In situ TEM studies of para—ferro phase transitions in NaNbO3

It is found from TEM observations in NaNbO3 crystals that the phase transition series from the P phase is P—R1—R2—S and that one, related to the Q phase is Q—Q1—Q2—S. It is shown that the R phase is composed of two subphases R1 and R2. The main difference between R1 and R2 is that the η values corresponding to these two phases in the superlattice spots [1/2, 1/2 ± η1/2, 0]* are 1/4 and 1/6, respectively. But the η values in the superlattice spots [1/2 ± η 1/2, 0]* corresponding to the Q1 and Q2 phases are 0 and 1/3, respectively. The main soft mode of the transition from phase S to R2 is determined as the zone boundary transverse optical mode Σ2 with wave vector q equal to [1/2, ξ, 0]*. The appearance of a superlattice spot at [1/2, 1/2, 0]* shows that the mode characterizing the tilt of octahedra is also softened due to mode—mode coupling.