This proposal seeks PEARL beam time to conduct the first high-pressure neutron diffraction study of the monovalent azides XDF2, where X = Na, ND4, Cs and Tl. In so doing, we will be able to contrast and compare the phase transition behaviour of the bihalides with that of the closely related monovalent azides, which are the subject of an ongoing PEARL study. In the case of NaDF2 and CsDF2 the main aim will be to collect data of sufficient quality to permit a full structure solution of three known high-pressure phases. For the ammonium salt, we will investigate the pressure evolution of the two forms of hydrogen bonding present in this system and attempt to isolate and solve the structure of ND4DF2 phase III. Finally, we will investigate the pressure response of the unusual cubic phase of TlDF2 and determine whether any high-pressure forms of this compound exist for pressures up 9 GPa.