Incommensurately modulated lanthanide coinage-metal diarsenides. I. CeAu(1-delta)As2 [delta = 0.015 (2)]--a new distortion variant of the HfCuSi2 type with irregularly stacked cis-trans chains of arsenic atoms.

Rare-earth metal coinage-metal diarsenides LnTAs2 (Ln = Y, La, Ce-Lu; T = Cu, Ag, Au) are known to crystallize in structures closely related to the HfCuSi2 type, which comprises a stacking sequence of puckered TAs layers and planar square As nets, separated by the Ln atoms. CeAu(1-delta)As2, with delta = 0.015 (2), shows an incommensurate positional modulation of the arsenic atoms in the planar As nets. Based on X-ray diffraction data on a twinned crystal, a structure model in the monoclinic superspace group P12(1)/m1(alpha0gamma)00 (No. 11.1) with basic unit-cell parameters of a = 5.804 (1), b = 5.814 (1), c = 10.179 (1) A and beta = 90.09 (8) degrees is presented. The components of the modulation wavevector q = alphaa* + 0b* + gammac* are alpha = 0.08 (1) and gamma = 0.39 (1). The structure comprises cis-trans chains and rectangles of As atoms and displays an intricate stacking sequence of the modulated arsenic nets. Rod groups and layer groups of the respective arrangements are identified and the arrangement of different motifs of the modulated As net is discussed.