  The results of XPS measurements (core levels and valence bands) of P+, Ca+,
P+Ca+ and Ca+P+ ion implanted (E=30 keV, D=1x1017 cm-2) commercially pure
titanium (cp-Ti) and first-principles density functional theory (DFT)
calculations demonstrates formation of various structural defects in titanium
dioxide films formed on the surface of implanted materials. We have found that
for double implantation (Ti:P+,Ca+ and Ti:Ca+,P+) the outermost surface layer
formed mainly by Ca and P, respectively, i.e. the implantation sequence is very
important. The DFT calculations show that under P+ and Ca+P+ ion implantation
the formation energies for both cation (P-Ti) and anion (P-O) substitutions are
comparable which can induce the creation of [PO4]3- and Ti-P species. For Ca+
and P+Ca+-ion implantation the calculated formation energies correspond to
Ca2+-Ti4+ cation substitution. This conclusion is in agreement with XPS Ca 2p
and Ti 2p core levels and valence band measurements and DFT calculations of
electronic structure of related compounds. The conversion of implanted ions to
Ca2+ and [PO4]3- species provides a good biocompatibility of cp-Ti for further
formation of hydroxyapatite.
