High-Pressure Study on Lead Fluorapatite

The compressional behavior of a synthetic lead fluorapatite (Pb9.35(PO4)6F2) has been investigated in situ up to about 16.7 GPa at 300 K, using a diamond-anvil cell and synchrotron X-ray diffraction. We find that the compressibility of lead fluorapatite is significantly different from that of fluorapatite (Ca10(PO4)6F2), chlorapatite (Ca10(PO4)6Cl2), and hydroxylapatite (Ca10(PO4)6(OH)2): lead fluorapatite is much more compressible, and elastically isotropic in the investigated pressure range. The pressure-volume data fitted to the third-order Birch-Murnaghan equation yield an isothermal bulk modulus (KT) of 54.3(18) GPa and the pressure derivative (KT') of 8.1(6). If KT' is fixed at 4, the obtained KT is 68.4(16) GPa, which is approximately only two-thirds of the isothermal bulk modulus of the calcium apatites.