Theα-α′ phase transition of hydrogen in elastically anisotropic cubic metals

The formulation of the statistical mechanics of the - phase transition in hydrogenmetal-systems is extended in such a way as to include the elastic anisotropy of the host crystal. The corresponding reduction of symmetry leads to qualitative modifications of the results. Depending on hydrogen concentration and temperature, phases with homogeneous,C ₃- andC ₂-symmetric equilibrium hydrogen distributions are formed. The transitions between these phases are investigated, and the relation of their orders and the corresponding changes in symmetry is studied. Differences between isotropic and anisotropic systems are discussed.     

Translational order parameter in the smectic-B phase of 4-hexyl-4′-[2-(4-isothiocyanatophenyl)ethyl]-1,1′-biphenyl

4-Hexyl-4′-[2-(4-isothiocyanatophenyl)ethyl]-1-1′-biphenyl, a liquid crystalline material shows smectic-B (SmB) and nematic phases, respectively, between 60.3–98.5°C and 98.5–130.8°C. X-ray diffraction patterns were recorded at different temperatures in the SmB phase and were used to compute translational order parameters. For this purpose, we have employed a theoretical model recently reported by Kapernaum and Giesselmann. The results obtained are discussed in terms of the basic understanding of the SmB phase.     

X-ray studies of the smectic B phase of the 4-bromobenzylidene-4′-alkoxyanilines

The results of X-ray studies of the nine members (4?÷?12) of the 4-bromobenzylidene-4′-alkyloxyaniline homologous series (nBBAA) in smectic B (SmB) and smectic A (SmA) phases are presented. Interestingly, no SmC phase was identified. The thickness of smectic layers in the SmA and SmB phases was estimated showing unexpectedly that in SmA it is bigger than in the SmB. For the SmB phase, orthorhombic unit cell parameters were determined as a function of temperature. The obtained data are discussed in relation to rotational dynamics.     

Oriental behaviour of some homologues of 4-n-pentyl-phenylthio-4′-n-alkoxybenzoate doped with dichroic dye

The long-range orientational order of three members of 4-n-pentylphenylthio-4′-n-alkoxybenzoate series doped with a dichroic dye was studied by means of electronic absorption and fluorescence measurements. The order parameters <P₂> and <P₄> as well as the orientational distribution function were determined on the basis of absorption and emission spectra of linearly polarized light recorded as a function of temperature in the nematic, smectic A, and smectic C phases. An influence of the dye molecular structure on the orientational order of the dye/liquid crystal mixture was observed. Moreover, the dependence of the order parameter values on the length of the alkoxy chain in liquid crystal molecules was found. It was shown that the order parameter <P₄>, obtained from fluorescence measurements, can be helpful in recognizing the second-order or weakly first-order transitions between various liquid crystalline phases.     

The properties of ferroelectric compound (S)-(+)-4-(1-methylheptyloxycarbonyl)phenyl 4′-(4-cyanoacetoxybut-1-oxy) biphenyl-4-carboxylate and its ability of an antiferroelectric phase induction in mixtures

Phase transitions and ferroelectric properties of (S)-(+)-4-(1-methylheptyloxycarbonyl)phenyl 4′-(4-cyanoacetoxybut-1-oxy)biphenyl-4-carboxylate will be described in this article. This compound exhibits the synclinic smectic C* phase and it has the ability to induce an antiferroelectric phase in mixtures with the other ferroelectric compounds. New systems with induced antiferroelectric phase and their properties are presented.     

Critical behaviour in the commensurate-incommensurate phase transition of pure and γ-irradiated LiRbSO4 crystals

The effect of γ-irradiation on the incommensurate phase of LiRbSO₄ crystals has been studied by measuring the specific heat C_p . As a result of γ-irradiation, the values of C_p decreased and the transition temperature T_c shifted to lower values. The values of ΔH and ΔS decreased with increasing dose rate. The results are explained on the basis of an energy change due to induced lattice defects.     

Temperature behaviour of the tetrafluoroaluminates K1−xRbxAlF4. Evolution of the shear structural phase transition

By DSC, X-ray diffraction and neutron powder diffraction the temperature behaviour of the K_1−xRb_xAlF₄ compounds is investigated in the concentration range x = 0 to x = 0.10. For large crystals it is shown that if x is greater than 0.02 the shear transformation of pure KAlF₄ splits into two structural phase transitions. The structures are determined by neutron powder profile refinement and the space groups P4/mbm, P2₁ and P2₁/m are attributed to the three successive phases. The high and low temperature phases are the same as in pure KAlF₄. The intermediate phase is derived from the T1AlF₄ type structure by AlF₆ octahedra rotations corresponding to the a¹_pb⁺_pc⁺ tilts system. It is shown that the temperature of the transition leading to the low temperature phase is sample size dependent.     

Pressure–temperature phase diagrams for four higher members (nonyl–dodecyl) of the homologous series of 4′-alkyl-4-isothiocyanato-biphenyl ( n BT)

Differential thermal analysis (DTA) method was applied to study the pressure–temperature phase diagrams for four members of the 4′-alkyl-4-isothiocyanato-biphenyl (nBT) homologous series (n?=?9–12). These compounds exhibit a smectic E polymorphism enriched by a smectic A phase for two longest members. Measurements were performed in the temperature range 25–110°C and the pressure range up to 165?MPa. For all substances studied the pressure-induced phases were established with one or two triple points in the examined pressure range. In addition, for the 11BT and the 12BT the phases existing under atmospheric pressure disappeared under relatively low pressures. The results are compared with recently studied smectogenic n-alkyl-cyanobiphenyl (nCB) compounds.     

The vibration spectrum and high temperature phase transition of NaBF_4 single crystal

The polarized Raman spectrum of NaBF_4 single crystal measured by use of different geometric arrangements are reported. The obvious anisotropy of the crystal results in the degradation of degenerated modes of BF_4. Experiments show that the vibration frequencies decrease with the increasing of temperature. But when it above the phase transition point the v_1 symmetric stretching vibration is restored. From this, the phase transition temperature can be estimated to be 250℃ (523K).     

The deforraable mirror method of adaptive phase correction

In this paper, a simple method of phase correction by using a micromachined deformable mirror (MMDM) is demonstrated. With correction of high-order phases due to propagating through medium, we obtained a clean pulse shape, flattened spectral phase and decreased the femtosecond laser pulse duration. It is shown by our experiment that the deformable mirror is an effective and easy method for adaptive phase correction.     

The phase structure of Einstein–Cartan theory

In the Einstein–Cartan theory of torsion-free gravity coupling to massless fermions, the four-fermion interaction is induced and its strength is a function of the gravitational and gauge couplings, as well as the Immirzi parameter. We study the dynamics of the four-fermion interaction to determine whether effective bilinear terms of massive fermion fields are generated. Calculating one-particle-irreducible two-point functions of fermion fields, we identify three different phases and two critical points for phase transitions characterized by the strength of four-fermion interaction: (1) chiral symmetric phase for massive fermions in strong coupling regime; (2) chiral symmetric broken phase for massive fermions in intermediate coupling regime; (3) chiral symmetric phase for massless fermions in weak coupling regime. We discuss the scaling-invariant region for an effective theory of massive fermions coupled to torsion-free gravity in the low-energy limit.     

The study of phase matching of nonlinear waves in microdroplets

In this paper, the internal field distribution of higher order MDRs of water droplets is calculated and the result shows that the higher order MDRs should dominate the nonlinear couple process. Furthermore, the phase matching required for four-wave mixing (FWM) process in the droplet is interpreted as the concept of angular momentum conservation. The slight mismatch in angular momentum leads to significant improvement of phase matching than that in optical cell.     

Ordering of hexagonal layers in phase β- and β′-Mg2Al3

Two phases, β and β′, in Mg₂Al₃ have been compared. Structural rod-like domains composed of sets of hexagonal layers have been described. The main set containing 11 layers does not change during β′–β transformation. The short sequences of layers on the β′ phase transform into clusters in the β-phase. The centres of the domains form a superstructure with a modulation vector equal to 3/22.     

XY theoritical phase diagrams of the thiospinels AB2xB′2−2xS4

Randomly diluted frustrated helimagnetic and ferromagnetic systems with spinel structure, ZnCr_2xAl_2−2xS₄ and CdCr_2xln_2−2xS₄, respectively, were studied in the framework of the XY model. Using the replica method, the variational principle and taking into account exchange integrals up to the fourth nearest neighbour (nn), J₄ in the case of HM and up to the second (nn) J₂ in the case of FM, the T vs. x magnetic phase diagrams were established. The dramatic effect of frustration is evidenced, which leads to the disappearance of long-range order and the appearance of spin-glass state for great dilution. The obtained results are in agreement with those obtained by magnetic measurements.     

The phase stability of Ca2TiO4 and related Ruddlesden–Popper phases

The Ruddlesden–Popper phases of the Ca–Ti–O system, Ca_n+1Ti_nO_3n+1, are investigated by means of atomistic simulations employing empirical pair potentials. The stability of the phases is examined in terms of various reaction schemes: the formation from the binary oxides, the addition of the perovskite oxide to a given phase, and the reaction between perovskite and rock-salt oxides. The energies of these reactions are compared with results previously obtained for the Ruddlesden–Popper phases of the Sr–Ti–O system. The importance of the disproportionation reaction of the various R–P phases in both Ca and Sr systems is also emphasized. The results obtained are in good agreement with experimental observations regarding both systems.     

Low-temperature phase behaviour of the binary system water–oxyethylated glycerol of polymerization degree n = 5 and intermolecular interactions in the system

In this paper, phase and glass transitions occurring in a binary system water–oxyethylated glycerol of polymerization degree n = 5 (OEG _{n =5}) are studied by differential scanning calorimetry (DSC) in the concentration range 0–100% (w/w) at temperatures lower than 273 K. The supplemented phase diagram of this system has been constructed for the first time. Concentration and temperature regions for the existence of a completely amorphous state and a heterogeneous crystalline/amorphous state have been determined. Processes of formation of crystalline and amorphous phases in the water–OEG _{n =5} system are studied by optical cryomicroscopy, both during cooling and subsequent heating. The cryomicroscopic data confirm the conclusions derived from the DSC data analysis. The number of strongly and weakly bound water molecules per OEG _{n =5} molecule has been calculated on the basis of the analysis of the supplemented phase diagram of the water–OEG _{n =5} system. The hydration number of an OEG _{n =5} molecule has been also determined by infrared spectroscopy.     

The elastic stiffness constants of β-NH4LiSO4 around its phase transition at 460 K

The complete tensor of the elastic stiffness constantsC _ij (i,j=1 to 6) of -NH₄LiSO₄ has been measured in the temperature range 290 K to 540 K including the ferroelectric phase transition at 460 K, by the ultrasonic pulse echo overlap method.Some ultrasonic attenuation coefficients were determined.The elastic stiffness constants were calculated using Landau Theory. The elastic stiffness constants are all well described within this theory with the exception ofC ₆₆, which can not be reproduced with coupling terms allowed by group theoretical arguments. This together with double peaks observed in the specific heat and in sound attenuation in some directions leads one to suspect an intermediate phase between the paraelectric and the ferroelectric phases.     

The spreading behaviour of perfluoropolyether droplets on solid surfaces

The spread of perfluoropolyether （PFPE） droplets on solid surfaces has been measured from the top-down view through a microscope system. Effects of substrates, molecular weight and end-group functionality on spreading of the PFPE droplets have been studied experimentally and the results were compared with those by molecular dynamics （MD） simulations. Silicon wafer and diamond-like carbon （DLC） substrates were used to study the effect of substrates on spreading. Two types of PFPE, Z-dol and Z-tetraol, with the same chain structure and various molecular weights （2000 and 4000 g/mol） were employed in experiments. Effect of molecular weight has been investigated through comparing the spreading of Z-dol 2000 and Z-dol 4000, and it is found that the increase of molecular weight will decrease the mobility of PFPE. Comparison between spreading of Z-dol and Z-tetraol of the same molecular weight proved that functional end group plays a significant role on the spreading of PFPE, which confirmed the MD simulation results.     

The prediction of the behaviour of Λ-hyperon electromagnetic form factors

The behaviour of the -hyperon electric and magnetic form factors is predicted within the modified three resonance ((783), (1020), (1680)) vector-meson dominance (VMD) model with correct analytic properties and the asymptotic behaviour predestinated by the quark model for baryons. The resonance couplings to are evaluated from the known couplings to nucleons by utilizing theSU(3) symmetry and normalization conditions for Dirac and Pauli form factors. The masses and widths of the considered vector mesons are fixed at the world averaged values. The optimal position of the effective inelastic threshold, common for Dirac and Pauli form factors, is determined from the fit of the only existing Orsay DM2 point on the cross section att=5·693 GeV². The mean square charge and magnetic radii of the -hyperon are found to be r _E ² =0·018 F² and r _M ² =–0·027 F², respectively.The authors are very much indebted to Prof. R. Baldini-Ferroli for calling their attention to the problem of -hyperon e.m. ff 's in connection with the FENICE experiment.