Figure 2: Fourier-transform inelastic X-ray scattering (FT-IXS) spectra for transverse two-phonon excitations along the bonding direction.

(a,c) Time-domain differential X-ray diffuse scattering intensities at various q coordinates along the approximate Γ to X (Δ) high-symmetry reduced wavevector direction in both the (1 3) Brillouin zone (BZ) and the (0 4) BZ respectively. This direction is shown in both BZs in Fig. 1a. The observed modulations stem from temporal coherences in the momentum-dependent phonon-phonon correlations. (b,d) Simultaneous frequency and momentum representations of the time-domain data in a,c, respectively, on a base-10 logarithmic scale, with two-phonon dispersion calculated from the INS data of ref. 28 overlaid. The trajectories of the momentum coordinates are displayed above each spectrum in reciprocal lattice units (rlu). Note the deviation of the q_x and q_z coordinates from the nominal Δ wavevector coordinates in panel (d). Clear matches with first overtone and combination transverse polarized phonon states are seen. Note that broadband behaviour seen near |q_y|∼0.2 in panel d is due to noisier statistics that arise from an average over a smaller solid angle, in comparison with data at other q coordinates.