Structures and Magnetic Ordering in Layered Cr Oxide Arsenides Sr 2 CrO 2 Cr 2 OAs 2 and Sr 2 CrO 3 CrAs.

Two novel chromium oxide arsenide materials have been synthesized, Sr 2 CrO 2 Cr 2 OAs 2 (i.e., Sr 2 Cr 3 As 2 O 3 ) and Sr 2 CrO 3 CrAs (i.e., Sr 2 Cr 2 AsO 3 ), both of which contain chromium ions in two distinct layers. Sr 2 CrO 2 Cr 2 OAs 2 was targeted following electron microscopy measurements on a related phase. It crystallizes in the space group P 4/ mmm and accommodates distorted CrO 4 As 2 octahedra containing Cr 2+ and distorted CrO 2 As 4 octahedra containing Cr 3+ . In contrast, Sr 2 CrO 3 CrAs incorporates Cr 3+ in CrO 5 square-pyramidal coordination in [Sr 2 CrO 3 ] + layers and Cr 2+ ions in CrAs 4 tetrahedra in [CrAs] - layers and crystallizes in the space group P 4/ nmm . Powder neutron diffraction data reveal antiferromagnetic ordering in both compounds. In Sr 2 CrO 3 CrAs the Cr 2+ moments in the [CrAs] - layers exhibit long-range ordering, while the Cr 3+ moments in the [Sr 2 CrO 3 ] + layers only exhibit short-range ordering. However, in Sr 2 CrO 2 Cr 2 OAs 2 , both the Cr 2+ moments in the CrO 4 As 2 environments and the Cr 3+ moments in the CrO 2 As 4 polyhedra are long-range-ordered below 530(10) K. Above this temperature, only the Cr 3+ moments are ordered with a Néel temperature slightly in excess of 600 K. A subtle structural change is evident in Sr 2 CrO 2 Cr 2 OAs 2 below the magnetic ordering transitions.