Studying the behavior with pressure and temperature of thermal expansivity data derived from (p,p,T) measurements of liquids, we found that a previously derived analytical equation of state is inapplicable under certain thermodynamic conditions. The main obstacle is that these conditions are determined by characteristic constants of the liquid under study. Following the same scheme used to derive the original equation of state, we propose a new analytical expression which removes this restriction. Three different substances have been used to test the ability of the new equation of state to represent the volumetric properties of compressed liquids,

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We analyse here the thermodynamic behaviour of the thermal expansion coefficient for a number of liquids. The purpose of this work is to provide some general rules to develop equations of state models meeting the following criteria: thermodynamic consistency, generality, predictive power and accuracy to represent derived properties over wide ranges of pressure and temperature. The liquids included into our analysis have been selected to meet two criteria: (1) available experimental data over wide ranges of pressure and temperature (from the melting point up to the critical point), and (2) liquids composed of molecules with different geometries and interactions.

We provide evidence of a universal equation of state for solids using a pseudospinodal hypothesis. A simple model to estimate the pseudospinodal curve is presented. This model combined with a previously reported ͑isothermal͒ volumetric equation ͓Baonza et al., Phys. Rev. B 51, 28 ͑1995͔͒ yields a complete equation of state applicable over the whole range of temperature. The resulting equation appears to be a well-behaved equation of state over the whole range of temperatures using a single reference thermodynamic state of the solid at atmospheric pressure as input data. Comparison with experimental results of molar volume, bulk modulus, and thermal-͑volumetric͒ expansion coefficient are presented. Comparison with previous equations of state are also presented and discussed. Our results imply that the thermodynamics of any solid are governed by its pseudospinodal curve.

We recently proposed an isothermal equation of state that was successfully applied to study the highpressure behavior of solids. Later, we developed a simple model to include temperature effects on the equation of state. In the present work we consider the prediction capabilities of the complete equation of state in the entire p-V-T surface of a solid. Our predictions have been compared with experimental data up to pressures of several GPa and temperatures between zero and temperatures rather higher than those of melting. We also compare the performance of our equation against the most successful equations of state proposed in the literature, with excellent results. The isothermal equation of state provides us an adequate representation of experimental pressure-volume data and a simple volume dependence for the Grüneisen parameter. The equation needs only four parameters evaluated at room pressure at a single reference temperature.

We propose a new Raman pressure scale based on the shift with pressure of the fundamental Raman band of micrometer-sized diamonds. First, we confirmed that the pressure slope of the triply degenerate diamond phonon behaves in a similar fashion to that of the bulk. Our measurements were calibrated Raman against the Sm:YAG fluorescence pressure scale up to 5 GPa using a gasketed sapphire anvil cell. The most relevant features regarding the design of the anvil cell are briefly outlined. Measurements were performed under hydrostatic conditions using 4 : 1 methanol-ethanol as pressure-transmitting medium. The calibration pressures according to the relationship p.GPa/ = 0.356[n.cm −1 / − 1332.3] are considered to be accurate within about 0.1 GPa. The convenience of using micrometer-sized diamonds as pressure sensors in Raman studies using gem anvil devices is demonstrated with several examples.

It has been argued that pressure tuning allows for unambiguous assignment of the nonperturbed bands involved in the Fermi coupling of molecular systems in the condensed phase. Here we study the pressure evolution of the Fermi resonance occurring in liquid methanol between the symmetric methyl-stretch fundamental and the methyl-bending overtones. Our analysis is based on Raman experiments in both stretching and bending fundamental regions, which are used to evaluate the effect of pressure on accidental degeneracies occurring in the vibrational spectra of liquid methanol. We emphasize that the difference in frequency of the Fermi doublet constitutes the governing quantity to determine the condition at which the exact degeneracy of the unperturbed modes occurs. Analysis based on the intensity ratio of the Fermi doublet must be disregarded. We confirm the necessity of measuring the full vibrational spectrum under pressure in order to obtain the Fermi coupling parameters unambiguously and to give a correct assignment of the bands involved in the resonance phenomenon. We also analyze the possible occurrence of several simultaneous resonance effects using a multilevel perturbation model. This model provides an appropriate description of the frequencies observed in the experiments over the whole pressure range if we consider that the main resonance occurs between nu3 and 2nu10, in contrast to previous assignments. Our global analysis leads to some general rules concerning measurement and interpretation of high-pressure vibrational spectroscopy experiments.

H u t CiipUcity ,I Light. ScatteringThe Rayleigh ratio has been obtained for the following mixtures of non-electrolytes that show "w-shape" CF vs. s curves: Nitrobenzene + n-Heptane, 1.4-Dichlorobutane t n-Heptane and Acetone + n-Dodecane. From them, the concentration-concentration correlation function S,,(O) has been calculated, as well as the excess Gibbs energy. It has been found that a clear tendency to homocoordination is present in all the mixtures studied. There is a rough correspondence between the concentration at which the maximum of the S,,(O) curves occur and those at which the negative curvature regions exist in the C: curves. Also the larger the value of max. S,,(O) the more intense thc "w-shape" effect. Despite size differences between the components and specific interactions which complicate the connection between S,,(O) and C;, an analysis in terms of simple mean field models supports the view that the systems which show "w-shape" C," curves still feel the eflects of the more or less distant UCST. Hence S,(O) is proposed as an indicator of the "w-shape" C:.Liquids / Thermodynamics