Chaleur spécifique d’alliages de structure A 15

The low temperature specific heat and the superconducting transition temperature have been measured in about fifty A 15-type binary alloys. The variation of the electronic specific heat are discussed in terms of the degree of atomic order.     

Phase hexagonale de L'ytterbium sous pression

Low temperature resistivity measurements have been used to map out the phase transition f.c.c.-hex in Ytterbium. The hexagonal phase is stable up to 2 kbar at helium temperature. According to the data of residual resistivity and T² variation law the density of carrier is higher in the hexagonal phase than in the f.c.c. phase.     

Transition de phase dans le nitrate de calcium Ca(NO3)2

The phase transition in calcium nitrate at 14° C has been investigated. Results of second harmonic generation (S.H.G.), dielectric measurements and other investigations are presented. S.H.G. experiments indicate absence of inversion center below 14° C. The phase transition is clearly distinguished by the disappearance of the SHG signal at 14° C. Dielectric measurements show a sharp maximum of ε′ at 14° C. Ferroelectric hysteresis loop is not observed below this temperature. Physical properties of calcium nitrate and cadmium nitrate are also compared.     

Etude des transformations liquide ⇄ solide ⇄ solide du chlorure de cesium divise

A study of the phase size effect upon the transition temperatures of cesium chloride has been carried out by thermal analysis and X-ray diffraction of samples obtained by condensation of the compound into porous materials of which the pore radii lie between 3.5 nm and 65 nm. The results show a decrease of the liquid ? solid transformation point with the pore radius for both rising and falling temperatures, while the polymorphic transition temperatures are decreased for cooling and increased for heating. The polymorphic transformation vanishes for CsCl in which the pore radii are smaller than 7.7 nm. The transition temperature variations are explained by homogeneous nucleation theory.     

Structure magnetique du sulfure de fer FeS de type blende

Zinc blende type FeS, which is cubic at room temperature and becomes orthorhombic, pseudo-tetragonal ($\text{c}\text{a}\text{< 1}$) at the magnetic transition, has been studied by neutron diffraction. The magnetic space group is F_c 2 2' 2' : ferromagnetic (001) planes are coupled antiferromagnetically, and the moments lie along the a axis, in agreement with previous Mössbauer measurements. The exchange integral between nearest neighbours in the ab plane is thus weaker than those between nearest neighbours in the ac and bc planes.     

Calcul semi-empirique de la temperature de transition des superconducteurs et de l'effet de la pression sur la transition

A method is developed to take into account the contribution of the Umklapprozesse to the electron-phonon interaction in polyvalent metals. This method is used to compute—from first principles—the high-temperature resistivity of non-transition metals as well as the transition temperature T_c of non-transition superconductors along the lines of , and 's theory of superconductivity (Phys. Rev. 108, 1175 (1957)). The effect of pressure on T_c is also computed and its dependence upon the band structure is stressed.     

Transitions de phase destructive et non destructive dans le cristal moleculaire bromo-2 naphtalene

The polymorphism of 2-bromonaphthalene has been studied by both crystallographic and energetical methods. Three crystalline forms have been characterized, respectively denoted Br_III, Br_II and Br_I. The first one is the stable form up to 319 K. At this temperature a first order phase transition occurs which breaks the single crystal and leads to a disordered structure Br_I isotypic with the naphthalene one. The Br_II form is a metastable one which appears when cooling the Br_I, form; when so formed, the Br_II phase can give again the Br_I one by annealing; so that the Br_I-Br_II transition is reversible. This second order transition extends over a large temperature range (275–319 K), it does not destroy the single crystal and corresponds to a substructural modification along the c axis of the monoclinic cell. The anomalies of the Cp curve and the variations of the principal components of the thermal expansion tensor suggest that there are two competitive temperature dependent molecular reorientations. The transition Br_II→Br_III which leads to the stable ordered form is spontaneous at room temperature, it takes place sluggishly and destroys the single crystal.Cell parameters of the Br_III form have been determined from powder data using an automatic indexing method: a = 9.578(1) Å, B = 9.601(1) Å, c = 10.303(1) Å, α = 100.79(1)°, β = 109.06(1)°, γ = 101.77(1)°. (Figure of merit M₂₀ = 86.)     

Etude par correlation angulaire γ-γ perturbee des aspects statiques et dynamiques de la structure du heptafluorohafniate de sodium

We have studied the temperature dependence of the perturbation of the differential γ-γ angular correlation of the ¹⁸¹Ta 133–482 keV cascade in sodium heptafluorohafnate. In the 20–300°C temperature region, we observed a drastic variation of the quadrupole interaction parameters when raising the temperature from 100 to 180°C. Such a variation is due to the existence of a phase transition corresponding to an orthorhombic-quadratic transformation of the lattice. In the low temperature phase (T < 120°C) the perturbation is static and the nuclear probe is located at two equally populated sites. In the high temperature phase (T > 180°C) the perturbation arises from the combination of static and dynamic interactions. The dynamic part of the interaction could be analysed within the isotropic perturbation model and was ascribed to jumps of the fluorine atoms onto the vertices of the cubic coordination polyhedra of the hafnium in the HfF₇^3? ion. The activation energy of the process is 0.13 ±0.03 eV. The idea of an order-disorder phenomena occuring in the course of the phase transition is consistently supported by these observations. Our results yield also valuable informations about the nature of the hafnium-fluor bond in this compound.     

Obtention des bandes de cameron par photo excitation de co en matrices d'argon de krypton et de xenon

CO trapped in solid matrices of argon, krypton and xenon has been excited by 10 eV photons. Cameron bands v′=0, v″=0,1,2,3,4 are observed during the irradiation. If red shifts are observed in argon and krypton matrices, the transition in xenon shows no differences with gas phase data.     

Etude de la structure du tellure liquide par diffraction de neutrons au voisinage de la temperature de fusion

The structure factor of liquid tellurium has been measured by neutron diffraction at the melting temperature and in the supercooled state. These results are a confirmation of the existence of an A₇ type stable configuration in a large range of temperature between 400 and 1100°C.     

Étude comparée de la première couche d'adsorption de méthane sur nitrure de bore et graphite

The adsorption of methane on crystalline boron nitride has been studied between 77 and 90 K by the volumetric method, especially in the first monolayer domain. It is shown that the thermodynamical properties of the layer are similar to those previously obtained on graphite. It can be inferred from the results that below 77 K, the layer undergoes two first-order phase transitions: a 2D gas-2D liquid transition and a 2D liquid-2D solid transition. In its solid state the localization of the layer is certainly the same on both substrates.     

Influence de l'adsorption métallique sur la résistivitéélectrique de films minces d'or

Precise electrical resistance and thickness measurements are used to study metallic adsorption on thin gold films under ultra-high vacuum conditions. The large variations of resistance observed for impurity coverage lower than a monolayer are interpreted by classical size effect. Adsorption of various “sp” and transition metals have been investigated. We report here some typical plots showing the resistance increase of gold films about 250 Å thick versus metallic impurity coverage. Different informations obtained from these curves are discussed. The adatom resistivity at very low coverage is determined experimentally; its variations versus valence impurity are similar to the bulk impurity resistivity; the role of the adsorption site is also examinated. In some cases the overlayer growth mechanism can be determined, specially when it is of the Frank-van der Merwe type; in this case, the influence of temperature on the structure of the first monolayer has been investigated. For coverage greater than a monolayer, some examples of intermetallic compound formation or intergranular migration revealed by electrical resistance measurements, X-rays and electron microscope observations are given.     

Etude par RMN de la relaxation spin réseau d'un syst`eme AB de noyaux en interconversion lente

Nuclear Overhauser effects allow us to determine all transition probabilities in a two-spin AB system, using a triple resonance apparatus we have constructed. Theoretical and experimental studies of the relaxation in the special case where the nuclei belong to a molecule interconverting slowly between two different conformations have been made, and the exchange ratio has been deduced.     

Etude d'un nouveau type de fluorure antiferromagnetique a caractere bidimensionnel: NaFeF4

A new type of two-dimensional antiferromagnetic structure has been investigated by magnetic susceptibility measurements, neutron diffraction and Mössbauer resonance. The magnetic cell of NaFeF₄ is doubled along the a axis and the spins lie along the b axis. The κ^-1 vs T curve shows a sharp minimum at about 105 K close to the three-dimensional transition temperature determined by Mössbauer spectrometry (111.5 K). A calculation of z. snfc;J/kz. snfc; has been performed using a high temperature series expansions technique (J/k = -23 K). The variation of the hyperfine field in the range 0.6 ?T/T_N? 1 gave the value of the critical exponant β = 0.25.     

Etude spectroscopique d'un jet de plasma d'argon hors d'equilibre thermodynamique local

We have studied the properties of an argon plasmajet seeded with a few percent of hydrogen. At atmospheric pressure, significant deviations from local thermodynamic equilibrium (L.T.E.) are shown. The plasma parameters (electrondensity N_e, temperature T) have been determined from the broadening and the absolute intensity of hydrogen lines. The transition probabilities and the broadening parameters (width and shift) of some AI lines between 4000 and 6000 Å have been measured.     

Effective exponents for de almeida-thouless line

The experimental irreversibility line at the magnetic phase diagram H-T of spin glasses is often approximated using a power law Hα (T_G-T)^φ/2. The mean field theory of de Almeida and Thouless predicts φ=3 for temperatures T close to the spin-glass transition temperature T_G . For a range of reduced temperature involved in an experiment one should use, however, effective exponents (φ>3) obtained by fitting the power law to the exact expression for de Almeida-Thouless line. These compare favorably with experimental values of φ obtained for different spin glasses. The increase of effective exponent up to φ=7 on approaching the spin-glass-ferromagnetism multicritical point is also considered.     

Shifting of transition temperature of ferroelectric liquid crystals due to addition of dye-An optical and dielectric study

In order to study the shifting of phase transition temperature of ferroelectric liquid crystals due to addition of dye molecules, we have investigated two ferroelectric liquid crystal materials (Felix 16/030 and Felix 16/100) and their five mixtures with Anthraquinone dye (1%, 2%, 3%, 4% and 5% wt/wt). The phase transition scheme has been investigated and analyzed by results obtained from the optical transmittance and the dielectric permittivity study with variation of temperature in the range of 30 to 100 °C. Both the samples clearly show the shifting of phase transition temperature with dye concentration, especially the SmC*-SmA phase transition temperature. It is also clear from the study that SmC*-SmA phase transition phenomenon also becomes stronger with the addition of dye molecules. A theoretical explanation has also been given for shifting of phase transition temperature. The amount of shift in transition temperature agrees well as obtained from optical and dielectric studies.     

Dielectric studies of nanoporous alumina films filled with the Rochelle salt

Dielectric studies of nanoparticles of the Rochelle salt embedded in pores of porous alumina have been performed in the temperature range from 80 K to the decomposition temperature of the bulk Rochelle salt. It has been revealed that the permittivity exhibits an anomaly corresponding to the lower structural transition to the paraelectric phase, whereas the upper ferroelectric transition is shifted above the decomposition temperature in agreement with the recently published data. The temperature of the lower transition for nanoparticles in pores is found to decrease by 10 K. Possible physical factors that can be responsible for the broadening of the region of existence of the ferroelectric phase have been discussed.     

Viscosity,glass transition and activation energy of solid cis-polyisoprene and trans-polyisoprene blends

Blends of cis-polyisoprene (CPI) and trans-polyisoprene (TPI) have been prepared by solution casting to study viscosity, glass transition temperature and activation energy for the glass transition. The viscosity of blends having different weight ratios has been obtained through a single experiment measuring storage and loss modulus using the dynamic mechanical analyser technique. The glass transition temperature is determined through the temperature at which the minimum of temperature derivative curve of viscosity falls. The activation energy of glass transition and fragility index have been obtained by employing the Vogel–Fulchar–Tammann (VFT) equation by assuming non-Arrhenius behaviour of viscosity of polymer blends. Results indicate that both glass transition and activation energy for the glass transition are influenced by composition and crosslink density of the blend.     

Entropy of Higher-Dimensional Charged de Sitter Black Holes and Phase Transition

From a new perspective, we discuss the thermodynamic entropy of (n+2)-dimensional Reissner-Nordströmde Sitter (RNdS) black hole and analyze the phase transition of the effective thermodynamic system. Considering the correlations between the black hole event horizon and the cosmological horizon, we conjecture that the total entropy of the RNdS black hole should contain an extra term besides the sum of the entropies of the two horizons. In the lukewarm case, the effective temperature of the RNdS black hole is the same as that of the black hole horizon and the cosmological horizon. Under this condition, we obtain the extra contribution to the total entropy. With the corrected entropy, we derive other effective thermodynamic quantities and analyze the phase transition of the RNdS black hole in analogy to the usual thermodynamic system.