Geoscience Reference
In-Depth Information
concentrations of H
2
PO
4
, HPO
2
4
, and/or Cl
2
ions. Quantitative NMR determina-
tion of proton content unequivocally determines the H
2
O and hydroxyl content,
whereas complementary IR absorbency studies of the 3300 cm
2
1
band confirm
the molar absorption coefficient of 81 lmol/cm originally determined for liquid
H
2
O by Thompson (1965)
[114]
. The dielectric constant variability of berlinite
prepared by different investigators is believed to result from (i) a variation in H
2
O
content in microscopic and macroscopic inclusions and (ii) unidentified crystalline
defects.
On the whole the research on the bulk crystal growth of berlinite has declined
significantly except for some recent publications on the special physical character-
istics like, for example, variations in the structure and molecular dynamics with
pressure, as it shows memory glass property under high pressure
[115]
. Besides,
berlinite attracts the attention of researchers as a substrate material for a variety of
nitrides
[116]
.
5.4 Gallium Phosphate
Gallium phosphate, like aluminum phosphate, belongs to the family of quartz and
quartz-like MXO
4
, where M
5
B, Al, Ga, Fe, Mn, and X
5
P, As
[117]
. Among
them, AlPO
4
and GaPO
4
have been studied extensively with reference to their
crystal growth, crystal chemistry, and physical properties, especially piezoelectric
properties
[82,118
123]
. Both AlPO
4
and GaPO
4
exhibit piezoelectric characteris-
tics which are superior
-quartz. As evident from
Table 5.10
, for the same AT-cut, GaPO
4
exhibits a larger coupling coefficient
which makes GaPO
4
a most promising material in the field of wide-band filters.
In addition to this, the
to those of conventional
α
phase transition for GaPO
4
occurs at 976
C, and
hence, it can be used in a very wide range of temperatures for filters and sensor
devices
[124
αβ
127]
. In fact, GaPO
4
is the most promising one and more exten-
sively being studied today among all other piezoelectric crystals.
Table 5.10 Comparison of Some Piezoelectric Characteristics
[117,123]
Parameter
Quartz
Aluminum Phosphate
(Berlinite)
Gallium Phosphate
Coupling coefficient
a
k (%)
8.5
11.0
.
16.0
Surtension coefficient
a
Q
3.10
6
10
6
.
5.10
4
αβ
Phase transition (
C)
573
584
No
a
AT-cut
