Structures and Magnetic Ordering in Layered Cr Oxide Arsenides Sr₂CrO₂Cr₂OAs₂ and Sr₂CrO₃CrAs

Two novel chromium oxide arsenide materials have been synthesized, Sr₂CrO₂Cr₂OAs₂ (i.e., Sr₂Cr₃As₂O₃) and Sr₂CrO₃CrAs (i.e., Sr₂Cr₂AsO₃), both of which contain chromium ions in two distinct layers. Sr₂CrO₂Cr₂OAs₂ was targeted following electron microscopy measurements on a related phase. It crystallizes in the space group P4/mmm and accommodates distorted CrO₄As₂ octahedra containing Cr²⁺ and distorted CrO₂As₄ octahedra containing Cr³⁺. In contrast, Sr₂CrO₃CrAs incorporates Cr³⁺ in CrO₅ square-pyramidal coordination in [Sr₂CrO₃]⁺ layers and Cr²⁺ ions in CrAs₄ tetrahedra in [CrAs]^- layers and crystallizes in the space group P4/nmm. Powder neutron diffraction data reveal antiferromagnetic ordering in both compounds. In Sr₂CrO₃CrAs the Cr²⁺ moments in the [CrAs]^- layers exhibit long-range ordering, while the Cr³⁺ moments in the [Sr₂CrO₃]⁺ layers only exhibit short-range ordering. However, in Sr₂CrO₂Cr₂OAs₂, both the Cr²⁺ moments in the CrO₄As₂ environments and the Cr³⁺ moments in the CrO₂As₄ polyhedra are long-range-ordered below 530(10) K. Above this temperature, only the Cr³⁺ moments are ordered with a Néel temperature slightly in excess of 600 K. A subtle structural change is evident in Sr₂CrO₂Cr₂OAs₂ below the magnetic ordering transitions.