We recently used high-resolution neutron diffraction on HRPD to observe the largest magneto-elastic coupling ever seen in a metallic magnet. The material studied was CoMnSi, a non-collinear antiferromagnet in which strong competing exchange interactions, rather than the spin-orbit interaction, seem to be the origin of giant magnetoelastic coupling. In this experiment, we propose to extend our study of materials with the same, MnP-based structure to NiMn(Ge,Si) as we believe that we will not only observe giant magneto-elastic coupling but also negative thermal expansion (NTE), again mediated by competing exchange.