The Ruddlesden-Popper (RP) compounds An+1BnO3n+1 exhibit a broad range of technologically interesting properties such as high-Tc superconductivity, colossal magnetoresistance, metal to insulator transitions and ferroelectric / multiferroic ground states. In a study of one of these compounds Ca3Mn2O7, we have recently made the remarkable discovery of significant negative thermal expansion. This has led us to propose a new mechanism and strategy for designing NTE materials. We request now two days on HRPD to study the closely related phase Ca2MnO4 which prior indication suggest also exhibits substantial NTE. This work will provide the community with a better understanding of this new NTE mechanism enabling the future design of new materials with enhanced properties.