Geoscience Reference
In-Depth Information
susceptibility versus temperature for Mn
2
VO(PO
4
)
2
H
2
O. This compound displays
Curie
20 K, at which point an antiferromagnetic
transition takes place. The high-temperature portion (150
Weiss behavior down to T
N
5
300 K) was fit to the
Curie
Weiss expression, yielding a Curie constant of C
9.0 emu/K/mol and a
5
Weiss constant of
μ
eff
5
8.5 BM. The
origin of the antiferromagnetic interaction most likely arises between Mn(2) and V
atoms, which are 3.150
˚
apart in their edge-sharing polyhedral arrangement.
θ
52
46 K. The effective magnetic moment
7.16 Hydrothermal Synthesis of Vanadates
The synthesis of vanadates is not as popular as silicates, phosphates, germanates, or
sulfates, because of their limited use in the modern technology unlike its analogous
materials silicates or phosphates. Further, the synthesis of vanadates is a relatively
recent field. The first systematic synthesis of vanadates was during the 1950s, when
ABO
4
(A
P, V, As) isostructural to AlPO
4
was obtained
[314]
. However, the solid-state synthesis was the most popular one followed by the
flux method. The main objective of the synthesis of AVO
4
(A
5
Al, Ga, B, Bi, Fe, etc.; B
5
Al, Cr, B, Fe, Ga,
Mn, etc.) was academic, and it was quite recently their technological potential was
explored. For example, FeVO
4
shows very interesting magnetic properties. This
compound shows successive phase FeVO
4
-I
5
FeVO
4
-IV
type structures with increasing pressures
[315]
. During the 1970s, a series of simple
and complex rare earth vanadates were synthesized with a motivation to develop new
luminophors, because a majority of these vanadates, particularly mixed rare earth
phosphates, contain isolated rare earth ions
[316]
. Whereas simple rare earth orthova-
nadates show very high thermal stability owing to their very high melting points and
also exhibit unique optical and magnetic properties
[317
!
FeVO
4
-II
!
FeVO
4
-III
!
319]
. All these simple rare
earth orthovanadate single crystals have been obtained by flux and melt techniques.
Byrappa et al.
[320]
attempted the synthesis of vanadates for the first time by hydro-
thermal technique, and they have obtained NH
4
Zr
2
V
3
O
12
crystals, which show
interesting proton conducting properties. However, during the 1990s, it was shown
that the hydrothermal technique helps in synthesizing a new class of compounds with
unique structures, especially from the phosphate, vanadate, and arsenate groups. This
made the hydrothermal technique popular for the synthesis of novel vanadates, which
exhibited interesting physicochemical properties owing to their unique structural
peculiarities. Similarly, the growth of simple rare earth orthovanadates doped with
active elements was realized by hydrothermal technique quite recently
[321]
.
Here, we describe the growth of simple rare earth orthovanadates and mixed
metal vanadates of recent importance by hydrothermal method.
7.16.1 Growth of R
Y, Gd)
Single crystals of yttrium orthovanadate, YVO
4
, with zircon structure, ZrSiO
4
,having
tetragonal space group 14/amd
[322]
find extensive applications owing to their most
MVO
4
(R
Nd, Eu; M
5
5
5
