SOLIDS MODELED BY AB-INITIO CRYSTAL-FIELD METHODS .12. STRUCTURE, ORIENTATION, AND POSITION OF A-TYPE CARBONATE IN A HYDROXYAPATITE LATTICE

Using ab initio methods, the position and the orientation of a CO32- i mpurity in a hydroxyapatite lattice is studied. The CO32- ion is orien ted in a plane that makes an angle of 7 degrees with the crystallograp hic c axis. The carbon atom and two oxygen atoms occupy positions clos e to the axis, whereas the third oxygen is oriented almost perpendicul ar to the symmetry axis. For the optimized structure, the vibrational characteristics are in reasonable agreement with the experimental data . The influence of experimentally observed variations of the lattice p arameters on the geometry of the CO32- impurity is negligible. A study of the potential energy surface indicates that the six symmetry-relat ed equilibrium positions are connected by rotation about the crystallo graphic c axis.