System The calculated phonon dispersion and atom-projected density of states (PDoS) curves for the three oxy-chalgoenides are shown in Fig. . All three dispersions are free from imaginary harmonic modes, indicating that the structures are dynamically stable. We note that this indicates the I4/mmm structure of Bi 2 SO 2 represents a minimum on the structural potential-energy surface. All three phonon dispersion curves are split into two regions, viz. a low-frequency region composed of modes dominated by motion of the heavier Bi and Ch atoms, and a higher-frequency region dominated by motion of the O atoms. The two regions are separated by a "phonon bandgap" of ∼ 1 -2.5 THz. As the mass of the chalcogen increases, the Ch peak in the PDoS occupies a narrower frequency range, which has the effect of flattening the associated low-frequency bands in the dispersion. Since the Bi PDoS extends over the whole of the lower-frequency region, this has a smaller effect on the overall bandwidth, although the bandwidth does fall from Bi 2 SO 2 to Bi 2 TeO 2 . More interestingly, whereas the dispersion of the O-based higher-frequency bands is similar in Bi 2 SO 2 and Bi 2 SeO 2 , the dispersion in Bi 2 TeO 2 has a markedly smaller bandwidth, which implies that the chalcogen layer exerts a significant influence on the oxide layer.