Abstract
An X-ray-diffraction study under pressure has shown that BaO, which has the NaCl-type structure at standard temperature and pressure, transforms to a tetragonal structure with a = 4.459 ± 0.008 A, c = 3.606 ± 0.006 A, and Z = 2 at 92 ± 3 kb and 23 ± 3 °C. At 140 ± 5 kb, it transforms to the tetragonal PH4I-type structure (space group P4/nmm). At 180 kb, the BaO in this structure has a = 4.397 ± 0.007 A, c = 3.196 ± 0.005 A, Z = 2, and u = 0.40. These two phase changes are reversible, and the volume changes at the transitions are approximately 5% and 7%, respectively. Using F. Birch's equation of state with an assumed value of 6 for (∂KT/∂P)T, the initial isothermal bulk modulus K0 has been calculated to be 0.69 ± 0.03 Mb for the phase with the NaCl-type structure. The axial ratios c/a for the high-pressure phases of BaO appear to be independent of pressure within experimental error.
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