Site occupancy wave and charge density wave in the modulated structure of Nd0.6Gd0.4Se1.85

摘要

Single crystals of Nd0.6Gd0.4Se1.85 have been prepared by chemical transport reactions starting from pre-annealed powder samples. Satellite reflections observed in X-ray and electron diffraction experiments indicate the presence of a two-dimensional lattice distortion. The origin of this is a site occupancy wave and, coupled to this, a charge density wave in the planar selenium layer of this compound. The modulated structure has been solved and refined from X-ray data using the superspace approach. Nd0.6Gd0.4Se1.85 can be described in the (3 + 2)-dimensional superspace group P4(alphabeta1/2) (beta - alpha1/2)00 with lattice parameters of a = 4.088 (1) Angstrom, c 8.336 (1) Angstrom. Diffuse scattering contributions have been observed around the satellite reflections which can be understood by the formation of domains with slightly different ordering patterns of Se atonts. Thus, the incommensurate translational parts of the modulation vectors are not constant over the whole crystal and can be described as alpha = 0.29 + k and beta - 0.29 - k with -0.05 less than or equal to k less than or equal to 0.05. (C) 2004 Elsevier Inc. All rights reserved.