In this experiment we propose to perform diffraction on isotopically substituted samples of Ca/Yb:ND/H3 and Ca/Yb:15ND3 at two concentrations (8 and 2 MPM) using the new NIMROD time-of-flight neutron diffractometer. Detailed structural information of the nearest neighbour N-D, N-N and Ca/Yb-N correlations will be obtained using difference analysis of these isotopically enriched samples. In addition, the extended low-Q limit of NIMROD gives us access to the regime of long-range correlations which fell off the coverage of the SANDALS instrument. This region is extremely important if we are to understand the longer-range interactions and correlations that lead to the onset of metalisation, and which give rise to a pre-peak below 0.3A-1. We also have the quite extraordinary possibility of looking directly at the onset of critical fluctuations as we approach metal-nonmetal phase separation.