Geoscience Reference
In-Depth Information
Li
3
B
5
O
8
(OH)
2
shows poor ionic conductivity and behaves as a dielectric mate-
rial below 498 K, with the electronic equivalent circuit representation indicating a
pure capacitor type behavior. Between 299 and 523 K, the material exhibits piezo-
electric behavior and at about 523 K, it shows superionic conducting behavior. The
complex impedance becomes almost zero at 10 MHz
[372]
.
The ionic conductivity has been studied by several workers. The ionic conduc-
tivity generally varies from 10
2
2
to 10
2
3
(ohm cm)
2
1
at about 300
C. The activa-
tion energy value is around 0.4 eV (380).
Alkali borates,
-LiBO
2
, crystals are doped with Mn
2
1
ions, and they show
weak yellow luminescence (UV excitation). New alkali-free borates of divalent
metals doped with Mn
2
1
ions show interesting luminescence properties. For exam-
ple, Mn-activated cadmium borates show yellow-green luminescence and
Zn
4
B
6
O
12
borate yields green luminescence
[373,374]
. similarly, Mn
2
1
-doped
LiZnBO
3
,2Li
2
O
α
ZnO
2B
2
O, and Na
2
Zn
2
MnB
4.67
O
11
show crimson lumines-
cence, and 4Li
2
O
3B
2
O
3
shows yellow luminescence
[374]
.
The complex coordinated complexes are best obtained under hydrothermal con-
ditions. Such complex structures have not been able to be obtained by other crystal
growth methods. Especially with the reduction in the PT conditions of synthesis,
the method is becoming more attractive for these complexes. New structures being
obtained are throwing light upon new and interesting physicochemical properties of
these compounds.
Thus among the complex coordinated compounds, today it is the mixed frame-
work structures from the rare earth phosphates, silicates, germanates, vanadates,
and borates, which have been studied extensively. However, in recent years, except
a few selected crystals of these mixed framework structures, the focus has been
shifted toward the MOF structures popularly known as hybrid framework structures
owing to their exceptional application potential, ease of their synthesis, chemically
tunable structures, and exceptionally high specific surface area. In spite of their
overwhelming popularity, their growth is limited only to the nanosize and not the
bulk single crystals. Readers can find more valuable information on these MOFs in
Refs.
[375
4ZnO
379]
References
[1] K. Byrappa, D.Yu. Pushcharovsky, Crystal chemistry and its significance on the growth
of technological materials: Part 1; silicates, phosphates and their analogs, Prog. Cryst.
Growth Charact. 24 (1992) 269
359.
[2] K.H. Wedeponl, Geochemistry, Holt, Reinhart, and Winston, New York, NY, 1971. p.
231.
[3] D.C.E. Corbridge, The structural chemistry of phosphorous. 4 preparative methods and
growth of rare earth phosphates, Bull. Soc. Fr. Crystallogr. 94 (1971) 271.
[4] K. Byrappa, Preparative methods and growth of rare earth phosphates, Prog. Cryst.
Growth Charact. 3 (1986) 163
198.
[5] D.C.E. Corbridge, Topics in Phosphorus Chemistry, Plenum Press, New York, NY,
1968.
