Compression of witherite to 8 Gpa and the crystal structure of BaCO3II

Natural witherite (Ba0.99Sr0.01CO3) has been studied by single-crystal X-ra y diffraction in the diamond anvil cell at eight pressures up to 8 GPa. At ambient pressure, cell dimensions are a = 5.3164(12) Angstrom, b = 8.8921(1 9) Angstrom, c = 6.4279(16) Angstrom, and the structure was refined in spac e group Pmcn to R(F) = 0.020 from 2972 intensity data. The unit cell and at om position parameters for the orthorhombic cell were refined at pressures of 1.2, 2.0, 2.9, 3.9, 4.6, 5.5. 6.2, and 7.0 GPa. The volume-pressure data are used to calculate equation of state parameters K-T0 = 50.4(12) GPa and K' = 1.9(4). At approximately 1.2 GPa, a first-order transformation to spa ce group P31c was observed. Cell dimensions of the high-pressure phase at 7 .2 GPa are a = 5.258(6) Angstrom, c = 5.64(1) Angstrom. The high pressure s tructure was determined and refined to R(F) = 0.06 using 83 intensity data, of which 15 were unique. This high-pressure phase appears to be more compr essible than the orthorhombic phase with an estimated initial bulk modulus (K-7.2GPa) of 10 GPa.