Mulliken symbols and frequencies of zone-centered optical phonon modes of Bi2OX2 (X = S, Se and Te). Raman or IR activity of each mode is also indicated by “Raman” and “IR”. The unit of the phonon frequency is cm−1.
| Symmetry | Bi2OS2 | Bi2OSe2 | Bi2OTe2 | Activity |
|---|---|---|---|---|
| Eu | 26.0 | 18.8 | 6.5 | IR |
| Eg | 30.7 | 25.9 | 20.0 | Raman |
| Eg | 63.2 | 55.8 | 41.1 | Raman |
| A2u | 63.9 | 54.6 | 50.6 | IR |
| A1g | 73.2 | 64.1 | 53.5 | Raman |
| Eu | 97.7 | 73.2 | 49.2 | IR |
| Eg | 111.4 | 80.5 | 54.8 | Raman |
| Eu | 126.6 | 89.4 | 83.5 | IR |
| A2u | 129.4 | 97.2 | 84.5 | IR |
| A1g | 132.2 | 88.6 | 75.1 | Raman |
| Eg | 138.4 | 100.2 | 101.1 | Raman |
| A1g | 149.7 | 138.6 | 123.9 | Raman |
| Eu | 262.1 | 228.1 | 183.7 | IR |
| A2u | 286.0 | 182.3 | 140.9 | IR |
| A1g | 346.5 | 217.9 | 163.2 | Raman |
| B1g | 363.5 | 342.2 | 311.2 | Raman |
| Eg | 415.0 | 376.0 | 321.9 | Raman |
| A2u | 466.0 | 420.0 | 372.1 | IR |