A Spectroscopic and Structural Study of M(3d) 2+ ‐Doped β‐Tricalcium Phosphate – the Binding Properties of Ni 2+ and Cu 2+ in the Pseudo‐Octahedral Ca(5)O 6 Host‐Sites

The pseudo‐octahedral Ca(5) 2+ position in β‐Ca 3 (PO 4 ) 2 is a highly interesting host site with unexpectedly short Ca–O bond lengths, which can be easily substituted by small 3d‐cations, such as Ni 2+ . Also Cu 2+ may fully replace Ca(5) 2+ , leading to remarkably large energetic and steric distortion effects due to E g ⊗ ϵ g Jahn–Teller coupling – with a ground‐state splitting of about 1.25 eV. The Cu(Ni)–O bond is strongly ionic according to a 3d‐mixing coefficient of the e g ‐electrons above 0.90, as revealed by EPR‐ and d–d spectroscopy. It is suggested that the flexibility of the Ca(5)O 6 host polyhedra, which lie isolated in the lattice, in respect to deformations and to substitutions by smaller cations is one reason that β‐TCP is a useful biomaterial.