Simple hydrides are of scientific interest due to their suitability for storage of hydrogen in the solid state. These materials also display a wealth of interesting physics, such as superconductivity under pressure, and high ionic conductivity at elevated temperatures. Recently BaH2 was observed to show a first order structural phase transition under applied pressure; as this was observed with X-rays, information on the positions of the hydrogen atoms was not observable. We propose to perform a high pressure neutron diffraction experiment on the isotopically similar BaD2 to determine the positions of the D atoms through the transition, providing valuable information on the nature of the high pressure phase.