Figure 2: Phonon spectrum measured near resonant crossings.

(a) Fits to the triple-axis (BT7) spectrum measured at 488 K for the crossing near the Γ point along Γ to X in the (200) zone (path is indicated in illustration to the right), where the red dashed-line peaks correspond to the fits to the local mode. (b) Fits to the spectrum measured at 688 K along the same Γ to X path, where the black dashed-line peaks correspond to the fits to the local mode. The blue shaded area indicates extra intensity not accounted for by the peak fit, which may correspond to a ‘waterfall’ effect associated with mode coupling, see text. (c) Fits to the time-of-flight chopper (ARCS) spectrum measured at 488 K along Γ to M in the (10) zone (the path is indicated in the illustration on the right). The arrows indicate an extra dispersionless peak superimposed on the TO mode. (d) Fits to the ARCS spectrum measured at 688 K along the same Γ to M path.