Biomedical Engineering Reference
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poorly crystalline and of submicron dimensions. They have a very
large specific surface area, typically 25-100 m
2
/g. On heating above
~700°C, dry CDHA with Ca/P = 1.5 will convert to β-TCP and that
with 1.5 < Ca/P < 1.67 will convert into a biphasic composite of HA
and β-TCP (see section 1.3.14 Biphasic (BCP) and triphasic calcium
(ortho)phosphates below) [312-323]. A reasonable solid-state
mechanism of a high-temperature transformation of CDHA into BCP
has been proposed [324, 325].
The variability in Ca/P molar ratio of CDHA has been explained
through different models: surface adsorption, lattice substitution
and intercrystalline mixtures of HA and OCP [326]. Due to a lack of
stoichiometry, CDHA usually contains other ions [82]. The extent
depends on the counter-ions of the chemicals used for preparation
(e.g., Na
+
−
). Direct determinations of the CDHA structures are still
missing and the unit cell parameters remain uncertain. However,
unlike that in ACPs (see section
, Cl
above), a LRO exists in
CDHA. The following lattice parameters were reported for formate
(HCO
1.3.8. ACP
2
−
) containing CDHA with Ca/P = 1.596 (ionic):
a
= 9.4729(20)
2+
and
c
= 6.8855(9) Å. A loss of Ca
ions happened exclusively from
Ca(2) sites, while the PO
tetrahedron volume and P-O bonds were
~4.4% and ~1.4% smaller, respectively, than those in HA [327].
A systematic study of defect constellations in CDHA is available in
literature [328]. As a first approximation, CDHA may be considered
as HA with some ions missing [329]. The more calcium is deficient,
the more disorder and imperfections are in CDHA structure [330].
Furthermore, a direct correlation between Ca deficiency and the
mechanical properties of the crystals was found: calcium deficiency
leads to an 80% reduction in the hardness and elastic modulus and
at least a 75% reduction in toughness in plate-shaped HA crystals
[331]. According to the chemical formula of CDHA (Table 1.1),
there are vacancies of Ca
4
2+
−
ions in
crystal structure of this compound [327, 329-334]. However, due
to Ca
(mainly on Ca(2) sites) and OH
vacancies in CDHA, the resulting negative charge might be
compensated by protonation of both an OH
2+
−
ion within the deficient
4
3−
calcium-triangle and a PO
ion in the nearest neighborhood of the
vacant calcium site. This results in the presence of some water in
CDHA structure: Ca
< 1)
[328]. According to this approach, there are no hydroxide vacancies in
CDHA, just a portion of OH
(HPO
)
(PO
)
(OH)
(H
O)
(0 <
x
10−
x
4
x
4
6−
x
2−
x
2
x
−
ions are substituted by water molecules.
Concerning possible vacancies of orthophosphate ions, nothing is
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