Computer modelling techniques have been employed to qualitatively and quantitatively investigate the
dehydration of
hydroxyapatite to oxyapatite and the defect chemistry of
calcium-deficient
hydroxyapatite, where a number of vacancy formation reactions are considered. The
dehydration of
hydroxyapatite into
oxyhydroxyapatite is calculated to be endothermic by E = +83.2 kJ mol(-1) in agreement with experiment, where thermal treatment is necessary to drive this process.
Calcium vacancies are preferentially charge-compensated by
carbonate ions substituting for
phosphate groups (E = -5.3 kJ mol(-1)), whereas charge-compensating reactions involving PO4 vacancies are highly endothermic (E > or =652 kJ mol(-1)). The exothermicity of the charge compensation of a Ca vacancy accompanied by a PO4/CO3 substitution agrees with their co-occurrence in natural bone tissue and tooth enamel. Our calculations of a range of defect structures predict (i) that
calcium vacancies as well as substitutional
sodium and
potassium ions would occur together with
carbonate impurities at
phosphate sites, but that other charge compensations by replacement of the
phosphate groups are unfavourable, and (ii) that the hydroxy
ions in the channel are easily replaced by
carbonate groups, but that the formation of water or
oxygen defects in the channels is thermodynamically unfavourable. Calculated elastic constants are reported for the defect structures.