We present new results on the microscopic nature of the ferroelectricity mechanisms in Ca 3 Mn 2 O 7 and Ca 3 Ti 2 O 7 . To the first approximation, we confirm the hybrid improper ferroelectric mechanism recently proposed by Benedek and Fennie for these Ruddlesden-Popper compounds. However, in Ca 3 Mn 2 O 7 we find that there is a complex competition between lattice modes of different symmetry which leads to a phase coexistence over a large temperature range and the “symmetry trapping” of a soft mode. This trapping of the soft mode leads to a large uniaxial negative thermal expansion (NTE) reaching a maximum between 250 and 350 K (3.6×10 −6 K −1 ) representing the only sizable NTE reported for these and related perovskite materials to date. Our results suggest a systematic strategy for designing and searching for ceramics with large NTE coefficients.