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[(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.