the sound velocity of the LA-phonon propagating along $[110]$. Owing to the neutron cross section and selection rules associated with phonon eigenvectors [@Harada70], sensitivity to both the TA1 and TA2 phonon modes is obtained with triple-axis measurements when the crystal is aligned in the $hk0$ scattering plane while alignment in the $hhl$ plane only affords measurements of the TA1 phonon. Time of flight measurements using chopper spectrometers allow momentum transfers out of the horizontal scattering plane to be measured and therefore afford sensitivity to both TA1 and TA2.

Typical data of TA1 and TA2 phonons are shown in Fig. \[fig:scans\] for temperatures $T = 10$ K (tetragonal phase) and $T = 80$ K (cubic phase). In panels (a) and (b) example $(4q0)$ (TA1) and $(4-q, 4+q, 0)$ (TA2) phonon data from IN22 are shown. Panels (c) and (d) depict typical $(44q)$ (TA1) and $(4-q, 4+q, 0)$ (TA2) slices from MERLIN. The TA1 mode does not show any $T$-dependence while the TA2 mode becomes softer in energy in the high temperature cubic phase. We selected the example $q$ values shown in Fig. \[fig:scans\] as they are in the region where the effect is strongest and where the peaks can be clearly 