Refinement of the P-T phase diagram of barium

The unusual c/a ratio of the Ba-II/``III'' hcp phase was investigated as a function of pressure at different temperatures. The largest deviations from an ideal c/a ratio for hcp was observed at room temperature just before the phase transition from the hcp to the complex Ba-IV phase. At higher temperatures this anomaly decreases and the c/a ratio approaches the ideal value of hcp Ba-V. This result together with the observation of a positive slope for the T-P phase boundary from Ba-II/``III'' to Ba-IV support the view that Ba-IV should be considered only as a low temperature phase which separates the first hcp phase usually labeled Ba-II/``III'' at moderate temperatures from the seemingly different second hcp phase Ba-V at higher pressures and ambient temperatures.