Magnetic Ordering in the Layered Cr(II) Oxide Arsenides Sr₂CrO₂Cr₂As₂ and Ba₂CrO₂Cr₂As₂

Sr₂CrO₂Cr₂As₂ and Ba₂CrO₂Cr₂As₂ with Cr²⁺ ions in CrO₂ sheets and in CrAs layers crystallize with the Sr₂Mn₃Sb₂O₂ structure (space group I4/mmm, Z = 2) and lattice parameters a = 4.00800(2) Å, c = 18.8214(1) Å (Sr₂CrO₂Cr₂As₂) and a = 4.05506(2) Å, c = 20.5637(1) Å (Ba₂CrO₂Cr₂As₂) at room temperature. Powder neutron diffraction reveals checkerboard-type antiferromagnetic ordering of the Cr²⁺ ions in the arsenide layers below T_{N1_Sr}, of 600(10) K (Sr₂CrO₂Cr₂As₂) and T_{N1_Ba} 465(5) K (Ba₂CrO₂Cr₂As₂) with the moments initially directed perpendicular to the layers in both compounds. Checkerboard-type antiferromagnetic ordering of the Cr²⁺ ions in the oxide layer below 230(5) K for Ba₂CrO₂Cr₂As₂ occurs with these moments also perpendicular to the layers, consistent with the orientation preferences of d⁴ moments in the two layers. In contrast, below 330(5) K in Sr₂CrO₂Cr₂As₂, the oxide layer Cr²⁺ moments are initially oriented in the CrO₂ plane; but on further cooling, these moments rotate to become perpendicular to the CrO₂ planes, while the moments in the arsenide layers rotate by 90° with the moments on the two sublattices remaining orthogonal throughout [behavior recently reported independently by Liu et al. [Liu et al. Phys. Rev. B 2018, 98, 134416]]. In Sr₂CrO₂Cr₂As₂, electron diffraction and high resolution powder X-ray diffraction data show no evidence for a structural distortion that would allow the two Cr²⁺ sublattices to couple, but high resolution neutron powder diffraction data suggest a small incommensurability between the magnetic structure and the crystal structure, which may account for the coupling of the two sublattices and the observed spin reorientation. The saturation values of the Cr²⁺ moments in the CrO₂ layers (3.34(1) μ_B (for Sr₂CrO₂Cr₂As₂) and 3.30(1) μ_B (for Ba₂CrO₂Cr₂As₂)) are larger than those in the CrAs layers (2.68(1) μ_B for Sr₂CrO₂Cr₂As₂ and 2.298(8) μ_B for Ba₂CrO₂Cr₂As₂) reflecting greater covalency in the arsenide layers.