ABSTRACT
Experimental evidences are presented showing unusually large and highly anisotropic vibrations in the "simple cubic" (SC) unit cell adopted by calcium over a broad pressure ranging from 30-90 GPa and at temperature as low as 40â K. X-ray diffraction patterns show a preferential broadening of the (110) Bragg reflection indicating that the atomic displacements are not isotropic but restricted to the [110] plane. The unusual observation can be rationalized invoking a simple chemical perspective. As the result of pressure-induced s â d transition, Ca atoms situated in the octahedral environment of the simple cubic structure are subjected to Jahn-Teller distortions. First-principles molecular dynamics calculations confirm this suggestion and show that the distortion is of dynamical nature as the cubic unit cell undergoes large amplitude tetragonal fluctuations. The present results show that, even under extreme compression, the atomic configuration is highly fluxional as it constantly changes.
ABSTRACT
Angle dispersive x-ray diffraction measurements of oxygen single crystals in a helium pressure transmitting medium have been performed up to 133 GPa at room temperature. The crystalline structure of metallic zeta oxygen, above 96 GPa, is shown to be associated with a continuous displacive structural transformation taking place in the ab plane of the monoclinic C2/m cell of the insulator epsilon phase. Although of first order, the change of crystalline structure to the metallic zeta phase from the epsilon phase is found to be isosymmetric, in agreement with recent calculations. Our Raman spectroscopy measurements on a single crystal of the zeta phase corroborate the present structure assignment. The interplay between metallization and structural changes is disclosed.
