Hydrothermal Growth of Some Selected Crystals - Hydrothermal Technology - page 191

400 C and log solubility (S 1 ) in NaOH (1 M) and in Na 2 CO 3 (1 M) versus 1/P.
These experimental results show that Na 2 CO 3 appears to be a better solvent than
sodium hydroxide in high-pressure domain for a temperature close to 400 C. From
Figure 5.11 ,

the enthalpy values have been calculated and the values are as

follows:

Δ H T 5 2395 5 cal = mole for NaOH ð 1M Þ ; 200 P 350 MPa

6 # #

Δ H T 5 4001 2 cal = mole for Na 2 CO 3 ð 1M Þ ; 150 P 350 MPa

6 # #

5.2.5 Defects Observed in Synthetic α -Quartz Single Crystals
The defects present in the synthetic -quartz crystals play an important role in

determining their quality and in turn their applications. Much of the recent works
on quartz is essentially focused on the defect studies. All the defects present in
quartz can be classified into two types:

α

1. The physicochemical defects
2. The structural defects.

Physicochemical defects are mainly induced by the nature of the nutrient or the
solvent and also by chemical contamination of the solution by the metal constitut-
ing the reaction vessel. Quite often, solid or liquid inclusions like NaFe 3 1 Si 2 O 6
(acmite) are inserted in the -quartz. The most important impurity in quartz is H 1 ,

which easily fits interstitially in the large (1 ˚ ) channels that lie parallel to the
c-axis to charge compensate for Al 3 1 which goes to an Si 4 1 . It enters the lattice

α

Effect of growth rate on purity (EPR results)

0.60

0.55

0.50

0.45

0.40

0.35

0.30

×

0.25

0.20

0.15

×

0.10

×

0.05 ×

×

5 10 15 20 25 30

Growth rate - MILS PER DAY

Figure 5.12 Effect of growth rate on Al 3 1 concentration in the Z-region of growth crystals [18] .