Raman study of structural relaxation and boson peak in amorphous films of adamantane

Low-frequency Raman scattering was used to study amorphous solid films of adamantane, a globular non-polar hydrocarbon molecule. As evidenced by its spectral characteristics this type of disorder is different from the orientational disorder found in the room temperature plastic phase by the absence of the translational order as well. This gives rise to the boson peak related to acoustic phonons which gradually disappears upon heating with simultaneous emerging of the phonon line at 50 cm-1 which characterizes the low-temperature ordered phase of adamantane. Adamantane dynamics resembles that of C60 fullerene although not in the same temperature range. All this makes adamantane an attractive system that could serve as a practical reference in molecular simulation studies of the glassy phase of model fluids.     

钛酸铅和钛酸钡振动性质及铁电相变的密度泛函理论研究

采用密度泛函赝势的方法, 研究了不同晶相的钛酸铅和钛酸钡的振动模式. 没有发现钛酸铅存在低温相变的证据, 而钛酸钡则存在四方-正交和正交-三角铁电相变. 振动频率随四方应变的变化关系表明, 随着四方应变的增大, 软模的频率增大, 在某一个临界点, 不稳定的软模转变为稳定的振模. 由于钛酸铅具有较大的四方应变, 使得其能够在四方相稳定下来, 而钛酸钡较小的四方应变是其仍能够发生低温铁电相变的一个重要原因.     

Orientational melting and reorientational motion in a cubane molecular crystal: a molecular simulation study

Detailed molecular simulations are carried out to investigate the effect of temperature on orientational order in cubane molecular crystal. We report a transition from an orientationally ordered to an orientationally disordered plastic crystalline phase in the temperature range 425-450 K. This is similar to the experimentally reported transition at 395 K. The nature of this transition is first order and is associated with a 4.8% increase in unit cell volume that is comparable to the experimentally reported unit cell volume change of 5.4% (Phys. Rev. Lett. 1997, 78, 4938). An orientational order parameter, eta(T), has been defined in terms of average angle of libration of a molecular 3-fold axis and the orientational melting has been characterized by using eta(T). The orientational melting is associated with an anomaly in specific heat at constant pressure (C(P)) and compressibility (kappa). The enthalpy of transition and entropy of transition associated with this orientational melting are 20.8 J mol(-1) and 0.046 J mol(-1) K(-1), respectively. The structure of crystalline as well as plastic crystalline phases is characterized by using various radial distribution functions and orientational distribution functions. The coefficient of thermal expansion of the plastic crystalline phase is more than twice that of the crystalline phase.     

Structural properties and phase transitions in a silica clathrate

Melanophlogite, a low-pressure silica polymorph, has been extensively studied at different temperatures and pressures by molecular dynamics simulations. While the high-temperature form is confirmed as cubic, the low-temperature phase is found to be slightly distorted, in agreement with experiments. With increasing pressure, the crystalline character is gradually lost. At 8 GPa, the radial distribution function is consistent with an amorphous state. Like pristine glass, the topology changes, plastic behavior, and permanent densification appear above ～12 GPa, triggered by Si coordination number changes. We predict that a partial crystalline and amorphous sample can be obtained by recovering the sample from a pressure of ～12-16 GPa.     

A high pressure FT-IR spectroscopic study of phase transitions in 1-chloro- and 1-bromoadamantane

The infrared spectra of two orientationally disordered adamantane derivatives, 1-chloro- and 1-bromoadamantane, have been obtained as a function of pressure by use of a diamond anvil cell. Phase transitions were detected from the splittings of vibrational bands and from changes in the slopes on plots of frequency versus pressure. In 1-chloroadamantane, a disorder-order phase transition was detected at approximately 5 kbar. This high-pressure phase is identical to the ordered phase obtained at low temperature. A second phase transition at high pressure was identified, for the first time, at approximately 17 kbar. The new high-pressure phase is an ordered phase. For 1-bromoadamantane, a semi-ordered to ordered phase transition was detected at about 5 kbar, and no other transitions were observed up to a pressure of 35 kbar. For both derivatives, the phase changes were reversible.     

High-pressure study of adamantane: variable shape simulations up to 26 GPa

We report simulations of adamantane by carefully combining ab initio and empirical approaches to enable simulations with internal degrees of freedom on crystalline adamantane up to a pressure of 26 GPa. Two sets of simulations, assuming the adamantane molecule as a rigid (RB) and flexible body (FB), have been carried out as a function of pressure up to 26 GPa to understand changes in the crystal structure as well as molecular structure. The flexible body simulations have been performed by including 6 lowest frequency internal modes (obtained from DFT calculations performed with Gaussian98) out of the total of 72. The calculated variation in c/a and V/V(0) from the RB and FB calculations as a function of pressure have been compared with the experimental curve. Other relevant thermodynamic and structural properties reported are the radial distribution functions, deviation in the position of a given type of atom with respect to its position at standard pressure, delta(s), cell parameters, volume, and energy. With an increase in pressure, three additional peaks are seen to develop gradually at three different pressures in the center of mass (com)-com radial distribution function (rdf). We attribute these changes to structural transformations (probably second-order phase transitions) which is consistent with the three phase transitions reported by Vijayakumar et al. for adamantane in the pressure range of 1 atm-15 GPa. Our simulations also show that these additional peaks in the rdf's are associated with the differences between opposite and parallel spin neighbors of Greig and Pawley as well as the crystallographic directional dependence of intermolecular distances in the first three shells of the neighbors. Also, the structural quantities from the RB calculation show considerable deviation from the FB calculation for pressures greater than 5 GPa, which suggests that the rigid body assumption for molecules may not be valid above this pressure.     

Ab initio prediction of the low-temperature phase diagrams in the systems KBr-NaBr, KX-RbX, and LiX-RbX (X=Cl,Br)

The authors have calculated the low-temperature phase diagrams for the ternary alkali halides KBr-NaBr, KX-RbX, and LiX-RbX (X=Cl,Br) systems on the ab initio level without any recourse to experimental information. Via global exploration of the enthalpy landscapes for many different compositions in these systems, candidates for both ordered stoichiometric modifications and crystalline solid solution phases have been identified. Next, their free enthalpies were computed on ab initio level, and the respective low-temperature phase diagram has been derived. They find miscibility gaps in the systems KBr-NaBr and KX-RbX (X=Cl,Br), while in LiX-RbX (X=Cl,Br) only crystalline ordered phases should be present, in agreement with available experimental data. Furthermore, they predict several new thermodynamically stable and metastable phases in these systems.     

Determination of static microstructure of dilute and concentrated suspensions of anisotropic particles by ultra-small-angle X-ray scattering

Ultra-small-angle X-ray scattering was performed on suspensions of anisotropic polystyrene particles of varying degrees of anisotropy. The wave vector dependence of particle form factors is well described by a model developed by Debye for the scattering from fused spheres. As volume fraction is raised, all suspensions undergo a disorder/order phase transition. The scattering from disordered and ordered suspensions of anisotropic particles is the same as that of spheres up to volume fractions of 0.45, suggesting that, in the dilute crystalline phase, the anisotropic particles order into a rotator or plastic crystal phase, where the particle centers of mass are ordered, but the particle directors are randomly distributed. Further increase in particle volume fraction leads to differences in scattering between homonuclear dicolloids and spheres, implying that the homonuclear dicolloids form a body-centered tetragonal phase with both positional and directional order. This conclusion is supported by real-space imaging of dried films of the particles.     

What does pressure decide to cook with orientationally disordered plastic phase of cubane: an orientational glass or crystal?

The effect of pressure on the structure and reorientational motion of molecules in orientationally disordered (OD) crystalline phase of cubane has been investigated in detail using variable shape molecular simulations in constant-pressure constant-temperature ensemble. Complete orientational ordering occurs at a pressure of 1.0 GPa and the OD phase transforms to an orientationally ordered phase at this pressure. The transition is associated with a kink in the variation of structural parameters such as cell parameters, unit-cell volume, and interaction energy. This transition is also associated with an anomaly in specific heat. Above this transition pressure, the structural quantities display only smaller changes with further increase in pressure. The structure of high-pressure orientationally ordered (HPOO) phase has been characterized using radial distribution functions and orientational distribution function. From detailed analysis of the structure of HPOO phase we conclude that it is isostructural with low-temperature orientationally ordered phase. The OD phase has four times larger compressibility than the HPOO phase.     

Structural development and kinetic analysis of PbTiO3 powders processed at low-temperature via new sol-gel approach

The synthesis of crystalline lead titanate powder by a generic low-temperature sol-gel approach is developed. Acetoin was added as ligand, instead of the commonly used alkanolamines, to ensure total dissolution of the precursor compounds. The feasibility of the acetoin-Ti isopropoxide complex as a new precursor of PbTiO₃ perovskite particles via sol-gel method has been demonstrated. No excess lead has been introduced. Nanometric PbTiO₃ crystallites have been formed at 400 °C under atmospheric pressure from titanium isopropoxide and lead acetate in alcoholic solution by remarkably low activation energy of crystallization process of 90 kJ mol⁻¹. The powders show tetragonal lattice and dendritic morphology. In addition to the effect of heat-treatment temperature, time, and atmosphere, the sol chemistry particularly influenced the phase composition, particle size, and particle morphology. The use of different ligands significantly modified powder morphology. The extent of the crystallization was quantitatively evaluated by differential thermal analysis and analyzed by Johnson-Mehl-Avrami approach. The crystallization followed two rate regimes depending on the interval of the crystallized fraction.     

Phase diagram of model anisotropic particles with octahedral symmetry

The phase diagram for a system of model anisotropic particles with six attractive patches in an octahedral arrangement has been computed. This model for a relatively narrow value of the patch width where the lowest-energy configuration of the system is a simple cubic crystal. At this value of the patch width, there is no stable vapor-liquid phase separation, and there are three other crystalline phases in addition to the simple cubic crystal that is most stable at low pressure. First, at moderate pressures, it is more favorable to form a body-centered-cubic crystal, which can be viewed as two interpenetrating, and almost noninteracting, simple cubic lattices. Second, at high pressures and low temperatures, an orientationally ordered face-centered-cubic structure becomes favorable. Finally, at high temperatures a face-centered-cubic plastic crystal is the most stable solid phase.     

Ab initio computation of low-temperature phase diagrams exhibiting miscibility gaps

A new methodology for the computation of the low-temperature part of phase diagrams without recourse to any experimental information is presented. A central element is a procedure for deciding whether formation of crystalline solid solution phases can take place in the chemical system. Via global exploration of the enthalpy landscapes for many different compositions in the system, candidates for ordered stoichiometric and crystalline solid solution phases are identified. Next, their free enthalpies are computed at ab initio level and a low-temperature phase diagram is derived. As examples, the low-temperature phase diagrams for the ternary alkali halides NaCl/LiCl NaBr/LiBr and NaCl/KCl are presented.     

Temperature dependence of self-diffusion in adamantane

Radiotracer experiments show that self-diffusion in the rotator crystalline phase of adamantane proceeds by a vacancy mechanism. The results are in good agreement with those determined from NMR experiments.     

Thermotropic phase behaviour of long-chain alkylmaltosides

The thermotropic phase behaviour and phase structure of crystalline and non-crystalline n-tetradecyl-beta-D-maltoside (C14G2) and n-hexadecyl-beta-D-maltoside (C16G2) have been investigated by means of differential scanning calorimetry and X-ray techniques. Upon lyophilisation, both compounds form a solid, lamellar phase comprising disordered head groups and hexagonally packed alkyl chains that are suggested to be tilted and interdigitated. This ordered lamellar phase melts into a metastable lamellar liquid crystal, which re-crystallises to a high-temperature crystalline polymorph comprising interdigitated, non-tilted alkyl chains. Remarkably, the high-temperature polymorph of C14G2 has the same melting point as that of C16G2, namely 105 degrees C for both surfactants. A low-temperature polymorph of anhydrous C14G2 crystallises from water at room temperature, whereas the hemihydrate of C14G2 crystallises at 6 degrees C from water, or from chloroform containing trace water. X-ray data suggest both these crystalline modifications to comprise interdigitated and tilted alkyl chains.     

A high-temperature neutron diffraction study of pure and scandia-stabilized zirconia

A high-temperature neutron diffraction apparatus has been used to study a section of the zirconia-scandia system; the purpose was to determine whether this technique can be generally applied to phase equilibria studies. The structure of the tetragonal form of zirconia at 1200°C has been confirmed, and the parameters obtained were z(4d) = 0.188 ± 0.002, B₀ = 2.50Ų, B_Zr = 0.80 Ų, and R = 0.045. The effect of substituting scandia into the tetragonal zirconia structure was studied and the transformation of the ordered low-temperature β-phase to a grossly nonstoichiometric fluorite phase was also observed.     

Stability of smectic phases in the Gay-Berne model

We present a detailed computer simulation study of the phase behavior of the Gay-Berne liquid crystal model with molecular anisotropy parameter kappa=4.4. According to previous investigations: (i) this model exhibits isotropic (I), smectic-A (Sm-A), and smectic-B (Sm-B) phases at low pressures, with an additional nematic (N) phase between the I and Sm-A phases at sufficiently high pressures; (ii) the range of stability of the Sm-A phase turns out to be essentially constant when varying the pressure, whereas other investigations seem to suggest a pressure-dependent Sm-A range; and (iii) the range of stability of the Sm-B phase remains unknown, as its stability with respect to the crystal phase has not been previously considered. The results reported here do show that the Sm-A phase is stable over a limited pressure range, and so it does not extend to arbitrarily low or high pressures. This is in keeping with previous investigations of the effect of molecular elongation on the phase behavior of Gay-Berne models. A detailed study of the melting transition at various pressures shows that the low-temperature crystalline phase melts into an isotropic liquid at very low pressures, and into a nematic liquid at very high pressures. At intermediate pressures, the crystal melts into a Sm-A liquid and no intermediate Sm-B phase is observed. On the basis of this and previous investigations, the reported Sm-B phase for Gay-Berne models appears to be a molecular solid rather than a smectic liquid phase.     

Plastic and glassy crystalline states of methane: A Monte Carlo simulation study

A Monte Carlo simulation study has been carried out on the glassy crystalline phases of methane obtained by annealing or quenching the plastic (orientationally disordered) phase. Different cooling rates lead to different states of the glass. Temperature variation of the reorientational parameter suggests the presence of a transition between the plastic and glassy crystalline phases.     

Ab initio prediction of the low-temperature phase diagrams in the systems CsX–LiX (X = F, Cl, Br, I)

We have calculated the low-temperature phase diagrams for the ternary alkali halides CsX–LiX (X = F, Cl, Br, I) at an ab initio level without any recourse to experimental information. The starting point of our general approach is the global exploration of the enthalpy landscapes for many different compositions in these systems. Candidates for both ordered stoichiometric modifications and crystalline solid-solution phases are identified, and their free enthalpies are computed at an ab initio level. From this the low-temperature phase diagrams are derived. We find that in all systems under investigation only crystalline ordered phases should be present, in agreement with available experimental data. Furthermore, we predict several new thermodynamically stable and metastable phases in these systems.     

In situ studies of phase transitions in thin discotic films

The crystalline to liquid crystalline (Cr-LC) phase transition in thin films of zone-cast hexa-peri-hexabenzocoronene sixfold substituted with dodecyl side chains (HBC-C12H25) has been studied in detail using grazing incidence X-ray diffraction (GID), electron diffraction (ED), and variable angle spectroscopic ellipsometry (VASE), When heating the material, a first minor transition is observed around 42 degrees C. This change is attributed to alterations of the crystalline alkyl chain packing, which only slightly changes the electronic properties of the material. At higher temperatures of about 90 degrees C, but still significantly below the previously reported transition temperature in bulk, the Cr-LC transition is observed. An accompanying large increase in optical anisotropy is compatible with the X-ray data, showing a transition from the as-cast herringbone-like crystalline state to a highly ordered discotic hexagonal columnar LC phase. The structural transition has the macroscopic effect of increasing the film thickness. The high structural order of the as-cast low-temperature phase is only partly recovered after cooling, and the phase transition exhibits a large hysteresis. From the ellipsometry data, the dielectric tensor of HBC-C12H25 was refined to unprecedented detail.     

Ab initio computation of the low-temperature phase diagrams of the alkali metal iodide-bromides: MBr(x)I1-x (0 </= x </= 1), Where M = Li, Na, K, Rb, or Cs

We have studied the low-temperature phase diagrams of the systems MBr-MI (M = Li, Na, K, Rb, or Cs) via global exploration of the enthalpy landscapes for many different compositions, leading to candidates for solid solution-like and ordered crystalline phases. For all of these candidates the free enthalpies are computed at the ab initio level, and the low-temperature phase diagrams of the five chemical systems are derived. We find not only the expected stable solid solution in the rocksalt structure type but also metastable solid solutions based on the CsCl type for the RbBr-RbI and CsCl-CsI systems. Furthermore, additional metastable structure candidates exhibiting ordered crystalline structures exist for several compositions. In the case of the LiBr-LiI system, the metastable solid solution based on the wurtzite type was generated, and the location of the miscibility gap was predicted.