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| Titel |
An IR and Calorimetric Investigation of the Structural, Crystal-Chemical and Thermodynamic Properties of Hydrogrossular |
| VerfasserIn |
C. A. Geiger, E. Dachs |
| Konferenz |
EGU General Assembly 2012
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 14 (2012) |
| Datensatznummer |
250059395
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| Zusammenfassung |
| The garnet class of phases is extremely broad in terms of composition and structural
properties. Garnet is found in nature and various synthetic garnet phases have a number of
important technical applications. There exist the rock-forming silicate garnets that are so
widespread geologically. An additional class is given by the so-called “hydrogarnets” in
which the tetrahedral site (Wyckoff position 24d) is empty. At relatively low temperatures
there is complete solid solution between Ca3Al2Si3O12 and Ca3Al2H12O12, for
example. The substitution mechanism can be written as O4H4 - SiO4. The latter,
pure OH-containing end-member, which has not been found in nature, is termed
katoite/hydrogrossular. Its structure has been investigated by various workers by X-ray and
neutron diffraction and by proton NMR, IR and Raman spectroscopic methods.
At ambient conditions the structure has the “standard” garnet cubic symmetry of
Ia-3d. At high pressures, and possibly at low temperatures, a different structure may
occur.
We measured the low temperature IR spectra and heat capacity of katoite in order to
understand its structural, crystal-chemical and thermophysical properties. A sample of
Ca3Al2H12O12 was synthesized hydrothermally in Au capsules at 250 °C and 3 kb water
pressure. X-ray powder measurements show that about 98-99% katoite was obtained.
Powder IR spectra were recorded between 298 K and 10 K. The measured spectra are
considerably different in the high wavenumber region, where O-H stretching modes occur,
between 298 K and 10 K. At room temperature the IR-active O-H band located
around 3662 cm-1 is broad and it narrows and shifts to higher wavenumbers and also
develops structure below about 80 K. Concomitantly, additional weak intensity
O-H bands located around 3600 cm-1 begin to appear and they become sharper
and increase in intensity with further decreases in temperature down to 10 K. The
spectra indicate that the vibrational behavior of individual OH groups and their
collective interactions measurably affect the lattice dynamic (i.e. thermodynamic)
behavior.
The low temperature heat capacity behavior was investigated with a commercially designed
relaxation calorimeter between 5 and 300 K on a mg-sized sample. The heat capacity data are
well behaved at T < 300 K and show a monotonic decrease in magnitude with decreasing
temperature. A standard third-law entropy value of So = 421.7 ± 1.6 J/mol-
K was calculated.
Using this new calorimetric-based So value and published standard enthalpy of
formation data for katoite, a calorimetric-based Gibbs free energy of formation at
298 K can be obtained as ΔG°f = -5021.2 kJ/mol. The Cp data show no evidence
for any phase transition as possibly expected by the change in OH-mode behavior
with decreasing temperature. We have no explanation for the appearance of the
additional modes. It is worth noting that the katoite crystal structure in terms of
lattice dynamic or thermodynamic behavior should be thought of having OH groups
and not O4H4 clusters or polyhedral units as is often written in the literature. The
single crystallographic OH group in katoite shows very weak, if any, hydrogen
bonding and the H atoms have large amplitudes of vibration. The weak H bonding
controls the nature of low energy OH-related vibrations and this leads to its large So
value. |
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