FUNDAMENTALS OF HYDROXYAPATITE AND RELATED CALCIUM PHOSPHATES - Advanced Biomaterials: Fundamentals, Processing, and Applications - page 23

OH

O

3/4

0.07 Ca

0.43

1/4

0.43 0.07

1/4

3/4

1/4

1/4 3/4

3/4 1/4

3/4

0.93
0.57

P

3/4

0.93
0.57

3/4

1/4

1/4

0.93
0.57

1/4

0.93
0.57

3/4

1/4 0.43

0.50
0.00

3/4

1/4

0.50
0.00

1/4

0.50
0.00

3/4

0.43

3/4 0.07

1/4

3/4 1/4

3/4

0.07

3/4

3/4

3/4

3/4

1/4

1/4

1/4 1/4 1/4

3/4

1/4

3/4

0.50
0.00

0.93
0.57

1/4

1/4

0.93
0.57

3/4 0.50

0.00

0.50
0.00

3/4

0.43

1/4 0.43

3/4 1/4

3/4

3/4

0.07

3/4

0.43 0.07

0.07

1/4 0.07

0.43

1/4

1/4

3/4 1/4

3/4

1/4 1/4

a

3/4

3/4

0.93
0.57

0.93
0.57

3/4

3/4

1/4

1/4

0.93
0.57

1/4

0.93
0.57

3/4

1/4

3/4

0.50
0.00

1/4

0.43

0.50
0.00

1/4 0.50

0.00

0.43

3/4 0.07

3/4

0.07

1/4 3/4

1/4

3/4

3/4

3/4

1/4

1/4

0.50
0.00

0.50
0.00

Figure 2.1. The arrangement of the atoms of calcium hydroxyapatite, Ca 10 (PO 4 ) 6 (OH) 2 , in a
hexagonal unit cell. The OH ions located in the corners of the unit-cell are surrounded by two
sets of Ca(II) atoms arranged in a triangle at appositions z = 1/4 and ¾, by two sets of PO 4
tetrahedra also in triangular positions; and by hexagonal array of Ca(I) atoms at the outermost
distance [124].

defi ned as Ca(I) or Ca (II) depending on their environment. Four Ca atoms
occupy the Ca (I) positions at levels z = 0 and two at z = ½ (0.5). Six Ca atoms
occupy the Ca(II) positions one set of three Ca atoms located at z = ¼ (0.25), the
other set of three at z = ¾ (0.75), surrounding the OH groups located at the cor-
ners of the unit cell. The six tetrahedral PO 4 groups are arranged in sets of three
at levels z = 0.25 and at z = .75. The network of PO 4 groups provides the skeletal
framework of the apatite and gives great stability to the apatite structure.
Calcium - defi cient apatite (CDA) is obtained by precipitation or hydrolysis
methods at temperatures 25 ° C to 95 ° C [34,57,64,102] . CDA differs from stoichio-
metric hydroxyapatite (HA) in several properties. Compared to HA, CDA has:

(a) a lower Ca/P molar ratio (ranging from 1.4 to 1.66 for CDA, 1.67 for
HA);
(b) lower crystallinity (refl ected by lower diffraction intensities and broader
diffraction peaks;