Title:Formation of Calcium Phosphate Apatite in System CaO-P2O5-H2O:
Equilibrium at 298 K Under a Nitrogen Atmosphere
Volume: 2
Author(s): Marina V. Chaikina and Natalia V. Bulina*
Affiliation:
- Institute of Solid State Chemistry and Mechanochemistry, Siberian Branch of Russian Academy of Sciences, 18
Kutateladze st., Novosibirsk 630117, Russia
Keywords:
Calcium phosphate system, equilibrium, hydroxyapatite formation, nitrogen atmosphere, apatite compounds, Miller- Kenrick method.
Abstract:
Aims: The formation of calcium phosphate apatite (hydroxyapatite, carbonate-containing
hydroxyapatite, fluorapatite and carbonate-containing fluorapatite) in aqueous systems has been studied
for over a century.
Background: However, in the region of low concentrations of liquid phases, the question of the nature,
composition and region of existence of apatite compounds remains controversial.
Objective: The results of studying the phase equilibrium in the system CaO-P2O5-H2O at 298 K in the
isotherm region from the invariant point of dicalcium phosphate and monocalcium phosphate monohydrate
to the lowest concentrations of the liquid phase components are presented.
Methods: Chemical analysis, thermogravimetry, IR spectroscopy and optical microscopy were used for
the analysis.
Results: Long-term monitoring of the establishment of equilibrium (up to 20 months) resulted in the determination
of regions of stable solid phases of calcium orthophosphates, calcium-deficient apatites, hydroxyapatite,
and apatite with (Ca/P)at >1.67. Two types of calcium-deficient apatite were identified that
differ in the (Ca/P)at ratio: the first type is 1.33 < (Ca/P)at ≤ 1.5; the second one is 1.5 ≤ (Ca/P)at < 1.67.
Conclusion: The invariant points of calcium orthophosphates and compounds with the apatite structure
were determined. The diagram was constructed using the Miller-Kenrick method based on obtained
experimental data, which confirms the established regions and invariant points of stable equilibrium
solid phases of the studied ternary system. The data obtained can be useful for understanding the processes
of formation and change of compounds with apatite structure.