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coordinated compounds is discussed in Part II (Chapter 8). The literature survey
shows that among all these compounds, the silicates and germanates have taken a
unique place in the hydrothermal research because of their diversified composi-
tions and structures. Here, these compounds will be discussed in two categories:
(i) rare earth silicates and phosphates and (ii) transitional metal silicates and
phosphates.
In this chapter, the main emphasis is on the crystal growth and crystal chemistry,
which in turn provides clues to grow a variety of complex coordinated compounds.
It is also well known that hydrothermal, solvothermal and supercritical hydrother-
mal methods produce very unusual complex structures, which cannot be obtained
by other synthesis methods. However, this chapter deals with only the growth of
bulk single crystals, and the micro- to nanosize crystals growth will be discussed in
Chapter 10.
7.2 Crystal Chemical Background
It is interesting to note that there is a close relationship between the crystal chemis-
try and crystal growth of these compounds. The primary requirement for any mate-
rial/crystal/mineral with a device potential is the understanding of its scientific
foundation, particularly the consequences of the submicroscopic crystal structure, i.
e., the spatial arrangement of atoms, ions, molecules, and the types of chemical
bonds between them. It is the chemical bond more than anything else that deter-
mines the structure and the properties of a crystal. A crystal chemical knowledge
helps in tailoring a given phase or a given structure that is to optimize chemical
classification, crystal structure, and defect structure with respect to the desired
properties. Therefore, the present authors briefly discuss the structural diversity of
silicates and the Si
O chemical bond.
Silicates are the compounds containing Si and O. Their main specific structural
feature is the presence of Si-atoms in a fourfold coordination with oxygen, which is
favored by the ratio rSi:rO
a
0.28. Table 7.1 gives the different types of coordina-
tions of cations as a function of ionic radii ratio. Bond lengths and bond angles in
Si-tetrahedra tend to be close to their mean values: d(Si
5
1.62 ˚ ; angle
a
O)
5
Table 7.1 The Different Types of Coordinations of Cations as a
Function of Ionic Radii Ratio [1]
CN
Coordination
Ionic Radii Ratio
2
Dumb-bells
, 0.15
3
Triangle
0.15 2 0.22
4
Tetrahedron
0.22 2 0.41
6
Octahedron
0.41 2 0.73
8
Cube
0.73 2 1.37
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