IR spectroscopy of hemimorphite between 82 and 373 K and optical evidence for a low-temperature phase transition

Abstract

Polarized IR absorption spectra of oriented hemimorphite single-crystals were recorded in the region of the O-H fundamentals and combination bands between 82 and 373 K. The almost exclusive orientation of the absorption bands in the (010) plane is in agreement with previous neutron structure refinements. The weak H bonds of the OH groups and water molecules are confirmed by the energies of the OH stretching fundamentals around 3350-3600 cm^(-1). However, even though two O-H stretching bands in both ɑ and c are predicted by factor group analysis, three strong bands in ɑ, and two in c are observed. This fact, as well as the occurrence of weak satellite bands in the a and c spectra, and a minor component parallel to the b axis direction, indicate complicated dynamics of the water molecules and OH groups within the structural channels. These motions can be partly explained by a twisting motion of the water molecule around its two-fold molecular axis. A much better interpretation for these motions, however, is found in dynamic proton disorder, which describes a hopping motion between disordered proton sites. A possible dynamic disorder-order phase transition at low temperatures was tested by determination of birefringence values (Δn) of hemimorphite between 82 and 373 K. A strong discontinuity in the slope of Δn_(x,γ) and Δn_(γ,z) was observed at 98(2) K. This, together with considerable energy shifts of IR bands upon cooling, provides evidence for a low-temperature second-order phase transition in hemimorphite.