Powder neutron diffraction study of the high temperature phase transitions in NaTaO₃

Neutron powder diffraction has been used to examine the structural phase transitions in the perovskite-like NaTaO₃, from room temperature to 933 K. The room temperature orthorhombic structure (Pbnm, a = 5.4768(1), b = 5.5212(1), c = 7.7890(2)) transforms to orthorhombic Cmcm at around 700 K, then to tetragonal P4/mbm at 835 K, and finally to cubic Pmm above 890 K. The structure in orthorhombic Cmcm is characterized by simultaneous tilting of the oxygen atom octahedron about two of its tetrad axes, the tilting of successive octahedra being out of phase along the b-axis, and in phase along the c-axis. The two tilt angles are comparable just above 700 K, but the out-of-phase tilt angle falls smoothly to zero as the transition to tetragonal is approached, in the manner suggestive of a tricritical transition. This results in an unusual variation of lattice parameters with temperature in the orthorhombic Cmcm phase. In the tetragonal phase the lattice parameters vary smoothly; however near the transition to cubic the in-phase tilt angle changes more rapidly with temperature than might be expected in a continuous phase transition.