The rate at which phonons move throughout a material depends on the character of the phonon modes. Historically, heat transport by phonons was thought to occur in a propagating manner on a length-scale much larger than interatomic distances (phonon-gas model). In disordered, anharmonic and structurally more complex solids, heat transport can alternatively be conducted by fundamentally different transport mechanisms, one of which is the diffuson. In contrast to the phonon-gas model, diffusons correspond to heat transport via local (atomic) scale random walk. This diffusive walk of heat means that non-propagating vibrations in a solid transfer thermal energy between adjacent diffuson modes, at a much smaller length scale compared to typical propagating modes (shown schematically in Figure ).