Geoscience Reference
In-Depth Information
120
(a)
100
80
O-H
(b)
60
P-O
40
O-P-O
Ga-O
20
Al-O
0
3600
2800
2000
1200
400
Wave number (cm
-1
)
Figure 5.34 FT-IR spectra of (a) AlPO
4
(wet) and (b) as-grown single crystals of GaPO
4
[122]
.
Source: Courtesy of the Gauthier-Villars/ESME, Paris.
characteristic GaPO
4
samples. It can be observed that the (OH)
2
content increases
with the temperature gradient.
The dielectric properties have been studied in detail by Hirano et al. (1990) for
GaPO
4
crystals
[119]
. According to the authors, the frequency independence of
ε
0
at room temperature in this crystal indicates that mainly electronic ionic polariza-
tions contribute to the dielectric constant of GaPO
4
. These two polarizations always
exist below 10
13
Hz.
Figure 5.35
shows the extrapolation of possible piezoelectric properties to other
quartz-like materials
[117,133]
.
Table 5.12
gives the extrapolation of AT-cut char-
acterizations from well-known quartz and quartz-like materials.
Cochez et al. (1994)
[126]
have well studied and extrapolated the relations that
exist between the crystal structure distortions, expressed in terms of the M
O
X
angle (where M
P, As) value and different physical
properties. For instance, piezoelectric characteristics of the AT-cut and dielectric
constant anisotropy of three known materials (SiO
2
,AlPO
4
, and GaPO
4
) have been
linearly related to their M
5
B, Al, Ga, Fe, Mn, and X
5
O
X angle value successfully. Conversely, knowledge
of the M
X value allows the prediction of physical properties for quartz-like
crystals that have not been synthesized. Consequently, using only M
O
O
X angles,
one can describe the structural strains, the
transition existence, and some phys-
ical properties, such as density, piezoelectric characteristics, and elastic constants.
Based on this reasoning, GaAsO
4
and GeO
2
appear to be the most promising
materials for piezoelectric applications, unfortunately, the concomitant increase in
structural distortions from quartz to GaAsO
4
and GeO
2
make their crystal growth
very difficult. Thus, from the crystal growth point of view, GaPO
4
seems to be the
αβ