FUNDAMENTALS OF HYDROXYAPATITE AND RELATED CALCIUM PHOSPHATES - Advanced Biomaterials: Fundamentals, Processing, and Applications - page 24

(c) greater a -axis dimension of CDA (9.438 to 9.461 A versus 9.422 A for
HA), probably due to the incorporation of HPO 4 (partial HPO 4 - for - PO 4
substitution) [ 64 , Table 2.2 ];
(d) appearance of IR absorption band at about 864 cm − 1 (attributed to
P-O-H vibration band of the HPO 4 group) and broad band at 3400 to
3600 (attributed to the H-O-H vibration band of adsorbed H 2 O ) .

In addition, heating or sintering CDA above 800 °C results in the transforma-
tion of CDA to - TCP

ratios), depending on the Ca/P of the CDA before sintering and on the sintering
temperature, 900 to 1100 ° C [57,64,84] .

β -TCP or mixtures of HA and β - TCP (with differing HA/ β

2.3.2 Substitutions in the Apatite Structure
The apatite structure, Ca 10 (PO 4 ) 6 (OH) 2 , is a very hospitable one allowing the sub-
stitutions of many other ions for the Ca 2+ , PO 3− , or OH − groups. Such substitu-
tions cause changes in the crystallographic, physical and chemical properties of
apatites: for example, lattice parameters ( a - and/or c - axis dimensions), spectral
properties, color, morphology (crystal size and shape), solubility and thermal sta-
bility. The extent of the change is proportional to the amount and size of the
substituting ion (Table 2.2). Substitution in the apatite also affects the in vitro cell
response [29,36,122,123].
Studies on properties of unsubstituted and substituted apatites have been
based on apatites prepared by precipitation, hydrolysis of other calcium phos-
phates (for example, DCPD, DCPA, OCP, -TCP), hydrothermal reactions or by

solid state reactions at high temperatures [3,7, 9,21,22,27,31,32,41,57,58,60,64,71,
73,76 - 81,83,87,89.91,92,103,106 - 109,115 - 118,121,126] .
Stoichiometric HA (Ca/P ratio = 1.67) is obtained by solid state reactions.
Biologic apatites and synthetic apatites obtained by precipitation or hydrolysis
(for example, DCPD or DCPA in NaOH solution) methods are usually calcium
defi cient (Ca/P ratio

α

< 1.67). Single crystals of HA can be obtained by hydro-

thermal reactions [3].

2.3.2.1 Fluoride or Chloride Incorporation. Substitution of F - for - OH or
Cl - for - OH does not signifi cantly change the atomic arrangements in the apatite
structure. However, while F-substitution does not change the hexagonal symme-
try of the apatite, Cl-substitution results in changing the symmetry from hex-
agonal to monoclinic symmetry because of the much larger Cl atom substituting
for the OH group, affecting the relative position of the Cl with respect to the Ca
triangle, as shown in Figure 2.2 [124]. F-for-OH (F ionic radius OH) substitu-

tion causes a contraction in the a -axis dimension with no signifi cant change in the
c -axis while Cl-for-OH substitution causes expansion in both a - and c - axis dimen-
sions compared to F- or Cl-free apatites (Table 2.2 and Table 2.3).
Full or partial F-for-OH substitution in synthetic apatites depends on the
F concentration in the solution [57,59,72]. Incorporation of F in synthetic or

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