Proton disorder in dehydrated hemimorphite - IR spectroscopy and X-ray structure refinement at low and ambient temperatures

Abstract

Polarized IR absorption spectra of dehydrated hemimorphite single-crystals [Zn_4Si_2O_7(OH)_2] were recorded between 82 and 373 K. The crystal structure, including the proton positions, was refined in space group Imml 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 ɑ. 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 ɑ and c. The disorder is also reflected by the large anisotropic displacement parameter U_(33) 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 (05 of the water molecule in hemimorphite) in the dehydrated phase.