Distortions in the cubic primitive high-pressure phases of calcium.

The superconductivity in highly compressed calcium involves the occurrence of closely related low-symmetry structural patterns with an exceptionally low coordination number. Earlier theoretical and experimental results are controversial and some findings are inconsistent with our later observations in the pressure range up to 60 GPa. This situation motivated the present concerted computational and experimental re-investigation of the structural arrangement of calcium slightly above the high-pressure limit of the bcc arrangement at low-temperatures.

On the stability of the disordered molecular alloy phase of ammonia hemihydrate.

The disordered-molecular-alloy phase (DMA) of ammonia hydrates [J. S. Loveday and R.

Europium-IV: an incommensurately modulated crystal structure in the lanthanides.

High-resolution x-ray powder-diffraction experiments were performed on europium metal at high pressure up to 50 GPa. At variance with previous reports, the hcp phase of Eu was observed to be stable not only to 18 GPa, but to 31.5 GPa.

Pressure-induced dehydration and the structure of ammonia hemihydrate-II.

The structure of the crystalline ammonia-bearing phase formed when ammonia monohydrate liquid is compressed to 3.5(1) GPa at ambient temperature has been solved from a combination of synchrotron x-ray single-crystal and neutron powder-diffraction studies. The solution reveals that rather than having the ammonia monohydrate (AMH) composition as had been previously thought, the structure has an ammonia hemihydrate composition.

Potassium under pressure: a pseudobinary ionic compound.

Experimentally, we have found that among the "complicated" phases of potassium at intermediate pressures is one which has the same space group as the double hexagonal-close-packed structure, although its atomic coordination is completely different. Calculations on this P6(3)/mmc (hP4) structure as a function of pressure show three isostructural transitions and three distinctive types of chemical bonding: free electron, ionic, and metallic. Interestingly, relationships between localized metallic structures and ionic compounds are found.