High-pressure Raman measurements of solid Kr have been performed to investigate the face-centered-cubic ͑fcc͒ to hexagonal-close-packed ͑hcp͒ phase transition up to 75 GPa at room temperature. The E 2g phonon mode, which is Raman active in the hcp phase, appears around P = 20 GPa at wave number of 90 cm −1 . The Raman frequency increases to 145 cm −1 at 75 GPa, which is in good agreement with analytical calculations based on the semiempirical many-body potential. The intensity of the Raman band increases with pressure, indicating the growth of the hcp phase. This result is compared with the previous x-ray study for the coexistence of fcc and hcp phases in Kr. Elastic shear modulus C 44 in the hcp phase was estimated from the vibrational frequency of the E 2g phonon, and compared with the recent theoretical results for solid Kr and Xe.
The multiple isotopic family of hydrogens (H 2 , HD, D 2 , HT, DT, T 2 ) due to large differences in the de Boer quantum parameter and inertia moments displays a diversity of pronounced quantum isotopic solid-state effects. The homonuclear members of this family (H 2 , D 2 , T 2 ) due to the permutation symmetry are subjects of the constraints of quantum mechanics which link the possible rotational states of these molecules to their total nuclear spin giving rise to the existence of two spin-nuclear modifications, ortho-and parahydrogens, possessing substantially different properties. Consequently, hydrogen solids present an unique opportunity for studying both isotope and spin-nuclear effects. The rotational spectra of heteronuclear hydrogens (HD, HT, DT) are free from limitations imposed by the permutation symmetry. As a result, the ground state of these species in solid state is virtually degenerate. The most dramatic consequence of this fact is an effect similar to the Pomeranchuk effect in 3 He which in the case of the solid heteronuclear hydrogens manifests itself as the reentrant broken symmetry phase transitions. In this review article we discuss thermodynamic and kinetic effects pertaining to different isotopic and spin-nuclear species, as well as problems that still remain to be solved.
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