Precipitates formed at an early stage (during the first 6 h) of the hydroxyapatite crystallization of a solution were studied. A nitrous synthesis was used (0.583M (NH(4))(2)HPO(4) and 0.35 M Ca(NO(3))(2).4H(2)O solutions at pH 11-12, 21 degrees C, fast mixing, lyophilization of aliquots). Although XRD patterns indicated an amorphous calcium phosphate (ACP), IR spectra revealed apatite nanocrystals in the precipitates. Some amount of free calcium was found in the mother solution by mass spectrometrical analysis of the aliquots. This amount considerably decreased as the synthesis proceeded, however, the decrease had a slight effect on the crystallinity of the precipitates. A new suggestion on the nature of delayed crystallization (under conditions as those in the present study) was proposed. The free calcium adsorbed by the nanoparticles from the solution formed a shell around a particle because the calcium diffusion into the bulk was poor at the low synthesis temperature. As such, the encapsulation delayed the crystallization of the nanoparticles. Evidence for this suggestion was given. New possibilities were proposed for preparation of bioactive materials of desired composition based on the structural and compositional peculiarities of the X-ray diffraction-amorphous calcium phosphates.

Lattice and surface impurity reactions and structural changes induced by them in slightly carbonated hydroxyapatite (SCHA) treated at 25-1100 degrees C were comprehensively studied. The SCHA was processed by a conventional wet synthesis at a high possible temperature(96 degrees C) using ammonium containing parent reagents. IR-spectroscopy, XRD, TG-DTA technique and mass spectrometric thermal analysis (MSTA) were employed for characterization of the samples. NH4+ with H3O+ in cationic-and CO3(2-) (A- and B-positions) with HPO4(2-) in anionic sites, and H2O, CO3(2-)(HCO3(-)) NO3(-), NxHy on the surface of particles were found and considered as impurity groups. Complicated changes in lattice constants of theSCHA stepwise annealed in air (for 2 h) were revealed; the changes were associated with reactions of the impurity groups. Filling the hexed sites with hydroxyl ions above 500 degrees C was shown to happen partly due to lattice reactions but was mainly owing to hydrolysis of the SCHA by water molecules in air. Decomposition of CO3(2-) groups proceeded through both thermal destruction and reactions with some of the impurity ions. The decarbonation in A-sites occurred at much lower temperatures (450-600 degrees C) than in B-sites (700-950 degrees C) and was first revealed to happen in two stages: due to an impurity reaction around 500 degrees C, and then through thermal destruction at 570 degrees C. A redistribution of CO3(2-) ions, decreasing in amount on the whole, was observed upon annealing above 500 degrees C. To avoid possible erroneous conclusions from TG-data, a sensitive method was shown to be required for monitoring gaseous decomposition products (such as the MSTA in this study), in case several impurity groups were present in a SCHA.

Nanocrystalline calcium phosphate was precipitated at 20 C from calcium nitrate and ammonium phosphate solutions at a pH of 9 -11. The nature of the mass loss steps in thermogravimetry of the product was revealed by in-situ mass spectrometry of released gases upon heating. Major amounts of nitrate-containing impurities like NH 4 NO 3 and Ca(NO 3 ) 2 were present which could only be removed by multiple washing with water. These impurities were not crystalline, but they were clearly detectable in the infrared spectrum. In conclusion, the high specific surface area of freshly precipitated calcium phosphate may cause the adsorption of considerable amounts of foreign compounds (molecules and ions) in the percent mass range.

The influence of foreign ions on the crystal lattice of hydroxyapatite (precipitated from aqueous solution by a conventional method at 96 C with a given ratio of Ca/P=1.67:1) was studied during heating in air in the temperature range of 25-1250 C. The lattice constants decreased to those of stoichiometric hydroxyapatite by a complex dependence during heating. This dependence was correlated to a simultaneous release of gaseous products. These products resulted from the decomposition of foreign ions which were incorporated into the hydroxyapatite lattice during precipitation, i.e. ammonium, nitrate and carbonate. Stoichiometric hydroxyapatite with correct lattice constants was obtained at 1100 C. Further heating to 1250 C resulted in only minor changes in the lattice characteristics.

Thermal evolution of amorphous calcium phosphate (ACP) powder from a fast nitrate synthesis with a Ca/P ratio of 1:1 were studied in the range of 20-980 °C. The powder consisted of amorphous dicalcium phosphate anhydrate (CaHPO) after heating to 200 °C. CaHPO gradually condensed to amorphous calcium pyrophosphate CaPO (CPP) between 200 to 620 °C. Amorphous CPP crystallized at 620-740 °C to a metastable polymorph α'-CPP of the high-temperature phase α-CPP and β-CPP. The α'-CPP/ β-CPP phase ratio reached a maximum at 800 °C (60 wt% α'-CPP/40 wt% β-CPP), and α'-CPP gradually transformed to β-CPP at a higher temperature. Some β-TCP occurred at 900 °C, so that a three-phasic mixture was obtained in the powder heated to 980 °C. The occurrence of metastable α'-CPP is attributed to Ostwald's step rule, and a mechanism for β-TCP formation is proposed. The advantages of prospective biomaterials from these powders are discussed.