Lattice Liquid Research Articles

Microscopic variational lattice liquid cluster model for ionic correlations inα−AgI

A mean-field model for mobile ion configurations in $\ensuremath{\alpha}\ensuremath{-}\mathrm{A}\mathrm{g}\mathrm{I}$ is proposed that permits the first simultaneous calculation of the site-disorder entropy and the ion diffusion activation energy. The model uses only one adjustable parameter, the mobile-ion effective charge, and it is in very good agreement with experiment. It shows that the mobile ions should be regarded as a lattice liquid and not a lattice gas.

Lattice parameter and57Fe isomer shift in PdHx: evidence for two new phases

Pd lattice parameter and 57Fe isomer shift (IS) measurements have been correlated in PdHx-Fe matrices (0.3 at.% Fe) over a wide range of hydrogen concentration in the alpha ' (lattice liquid), and in the beta and alpha phases detected previously by Mossbauer measurements. The authors find that the IS is proportional to the logarithm of the unit cell volume, IS varies as 3 ln a0, just as in IS measurements under pressure. The experimental value of the slope, +3.92 mm s-1, is however much larger than that found by pressure measurements in similar matrices. This shows that the effect in PdHx is of electronic origin, and gives support to the band calculations which have predicted partial localisation of hydrogen electrons in a Pd-H bond.

Local moment hyperfine studies of the Kondo system PdHxFe0.003

The effect of hydrogen on the Fe giant moment in dilute PdFe and the resulting single moment Kondo and RKKY interactions in PdHxFe is investigated by Mössbauer hyperfine techniques in an external magnetic field. The results show that within a large range of composition within the α′ phase (hydrogen lattice liquid), μ(Fe) remains constant at about 3.9μB, smaller than the 12.6μB in dilute PdFe. The Kondo temperature TK is found to increase with hydrogen, as determined from the usual X=μH/k(T+ϑ) modification to the Brillouin function B1(X) where ϑ=4.5 TK. The dynamic part of the impurity interactions leads to a spin-glass–type spin-spin relaxation, which produces a line broadening in a magnetic field much larger for the outer Mössbauer lines and with a maximum in field at about X=1. At higher concentrations (x=0.76), the saturation HF field decreases, indicating a possible orbital component on μ(Fe); this conforms with certain recent calculations. Above x=0.8, new Mössbauer lines are found, indicating the phase transitions and two-phase regions α′ → β and β → α″ with increasing hydrogen content.

Abstract: Local moment hyperfine studies of the Kondo system PdHxFe0.3%

The effect of hydrogen on the Fe giant moment in dilute Pd Fe and the resulting single moment Kondo and RKKY-interactions in Pd Hx Fe is investigated by Mössbauer hyperfine techniques in an external magnetic field. The results show that within a large range of composition within the α′ phase (hydrogen lattice liquid), Fe remains constant at about 4.2μB smaller than the 12.6 μB in dilute Pd Fe. The Kondo temperature TK is found to increase with hydrogen, as determined from the usual X=μH/k (T+ϑ) modification to the Brillouin function B1 (X) where ϑ=4.5 TK. Deviations from this modified B1 (X) have been related to the static part of the impurity interactions, and a molecular field approximation spin glass calculation is done to compensate for this. The dynamic part of the impurity interactions leads to a spin glass-type spin-spin relaxation, which produces a line broadening in magnetic field much larger for the outer Mössbauer lines and with a maximum in field at about X=1. At higher concentrations (x=0.75), the saturation HF field decreases, indicating a possible orbital component on μ (Fe), conform with certain recent calculations. Above x=0.8, new Mössbauer lines are found, indicating the phase transitions and two-phase regions α′→β and β→α″ with increasing hydrogen content. The associated changes in line position are very large, about 0.18 and 0.22 in mm/sec, in the sense of lattice expansion. The advantage of hyperfine measurements is shown by the detail of the information obtained, and especially in the possibility of separating single moment from collective effects.

Argon Adsorbed on a Graphitic Surface. The Heat of Adsorption from Lattice Liquid Theory

A model is proposed to account for the differential or isosteric heat of adsorption which has been observed for argon on graphon at concentrations between 0.5 and 2.0 monolayers for temperatures in the neighborhood of the normal boiling point of argon. The energy of a molecule on the surface is considered to be the sum of two contributions: (1) a site energy for the isolated molecule and (2) an interaction energy with neighbors in the same layer. The site energy for a molecule is calculated assuming atoms and molecules in lower layers fixed with respect to both a configuration in a hexagonally packed planar lattice and the distance between layers. The interaction energy is calculated assuming the behavior in any layer is represented by a two dimensional lattice liquid model utilizing the so-called Ono approximation for the variation of the generalized free volume with vacancies in the lattice. The distribution of adsorbed molecules in any layer is considered to be completely random. On the basis of this model, an interpretation is made of the two maxima in the isosteric heat which have been observed experimentally.