We request 2 days on HRPD to perform a detailed study of the low temperature behaviour of γ-Mg(11BD4)2 in order to measure the change lattice parameter as a function of temperature. We will then use these data to model the phonon density of states responsible for the negative thermal expansion observed between 2K and 300K. Understanding this process will allow us to establish a clear link between the low energy rigid body movement and the bulk properties of the material. When combined with Inelastic Neutron Spectroscopy and ab initio DFT calculations, we will be able to build a clear model for the low temperature behaviour of γ-Mg(11BD4)2.