no obvious phase changes were noticed in
the XRD patterns of the samples with Ce-addition ranging from 0.5 to
2 at.%. For all the samples, the amount of HAp exceeded 92%, while
the amount of admixture β-TCPwas always below8% (Table 1). The numerical
values of a crystallinity degree (XC)were calculated according to
Landi et al. [29] and appeared to be in the range of 84–89% (Table 1).
This finding pointed out to the well crystalline structure of all samples.
Furthermore, the crystallinity degrees of all Ce-substitutedHAp samples
were found to be slightly higher than that of undoped HAp. Thus, addition
of small amounts of Ce helped crystallization of HAp. However,
since the highest degree of crystallinity was found for sample H2 containing
the minimum level (0.5 at.%) of Ce, further increase in the
amount of Ce resulted in the decrease in crystallinity degree, which is
in a good agreement with the literature data [28]. Probably, this might
be due to the charge differences between Ca2+ and Ce3+ ions. Namely,
since HAp easily forms non-stoichiometric Ca-deficient compounds [3],
small amounts of Ce3+ ions replacing small amounts of Ca2+ ions might
result in better stoichiometry and, therefore, better crystallinity, while
the increased amounts of incorporated Ce3+ ions result in decreased
crystallinity due to charge imbalances.