Geoscience Reference
In-Depth Information
analogs was extremely popular. The interest in niobates and tantalates is connected
to their excellent ferroelectric and piezoelectric properties. In fact, the hydrothermal
method is the only perspective method of obtaining the monocrystals of niobates
and tantalates. Compared to the majority of the inorganic compounds, these materials
have very low solubility. In aqueous solutions, both the elements form the polymeric
oxyanions of type [H
2
Nb
6
O
19
]
6
2
,[HNb
6
O
19
]
7
2
,[Nb
6
O
19
]
8
2
,[Ta
6
O
19
]
6
2
, etc., how-
ever, most of these polymeric oxyanions are not stable and, with the variation in pH
and with an increase in the growth temperature, they form hydrated pentoxides. These
complexes are readily soluble in strong hydrofluoric acid or strong alkaline solutions
[117
119]
. In weak acidic solutions, the formation of anions like [NbOF
6
]
3
2
and
[TaOF
6
]
3
2
takes place, so that the synthesis of niobates and tantalates is possible,
always within limited and narrow boundaries; thus, the synthesis of these compounds
is always a challenging one. Marshall and Laudise
[120]
have studied the systems
K
2
O
H
2
Oat400
C and approximately 560 atm.
Mixed crystals of K(Ta, Nb)O
3
have been grown on a seed crystal. The rates of
growth of the face reached 0.25 mm/day in 15 M KOH at 650
C,
a
Ta
2
O
5
a
H
2
OandK
2
O
a
Nb
2
O
5
a
40
C, and
Δ
T
5
B
l100 atm
[99]
. K(Ta, Nb)O
3
crystals with dimensions of up to 1 cm were grown.
Rare earth orthotantalates, LnTaO
4
(Ln
1500
C,
5
La
Lu) have been obtained at T
5
10
9
Pa. These are the high-temperature and -pressure compounds and exhibit
intense polymorphism
[120
P
5
8
3
122]
.
The discovery of piezo- and ferroelectric properties in the natural mineral stibio-
tantalite generated a great interest in obtaining a series of synthetic monocrystals of
the group ABO
4
, where A
Nb, Ta, Sb
5
1
. Their structures are made
up of weakly formed oxygen octahedra and, in the middle of these octahedra, the
cations A and B are located. These crystals can be obtained essentially by the
hydrothermal technique. Although some attempts have been made to obtain these
crystals by other techniques like flux and melt, good quality crystals are obtained
only by the hydrothermal technique.
The hydrothermal growth technology for ABO
4
crystals has been worked out in
great detail by Popolitov. Usually, alakaline solutions like MOH, MF, MHF
2
,or
H
2
O
2
(where M
Sb, Bi; B
5
5
Li, Na, K, Rb, Cs) are used as mineralizers in the hydrothermal
growth of these crystals. The growth of these compounds is carried out in the
systems A
2
O
3
a
5
B
2
O
5
a
R
a
H
2
O and ABO
4
a
R
a
H
2
O (where R
LiF, NaF, KF,
5
Table 8.4 Optimum Experimental Conditions of Crystallization of HgSe and HgTe
[111]
T
(
C)
ΔT
(
C)
Fill
(%)
Experimental Duration
(days)
HCl
Concentration
(M)
Transport of
Nutrient (%)
300
100
82.92
7
12
64
316
78
82.92
10
12
82
318
82
83.34
12
12
100
300
50
85.3
6
12
22.5
350
50
80.43
6
12
29
