  We report investigation of structural phase transitions in technologically
important material sodium niobate as a function of temperature on heating over
300-1075 K. Our high resolution powder neutron diffraction data show variety of
structural phase transitions ranging from non-polar antiferrodistortive to
ferroelectric and antiferroelectric in nature. Discontinuous jump in lattice
parameters is found only at 633 K that indicates that the transition of
orthorhombic antiferroelectric P (space group Pbcm) to R (space group Pbnm)
phase is first order in nature, while other successive phase transitions are of
second order. New superlattice reflections appear at 680 K (R phase) and 770 K
(S phase) that could be indexed using an intermediate long-period modulated
orthorhombic structure whose lattice parameter along <001> direction is 3 and 6
times that of the CaTiO3-like Pbnm structure respectively. The correlation of
superlattice reflections with the phonon instability is discussed. The critical
exponent ({\beta}) for the second order tetragonal to cubic phase transition at
950 K, corresponds to a value {\beta}$\approx 1/3$, as obtained from the
temperature variation of order parameters (tilt angle and intensity of
superlattice reflections). It is argued that this exponent is due to a second
order phase transition close to a tricritical point. Based on our detailed
temperature dependent neutron diffraction studies, the phase diagram of sodium
niobate is presented that resolves existing ambiguities in the literature.
