Figure 9.

Inelastic scans measured at a q_∥-value of q_∥=1.37 Å ⁻¹. The spectra can be fitted using different models: all models contain an elastic line, centred at energy transfer 0 (described by a Gaussian peak profile) and a Lorentzian peak also centred at ‘0’ to account for the incoherent scattering. (a) A fit including three phonon excitations, which shows a good agreement with the measured data. It was also attempted to fit the spectra with two excitations, and one excitation only (c). By comparing the resulting χ²-values, the fit with three phonons provides the best model to the data. This is also supported by the results of an F-test in table 3.