Hydrothermal Growth of Some Selected Crystals - Hydrothermal Technology - page 196

special characteristics (such as quartz structural channels) seems very interesting. It
is possible to obtain different types of structures, metallic and nonmetallic with spe-
cial properties [65] . Besides, quartz substrates are popularly used for the growth of
thin films of various compounds for special applications [66] .
Thus, any work on quartz is still a promising field, and it will have some impor-
tant consequential effects on hydrothermal growth, in general, and facilitate the
development of other materials production.

5.3 Berlinite

Berlinite, AlPO 4 , is named after N.J. Berlin (1812), a pharmacologist of the
University of Lund. Its physical properties are given in Table 5.4 [67] . Berlinite
replaces quartz in electronic devices because its large mechanical coupling factors are
greater than

-quartz, and its resonant frequency is nearly independent of temperature
for certain orientations [68

α

70] . Berlinite is also interesting because its presence in
Ca aluminum phosphate bioglass ceramics is believed to lead to higher bioactivity of
bone implants [71

73] . Table 5.5 gives a tentative comparison with quartz.
Berlinite occurs in nature associated with augelite, attacolite, and other phos-
phates as small crystals at the Westana Iron mines near Nasum, Kristiaanstad,
Sweden [73] . Always berlinite occurs in association with metamorphic or hydro-
thermally altered rocks. Exceptionally, for the first time occurrence and formation
of berlinite have been reported under sedimentary conditions was found in a highly
unusual cave setting of Cioclovina Cave, Romania [74] . However, the synthetic
berlinite is more popular and abundant, and it can be obtained as fairly big crystals.
The first successful growth of berlinite crystals was by Jahn and Kordes (1953)
[75] , followed by Stanely (1954) [76] . Stanely carried out the experiments in a
sealed borosilicate glass vessel placed in a low-pressure environment. Mason
(1950) [77] carried out the piezoelectric measurements for a small crystal of berli-
nite. However, the importance of berlinite only came into light in 1976 when
Chang and Barsch [68] studied the piezoelectric properties of berlinite and reported
large mechanical coupling constants and that its resonant frequency was nearly
independent of temperature for certain orientations.
The main problem connected with the growth of berlinite is the negative TC of
solubility, which insists on some special growth conditions, because of the lower
solubility at higher temperatures and higher solubility at lower temperatures. One
of the principal problems in a systematic growth of AlPO 4 crystals is the growth
rate relationship in H 3 PO 4 and the lack of good solubility data (because of the
negative TC of solubility) at the constant fill conditions, which are essential for
successful growth [78] .
Although much of progress has been achieved in the growth of berlinite crystals,
our knowledge on AlPO 4 is still comparable to that of quartz some 30 years ago,
particularly with reference to solubility, reproducibility, crystal perfection, phase
equilibria data, crystal size, and so on.