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

The synthetic solid solutions between lead fluorapatite and lead fluorvanadate apatite, Pb-10[(PO4)(6-x) (VO4) (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 PO4 (3-) cations by the VO4 (3-) cations progresses, the isothermal bulk modulus steadily decreases, with a maximum reduction of about 16% (from 68.4(16) GPa for Pb-10(PO4)(6)F-2 to 57.2(28) GPa for Pb-10(VO4)(6)F-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[(PO4)(6-x) (VO4) (x) ]F-2 become more elastically anisotropic.Materials Science, MultidisciplinaryMineralogySCI(E)EI0ARTICLE3219-2263