Incommensurately modulated lanthanide coinage-metal diarsenides. II. GdCuAs2, GdAu1−δAs2 and TbAu1−δAs2 – new distortion variants of the HfCuSi2 type with irregularly stacked zigzag chains of arsenic atoms

Abstract

GdCuAs2, GdAu1−δAs2 and TbAu1−δAs2 crystallize as incommensurately modulated variants of the HfCuSi2 type. Structure models have been developed in the monoclinic superspace group P121/m1(α0γ)00 (No. 11.1). The components of the modulation wavevectors q = αa* + 0b* + γc* are α = 0.04 (1) and γ = 0.48 (1) for GdCuAs2, α = 0.03 (1) and γ = 0.48 (1) for GdAu1−δAs2 and α = 0.02 (1) and γ = 0.46 (1) for TbAu1−δAs2. The predominant effect of the positional modulation is the distortion of a square net of arsenic atoms, which results in planar zigzag chains. Rod groups and layer groups of the respective structure motifs are identified and discussed.