Geoscience Reference
In-Depth Information
Figure 7.27 (a) Hydrothermally grown BSO (1 wt% SiO
2
); (b) Hydrothermally grown BSO
(3 wt% SiO
2
).
Source: Photos courtesy: Changlong Zhang.
with 4 N NaOH solution and the external void was filled with a sodium-silicate
pressure-balancing solution. The following were the experimental conditions:
Dissolution temperature: 390
C
Crystallization temperature: 385
C
Δ
T: 5
C
Pressure: 6000
12,000 psi
Growth duration: 30
40 days
Growth rate: 0.1
0.3 mm/day.
Little or no growth occurred at NaOH normalities less than 4 N or nutrient zone
temperatures less than 390
C. Excessive, unoriented growth on seeds and spontane-
ous nucleation on the walls of the liner occurred with normalities above 5
C.
Experiments using Czechrolski-grown nutrient produced better crystals. Limited
experimental data indicates that the preferred growth orientation is [100]. Most of
the crystals were colorless in contrast to the straw color of Czechrolski crystals.
Some were pale green, perhaps as a result of minor leaks in the platinum liner
[133]
.
Figure 7.28
shows the absorption spectra of various BSO samples. The solid
curves represent the absorption spectra taken at 10 K of a hydrothermally grown
sample with those of the Cz-grown undoped and Al-doped samples. The deep-
donor absorption shoulder is present in the curve for the undoped-Cz sample. The
dashed curves represent the spectra taken on the three samples after they were
exposed to intense monochromatic light (0.2 mw/cm
2
) for 1 h at 10 K; 3 and
3.3 eV lights were used for the undoped and Al-doped Cz samples, respectively.
The additional photochromic absorption in the undoped and Al-doped Cz-BSO
samples is caused by the transfer of electrons to as yet unidentified traps. The
hydrothermally grown BSO crystals do not produce any photochromic absorption.
