Solid solutions between lead fluorapatite and lead fluorvanadate apatite: Compressibility determined by using a diamond-anvil cell coupled with synchrotron X-ray diffraction

摘要

The synthetic solid solutions between lead fluorapatite and lead fluorvanadate apatite, Pb _(10)[(PO _4) _(6-x)(VO _4) _x]F _2 with x equal to 0, 1, 2, 3, 4, 5, and 6, were compressed up to about 9 GPa at ambient temperature by using a diamond-anvil cell coupled with synchrotron X-ray radiation. A second-order Birch-Murnaghan equation of state was used to fit the data. As the substitution of the PO _4 ~(3-) cations by the VO _4 ~(3-) cations progresses, the isothermal bulk modulus steadily decreases, with a maximum reduction of about 16% (from 68.4(16) GPa for Pb _(10)(PO _4) _6F _2 to 57.2(28) GPa for Pb _(10)(VO _4) _6F _2). For the entire composition range, the a-axis dimension remains more compressible than the c-axis dimension, with the ratio of the axial bulk moduli (K _(T-c):K _(T-a)) larger than 1. The ratio of K _(T-c) to K _(T-a) increases from about 1.04(4) to 1.23(14) as the composition parameter x increases from 0 to 6, suggesting that the apatite solid solutions Pb _(10)[(PO _4) _(6-x)(VO _4) _x]F _2 become more elastically anisotropic.