Isomorphous substitutions in beta-tricalcium phosphate: The different effects of zinc and strontium

X-ray diffraction and infrared absorption analyses have been carried out on zinc-substituted and strontium-substituted beta-tricalcium phosphate prepared by solid-state reaction. Zinc can substitute calcium up to 20 atom %, inducing a nonlinear variation of the lattice constants and an increase in degeneracy of the PO43- infrared absorption bands. On the other hand, up to 80 atom % of strontium can enter into the crystal structure of beta-tricalcium phosphate, causing a linear enlargement of the unit cell, in agreement with its greater ionic radius compared to that of calcium. Furthermore, strontium incorporation provokes the shift of the PO43- absorption bands toward lower frequencies. On the basis of the data previously obtained on magnesium-substituted beta-tricalcium phosphate, the different behaviours exhibited by zinc and strontium could be attributed to a different distribution into the cationic sites of the beta-tricalcium phosphate structure. The results allow us to relate the effect of bivalent ions on the structure and relative stability of calcium phosphates with their ionic radius, and can be utilized to interpret the role of ionic composition on the properties of biological phosphates. (C) 1997 Elsevier Science Inc.