IR SPECTROSCOPY OF HEMIMORPHITE BETWEEN 82-K AND 373-K AND OPTICAL EVIDENCE FOR A LOW-TEMPERATURE PHASE-TRANSITION

Polarized IR absorption spectra of oriented hemimorphite single-crysta ls were recorded in the region of the O-H fundamentals and combination bands between 82 and 373 K. The almost exclusive orientation of the a bsorption bands in the (010) plane is in agreement with previous neutr on structure refinements. The weak H bonds of the OH groups and water molecules are confirmed by the energies of the OH stretching fundament als around 3350-3600 cm(-1). However, even though two O-H stretching b ands in both a and c are predicted by factor group analysis, three str ong bands in a, and two in c are observed. This fact, as well as the o ccurrence of weak satellite bands in the a and c spectra, and a minor component parallel to the b axis direction, indicate complicated dynam ics of the water molecules and OH groups within the structural channel s. These motions can be partly explained by a twisting motion of the w ater molecule around its two-fold molecular axis. A much better interp retation for these motions, however, is found in dynamic proton disord er, which describes a hopping motion between disordered proton sites. A possible dynamic disorder-order phase transition at low temperatures was tested by determination of bi-refringence values (Delta n) of hem imorphite between 82 and 373 K. A strong discontinuity in the slope of Delta n(X,Y) and Delta n(Y,Z) was observed at 98(2) K. This, together with considerable energy shifts of IR bands upon cooling, provides ev idence for a low-temperature second-order phase transition in hemimorp hite.