On the lattice parameters of sodium niobate at room temperature and above

Sodium niobate undergoes a complicated sequence of phase transitions on heating from room temperature, before becoming cubic above 641 degrees C. In all five non-cubic phases exhibited, the octahedra are tilted about < 100 > pseudocubic directions. The phases stable between - 80 degrees C and 360 degrees C, and between 360 degrees C and 480 degrees C are antiferroelectric with the niobiums displaced from the centres of the octahedra. The combination of octahedral tilts and cation displacements leads to the edge lengths of the psedocubic cell of each phase becoming a multiple of the cubic cell found above 641 degrees C. We have re-examined these phases using HRPD at ISIS, Rutherford Appleton Laboratory, UK, and find that the multiplicities of the pseudocubic cells describing the phases stable in the ranges 360-480 degrees C and 480-520 degrees C are both 2 x 4 x 6, and not 2 x 2 x 6 and 2 x 2 x 2 as reported in the literature. Furthermore, the high resolution used in the scattering experiment has revealed that the phase stable at room temperature has symmetry which is lower than orthorhombic. (C) 1999 Elsevier Science B.V. All rights reserved.