Effect of strontium substitution on the morphology of calcium phosphates crystals and polymorphism of the hydroxyapatite hexagonal (P63/m) / monoclinic (P21/c)

摘要

The production of calcium phosphate (bioceramics) by chemical precipitation is widely known. Over the past 20 years, researches have been done in the area of doping. Among these, there is research using ion strontium (Sr) dating back to the last 7 years. The growth of calcium phosphate crystals has high importance in biomineralization. This study aimed, through incorporation of strontium ions (Sr), control synthesis parameters on the formation of different morphologies and crystallography phases. For synthesis, by chemical precipitation of improved solutions with Ca/P ratio=1.67 and 19 at%, 40 at% and 53 at% of Sr ions, made in basic to acid in pH = 6,7 and 8. By XRD analysis found the brushite crystallographic phases (DCPD) monoclinic (Cc), monetite (DCP) triclinic (P-1), hydroxyapatite (HAP) monoclinic (P21/c) and hexagonal (P63/m). It was noted that the formed phase depends on the concentration of Sr in the synthesis and the influence of pH on the ionic substitutions between Sr and Ca, which result in different morphologies (plates, petals and hexagons). In summary, for both pH = 6 with no addition of Sr, 19, and 40 at% and pH 7 and 8 with addition of 40 and 53% have DCPD phases and pure or doped DCP with Sr of ～4 to ～7 at%, plates or petals and their ionic ratios of the ～1.42 at ～1.23. At pH 7 and 8, without the addition of S+ and 19% at present HAP phase (P63/m) pure or doped with inclusion of ～11 to ～14 at% as unshaped grains and ionic ratios of the ～1.54 at ～1.85, among these there are nanosphere at pH 8 to 19 at% with the inclusion of at ～14% and ratio ～1.76. In addition to these at all pH with addition of Sr wit 40 and 53 at% formed hexagonal crystals with high symmetry and advanced nucleating polymorphic phase HAP monoclinic (P21/C) with inclusions of ～18 to ～34 at % and ratio Ca+Sr/P ～1.39 to ～1.76. The relationship of the two polymorphs HAP (P63/m) and HAP (P21/c) is a complex issue related to the actual structure of pure phase (P63/m), since the geometry of twinning phase (P21/c) is only announced in the literature, but nothing is known about the relationship between these polymorphs and growth morphologies is not known experimentally. Theoretical investigations show that the polymorph P21/c is more ordered and energetically more stable than P63/m. In the samples with additions of Sr from 40 at% to 53 at% have structural stability and symmetry during the crystal growth, resulting in nucleation by superposition layers with preferential orientation in the (020) crystal lattice of HAP monoclinic (P21/c), forming a highly regular hexagonal morphology and occasionally biomimetic hexagonal crystals found in nature with high mechanical performance. The form of perfect hexagonal crystal externally disguises the complex process of its growth, wherein the polymorph (P21/c) is dominated since the nucleation phase, represented not as a single crystal, but a fine twinning texture through small areas of crystals growth which are displayed in points of pseudo-hexagonal symmetry, with an initial phase characterized by an excessive growth of twinned monoclinic polymorphs while in the second phase, there is complete formation of the single crystal to hexagonal structure. This work was achieved control on the atomic substitution process in supersaturation of Sr replacing the sites of Ca, leading to stability and structural homogeneity of the "bulk" as well as the walls of HAP monoclinic crystals (P21/c) nucleated.