Volume 29, Issue 9, September 2003
Index of content:
- HIGH-PRESSURE STUDIES
29(2003); http://dx.doi.org/10.1063/1.1614172View Description Hide Description
At low pressures the ortho-para conversion in and is a slow process governed by the magnetic dipole interaction of the nuclear magnetic moments,phonons being the main energy sink. As the pressure is raised to a few GPa and the Debye temperature increases substantially, the conversion energy finds itself in an area where phonon states are depleted and conversion slows down. The recent Raman and NMR experiments showed that the conversion rate in after an initial slowdown predicted by theory, increases immensely. As to solid the conversion rates have apparently not yet been directly measured under pressure. In order to explain the anomaly observed in we have suggested a new conversion mechanism, in which the basic conversion-producing interaction only initiates conversion, whereas the energy is removed by rotational excitations via the stronger electric quadrupole–quadrupole interaction. Estimated conversion rates are in good qualitative agreement with available experimental observations. Here we extend the theory to solid taking into account the differences between and in the molecular and solid-state parameters. The new libron-mediated channel is predicted to result in conversion rates for under pressure that are an order of magnitude larger than at
29(2003); http://dx.doi.org/10.1063/1.1614173View Description Hide Description
The electronic energy bands in structures whose primitive cell contains up to four molecules are studied with full optimization of the structures on the basis of GGA and LDA band calculations. Above 250 GPa, the eventual optimal structure obtained by the GGA or the LDA calculation is which is a layered structure with the molecular bonds lying in planes and which has a metallic band structure with no band gaps. The metallic property of the band structure remains unchanged even if the molecular bonds in the plane of the are inclined so that the atoms in the molecule lie out of the plane. The electronic bands of the structure and those of some other candidate structures are discussed in the light of recent experimental results. Effects of the occupation of electronic states on the predicted optimal structures are also studied.
Influence of vibrational anharmonicity and vacancies on the thermodynamic properties of rare gas crystals29(2003); http://dx.doi.org/10.1063/1.1614174View Description Hide Description
A statistical method is used to calculate thermodynamic properties of Ar, Kr and Xe (isobaric and isochoric heat capacity,bulk modulus,thermal expansion coefficient, interatomic distances, Grüneisen parameter), and good agreement with experimental values is observed. It is shown that at high temperature, slightly above the melting point of the rare gas crystals, an instability of the crystalline state occurs. As the temperature approaches this instability, the isobaric heat capacity and the thermal expansion coefficient show strong increases similar to the experimentally observed anomalies.