Orientational Order-disorder Transition Research Articles

Topological defects as nucleation points of the nematic-isotropic phase transition in liquid crystal shells.

The transition from a nematic to an isotropic state in a self-closing spherical liquid crystal shell with tangential alignment is a stimulating phenomenon to investigate, as the topology dictates that the shell exhibits local isotropic points at all temperatures in the nematic phase range, in the form of topological defects. The defects may thus be expected to act as nucleation points for the phase transition upon heating beyond the bulk nematic stability range. Here we study this peculiar transition, theoretically and experimentally, for shells with two different configurations of four +1/2 defects, finding that the defects act as the primary nucleation points if they are co-localized in each other's vicinity. If the defects are instead spread out across the shell, they again act as nucleation points, albeit not necessarily the primary ones. Beyond adding to our understanding of how the orientational order-disorder transition can take place in the shell geometry, our results have practical relevance for, e.g., the use of curved liquid crystals in sensing applications or for liquid crystal elastomer actuators in shell shape, undergoing a shape change as a result of the nematic-isotropic transition.

Prediction and understanding of barocaloric effects in orientationally disordered materials from molecular dynamics simulations

Due to its high energy efficiency and environmental friendliness, solid-state cooling based on the barocaloric (BC) effect represents a promising alternative to traditional refrigeration technologies relying on greenhouse gases. Plastic crystals displaying orientational order-disorder solid-solid phase transitions have emerged among the most gifted materials on which to realize the full potential of BC solid-state cooling. However, a comprehensive understanding of the atomistic mechanisms on which order-disorder BC effects are sustained is still missing, and rigorous and systematic methods for quantitatively evaluating and anticipating them have not been yet established. Here, we present a computational approach for the assessment and prediction of BC effects in orientationally disordered materials that relies on atomistic molecular dynamics simulations and emulates quasi-direct calorimetric BC measurements. Remarkably, the proposed computational approach allows for a precise determination of the partial contributions to the total entropy stemming from the vibrational and molecular orientational degrees of freedom. Our BC simulation method is applied on the technologically relevant material CH3NH3PbI3 (MAPI), finding giant BC isothermal entropy changes (∣ΔSBC∣ ~ 10 J K−1 kg−1) under moderate pressure shifts of ~0.1 GPa. Intriguingly, our computational analysis of MAPI reveals that changes in the vibrational degrees of freedom of the molecular cations, not their reorientational motion, have a major influence on the entropy change that accompanies the order-disorder solid-solid phase transition.

Pattern formation and phase transition in the collective dynamics of a binary mixture of polar self-propelled particles.

The collective behavior of a binary mixture of polar self-propelled particles (SPPs) with different motile properties is studied. The binary mixture consists of slow-moving SPPs (sSPPs) of fixed velocity v_{s} and fast-moving SPPs (fSPPs) of fixed velocity v_{f}. These SPPs interact via a short-range interaction irrespective of their types. They move following certain position and velocity update rules similar to the Vicsek model (VM) under the influence of an external noise η. The system is studied at different values of v_{f} keeping v_{s}=0.01 constant for a fixed density ρ=0.5. Different phase-separated collective patterns that appear in the system over a wide range of noise η are characterized. The fSPPs and the sSPPs are found to be orientationally phase synchronized at the steady state. We studied an orientational order-disorder transition varying the angular noise η and identified the critical noise η_{c} for different v_{f}. Interestingly, both the species exhibit continuous transition for v_{f}<100v_{s} and discontinuous transition for v_{f}>100v_{s}. A new set of critical exponents is determined for the continuous transitions. However, the binary model is found to be nonuniversal as the values of the critical exponents depend on the velocity. The effect of interaction radius on the system behavior is also studied.

Collective pattern formation in a binary mixture of self-propelled particles

We have studied the collective behaviour of a binary mixture of self-propelled particles (SPPs) with non-identical motile properties. The two types of SPPs with low and high velocity is modelled in a two-dimensional system with periodic boundary conditions (PBCs) at a fixed particle density. The two types have two widely different velocities. The alignment interaction rule and angular noise similar to Vicsek Model (VM) is implemented in this model. Both inter and intra-particle interactions are considered among the SPPs. The system exhibits various self-organised pattern formation and phase segregation which mainly depends on the angular noise present in the system. Without any restriction of interaction or repulsion among the particles, the SPPs phase-separated into dense structures. The orientational order-disorder phase transition is studied, and nontrivial results are obtained. Cluster size distribution (CSD) for the entire system is also studied at the transition region. The CSD exhibits a power-law behaviour at the transition, with an exponent that of the standard percolation.

Phase Transitions in Two-Dimensional Systems of Janus-like Particles on a Triangular Lattice.

We studied the phase behavior of two-dimensional systems of Janus-like particles on a triangular lattice using Monte Carlo methods. The model assumes that each particle can take on one of the six orientations with respect to the lattice, and the interactions between neighboring particles were weighted depending on the degree to which their A and B halves overlap. In this work, we assumed that the AA interaction was fixed and attractive, while the AB and BB interactions varied. We demonstrated that the phase behavior of the systems considered strongly depended on the magnitude of the interaction energies between the AB and BB halves. Here, we considered systems with non-repulsive interactions only and determined phase diagrams for several systems. We demonstrated that the phase diagram topology depends on the temperature at which the close-packed systems undergo the orientational order–disorder transition.

Understanding colossal barocaloric effects in plastic crystals

Plastic crystal neopentylglycol (NPG) exhibits colossal barocaloric effects (BCEs) with record-high entropy changes, offering exciting prospects for the field of solid-state cooling through the application of moderate pressures. Here, we show that the intermolecular hydrogen bond plays a key role in the orientational order of NPG molecules, while its broken due to thermal perturbation prominently weakens the activation barrier of orientational disorder. The analysis of hydrogen bond strength, rotational entropy free energy and entropy changes provides insightful understanding of BCEs in order-disorder transition. External pressure reduce the hydsrogen bond length and enhance the activation barrier of orientational disorder, which serves as a route of varying intermolecular interaction to tune the order-disorder transition. Our work provides atomic-scale insights on the orientational order-disorder transition of NPG as the prototypical plastic crystal with BCEs, which is helpful to achieve superior caloric materials by molecular designing in the near future.

Neutron scattering study of the orientational disorder in potassium cyanide

We report the results of a combined neutron diffraction and total scattering study of the orientational order-disorder phase transitions in potassium cyanide, KCN. The diffraction data are analysed in terms of the spontaneous strains that accompany the phase transitions. The total scattering data are analysed using the Reverse Monte Carlo method, which gives direct access to the distribution of atomic positions and hence molecular orientations in each phase. Incorporating information from diffuse scattering in this way provides a means to measure the coefficients of the orientation distribution function up to almost arbitrarily high orders, and furthermore has the advantage that this function is naturally positive everywhere. The results for the cubic phase show that the distribution of orientations never exceeds 25% difference from an isotropic distribution.

Chemical potential and solid-solid equilibrium of near-spherical Lennard-Jones dumbbell crystal

We studied the orientational order-disorder transition of crystals made up of near-spherical Lennard-Jones dumbbells, of which reduced bond lengths are 0.225, 0.250 and 0.275. Various techniques of Monte Carlo (MC) simulations are used to calculate the chemical potentials of ordered and disordered crystals, and thereby to predict orderdisorder phase transition. First, we performed NPT MC simulations to determine crystal structure, equilibrium positions and orientations of the molecules. We then calculated the free energies of the crystals using the expanded ensemble MC simulations combined with the Einstein-molecule method and the thermodynamic integration method. The solid-solid phase equilibrium is determined from the free energy profiles of the individual phases by equating the chemical potential. The predictions of phase transition obtained from the conventional NPT MC simulation and the free energy simulation were in excellent agreement with each other, which confirms the validity of the present method of calculating the chemical potential of crystal. In addition, the Gibbs-Duhem integration was performed to obtain a complete coexistence curve between the two crystal phases. Orientational probability distributions of molecular axes were analyzed to find the characteristic behavior of rotational motion of molecule in the crystal. At sufficiently low temperature, flipping rotation of molecule in the ordered crystal is suppressed. In contrast, the flipping rotation occurs at higher temperature close to the transition while orientationally ordered structure is still maintained. In the freeenergy calculation, such a unique rotational behavior requires to use a suitable form of external rotational potential with proper symmetry number. The present study demonstrates how one can judiciously choose a correct simulation scheme for the calculation of chemical potentials of molecular crystals.

Terahertz spectroscopic study on order–disorder phase transition of nonadecane

Terahertz transmission spectra of nonadecane (C19H40) in the crystalline and rotator phases were measured over the frequency range of 1–5.5THz. The absorption peak at 2.2THz, related to intermolecular interaction, was observed only in the crystalline phase. Using the absorbance at this frequency, the transition between the two phases was investigated. A model of the orientational order–disorder transition of the molecule is presented to qualitatively explain the transition rate.

Dramatic softening of the negative thermal expansion material HfW2O8 upon heating through its WO4 orientational order-disorder phase transition

HfW2O8 undergoes a dramatic softening where the average bulk modulus (P = 52–414 MPa) drops from 69 GPa at 298 K to 48 GPa at 430 K as the temperature of the WO4 orientation order-disorder transition is approached. This is accompanied by increasingly negative thermal expansivity (−10 ppm·K−1 to −15 ppm·K−1) and reversible WO4 orientational disordering upon compression in α-HfW2O8. Additionally, α-HfW2O8 becomes elastically softer upon compression at constant temperature. The α→β phase transition temperature decreases by ∼30 K between 52 and 414 MPa. Above this phase transition, no further temperature-dependent softening or pressure-dependent changes in the coefficient of thermal expansion occurred.

Phase behaviour of the confined lattice gas Lebwohl-Lasher model

The phase behaviour of the Lebwohl-Lasher lattice gas model (one of the simplest representations of a nematogenic fluid) confined in a slab is investigated by means of extensive Monte Carlo simulations. The model is known to yield a first order gas-liquid transition in both the 2D and 3D limits, that is coupled with an orientational order-disorder transition. This latter transition happens to be first order in the 3D limit and it shares some characteristic features with the continuous defect mediated Berezinskii-Kosterlitz-Thouless transition in 2D. In this work we will analyze in detail the behaviour of this system taking full advantage of the lattice nature of the model and the particular symmetry of the interaction potential, which allows for the use of efficient cluster algorithms.

Large Electrocaloric Effect from Electrical Field Induced Orientational Order-Disorder Transition in Nematic Liquid Crystals Possessing Large Dielectric Anisotropy

ABSTRACTLarge electrocaloric (EC) effects in ferroelectric polymers and in ferroelectric ceramics have attracted great attention for new refrigeration development which is more environmental friendly and more efficient and thus could be an alternative to the existing vapor-compression refrigerators which consume large energy and release large amount of green house gas. However in the past, all EC effects investigations have been focused on solid state dielectrics. It is interesting to ask whether a large EC effect can also be realized in dielectric fluids. A dielectric fluid with large EC effect could lead to new design of cooling devices with simpler structures than these based on solid state EC materials, for example, they can be utilized as both the refrigerant and heat exchange fluid. Here we present that a large EC effect can be realized in the liquid crystal (LC) 5CB near it's nematic-isotropic (N-I) phase transition. The LC 5CB possesses a large dielectric anisotropy which can induce large polarization change from the isotropic phase to the nematic phase near the N-I transition. An isothermal entropy change of more than 23 Jkg-1K-1 was observed near 39 oC that is just above the N-I transition.

A specific heat anomaly in multiwall carbon nanotubes as a possible sign of orientational order–disorder transition

We present micro-calorimetric specific heat measurements on different multiwall carbon nanotubes of large outer diameter, as a function of temperature in the range 10–120 K. A clear anomaly at 60 K with the shape of a peak is present in two of the samples, and both the height and the characteristic temperature of the peak are independent of magnetic field and do not exhibit thermal hysteresis, discarding magnetic degrees of freedom related to Fe seeds or contributions from adsorbed gases. These features suggest that the anomaly may be caused by a structural change. As the anomaly is also unaffected by induced intertube disorder, it may be related with a melting of orientational dislocations of individual tubes within multiwall nanotubes, an effect that was theoretically predicted to occur in carbon nanotubes and represents a distinctive feature of asymmetric molecular systems.

Thermal behavior of disordered phase of caffeine molecular crystal: Insights from Monte Carlo simulation studies

We have studied the thermal behavior of orientationally disordered phase of caffeine molecular crystal using variable shape variable size Monte Carlo simulations in isothermal-isobaric ensemble. We have investigated the structure, especially the nature of orientational disorder of caffeine molecules as a function of temperature in the range of 400-550 K. Experimentally this system is known to undergo a phase transition at 426 K (considered to be an orientational order-disorder transition) and melt at 512 K. Our simulations reproduce these two transitions in excellent agreement with experiment. We find that the in-plane reorientational motion of molecules is restricted to small angles below 425 K, and above this temperature, molecules undergo essentially free rotations in molecular plane, and we find the melting to occur between 525 and 550 K. In the high temperature disordered phase, the disorder is mostly attributable to the in-plane orientational motion of the molecules. The potential energy profile for the in-plane reorientational rotation has six wells as a consequence of specific packing of molecules in the ab crystallographic plane. Also we find considerable out-of-plane reorientational disorder for the molecules in the high temperature disordered phase. We have also studied the structure and orientational disorder of the system that is quenched from 450 to 300 K. We find that in the quenched phase, the molecular orientational arrangement remains partially frozen.

Friction and Adhesion between C60 Single Crystal Surfaces and AFM Tips: Effects of the Orientational Phase Transition

We have investigated the nanotribological properties of C60 single crystal (111) and (100) surfaces around its orientational order-disorder phase transition temperature, approximately 260 K, by atomic force microscopy and frictional force microscopy (AFM/FFM) in high vacuum. Results show that for both surfaces across the phase transition temperature, the friction force and the adhesive force between a C60 coated AFM tip and the C60 crystal surfaces exhibit discontinuous behavior. The friction force within the applied external load range in the low temperature phase is significantly larger than that in the high temperature phase, with no obvious change in the slope of the friction force curves (the friction coefficient) in the low and high temperature phases. The abrupt change in friction was found to be caused mainly by the abrupt change in adhesion, which, in turn, can be qualitatively understood through changes in the van der Waals interaction and the short-range Coulomb interaction associated with the structural changes across the phase transition. Compared to most other degrees of freedom, the rotation of C60 molecules was found to have little effect on friction and is an ineffective energy dissipation channel.

Solid C60 doped with C70: Order-to-disorder transition studied with dielectric and 13C nuclear magnetic resonance spectroscopies

C 60 crystals containing C70 at various dopant levels from 0% to 10% have been grown via a method of “solvent dilution.” Their crystallographic structures as revealed by x-ray diffraction exhibit an extra hcp lattice, and this stacking fault character increases with C70 concentration. The same trend applies to the shift of transition temperature in dielectric spectra, and in 13C nuclear magnetic resonance spectra, an additional resonance appeared besides the primary resonance in doped samples. We explain such phenomena as follows. The additional resonance arises from a diamagnetic ring-current effect in C70. C60 and C70 molecules in an hcp phase undergo a cylindrical motion around their threefold axis, which is parallel to the crystallographic sixfold axis, instead of “spherical” motion in fcc C60 crystal. The lower molecular rotations correspond to lower free energy of the high-temperature phase. The free energy barrier between the high- and the low-temperature phases thus decreases, accounting for the suppression of orientational order–disorder transition temperature.

Effect of C60 molecular rotation on nanotribology.

The effect of C60 molecular rotation on the nanotribological properties of C60 single crystal surfaces has been studied by atomic/frictional force microscopy. The orientational order-disorder phase transition, in which the high temperature C60 free rotation is reduced to a low temperature hindered rotation, is shown to give rise to an abrupt change in friction and adhesion. This change in frictional force is quantitatively consistent with the observed change in adhesion. The similar slopes of the friction versus load curves in both phases indicate that the friction coefficient in the two phases remains about the same. Hence the C60 rotation does not provide an additional energy dissipation channel in the friction process.

Two-Dimensional Orientational Order-Disorder Phase Transition in NH 4 AlF 4

In NH 4 AlF 4 the NH 4 + molecules form a layered structure where the interlayer distance, 6.367 Å, is about twice as large as the intralayer nearest neighbours distance, 3.597 Å. There is an orientational order-disorder transition at T c = 156.9 - 0.1 K, explained by the Ising model, with first and second neighbours and a weak interlayer coupling. Monte Carlo simulations are presented based on this model.

Two-Dimensional Orientational Order-Disorder Phase Transition in NH 4 AlF 4

In NH 4 AlF 4 the NH 4 + molecules form a layered structure where the interlayer distance, 6.367 Å, is about twice as large as the intralayer nearest neighbours distance, 3.597 Å. There is an orientational order-disorder transition at T c = 156.9 - 0.1 K, explained by the Ising model, with first and second neighbours and a weak interlayer coupling. Monte Carlo simulations are presented based on this model.

Uniaxial Orientational Order–Disorder Transitions in Diammine Magnesium Halides, Mg(ND3)2Cl2 and Mg (ND3)2Br2, Investigated by Neutron Diffraction

Neutron powder diffraction on Mg(ND3)2Cl2 and Mg(ND3)2Br2 revealed as a function of temperature uniaxial orientational order–disorder behavior of the ND3 groups. The crystal structures of both compounds are built up from chains of octahedra 1∞[Mg(NH3)2X4/2] with X=Cl and Br arranged in different ways relative to each other. At ambient temperatures (X=Cl) and 270 K (X=Br) the ND3 groups are disordered with respect to a rotation about the bond Mg–N. The D atom density is well described by a fourfold split position, each D “site” connecting an N with an X atom: Mg(ND3)2Cl2, Cmmm, a=8.1828(6) Å, b=8.2007(6) Å, c=3.7543(2) Å, R(F2)Bragg=5.9%; Mg(ND3)2Br2, Pbam, a=5.9714(2) Å, b=11.9175(3) Å, c=3.98477(8) Å, R(F2)Bragg=7.9%. In both cases the c axis corresponds to the direction of the chains 1∞[Mg(NH3)2X4/2]. At low temperatures (8 K (X=Cl) and 1.5 K (X=Br)) both compounds are ordered with respect to the ND3 groups: They are arranged antiferroelectrically on either side of the chains 1∞[Mg(NH3)2X4/2]. The symmetry is lowered compared to the situation at ambient temperatures and 270 K respectively, which involves in both cases a doubling of the orthorhombic c axis: Mg(ND3)2Cl2, Ibmm, a=8.1319(3) Å, b=8.1338(3) Å, c=7.4410(2) Å, R(F2)Bragg=5.9%; Mg(ND3)2Br2, Pnam, a= 5.92837(8) Å, b=11.8448(2) Å, c=7.9117(1) Å, R(F2)Bragg=5.0%. Detailed evaluation of neutron diffraction data of Mg(ND3)2Cl2 as a function of temperature (50 K<T<150 K) characterizes the phase transition as continuous with Tt≈135 K.