylbenzene, and pyridine and pyridine derivatives. The anion exchange membranes obtained showed excellent electrochemical properties in electrodialysis. The transport numbers of sulfate ions, bromide ions, nitrate ions, and fluoride ions relative to chloride ions were evaluated in connection with the species of a substituent and the position of the substituent in the pyridinium groups. In general, when a hydrophilic substituent (methanol groups) existed at the 2-position of the pyridinium groups, nitrate ions and bromide ions, which are less hydrated, permeated through the membranes with difficulty, and sulfate ions permeated selectively through the membranes. On the other hand, when hydrophobic groups, for example, ethyl groups, existed at the 2-position of the pyridinium groups, bromide ions and nitrate ionspermeated selectively through the membranes and fluoride ions had difficulty permeating through the membranes. The carbon number of the alkyl chain of 4-alkyl pyridinium groups also affected permeation of nitrate ions and bromide ions due to the change in hydrophilicity of the membranes. Though the hydration of the anions and the species of the substituent at the 2-position of the pyridinium groups were related to selective permeation of the anion through the membranes, permeation of sulfate ions was not as sensitive to the hydrophilicity of the membranes. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 49–58, 1998     

A Chemically Triggered and Thermally Switched Dielectric Constant Transition in a Metal Cyanide Based Crystal

A dielectric constant transition is chemically triggered and thermally switched in (HPy)₂[Na(H₂O)Co(CN)₆] ( 2 , HPy=pyridinium cation) by single‐crystal‐to‐single‐crystal transformation and structural phase transition, respectively. Upon dehydration, (HPy)₂[Na(H₂O)₂Co(CN)₆] ( 1 ) transforms to its semi‐hydrated form 2 , accompanying a transition from a low‐dielectric state to a high‐dielectric state, and vice versa. This dielectric switch is also realized by a structural phase transition in 2 that occurs between room‐ and low‐temperature phases, and which corresponds to high‐ and low‐dielectric states, respectively. The switching property is due to the variation in the environment surrounding the HPy cation, that is, the hydrogen‐bonding interactions and the crystal packing, which exert predominant influences on the dynamics of the cations that transit between the static and motional states.     

Temperature-triggered phase transition in pyridazine hexafluorophosphate

The pyndazine hexafluorophosphate[C₄H₅N₂]⁺[PF₆]^-（1） undergoes a reversible phase transition around140 K,which was confirmed by the DSC measurement.Variable-temperature crystal structures determined at 293 K and 93 K show that the compound crystallizes in the same space group P2₁/c,indicating that 1 undergoes an iso-structural phase transition.As the temperature decreases,dielectric measurement of the title compound shows no significant change around the phase transition temperature.Classic hydrogen bonds are found between molecules at 293 K and 93 l< with similar packing arrangement.The most distinct difference between the low temperature and room temperature structures is the order-disorder transition of the hexafluorophosphate anion,which is probably the driving force of the phase transition.     

Heat capacity and magnetic phase transition of two-dimensional metal-assembled complex (NEt<Subscript>4</Subscript>)[{Mn<Superscript>III</Superscript>(salen)}<Subscript>2</Subscript>Fe<Superscript>III</Superscript>(CN)<Subscript>6</Subscript>]

Summary Heat capacity measurements of the two-dimensional metal-assembled complex, (NEt₄)[{Mn^III(salen)}₂Fe^III(CN)₆] [Et=ethyl, salen= N,N’-ethylenebis(salicylideneaminato) dianion], were performed in the temperature range between 0.2 and 300 K by adiabatic calorimetry. A ferrimagnetic phase transition was observed at T_c1=7.51 K. Furthermore, another small magnetic phase transition appeared at T_c2=0.78 K. Above T_c1, a heat capacity tail arising from the short-range ordering of the spins characteristic of two-dimensional magnets was found. The magnetic enthalpy and entropy were evaluated to be ΔH=291 J mol^-1 and ΔS=27.4 J K^-1 mol^-1, respectively. The experimental magnetic entropy agrees roughly with ΔS=Rln(5·5·2) (=32.5 J K^-1 mol^-1; R being the gas constant), which is expected for the metal complex with two Mn(III) ions in high-spin state (spin quantum number S=2) and one Fe(III) ion in low-spin state (S=1/2). The heat capacity tail above T_c1 became small by grinding and pressing the crystal. This mechanochemical effect would be attributed to the increase of lattice defects and imperfections in the crystal lattice, leading not only to formation of the crystal with a different magnetic phase transition temperature but also to decrease of the magnetic heat capacity and thus the magnetic enthalpy and entropy.     

Electrical properties of the CuCr2O4 spinel catalyst

Electrical resistivity of CuCr₂O₄ spinel obtained by a coprecipitation method was measured at various temperatures from 350 to 923K in air. Oxidation of its surface chromium ions at various temperatures was determined by an iodometric method. The temperature dependence of resistivity and the presence of higher-valent chromium ions at higher temperatures indicated CuCr₂O₄ to be an extrinsic semiconductor. Log ? vs $\text{1}\text{T}$ plots yielded straight lines in the whole temperature range with two breaks, one around 550K and the other around 730K. The first break was attributed to the saturation of the surface with higher-valent chromium, while the second break was attributed to tetragonal-to-cubic phase transition in CuCr₂O₄. Log ? vs $\text{1}\text{T}$ plots obtained while cooling exhibited hysteresis near the second break, confirming this break to be due to a first-order diffusionless transition.     

Infrared studies of phase transitions in ferroelectric (C5H5NH)5Bi2Br11

Infrared spectra (3500–500 cm⁻¹) of polycrystalline (C₅H₅NH)₅Bi₂Br₁₁ samples were investigated within the temperature range 27–456 K. The assignments of the observed bands in the spectra measured at 27, 310 and 456 K 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 403 K. 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.     

Design of anhydrous electrorheological (ER) suspensions based on I2-doped poly(pyridinium salt)

A rigid-rod aromatic poly(pyridinium salt) was synthesized and doped with iodine (I₂) for making anhydrous electrorheological (ER) fluids. The I₂-doped particles were further processed into ones having insulating skins. Dielectric properties and current densities of the suspensions containing these particles were studied to elucidate the roles of conductivity of the dispersed phase in the ER suspension. © 1994 John Wiley & Sons, Inc.     

Synchrotron and neutron powder diffraction study of phase transition in weberite-type Nd3NbO7 and La3NbO7

La₃NbO₇ and Nd₃NbO₇ are insulating compounds that have an orthorhombic weberite-type crystal structure and undergo a phase transition at about 360 and 450 K, respectively. The nature of the phase transitions was investigated via heat capacity measurements, synchrotron X-ray and neutron diffraction experiments. It is here shown that above the phase transition temperature, the compounds possess a weberite-type structure described by space group Cmcm (No. 63). Below the phase transition, the high temperature phase transforms into a weberite-type structure with space group Pmcn (No. 62). The phase transformation primarily involves the off-center shifting of Nb⁵⁺ ions inside the NbO₆ octahedra, combined with shifts of one third of the Ln³⁺ (Ln³⁺=La³⁺ and Nd³⁺) ions at the center of the LnO₈ polyhedra towards off-center positions. The phase transition was also proven to have great impacts on the dielectric properties.     

镉基杂化晶体的介电相变和蓝白光致发光双重特性

通过采用容易无序的胺,我们合成了2种有机无机杂化晶体,分别为基于bempy （bempy=1-甲基-1-溴乙基吡咯烷阳离子）的溴盐化合物（bempy） Br （1）和镉基溴化物（bempy）₂CdBr₄（2）,并对其结构相变、介电相变和蓝白荧光进行了详细的表征分析。化合物1在测试温度范围内未观察到可逆相变,化合物2为高温介电相变,介电和差示扫描量热法测试表征其相变温度为357 K。同时,化合物1和2均具备蓝白光致发光特性,荧光测试表明,化合物1和2分别在538 nm和547、750 nm处存在发射峰。化合物2具备介电相变和蓝白光致发光的双重特性。     

Phase Transitions in Solid Methanol

Neutron powder diffraction patterns were measured for the ordered α-phase and the disordered β-phase of deuterated methanol. The structure of the α-phase at 160 K is in agreement with that found earlier at 15 K. A complete refinement of the structure of the β-phase at 170 K was also carried out. The space groups are α-P2₁2₁2₁ and β-Cmcm. For the disordered phase, the thermal parameters indicate that the molecules are localized rather than being in free rotation. A transformation matrix was found that relates the unit cells of the two phases. The transition involves mainly the large-angle rotation of molecules in a plane. In a second experiment, the α- to β-phase transition in both deuterated and undeuterated solid methanol was examined using Raman spectroscopy and a metastable phase was produced, for the undeutered sample, by rapid quenching through the phase transition. Only two modes of the methyl groups in this metastable phase differ from the internal modes of the stable α-phase.     

Chichibabin pyridinium synthesis

Chichibabin pyridine synthesis involves the reaction of three aldehydes and ammonia to form 2,3,5-trisubstituted pyridines. This study examined the synthesis of tetrasubstituted pyridinium from aldehydes and an amine hydrochloride in the presence/absence of Pr(OTf)₃. Important insights into the reaction mechanisms of Chichibabin pyridinium synthesis were proposed through the investigation of reaction intermediates along with quantitative GC–MS analysis.     

Nuclear magnetic resonance study of the ferroelastic phase transition of order-disorder type in [N(C2H5)4]2CdCl4

This study uses nuclear magnetic resonance (NMR) techniques to examine the detailed changes in [N(C₂H₅)₄]₂CdCl₄ around its phase transition at the temperature T_C = 284 K. The chemical shifts and spin-lattice relaxation times in the rotating frame (T_1ρ) were determined from ¹H magic angle spinning (MAS) NMR and ¹³C cross-polarization (CP)/MAS NMR spectra. The two sets of inequivalent ¹H and ¹³C nuclei in CH₃ and CH₂ were distinguished. A ferroelastic phase transition was observed at T_C, without structural symmetry change. The phase transition is mainly attributed to the orientational ordering of the [N(C₂H₅)₄]⁺ cations, and the spectral splitting at low temperature is associated with different ferroelastic domains.