Synthesis, Structure, and Properties of the Layered Oxide Chalcogenides Sr₂CuO₂Cu₂S₂ and Sr₂CuO₂Cu₂Se₂

The structures of two new oxide chalcogenide phases, Sr₂CuO₂Cu₂S₂ and Sr₂CuO₂Cu₂Se₂, are reported, both of which contain infinite CuO₂ planes containing Cu²⁺ and which have Cu⁺ ions in the sulfide or selenide layers. Powder neutron diffraction measurements show that Sr₂CuO₂Cu₂Se₂ exhibits long-range magnetic ordering with a magnetic structure based on antiferromagnetic interactions between nearest-neighbor Cu²⁺ ions, leading to a √2 a × √2 a × 2 c expansion of the nuclear cell. The ordered moment of 0.39(6) μ_B on the Cu²⁺ ions at 1.7 K is consistent with the value predicted by density functional theory calculations. The compounds are structurally related to the cuprate superconductors and may also be considered as analogues of the parent phases of this class of superconductor such as Sr₂CuO₂Cl₂ or La₂CuO₄. In the present case, however, the top of the chalcogenide-based valence band is very close to the vacant Cu²⁺ 3d states of the conduction band, leading to relatively high measured conductivity.