Incommensurately modulated lanthanide coinage-metal diarsenides. I. CeAu1−δAs2 [δ = 0.015 (2)] – a new distortion variant of the HfCuSi2 type with irregularly stacked cis–trans chains of arsenic atoms

Abstract

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. CeAu1−δAs2, with δ = 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 P121/m1(α0γ)00 (No. 11.1) with basic unit-cell parameters of a = 5.804 (1), b = 5.814 (1), c = 10.179 (1) Å and β = 90.09 (8)° is presented. The components of the modulation wavevector q = αa* + 0b* + γc* are α = 0.08 (1) and γ = 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.