Phase stability and structural temperature dependence in sodium niobate: A high-resolution powder neutron diffraction study

We report an investigation of structural phase transitions in the technologically important material sodium niobate as a function of temperature on heating over 300-1075 K. Our high-resolution powder neutron diffraction data show a variety of structural phase transitions ranging from nonpolar antiferrodistortive to ferroelectric and antiferroelectric in nature. A discontinuous jump in lattice parameters is found only at about 680 K. This observation along with the reported thermal hysteresis in the dielectric anomaly indicate that the transition of the orthorhombic P to R phase is first order in nature, while other successive phase transitions are of second order. Additional 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 the < 001 > direction is three and six times, respectively, that of the CaTiO3-like Pbnm structure. 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 about 950 K corresponds to a value beta approximate to 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, a phase diagram of sodium niobate is presented that resolves existing ambiguities in the literature.