Structure of a Calcium-deficient Apatite

Objectives: Bones and tooth mineral have variable Ca/P ratios ascribed to phosphate ions adsorbed on crystal surfaces or loss of calcium ions from the crystal lattice. The aim was to compare the structure of a well-crystallised calcium-deficient apatite (Ca-def Ap), Ca/P mol ratio 1.5, with that published for hydroxyapatite (Ca/P=1.667). Methods: Nine samples of Ca-def Ap were prepared from suspensions of CaHPO₄ at 90°C by raising the pH (initially ~4) through slow release (72h) of NH₃ from urea. Products were dried at 100°C for 24h and studied by chemical analysis, X-ray powder diffraction (and Rietveld analysis of this data), SEM, He pycknometry, ¹H and ³¹P MAS NMR spectroscopy and Raman and FTIR spectroscopy. Results: All samples contained apatite, but three also contained CaHPO₄. IR and Raman confirmed the presence of HPO₄^2- and absence of carbonate. Mean results for the six pure samples were: a=9.4320(40), c=6.8751(31)Å (hydroxyapatite a=9.4243(55), c=6.8856(35)Å; unit cell formula neglecting protonation of phosphate ion, Ca_9.303(PO₄)₆(OH)_0.606.2.0H₂O, theoretical density 3.10g cm^-3; experimental density (mean for 3 samples) 3.15g cm^-3: Ca/P mole ratio from chemical analysis and phase analysis after heating to 900°C, 1.550(8) and 1.550(2) respectively. Occupancies from Rietveld structure refinements indicated a preferential loss of Ca from Ca2 sites compared with Ca1, but losses were substantially smaller than expected from chemical analyses. We suggest that imperfect modelling of the structure in the refinement, particularly disorder associated with Ca2 sites resulted in errors in Ca2 occupancies. The P-O bonds were slightly shorter than in OHAp, rather than longer as might be expected from protonation of phosphate tetrahedra. However, known acid phosphate structures indicate it is unlikely that increases in P-O lengths would be detected. The observed decrease is tentatively assigned to the presence of Ca²⁺ ion vacancies. Conclusions: Ca-def Aps differ structurally from hydroxyapatite in specific detail.