k=2π/a [1/21/21/2] zero-field ordering in the intermediate phase of CeB6 observed by X-ray scattering:: what orders?

A number of superstructure reflections (h/2 k/2 l/2) with h, k, l = 2n + 1 are observed in the intermediate phase II of CeB6 by resonant X-ray scattering at the L-II and L-III absorption edges of Ce. This gives evidence of a zero-field ordering with wave vector k(0) = 2 pi /a [1/2 1/2 1/2]. The intensities of the reflections (5/2 1/2 1/2), (5/2 3/2 3/2) and (7/2 1/2 1/2) in nonresonant conditions, where reliable calculations can be made, are consistent neither with the Jahn-Teller pair distortion nor with the Gamma (3) quadrupolar ordering proposed in the literature as possible zero-field ordering mechanisms. Apparently, the rs quadrupolar ordering accompanied by displacements of the boron atoms should also be considered, but the experimental data do not allow to draw definite conclusions. (C) 2001 Elsevier Science B.V. All rights reserved.