Proton disorder in dehydrated hemimorphite -
IR spectroscopy and X-ray structure refinement at low
and ambient temperatures
Eugen Libowitzky1,2,
Thomas Kohler3, Thomas Armbruster3,
and George R. Rossman2
1Institut
für Mineralogie und Kristallographie, Universität
Wien - Geozentrum,
2 Division of Geological and Planetary Sciences, California Institute
of Technology,
Pasadena, CA 91125, USA*
3Laboratorium
für Chemische und Mineralogische Kristallographie,
Universität Bern,
Ereiestr. 3, CH-3012 Bern. Switzerland
Abstract:
Polarized IR absorption spectra of
dehydrated hemimorphite single-crystals [Zn4Si2O7(OH)2]
were recorded between 82 and 373 K. The crystal structure, including
the proton positions, was refined in space group Imm2 from
single-crystal X-ray data measured at 110 and 298 K. Both methods, as
well as electrostatic and crystal chemical considerations, show that
the protons of the hydroxide groups are disordered in the structural
channels. One proton is located at the line between two O3 atoms
(O3···O3' = 2.99-3.02 Å)
thus building
up a weak H bond. The corresponding O-H stretching band occurs at ~3530
cm-1 and is polarized parallel to a. Due to
proton-proton repulsion, the O-H vector of the other H atom deviates
from the O3···O3' line. As a
consequence, the corresponding IR stretching vibration is observed at
~3600 cm-1 and components of this band occur
parallel to a and c. The disorder is also
reflected by the large anisotropic displacement pa rameter U33
of the O3 atom.
To test for a possible low-temperature
phase transition (similar to that in natural hemimorphite),
birefringence values were determined in (010) and (001) sections
between 83 and 373 K. However, the smooth curves do not indicate a
transition. This is explained by the lack of an additional proton
acceptor (O5 of the water molecule in hemimorphite) in the dehydrated
phase.
European Journal of Mineralogy 9, 803-810.