Structural interplay between strontium and calcium in α-CaHPO4 and β-SrHPO4

The ability of strontium ion to inhibit the abnormally high bone resorption, which occurs in pathologies characterized by loss of bone substance, has stimulated a number of research on strontium substituted/doped calcium phosphates. However, no information was available up to now on strontium substitution to calcium in the structure of α-CaHPO4, monetite, in spite of the involvement of this phosphate in the composition of biomaterials for hard tissue substitution/repair and although it is isomorphous with α-SrHPO4. Herein, we investigated the substitution of strontium to calcium in the structure of α-CaHPO4, as well as the replacement of calcium for strontium in the structure of a further polymorph of SrHPO4, namely β-SrHPO4. To this purpose, monetite at increasing degree of strontium substitution for calcium was synthesized by means of direct synthesis in aqueous solution, as well as through thermal treatment of strontium-substituted brushite; whereas the synthesis of β-SrHPO4 was carried out at low temperature. The results of structural refinements, spectroscopic analysis and electron microscopy investigation indicate that the method of synthesis has a great influence on the range of strontium incorporation into α-CaHPO4, which can reach 100 at%. The morphology of the synthesized materials is also remarkably dependent on composition. The analysis of the products synthesized at low temperature shows that the upper limit of possible substitution of calcium to strontium in the structure of β-SrHPO4 is much more limited, just up to about 20 at%. Moreover, powder X-ray analysis of β-SrHPO4 states that it crystallizes with a monoclinic cell in the space group P21/c.