Phase transitions in molecular crystals of n-alkanes: Tri-, tetra-, and pentacosane

Phase transitions in normal alkanes, such as tricosane (C₂₃H₄₈), tetracosane (C₂₄H₅₀), and pentacosane (C₂₅H₅₂), were studied by differential scanning calorimetry. The elimination of some procedural errors provided the possibility to obtain true values for the thermodynamic parameters of phase transitions and ascertain their nature. The comparative analysis of heat capacity jumps was performed on the basis of the theory of diffuse first-order phase transitions.

     

Phase transitions in molecular crystals of <Emphasis Type="Italic">n</Emphasis>-alkane alcohols

A comparative analysis of the phase transitions in molecular crystals of n-alkane alcohols with different lengths of the (C _n H_{2n + 2}OH) chain was performed using differential scanning calorimetry (DSC). Investigation of the temperature hysteresis made it possible to reveal a number of new effects associated with the specific features of the first-order phase transitions. A quantitative analysis of the temperature dependence of the heat capacity was carried out in the framework of the theory of diffuse (Λ-shaped) first-order phase transitions.     

Anomalies in the Magnetic Susceptibility in the Second-Order Phase Transitions beyond the Curie Point

Taking into account the inexhaustible interest in studying the peculiarities of physical properties in the neighborhood of phase transitions and the growth of experimental investigations of cobalt fluoride, we have studied the peculiarities of magnetic susceptibility in the vicinity of the critical field H_C at which cobalt fluoride performs the second-order phase transition from the antiferromagnetic phase to the angular phase. It is discovered that in the magnetic field H ‖ C₄, the magnetic susceptibility becomes infinite at H → H_C. It is shown that as the magnetic field direction deviates from the C₄ axis, the magnetic susceptibility in the critical field H_C proves to be finite. It is also shown that the change in the magnetic susceptibility with the change in the magnetic field considerably decreases at extremely insignificant deviations of the field H from the C₄ axis. Since the calculations are performed in terms of the Landau theory of phase transitions, we pay attention to the similarity and difference between the obtained results and those in the vicinity of the Curie point obtained by using the Landau theory of phase transitions.     

A CCD based detector for quick detection of pressure induced phase transitions

Abstract

A high sensitivity CCD based two dimensional angle dispersive X-ray are a detector has been developed for quick detection of pressure induced phase transitions for a laboratory X-ray source such as a rotating anode generator. The performance of this detector was tested by successfully carrying out powder X-ray diffraction measurements on element Pd, intermetallics AuIn², AuGa² and low Z scatterer adamantane (C₁₀H_l6) at ambient conditions. Its utility for quick detection of phase transitions at high pressures with diamond anvil cell (DAC) is demonstrated by reproducing the known pressure induced structural phase transitions in RbI and KI. The importance of this detector system in search of unknown phase transitions has been established by observing new structural phase transitions in In_0.25Sn_0.75 and AuGa₂. Various softwares have also been developed such as interactive location of centre of diffraction rings, radial integration and image enhancement to analyze data from this detector.     

Linear and nonlinear dielectric properties of nanocomposites based on the organic ferroelectric of diisopropylammonium bromide

ABSTRACT

The present work aims at investigating linear and nonlinear dielectric properties of nanocomposites based on diisopropylammonium bromide (C₆H₁₆NBr, DIPAB) embedded into Al₂O₃ films having honeycomb structure with pores of 100?nm in diameter, and into opal matrices with three-dimensional structure containing pores of 100 and 60?nm. The obtained results indicated the shift of Curie point toward lower temperatures for DIPAB in porous aluminum oxide and the appearance of two phase transitions, detected upon heating and cooling for DIPAB in opal matrices. In addition, a ferroelectric phase was found to form between these two phase transitions without significant change of Curie temperature.     

Temperature evolution of the intra-ion absorption spectra of (NH2(C2H5)2)2CoCl4 crystals in the region of their phase transitions

On the basis of spectroscopic studies of (NH₂(C₂H₅)₂)₂CoCl₄ crystals, the absorption bands corresponding to the internal electronic transitions in the Co²⁺ ion were identified. The values of the crystal field and Racah parameters were calculated. The temperature evolution of the absorption spectra of (NH₂(C₂H₅)₂)₂CoCl₄ crystals reveals the anomalies of their parameters at the points of phase transitions. The corresponding changes of the absorption spectra were discussed in terms of distortion of the metal-halogen complex. The temperature dependences of the absorption spectra of (NH₂(C₂H₅)₂)₂CoCl₄ crystals confirm the presence of the thermochromic phase transitions at 255 and 330?K.     

Infrared and Dielectric Studies of [(C2H5)4N]2SiF6

The infrared spectrum of [(C₂H₅)₄N]₂SiF₆ was recorded and discussed in relation to its crystal structure. This spectrum indicates that cations and anions are distorted and are not hydrogen bonded. Two structural phase transitions were observed in the tetraethylammonium compound [(C₂H₅)₄N]₂SiF₆ by means of dielectric measurements. High-frequency dielectric dispersion phenomena in this compound were also analysed. The evolution of the dielectric constant with the temperature indicates the presence of two phase transitions, at high temperatures, which are of order-disorder character.     

Low-frequency critical dynamics in (CnH2n+1NH3)2SnCl6 model biomembrane systems

Nuclear magnetic resonance has been employed as a probe for the collective hydrocarbon chain dynamics in the organic–inorganic model biomembranes (C_nH_2n+1NH₃)₂SnCl₆, undergoing order–disorder and conformational phase transitions. No anomalies were observed in the laboratory-frame spin–lattice relaxation measurements at the order–disorder phase transitions, whereas a discontinuity was manifest at the conformational phase transitions characteristic of a first-order phase transition. On the other hand, our rotating frame spin–lattice relaxation measurements revealed a low-frequency critical collective chain dynamics in the kilohertz regime associated with the order–disorder phase transition.     

Microwave spectrum of formic acid and its isotopic species in D, 13C and 18O. Study of Coriolis resonances between ν7 and ν9 vibrational excited states

In the investigation of the 8 → 280 GHz region, 241 and 57 transitions of H¹²COOH and DCOOH, respectively, have been assigned to the ν₇ and ν₉ vibrational states coupled by a strong Coriolis resonance. The numerical analysis based on Watson's theory of centrifugal distortion coupled with the addition of Coriolis interaction allows us to obtain a set of parameters which fits the transitions well. The rotational spectra of the isotopic species HCOOD and DCOOD have also been investigated. In this investigation 55 and 67 transitions have been assigned to the ν₇ and ν₉ vibrational states of these two molecules, respectively. A very weak Coriolis resonance was detected. Two non-rigid independent rotors were thus employed and gave us a set of parameters which fits the transitions quite well. The rotational spectrum of the ground state of H¹²COOH, H¹³COOH, HCOOD, DCOOH, H¹²C¹⁶O¹⁸OH, and H¹²C¹⁸O¹⁶OH have been reinvestigated and a set of improved parameters was obtained for each species.     

On the theory of noncollinear magnetic structures of CoF2 in an external field

A quantum theory of spin-orientational transitions in a tetragonal two-sublattice antiferromagnet with large single-ion anisotropy is developed. It is shown that the values of observable (physical) parameters that define the crystal properties for H?C₄ and H⊥C₄, can greatly differ not only from each other but from bare spin-hamiltonian constants.     

Observation of critical behaviors in squaric acid by H nuclear magnetic resonance

¹H nuclear magnetic resonance (NMR) was employed in order to investigate the phase transitions in a two-dimensional antiferroelectric system, squaric acid (H₂C₄O₄). As a result, in addition to the critical behaviors around the long range order phase transition, similar behaviors were observed at higher temperatures where a short range order phase transition presumably takes place.     

Solid-solid phase transitions in (CnH2n+1NH3)2 CdCl2Br2 compounds

(C_nH_2n+1NH₃)₂ CdCl₂Br₂ compounds with n≥10 show several solid-solid phase transitions before descomposition takes place at 436 K. The results show a different behaviour between odd or even members of the series studied. The sequence of phase -- transitions can be interpreted as a dynamic order-disorder mechanism followed by a melting of the alkyl chains.     

Phase transitions in a metal–organic coordination polymer: [Zn2(C8H4O4)2(C6H12N2)] with guest molecules. Thermal effects and molecular mobility

Thermal effects of a series of [Zn₂(C₈H₄O₄)₂(C₆H₁₂N₂)] porous compound with the guest molecules located in the pores were studied using differential scanning calorimetry combined with solid-state ¹H nuclear magnetic resonance spectroscopy. The intercalation of the molecules was shown to produce various thermal anomalies and phase transitions, which were characterized and analyzed.     

Computational reconstitution of crystal structures and (p,T) phase diagrams of benzene C6H6 and hexachlorobenzene C6Cl6

Abstract

With a simplified dynamical model for molecular packing analysis, crystal and molecular structures of benzene C₆H₆ and hexachlorobenzene C₆H₆ were investigated. This model includes thermal motion and molecular deformation effects. Several crystalline structures have been found by the calculation. They are compared to experimental structures determined at various temperatures and pressures, by X-ray diffraction or neutron scattering. A schematic (p, T) phase diagram is suggested for each compound.     

Phase transitions and temperature changes of the optical absorption edge in (NH2(C2H5)2)2CoCl4 layered crystal

This work was devoted to X-ray diffraction study and investigations of temperature changes of the optical absorption edge of (NH₂(C₂H₅)₂)₂CoCl₄ crystals in the region of possible phase transitions. The X-ray powder diffraction data revealed the monoclinic phase at room temperature – space group P2/n. The cobalt atom was found to be square-plane coordinated by four chlorine atoms resulting [CoCl₄]^2– anion, which is surrounded by two DEA⁺ cations. It was shown that the low-energy tail of the absorption edge in these materials possesses an exponential shape. In the temperature range above 255?K it follows the empirical Urbach’s rule. The obtained experimental data confirmed the existence of the ferroelastic phase in (NH₂(C₂H₅)₂)₂CoCl₄ in the temperature range between 255 and 326?K. The anomalous behaviour of the investigated parameters observed at the temperatures below 255?K would be related to earlier unknown phase transitions.     

Quadrupolar susceptibility and magnetic phase diagram of PrNi2Cd20 with non-Kramers doublet ground state

ABSTRACT

In this study, ultrasonic measurements were performed on a single crystal of cubic PrNi₂Cd₂₀, down to a temperature of 0.02?K, to investigate the crystalline electric field ground state and search for possible phase transitions at low temperatures. The elastic constant (C₁₁?C₁₂)/2, which is related to the Γ₃-symmetry quadrupolar response, exhibits the Curie-type softening at temperatures below ～30?K, which indicates that the present system has a Γ₃ non-Kramers doublet ground state. A leveling-off of the elastic response appears below ～0.1?K toward the lowest temperatures, which implies the presence of level splitting owing to a long-range order in a finite-volume fraction associated with Γ₃-symmetry multipoles. A magnetic field–temperature phase diagram of the present compound is constructed up to 28?T for H || [110]. A clear acoustic de Haas–van Alphen signal and a possible magnetic-field-induced phase transition at H ～26?T are also detected by high-magnetic-field measurements.     

New phase transition in (CnH2n+1NH3)2MnCl4

(C_nH_2n+1NH₃)₂MnCl₄ consist of perovskite-type layers sandwiched between hydrocarbon radical layers. New phase transitions were found between liquid nitrogen and decomposition temperature in these compounds by differential thermal analysis and hot stage microscopy. A systematic research of the lattice dynamics, the transition mechanisms, their influence on physical properties and their dependence on the various possibilities of molecular engineering offered in these quasi two-dimensional perovskites was started.     

Microwave spectra of the ground vibrational state of formaldehyde substituted with 17O and 18O

Pure rotational transitions in the ground vibrational state have been measured for H₂¹²C¹⁸O, H₂¹²C¹⁷O, H₂¹³C¹⁸O, and H₂¹³C¹⁷O in the frequency region 8–75 GHz. These have included both Q- and R-branch transitions, and have permitted accurate evaluation of rotational constants and several quartic centrifugal distortion constants for each species. These in turn have permitted the prediction of several transitions of possible use in radioastronomy.     

Magnetic resonance study of the two-dimensional spin diffusion in the Heisenberg paramaget (C18H37NH3)2MnCl4

¹H nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR) techniques were employed to study the perovskite-type layered structure compound (C₁₈H₃₇NH₃)₂MnCl₄ undergoing structural phase transitions. The spin relaxation was found to sensitively reflect the two-dimensional electron spin diffusion.     

Further examples of 19e− sandwiches of Fe(I) studied by Mössbauer spectroscopy: Per-ethylated benzene complexes

The organometallic complexes C₅H₅ Fe(I) C₆(C₂H₅)₆ and C₅(CH₃)₅ Fe(I)C₆ H(C₂H₅)₅ have been studied by Mössbauer spectroscopy. Hyperfine data are typical for 19e^? systems with one electron on the e ₁ ^* orbital. The thermal variation of the quadrupole splittings indicates vibronic reduction of the spin-orbit constant, in agreement with previous analyses of similar 19e^? systems. The quadrupole splitting and recoil-free fraction data give evidence for phase transitions, which are discussed with respect to cooperative Jahn-Teller distortion and hindered rotations of the ethyl groups.