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

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 weak H bonds of the OH groups and water molecules are confirmed by the energies of the OH stretching fundamentals around 3350-3600 cm (super -1) . Two O-H stretching bands in both a and c are predicted by factor group analysis, but the bands observed are three strong bands in a and two in c: these results indicate complicated dynamics of the water molecule and OH groups within the structural channels. Although these motions can be explained by a twisting motion of the water molecule around its two-fold molecular axis, it is considered that a much better interpretation can be found in dynamic proton disorder, which describes a hopping motion between disordered proton sites. A possible dynamic disorder-order phase transition at low T was tested by determination of the birefringence values (Delta n) of hemimorphite between 82 and 373 K; a strong discontinuity in the slope of Delta n (sub XY) and Delta n (sub YZ) was noted at 98 K which, together with considerable energy shifts of IR bands on cooling, provides evidence for a low-T second-order phase transition in hemimorphite.