Fig. 2.

INS measurements reveal two behaviors in different portions of the acoustic phonon branch. (A–D) S($\left|\mathbf{Q}\right|,E$) maps measured with $E_i$ = 12 meV at the CNCS in the ordered phase, ($A$) 300 K and ($B$) 460 K, and the superionic phase, ($C$) 530 K and ($D$) 600 K. The horizontal band around 3.5 meV at 300 K shows the nondispersive phonons. This mode damps and merges into the quasielastic lines at $T$ $>$ $T_{od}$. The vertical streaks, which intensify with increasing temperature, represent the dispersive acoustic phonons especially at $\left|\mathbf{Q}\right|$ = 3.3 ${\AA }^{-1}$. ($E$) $S\left(E\right)$ cuts from $E_i$ = 4 and 12 meV data, integrated over the range of 0.7 $\le$ $\left|\mathbf{Q}\right|$ $\le$ 2.5 and 1 $\le$ $\left|\mathbf{Q}\right|$ $\le$ 4 ${\AA }^{-1}$, respectively. The two datasets are stitched by matching intensities at $E\hspace{0.17em}=$ 2 meV. The 3.5-meV peak broadens with temperature increase and smears out at $T$ $>$ $T_{od}$ and the quasielastic width increases. ($F$) Model S($E$) from the DHO with energy $E_0$ and damping width $\mathrm{\Gamma }$, convoluted with the instrument resolution (FWHM = 1.5 meV). Elastic line is included. Purple, blue, and red represent S($E$) at 300, 460, and 750 K.