Figure 3: Phonons measured using a time-of-flight chopper spectrometer.

(a) Intensity slice of the data at 688 K in the (00) zone at the energy of the localizing mode; E=[11.5, 12] meV. Transverse scattering geometry selects the TO mode preferencially along Q=[0, K, 0] between Γ and X. The locations of the high-symmetry points in the zone are shown on the right of the figures. The small white box indicates the location and the K and H integration range of the energy cut in b. (b) Energy cut at Q=[−2, 0.37, 0] confirming the localizing mode feature found in Fig. 1. (c) Intensity slice of the data in the (20) zone at the energy of the localizing mode with the same energy integration range as in a. The small white box indicates the location and the K and H integration range of the energy cut in d. (d) Energy cut at Q=[−2.3, 1.7, 0] showing that the localizing mode feature appears along the [110] direction at the same energy as along the [100] direction. (e) Phonon dispersion at 688 K along Q=[−2+H, 2+H, 0] in the (10) zone. (f) Same as e, except at 488 K. The indicated second crossing is the point where the localizing mode intersects the TO mode near the M point at the zone edge, as illustrated on the right.