Geoscience Reference
In-Depth Information
7
Hydrothermal Synthesis and
Growth of Coordinated
Complex Crystals (Part I)
7.1 Introduction
Hydrothermal synthesis and growth of complex coordinated crystals are not new
especially for silicates. In fact, the hydrothermal technology was clubbed under sili-
cate technology in the earlier days. Silicates have drawn the attention of crystal
chemists right from the earliest days owing to their abundance, structural diversity,
and widespread applications in various technologies. Thus, the synthesis of com-
plex coordinated complexes began in the nineteenth century. In fact, hydrothermal
technique was considered as a most powerful tool to grow these complex coordi-
nated crystals. During the 1960s and 1970s, the importance of the hydrothermal
technique for the synthesis of inorganic materials without natural analogs was real-
ized. Since then a large variety of complex coordinated complexes are being
obtained with very complex crystal structures. More recently, the hybrid or mixed
metal-organic framework (MOF) structures, which are described as crystalline
compounds consisting of metal ions or clusters coordinated to often rigid organic
molecules to form one-, two-, or three-dimensional structures that can be porous,
and can be used for a variety of novel applications like gas separation, gas purifi-
cation, catalysts, sensors, and hydrogen storage. However, these hybrid MOFs
cannot be grown as large bulk crystals, but they are frequently processed as submil-
limeter size crystalline materials. Although these mixed hybrid framework struc-
tures are very popular in modern technology, they cannot be widely synthesized
using any processing technique, except hydrothermal and solvothermal techniques.
The complex coordinated crystals covers silicates, phosphates, vanadates, arsenates,
molybdates, tungstates, fluorides, sulfates, selenides, borates, and so on, with a
great variety of cations starting from alkali, alkaline earth, transitional to rare earth
metals. The number of all such compounds exceeds several hundreds owing to
their academic and technological interest and here it is impossible to discuss each
and every compound. Therefore, we have discussed these compounds in two parts
for the sake of the reader's convenience. Part I covers silicates, germanates, phos-
phates, vanadates, and borates (this chapter). The remaining group of complex
