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

Abstract

Sr2CrO2Cr2As2 and Ba2CrO2Cr2As2 with Cr 2+ 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 Cr 2+ ions in the arsenide layers below TN1_Sr, of 600(10) K (Sr2CrO2Cr2As2) and TN1_Ba 465(5) K (Ba2CrO2Cr2As2) with the moments initially directed perpendicular to the layers in both compounds.Checkerboard-type antiferromagnetic ordering of the Cr 2+ 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 d 4 moments in the two layers. In contrast, below 330(5) K in Sr2CrO2Cr2As2 the oxide layer Cr 2+ 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. 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 Cr 2+ 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 Cr 2+ moments in the CrO2 layers (3.34(1) B (for Sr2CrO2Cr2As2) and 3.30(1) B (for Ba2CrO2Cr2As2)) are larger than those in the CrAs layers (2.68(1) B for Sr2CrO2Cr2As2 and 2.298(8) B for Ba2CrO2Cr2As2) reflecting greater covalency in the arsenide layers.

Exploring unconventional ferromagnetism in hole-doped LaCrAsO: insights into charge-transfer and magnetic interactions

“…In LaMnAsO (material analogue to LaCrAsO), the local moment on Mn 2+ was observed to increase slowly from 2.43 μ B at 290 K to 3.34 μ B at 2K. 55 It is reported that the saturated magnetic moment of the [CrAs] 1− sublayer in Sr 2 Cr 3 Sr 2 O 2 56 and Sr 2 Cr 2 Sr 1 O 3 57 is 2.68 and 3.37 μ B /Cr, respectively, which should be a better benchmark. In this sense, our calculated saturated magnetic moment is reasonable.…”

Advancing Heteroanionicity in Zintl Phases: Crystal Structures, Thermoelectric and Magnetic Properties of Two Quaternary Semiconducting Arsenide Oxides, Eu8Zn2As6O and Eu14Zn5As12O

“…Over the past decade, the exploration of novel heteroanionic oxypnictides has turned up many unique materials showing high-temperature superconductivity, , interesting magnetism, − topological properties, magnetoresistance, , and potential for thermoelectric energy conversion . The 2008 discovery of high-temperature superconductivity in iron arsenide oxides significantly heightened interest in this class of compounds, albeit inadvertently overshadowing their tremendous potential as narrow band gap semiconductors. ,− …”