  Neutron diffraction studies using powder samples have been used to understand
the complex sequence of low temperature phase transitions of NaNbO3 in the
temperature range from 12 K-350 K. Detailed Rietveld analysis of the
diffraction data reveal that the antiferroelectric to ferroelectric phase
transition occurs on cooling around 73 K while the reverse ferroelectric to
antiferroelectric transition occurs on heating at 245 K. However, the former
transformation is not complete till down to 12 K and there is unambiguous
evidence for the presence of the ferroelectric R3c phase coexisting with an
antiferroelectic phase (Pbcm) over a wide range of temperatures. The coexisting
phases and reported anomalous smearing of the dielectric response akin to
dipole glasses and relaxors observed in the same temperature range are
consistent with competing ferroelectric and antiferroelectric interactions in
NaNbO3. We have carried out theoretical lattice dynamical calculations which
reveal that the free energies of the antiferroelectric Pbcm and ferroelectric
R3c phases are nearly identical over a wide range of temperature. The small
energy difference between the two phases is of interest as it explains the
observed coexistence of these phases over a wide range of temperature. The
computed double well depths and energy barriers from paraelectric Pm m to
antiferroelectric Pbcm and ferroelectric R3c phases in NaNbO3 are also quite
similar, although the ferroelectric R3c phase has a slightly lower energy.
