Neutron and X-ray precession studies of the incommensurate reflections near the 300°C transition in barium sodium niobate.

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Abstract
Reflections at positions incommensurable with respect to the basic tetragonal lattice are disclosed for the first time in Ba2NaNb5O15 in the region of the tetragonal to orthorhombic transition, near 300°C, through neutron scattering and X-ray precession experiments. The satellite reflections are observed at ± (1 + δ) (a* + b*)/4+ c*/2 with δ ⋍ 0.1 and a strong temperature dependence. In the orthorhombic phase these reflections “lock-in” at (± (a*+b*)/4 + c*/2). In the transition region we observed diffused scattering of neutrons and X-ray as well as a soft phonon. The probable occurence of an incommensurate phase is discussed. The observations are consistent with a 4 dimensional order parameter at (a*/4, b*/4, c*/2).