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 devia tions 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 ph ase. At higher temperatures this anomaly decreases and the c/a ratio a pproaches the ideal value of hcp Ba-V. This result together with the o bservation 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 dif ferent second hcp phase Ba-V at higher pressures and ambient temperatu res.