This proposal seeks to conduct total-scattering studies on thallium and caesium azides to investigate the effect of temperature on the atomic motions, particularly of the azide anions. This total-scattering study will complement previous diffraction and computational studies by providing a conclusive experimental account of the rotational disorder of the azides. This will provide an unrivalled insight into the structural re-arrangements that occur during, and immediately preceding, phase transitions in the metal azides. Furthermore, the structural data obtained in this work will be valuable to those who seek to model high-energy processes (e.g. explosive decomposition) at extreme conditions.