Vibrational excitations ofAs2O3. II. Crystalline phases

Infrared absorption and polarized Raman spectra of monoclinic ${\mathrm{As}}_2$ ${\mathrm{O}}_3$ and the room temperature Raman spectrum of cubic ${\mathrm{As}}_2$ ${\mathrm{O}}_3$ have been recorded. A number of vibrational features possessed by the crystalline modifications of arsenic trioxide are common to crystals of ${\mathrm{As}}_2$ ${\mathrm{S}}_3$ and ${\mathrm{As}}_2$ ${\mathrm{Se}}_3$, but have been resolved in greater detail. In particular, the layered monoclinic phase, claudetite, exhibited rigid layer modes at Raman shifts of 30, 38, and 49 ${\mathrm{cm}}^{-1\mathrm{}}$, and numerous Davydov doublets, all of which appeared as a consequence of weak layer-layer coupling. Oriented samples of claudetite yielded polarized Raman measurements which did not obey the symmetry-determined selection rules of the $C_{2h}^5-\frac{P{2}_1}{n}$ space group for reasons which can be traced to the fact that claudetite is biaxial. However, the vibrational frequencies of claudetite scale consistently to those of monoclinic ${\mathrm{As}}_2$ ${\mathrm{S}}_3$ and ${\mathrm{As}}_2$ ${\mathrm{Se}}_3$ by scaling factors of 0.82 and 0.58, respectively. Thus, many claudetite lines are related by the empirical scaling result to ${\mathrm{As}}_2$ ${\mathrm{S}}_3$ modes belonging to vibrational species which have been identified by infrared reflectivity measurements. Recurring bands in the ordered and disordered phases of ${\mathrm{As}}_2$ ${\mathrm{O}}_3$ are noted and structural similarities between the different phases are evidenced by vibrational features that are comparable in frequency and symmetry. Accordingly, layered structure in arsenic trioxide glass is inferred.