Anomalous viscoelasticity near the isotropic-nematic phase transition in liquid crystals

Recent optical Kerr effect experiments have shown that orientational relaxation of nematogens shows a pronounced slow down of the response function at intermediate times and also a power law decay near the isotropic-nematic (I-N) transition. In many aspects, this behavior appears to be rather similar to the ones observed in the supercooled liquid near-glass transition. We have performed molecular dynamics simulations of model nematogens (Gay-Berne with aspect ratio 3) to explore the viscoelasticity near the I-N transition and also investigated the correlation of viscoelasticity (if any) with orientational relaxation. It is found that although the viscosity indeed undergoes a somewhat sharper than normal change near the I-N transition, it is not characterized by any divergence-like behavior (like the ones observed in the supercooled liquid). The rotational friction, on the other hand, shows a much sharper rise as the I-N transition is approached. Interestingly, the probability distribution of the amplitude of the three components of the stress tensor shows anisotropy near the I-N transition-similar anisotropy has also been seen in the deeply supercooled liquid. Frequency dependence of viscosity shows several unusual behaviors: (a) There is a weak, power law dependence on frequency [eta(')(omega) approximately omega(-alpha)] at low frequencies and (b) there is a rapid increase in the sharp peak observed in eta(')(omega) in the intermediate frequency on approach to the I-N transition density. These features can be explained from the stress-stress time correlation function. The angular velocity correlation function also exhibits a power law decay in time. The reason for this is discussed.     

Unusual magnetic properties of one-dimensional molecule-based magnets associated with a structural phase transition

Three ion-pair complexes, [RbzPy](+)[Ni(mnt)(2)](-) (mnt(2)(-) = maleonitriledithiolate; [RbzPy](+) = 4-R-benzylpyridinium; R = Br (1), Cl (2), and NO(2) (3)), with unusual magnetic properties have been synthesized and characterized. The crystal structures of 1 and 2 have been solved. The two complexes belong to the P2(1)/c space group with Z = 4 and C(20)H(11)BrN(5)NiS(4), a = 12.0744(17) A, b = 26.369(4) A, c = 7.440(3) A, and beta = 102.63(3) degrees for 1 and C(20)H(11)ClN(5)NiS(4), a = 12.105(2) A, b = 26.218(4) A, c = 7.374(2) A, and beta = 102.55(2) degrees for 2, respectively. The [Ni(mnt)(2)](-) anions in 1-3 form uniformly spaced one-dimensional (1-D) magnetic chains of s = 1/2 at room temperature. The temperature dependences of the susceptibility for 1-3 show that they undergo phase transitions. All three complexes are paramagnetic in their high-temperature (abbreviation HT) phase and diamagnetic in the low-temperature (abbreviation LT) phase because of strong dimerization along the stacking direction. The results of thermal analysis (DSC) further confirm that the phase transition for 1 and 2 is first-order but maybe second-order for 3. The phenomena observed in this study are similar to those of the 1-D radical systems.     

Anomalous structural and mechanical properties of solids confined in quasi-one-dimensional strips

We show using computer simulations and mean field theory that a system of particles in two dimensions, when laterally confined by a pair of parallel hard walls within a quasi-one-dimensional channel, possesses several anomalous structural and mechanical properties not observed in the bulk. Depending on the density rho and the distance between the walls Ly, the system shows structural characteristics analogous to a weakly modulated liquid, a strongly modulated smectic, a triangular solid, or a buckled phase. At fixed rho, a change in Ly leads to many re-entrant discontinuous transitions involving changes in the number of layers parallel to the confining walls depending crucially on the commensurability of interlayer spacing with Ly. The solid shows resistance to elongation but not to shear. When strained beyond the elastic limit, it fails undergoing plastic deformation but surprisingly, as the strain is reversed, the material completely recovers and returns to its original undeformed state. We obtain the phase diagram from mean field theory and finite size simulations and discuss the effect of fluctuations.     

Low temperature structural phase transition of Ba3NaIr2O9

Single crystal X-ray and synchrotron X-ray powder diffraction have been used to probe the structure of Ba₃NaIr₂O₉ from 300 K down to 20 K. Ba₃NaIr₂O₉ is found to undergo a structural transition from hexagonal symmetry, P6₃/mmc, at ambient temperature to monoclinic symmetry, C2/c, at low temperature. The evolution of the unit cell volume upon cooling is indicative of a higher order structural transition, and the symmetry breaking becomes apparent as the temperature is decreased. The low temperature monoclinic structure of Ba₃NaIr₂O₉ contains strongly distorted [NaO₆] and [IrO₆] octahedra in comparison to the room temperature hexagonal structure.     

Pressure-induced structural phase transition in the half-Heusler compound CaAuBi

We report a structural phase transition of the ternary compound CaAuBi under pressure, from the known cubic half-Heusler phase to a hexagonal LiGaGe type phase, based on synchrotron X-ray diffraction patterns taken under pressures up to 18 GPa. We report lattice parameters and atomic coordinates, and perform total energy calculations for both the cubic and hexagonal phases under different pressures. Finally, we present a structure map that places CaAuBi in the context of related 18 electron XYZ ternary systems.     

The structural phase transition in SrV6O11

Single‐crystal X‐ray diffraction and specific heat studies establish that strontium hexavanadium undecaoxide, SrV₆O₁₁, undergoes a P6₃/mmc to inversion twinned P6₃mc structural transition as the temperature is lowered through 322 K. The P6₃/mmc and P6₃mc structures have been determined at 353 K and at room temperature, respectively. For the room‐temperature structure, seven of the ten unique atoms lie on special positions, and for the 353 K structure all of the seven unique atoms sit on special positions. The P6₃/mmc to P6₃mc structural phase transition, accompanied by a magnetic transition, is a common characteristic of AV₆O₁₁ compounds, independent of the identity of the A cations.     

Low-temperature structural phase transition in deuterated and protonated lithium acetate dihydrate

Heat capacity measurements of protonated lithium acetate dihydrate show a structural phase transition at T = 12 K. This finding is in contrast to earlier work, where it was thought that only the deuterated compound undergoes a low temperature structural phase transition. This finding is confirmed by low temperature ultrasound spectroscopy, where the structural phase transition is associated with a velocity decrease of the ultrasonic waves, i.e. with an elastic softening. We compare the thermodynamic properties of the protonated and deuterated compounds and discuss two alternatives for the mechanism of the phase transition based on the thermal expansion measurements.     

Pressure induced structural phase transition in triglycine sulfate and triglycine selenate

To elucidate the cause of destruction of ferroelectricity with pressure in triglycine sulfate and triglycine selenate, we have investigated these compounds with the help of Raman measurements as well as first principles total energy and structural optimization calculations. Our results show that, beyond the critical pressures, the loss of ferroelectricity in these compounds is due to the conformational change in one of the three glycine ions of these crystals. Our studies suggest that pressure induced phase transition might be of displacive nature unlike the temperature induced ferroelectric phase transition in these crystals which is known to be of order-disorder type.     

Negative thermal expansion emerging upon structural phase transition in ZrV2O7 and HfV2O7

ZrV(2)O(7) and HfV(2)O(7), which show negative thermal expansion (NTE) in the high-temperature phase, were investigated using X-ray diffraction and heat capacity calorimetry. Two sharp anomalies due to successive phase transitions were observed in the temperature dependence of heat capacity at 345.5 K and 373.4 K for ZrV(2)O(7) and 341.8 K and 370.3 K for HfV(2)O(7). The smallness of their combined entropies of transition suggested that the phase transitions are of displacive type. Effective phonon densities of states (DOS) described by a simple model, and mode-Grüneisen parameters of the low-temperature phase were obtained through the spectrum analyses of heat capacities of ZrV(2)O(7) and HfV(2)O(7). Their effective phonon DOS's show the three features common to NTE compounds: low-energy phonon mode, high-energy phonon mode, and a wide phonon gap in between. The mode-Grüneisen parameter of low-energy modes corresponding to translational and librational vibrations of the constituent polyhedra is negative but with a small absolute value due to the distortion of V(2)O(7) group in the low-temperature phase, resulting in positive thermal expansion. It is revealed that the release of the structural distortion upon the successive phase transitions with large volume increase leads to the NTE of ZrV(2)O(7) and HfV(2)O(7) in the high-temperature phase.     

Anomalous properties of flavonoids in reversed phase high performance liquid chromatography

It is shown through reversed phase high performance liquid chromatography that a characteristic feature of such abundant natural flavonoids as flavon-3-ols is an anomalously strong antibate dependence of their retention indices (RI) on the organic solvent concentration (C) in the eluent, dRI/dC < 0. In order to interpret this anomaly, the specific optical rotation values [α]_D²⁰ of natural (+)-(2R,3R)-dihydroquercetin in different solvents are compared, confirming the reverse formation of hydrated flavonoids in aqueous solutions.     

Pressure induced structural phase transition of OsB2: First-principles calculations

Orthorhombic OsB₂ was synthesized at 1000 °C and its compressibility was measured by using the high-pressure X-ray diffraction in a Diacell diamond anvil cell from ambient pressure to 32 GPa [R.W. Cumberland, et al. (2005)]. First-principles calculations were performed to study the possibility of the phase transition of OsB₂. An analysis of the calculated enthalpy shows that orthorhombic OsB₂ can transfer to the hexagonal phase at 10.8 GPa. The calculated results with the quasi-harmonic approximation indicate that this phase transition pressure is little affected by the thermal effect. The calculated phonon band structure shows that the hexagonal P 6₃/mmc structure (high-pressure phase) is stable for OsB₂. We expect the phase transition can be further confirmed by the experimental work.     

Low-temperature structural phase transition in synthetic libethenite Cu2PO4OH

Low-temperature structural properties of the synthetic mineral libethenite Cu₂PO₄OH were investigated by single-crystal X-ray diffraction, synchrotron X-ray powder diffraction, specific heat measurements, and Raman spectroscopy. A second-order structural phase transition from the Pnnm symmetry (a=8.0553(8) Å, b=8.3750(9) Å, c=5.8818(6) Å at 180 K) to the P2₁/n symmetry (a=8.0545(8) Å, b=8.3622(9) Å, c=5.8755(6) Å, β=90.0012(15) at 120 K) was found at 160 K during cooling. At 120 K, the monoclinic angle is 90.0012(15) from single crystal X-ray data vs 90.083(1) from powder X-ray diffraction data. The P2₁/n–to–Pnnm transition may be a general feature of the adamite-type compounds, M₂XO₄OH.     

Dielectric properties near the isotropic-smectic-C* phase transition

The dielectric properties in the vicinity of the isotropic-smectic-C* phase transition in ferroelectric liquid crystals are discussed. The temperature and frequency dependence of the complex dielectric permittivity in both the phases of the transition are calculated. The theoretical phase diagram of the dielectric permittivity is studied.     

Synthesis,structural phase transition,and characterization of potassium hydrogen bis-dichloroacetate

Potassium hydrogen bis-dichloroacetate (1) was synthesized and separated as crystals. Differential scanning calorimetry (DSC) measurement reveals that this compound undergoes a reversible phase transition at about 259 K with a heat hysteresis of 23.5 K. Dielectric anomaly observed at 260 K in the heating process further confirms the phase transition. The room temperature X-ray single-crystal structure determination indicates that 1 crystallizes in the monoclinic crystal system with a centrosymmetric space group P2₁/c, and cell parameters are a =?6.240(1), b =?23.177(4), c =?7.335(1) Å, β =?106.938(1)°, V =?1014.8(3) ų, and Z =?4. In the low temperature phase, 1 also crystallizes in monolinic with space group P2₁/c, and cell parameters are a =?6.180(1), b =?22.988(2), c =?7.200(1) Å, β =?108.098(1)°, V =?972.4(1) ų, and Z =?4. The structural phase transition is dominating caused by the torsion of bond angles.     

Electron-induced phase transition in hydrogen-bonded solid-state 2-pyridone

This paper presents the results of experimental studies of hydrogen-bonded 2-pyridone crystal IR spectra. Spectral studies have demonstrated the existence of two anhydrous solid-state phases of each compound, namely the α and the β phases. Hydrogen bonds in the high-temperature α phase of these crystals have been estimated to be 40% stronger than the hydrogen bonds in the β phase, which are stable at room temperature. The mechanism of the phase transition in the solid-state 2-pyridone is proposed on the basis of the IR spectral data. This was possible by taking into account small changes in the geometry of heterocyclic molecular skeletons, which accompany the electron density redistribution in the hydrogen bonds occurring during the transition. The phase transition is connected with a partial change in the hydrogen bond nature from the N(+)-H···O(-) in the α phase, to the N-H···O hydrogen bonds in the β phase crystals.     

Glass transition and structural properties of glycidyloxypropyl-heptaphenyl polyhedral oligomeric silsesquioxane-epoxy nanocomposites

We have used molecular simulations to study the properties of nanocomposites formed by the chemical incorporation of polyhedral oligomeric silsesquioxane (POSS) particles in the cross-linked epoxy network. The particular POSS molecule chosen—glycidyloxypropyl-heptaphenyl POSS—can form only one bond with the cross-linker and thus was present as a dangling unit in the network. Four epoxy-POSS nanocomposites containing different fractions (up to 30 mass/%) of POSS particles were studied in this work. Well-relaxed atomistic model structures of the nanocomposites were created and then molecular dynamics simulations were used to characterize the density, glass transition temperature (T _g), and the coefficient of volume thermal expansion (CVTE) of the systems. In addition to the effect of nanoparticle loading, the effect of nanoparticle chemistry on the nanocomposite properties was also characterized by comparing these results with our previous results (Lin and Khare, Macromolecules 42:4319–4327, 2009) on neat cross-linked epoxy and a nanocomposite containing a POSS nanoparticle that formed eight bonds with the cross-linked network. Our results showed that incorporation of these monofunctional POSS particles into cross-linked epoxy does not cause a measurable change in its density, glass transition temperature, or the CVTE. Furthermore, simulation results were used to characterize the aggregation of POSS particles in the system. The nanofiller particles in systems containing 11, 20, and 30 mass/% POSS were found to form small clusters. The cluster-size distribution of nanoparticles was also characterized for these systems.     

Probing the structural and magnetic properties of transition metal-benzene anion complexes

Two types of transition metal-benzene anion complexes, (titanium)(n)(benzene)(m)? and (cobalt)(n)(benzene)(m)? (n ≤ 2, m ≤ 3) have been determined using density functional theory. The photoelectron spectra of Ti(n)Bz(m)? and Co(n)Bz(m)? (n ≤ 2, m ≤ 3) were discussed from the perspective of quantum chemical calculations of the vertical detachment energies (VDEs) of several low-energy isomers obtained by the structural optimization procedure. The binding of Ti and Co atoms to benzene molecules is accounted by 3d-π bonds, as revealed by the molecular orbitals. The topology of the electronic density has been analyzed, suggesting that the C-C bonds were weakened in the transition metal-benzene complexes in comparison to those in free benzene. Spin density distribution results show the spin densities for Ti(n)Bz(m)? and Co(n)Bz(m)? (n ≤ 2, m ≤ 3) reside mainly on the metal Ti and Co centers (70%-90%). A shift to lower magnetic moment with respect to the pure titanium/cobalt cluster anions indicates the solvent benzene molecule acts to demagnetize the bare titanium/cobalt cluster anions.