Geoscience Reference
In-Depth Information
valuable unusual magnetic, optical, and luminescent properties. Hence, they are used
as highly efficient laser diode-pumped microlasers
[323]
, an efficient phosphor
[324]
as very attractive polarizer material
[325]
, and low-threshold laser host
[326]
.The
laser emission cross section of Nd:YVO
4
at 1064 nm is 2.7 times greater than that of
Nd:YAG
[327]
. As a laser-host material, Nd:YVO
4
offers attractive properties such as
highly polarized emission, large absorption cross section, high thermal fracture limit,
and good mechanical strength. Together, these characteristics are favorable for a broad
range of diode-pumped laser applications including visible lasers. Single crystals of
yttrium orthovanadates have been obtained by various techniques like Czochralski
[328,329]
,zonemelting
[318,330]
,Verneuilmethod
[329]
, dissolution in melts
[331]
,
from flux
[331,332]
,andtopseededsolutiongrowth
[333]
. However, there are no
reports dealing with the hydrothermal synthesis of these crystals. This is probably con-
nected to the high-temperature and pressure conditions involved in their growth and
also to the strong belief that their growth under mild hydrothermal conditions will
introduce water into the structure or to the interstices. Further, these crystals are
known to have high melting points and very low solubility. Therefore, the high-
temperature solutions and melt techniques were preferred over the others. In fact,
several problems have been encountered in the growth of yttrium orthovanadates by
the above said conventional methods. These problems might be connected to the insta-
bility of the pentavalent vanadium at high temperatures and the loss of oxygen through
surface encrustation by the reaction of the melt with the crucible material. The
unstable melt behavior of YVO
4
leads to the appearance of zoning in crystals due to
the variation in the dopant concentration. Recently, it has been observed that YVO
4
crystals are very difficult to grow with a stoichiometric or near stiochiometric compo-
sition. Further, the most critical aspect is the appearance of metastable phases which
precipitate within YVO
4
crystals
[334]
. The detection of correct Y
O stoichiome-
try in YVO
4
crystals is difficult
[335]
. Moreover, as the temperatures involved in the
flux growth is about 1300
C and in melt growth is above 1800
C, there is a high prob-
ability of vanadium attacking the container. In order to overcome most of these
difficulties in melt and high-temperature solution techniques, Byrappa et al.
[321]
have preferred hydrothermal technique as a solution to this problem. Since the experi-
ments in hydrothermal technique are carried out in a closed system, the loss of oxygen
could be readily prevented. In addition, the experimental conditions adopted by the
authors involve the use of Teflon liners for the autoclaves and the interaction between
the container and vanadium is avoided.
The experiments on the hydrothermal growth of R:MO
4
(R
a
V
a
Y,
Gd) were carried out using the Morey type of autoclaves provided with Teflon
liners. The use of Teflon liners has helped in overcoming the entry of inclusions
from the autoclave material. The schematic diagram of hydrothermal autoclave used
in the present work is shown in
Figure 7.74
. The starting materials such as Y
2
O
3
,
V
2
O
5
, and R
2
O
3
(where R
Nd, Eu; M
5
5
Nd, Eu,) were weighed in appropriate proportions in
Teflon liners. A suitable mineralizer solution of known concentration was added
into the Teflon liners and the entire mixture was stirred well until a homogeneous
and relatively viscous solution was obtained. The temperature was kept constant at
240
C and the autogeneous pressure generated depended on the fill, usually
5
