Geoscience Reference
In-Depth Information
light green in color. Rutile synthesized by the hydrothermal method has properties
closer to the natural rutile. In pure water and aqueous solutions of NaOH or
Na
2
CO
3
at temperatures below 350
400
C, the anatase phase of TiO
2
crystallizes
and, at temperatures further down, the brookite phase crystallizes
[86]
.
A rutile crystal grown by the Verneuil method was used as a seed and experi-
ments were carried out at a solubility zone temperature 550
600
C, with respec-
35
C)
[86]
. Under these
conditions the growth rate of the (100) and (110) faces was 0.15
tive autoclave filling coefficients of 0.6
0.5 and (T
20
5
0.21 mm/day in
the 5-day experiments. Typical conditions for crystallization were 570
C autoclave
filling coefficient
25
C. The given directions are not directions of
rapid growth. Judging by the morphology of the spontaneous rutile crystals (they
often have a typical long-prismatic form), the growth rate along (001) should be
higher. In recent years, the growth of nanosize TiO
2
both as anatase and rutile is
very popular for a variety of applications, and its growth is more popular in this
size range than the growth of TiO
2
as bulk single crystals. The growth of nanosize
TiO
2
crystals will be dealt in detail in Chapter 10.
0.6,
Δ
T
5
5
ZrO
2
(Baddeleyite)
Both ZrO
2
and HfO
2
adopt the monoclinic P2
1
/c baddeleyite structure, which is a
defect fluorite structure type
[87]
. The small ionic radii of tetravalent zirconium and
hafnium are not capable of supporting the eight coordinate environment associated
with the fluorite (CaF
2
) structure. Instead, a coordination sphere of seven oxygen
ligands surrounds the metal center. Structurally very similar, both zirconia and hafnia
undergo phase transitions to a tetragonal structure when heated above 1200
Cand
1750
C, respectively
[88]
. Finally, above 2600
C, they both adopt the cubic fluorite
structure and remain stable until melting at 2710
C for zirconia and 2900
C for haf-
nia. However, the high-temperature cubic phases of zirconia and hafnia can be stabi-
lized at room temperature by incorporating additives such as rare earth and alkaline
earth metals into the crystal lattice
[89]
.Kuznetsov
[82]
was the first to synthesize
ZrO
2
crystals under hydrothermal conditions using fluoride solutions NaF, or KF and
NH
4
F with concentration 7
690
C. In KF solutions, ZrO
2
is transported only at temperatures over 600
C, and small crystals of 0.5 mm size are
formed in the upper zone of the autoclave. Below 600
C, all that happens is that the
original material is made coarser, and lamellar crystals up to 0.3 mm in size are
formed in the lower zone of the autoclave. In NH
4
F solution, there occurs an intensive
recrystallization of ZrO
2
at relatively low temperatures. However, there are some prac-
tical difficulties in the growth of ZrO
2
due to the retrograde solubility in NH
4
Fsolu-
tions. In order to simplify the problem, a furnace with a horizontal autoclave, a “hot”
zone, was created at the obturator. By this means, ZrO
2
crystals in the form of thin
plates 2.5
10% at temperatures 520
0.5 mm with (001), (010), and (110) faces were obtained. Increase
in the temperature gradient raises the growth rate, and the crystals become thicker and
form twinning. Growth rate is maximum in the c-direction and minimum in the
a-direction and slower in the b-direction. The growth rate of the (100) face is roughly
10 times lower than that of the (001). The basic properties of this synthetic ZrO
2
are
3
2.5
3
