Fig. 3. Phonon dispersions up to 6 meV.

In a–d we show representative examples of [VO(acac)${}_2$] phonon dispersions measured on LET with $E_i$ = 7.3 meV and $T$ = 5 K. This incident energy allowed us to resolve acoustic modes up to 6 meV. Dispersions curves have been obtained integrating over two out of the three components of the neutron scattering vector $\mathbf{Q}=\left[\eta ,\zeta ,\xi \right]$ (expressed in terms of reciprocal lattice vector units) around different Bragg peaks. a Shows longitudinal acoustic modes along the symmetry direction $\mathrm{\Gamma }$–Z around [0, 0, −2]. b–d Report transverse acoustic modes along three principal symmetry directions: $\mathrm{\Gamma }$–Z around [−1, 0, −3], $\mathrm{\Gamma }$–X around [0, 0, −2] and $\mathrm{\Gamma }$–Y around [0, 0, −2] and [0, 1, −2]. The colour bar reports the intensity normalised for the maximum in each panel. Simulated dispersion curves along the same directions are reported in e–h and were calculated with Eq. (3) (see Methods), with phonon energies and polarisation vectors obtained from DFT.