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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
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