Structures and Magnetic Ordering in Layered Cr Oxide Arsenides SrCrOCrOAs and SrCrOCrAs.

Two novel chromium oxide arsenide materials have been synthesized, SrCrOCrOAs (i.e., SrCrAsO) and SrCrOCrAs (i.e., SrCrAsO), both of which contain chromium ions in two distinct layers. SrCrOCrOAs was targeted following electron microscopy measurements on a related phase. It crystallizes in the space group 4/ and accommodates distorted CrOAs octahedra containing Cr and distorted CrOAs octahedra containing Cr. In contrast, SrCrOCrAs incorporates Cr in CrO square-pyramidal coordination in [SrCrO] layers and Cr ions in CrAs tetrahedra in [CrAs] layers and crystallizes in the space group 4/. Powder neutron diffraction data reveal antiferromagnetic ordering in both compounds. In SrCrOCrAs the Cr moments in the [CrAs] layers exhibit long-range ordering, while the Cr moments in the [SrCrO] layers only exhibit short-range ordering. However, in SrCrOCrOAs, both the Cr moments in the CrOAs environments and the Cr moments in the CrOAs polyhedra are long-range-ordered below 530(10) K. Above this temperature, only the Cr moments are ordered with a Néel temperature slightly in excess of 600 K. A subtle structural change is evident in SrCrOCrOAs below the magnetic ordering transitions.