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Neutron diffraction data from sub- and supercritical CO2, collected over three different experimental runs, are analysed together, in order to investigate the evolution of the microscopic structure with the thermodynamic parameters. The relative orientations of neighbouring molecules are analysed also in terms of the angular radial correlation function, obtained by molecular dynamics simulation runs compatible with the total neutron weighted radial distribution function. Orientational correlations between neighbouring molecules are seen to persist across the whole thermodynamic range investigated. However, these are limited to the first neighbouring shell, and mainly ascribable to the quadrupolar interaction.

In this paper we present a thorough study of the local microscopic structure of carbon dioxide in a wide thermodynamic region, covering both undercritical and supercritical states. From neutron diffraction measurements we derive accurate information on the neutron weighted distribution function,gnw (τ), which allows us to single out a realistic interaction potential to be used in Molecular Dynamics (MD) simulations. These MD simulations are then performed to supplement the information obtained from the neutron scattering experiments. In such a way we determine appropriateaverages of the molecular distribution function,g(R, ω1,ω2). The presence of preferred relative orientations between neighbouring molecules is evident. We find that, among first neighbouring molecules,T-shaped configurations are largely preferred in all the investigated thermodynamic states and that electrostatic interactions play a crucial role in determining the local structure of the fluid.

The study of spatial features in diatomic liquids and gases is reviewed with a specific emphasis on experimental measurements using neutron and X-ray diffraction. The formalism for defining the pair-correlation function is described and related to the liquid structure factor which can be extracted from the diffraction measurements. Different molecular systems can be classified in terms of the intermolecular potential which depends on various factors related to geometrical conformation, electron density distribution and anisotropy. The increased complexity over monoatomic systems gives rise to new phenomena associated with the orientational correlation of molecular axes. The techniques of neutron and X-ray diffraction are critically reviewed and a full list of published work (plus some unpublished results) is presented. The experimental measurements are analysed and compared with the results of computer simulations using model potentials. It is noted that several systems (e.g. N2, O2, F2, CO) have a relatively simple behaviour but others (e.g. Cl2, Br2, I2) are only just on the point of being properly understood, while the hydrogen-bonded liquids (e.g. HF, HCl) still pose many problems that currently remain unresolved. The review ends with a brief consideration of likely developments in the foreseeable future.

A new experimental method which allows very accurate and complete determination of phase diagram in pure components and mixtures is presented. It employs the technique of absorption spectroscopy by a polychromatic neutron beam from a pulsed source. This method is applied to the measurement of the coexistence curve in pure Xe and in a 3 HeXe mixture near the gas-gas demixing point.

We report neutron diffraction measurements for two Ne-Xe mixtures at $T=275$ K and $P=26$ MPa, having a Xe molar fraction of 0.81 and 0.45, respectively. These thermodynamic points are chosen to be in the single phase region, close to the gas-gas demixing surface. Total structure factors and neutron weighted pair distribution functions have been extracted. Partial pair distribution functions ${g}_{\mathrm{ij}}(r)$ have been calculated by means of molecular dynamics simulation using Lennard-Jones interaction potentials and the Lorentz-Berthelot mixing rules, obtaining a good agreement with the experimental data. These partial pair distribution functions have been analyzed in terms of density expansion and also compared with those derived in a previous experiment on a He-Xe mixture. The main difference found between the structural properties of the two mixtures is that, in the Ne containing mixture, a definitely larger fraction of atoms of the lighter species occupies the Xe-Xe first neighbor shell. The model already used in interpreting the thermodynamic behavior of the He-Xe mixture in terms of microscopic properties suggests that this structural difference is related to the different behavior of the pressure-temperature projection of the critical line in the two mixtures.

Neutron-diffraction experiments on liquid DI, HI, and on an equimolar mixture of HI and DI are presented. All these three samples were in the same thermodynamic state corresponding to the orthobaric liquid at T=253 K. The three partial structure factors ${\mathit{S}}_{\mathrm{II}}$(Q), ${\mathit{S}}_{\mathrm{HI}}$(Q), ${\mathit{S}}_{\mathrm{HH}}$(Q) are derived exploiting the standard isotopic substitution procedure. The corresponding pair correlation functions ${\mathit{g}}_{\mathrm{II}}$(r), ${\mathit{g}}_{\mathrm{HI}}$(r), and ${\mathit{g}}_{\mathrm{HH}}$(r) are evaluated and compared with those given by a model that neglects all orientational correlations. Our data indicate that ${\mathit{g}}_{\mathrm{II}}$(r) (which is essentially the center-center correlation function) is well reproduced by the pair distribution function of a monatomic Lennard-Jones fluid and that ${\mathit{g}}_{\mathrm{HI}}$(r) (which should be sensitive to the correlations between molecular and intermolecular axes) is very similar to the one derived neglecting orientational correlations. On the contrary, orientational correlations between the molecular axes are clearly present in the ${\mathit{g}}_{\mathrm{HH}}$(r), which deviates significantly from the uncorrelated model results. These facts are consistent with the idea that H bonding is not present in liquid HI and indicate also that the only relevant terms of the anisotropic intermolecular potential are those due to the electric multipolar interactions.

Neutron diffraction measurements on orthobaric liquid chlorine at four thermodynamic states, almost equally spaced from triple up to critical point, are reported. The data are analyzed to extract information on the orientational correlations. The orientational structure is discussed with respect to the best available intermolecular potential. Comparison with other diatomic liquid is presented.

The quantum effect on the radial distribution function of a liquid has been analyzed by means of the corresponding states principle. Also the best mode has been discussed for describing liquid Helium.

Magnetization‐vs‐temperature curves were determined in MnNi3 for different states of order in the temperature range −196° to 375°C and for fields up to 4000 Oe. Neutron‐diffraction analysis has been performed on the same specimens in order to determine both the average‐order parameter S and the size of ordered regions. The results are compared with the ones available in the literature and discussed in terms of details of the state of order.

The diffusion coefficient of A and HT in liquid N2 have been measured by using the capillary method, at low concentrations, at constant pressure (about 70 cm Hg), within a range of temperature from 67 to 76oK. The results can be expressed by the following equations $$\begin{gathered} D\left( {A - N_2 } \right) = 730{\text{ exp}}\left[ { - \frac{{260}}{T}} \right] \cdot 10^{ - 6} cm^2 /s, \hfill \\ D\left( {HT - N_2 } \right) = 1260{\text{ exp}}\left[ { - \frac{{260}}{T}} \right] \cdot 10^{ - 6} cm^2 /s, \hfill \\ \end{gathered} $$ These results are discussed in terms of the dependence of the diffusion coefficient upon the molecular parameters of the different substances.

A simple expression of the diffusion coefficient as function of the temperature at constant pressure for mixtures of perfect liquids previously suggested by us has been checked in the case HT-liquid Argon. The value obtained by the capillary method adopted for low temperature work isDcxp(HT-A)=(3.17 ± 0.15)10−5 cm2/s atT=84.5°K in satisfactory agreement with the calculated valueDca’c (HT-A)=2.94 10−5 cm2/s.

Measurements of the mobility of positive and negative charges in the $^{3}\mathrm{He}$ critical region are reported. The experimental data show that for the two probes there is a definite lowering of the mobility at the critical point. However, within the limits of our approach to the critical point ($\frac{\ensuremath{\Delta}T}{{T}_{c}}\ensuremath{\simeq}6\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$; $\frac{\ensuremath{\Delta}p}{{p}_{c}}\ensuremath{\simeq}3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}$), no evidence of dramatic decrease in the mobility is observed. The decrease in the mobility is $\ensuremath{\approx}30%$ for either probes; however, the influence of the critical phenomena on the shape of the mobility curve differs for the two probes probably because the positive and negative carriers differ in their effective radius. Some indication of the correct value of the $^{3}\mathrm{He}$ critical temperature is also reported.

Transport phenomena are the irreversible decay processes that occur spontaneously when a system, pertubed to a non equilibrium state by some external or internal disturbance, advances through a series of non equilibrium states until equilibrium is reached. A system in absence of external forces is in equilibrium the intensive state variable (for example density, local velocity, temperature, composition etc.) are constant at all times and have the same magnitude at all position in the system. Therefore at equilibrium there are no gradients of the state parameters in the system and there is no net flux of matter or momentum or energy through the system. If the system is perturbed to a non equilibrium state at least one state parameter becomes function of position and spontaneously fluxes of the appropriate thermodynamic quantities are generated by these gradients. In these lectures we will be restricted to the transport process called isothermal isobaric diffusion i. e. the transport process we have in a multicomponent system, in absence of external forces at mechanical equilibrium and at uniform temperature, when a composition gradient is present.

The structure factors for two thermodynamic states of the mixture He-Xe have been measured by neutron diffraction. These two thermodynamic states are the coexisting ones at the phase transition of the gas-gas demixing process. The Xe-Xe correlations, in the mixture, appear to be similar to those in pure Xe near the coexistence curve and having the same Xe number densities. These results suggest that this phase transition is of the liquid-gas type.

The self‐diffusion coefficient of liquid argon has been measured by the capillary technique at 84.56±0.04 °K and at about 70 cm Hg. The average value of D(A37—A40) is (1.53±0.03) 10−5 cm2/sec.

Polarized neutrons were used to determine the magnetic moments of the MnNi3 alloy. The ratio of the magnetic to the nuclear scattering amplitude was determined from the polarization ratio for a given reflection. The magnetic moments of Mn and Ni were calculated by using the magnetic form factors of metallic Ni and the Mn2+ ion. The average magnetic moment per atom was found to be approx equal l mu BETA, in agreement with experimental results.

Neutron diffraction data on liquid H2S, D2S and an equimolar mixture of the two are presented for two thermodynamic states along the liquid—vapour coexistence curve. Comparison of the site-site radial distribution functions obtained with those of a model fluid where orientational correlations are absent and with those of water demonstrates that, although hydrogen sulphide does not form hydrogen bonds, nevertheless weak orientational correlations between first neighbouring molecules are detectable in both thermodynamic states. An analysis of the molecular configurations compatible with the experimental data, as calculated by applying the empirical potential structural refinement, is presented also.