High-pressure structural study of barium to 90 GPa

The crystal structure of Ba has been studied under pressures of up to 90 GPa at room temperature using powder x-ray-diffraction techniques. Phases I and II have been perfectly explained by the bcc and hcp structures, respectively. However, we obtained no evidence for the existence of phase III, which has been previously reported between 8 and 12.6 GPa at room temperature. The powder pattern of phase IV was very spotty and its structure has remained unsolved. The structure of phase V above 45 GPa has been confirmed to be hcp. Ba therefore has two hcp phases under pressure: phase II (5.5--12.6 GPa) and phase V (above 45 GPa). The c/a ratio of phase II decreases dramatically with pressure, while that of phase V is pressure-independent with a value of 1.575. The mechanisms to stabilize the two hcp structures are expected to be very different.