Biomedical Engineering Reference
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crystalline [98]. The X-ray diffraction pattern of this rapidly
precipitated phase showed only two very broad and diffuse peaks,
typical for substances that lack the periodic LRO [196]. The extreme
diffuseness of the synthetic ACP pattern also provided a basis for
interpreting the reduced intensity of the apatitic X-ray diffraction
patterns of bones as being due to the mineral having an amorphous
component [101]. The diffracted X-ray energy from this component
was so uniformly dispersed that it could not be separated from the
subtracted background intensity [20].
Initially, there were suggestions that a synthetic ACP was, in fact,
HA of such small crystal dimensions that its X-ray diffraction pattern
was widely broadened to appear amorphous in character. However,
calculated X-ray diffraction patterns assuming that ACP consisted of
small groups of HA unit cells, or even a single HA unit cell, did not
match the observed ACP diffraction data [221]. The NMR spectra of
ACP are also sufficiently different from those of HA to suggest that
they do not have the same structural motif [222]. Then, a probability
that ACP was structurally distinct from HA led to a study of this
material by the X-ray radial distribution method, which showed that
its characteristic diffuseness resulted from a breakdown in atomic
LRO within the interior of the synthetic ACP [223-225]. The major
peak positions were found to occur at distances of 2.55, 3.75, and 6.40
Å with an evidence of a peak at 2.9 Å [223]. Furthermore, any orderly
atomic arrangements did not extend beyond ~0.95 nm in diameter
that corresponded to the smallest values of a far-MRO. A contiguous
periodic regularity in the distribution of these domains typical of
crystalline materials was absent. Extended X-ray absorption fine
structure (EXAFS) spectroscopy [116, 226] indicated that possible
regularities in the local environment around individual Ca
2+
ions
were even more circumscribed, not extending beyond distances of
~0.3 nm (Fig. 2.9). This is a SRO scale. In addition, an infrared analysis
showed a similar lack of crystalline order for orthophosphate anions
in the ACP structure [31, 129, 140, 228]. The method is based on the
observation that a splitting of the P-O anti-symmetric bending mode
at 550-600 cm
−1
(Fig. 2.10) increases as crystallinity increases. This
apparent lack of crystalline regularity is one of the striking features
of ACP that distinguishes it from other calcium orthophosphates and
provides the structural basis for its name [20].
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