Long Range Magnetic Order in 6H Hexagonal Perovskites Containing Chromium and Rhenium

Polycrystalline samples of Ba₃Cr₂ReO₉ and Ba₂CrReO₆ were synthesized using solid-state methods and characterized using laboratory X-ray powder diffraction (XRPD), magnetometry, specific heat, and variable temperature neutron powder diffraction (NPD). Ba₃Cr₂ReO₉ and Ba₂CrReO₆ both crystallize in the 6H hexagonal structure with space group symmetry P6₃/mmc. The transition metal ions in Ba₃Cr₂ReO₉ are ordered with Re⁶⁺ ions preferentially occupying the octahedra that share corners, and Cr³⁺ ions preferentially occupying sites in the face-sharing bioctahedra. Though there is a small amount of antisite mixing between the two ions. Ba₂CrReO₆ adopts a very similar crystal structure with a substantially higher fraction of Re⁵⁺ in the face-sharing bioctahedra. Both Ba₃Cr₂ReO₉ and Ba₂CrReO₆ exhibit long-range antiferromagnetic ordering. The magnetic structures feature antiferromagnetic Re-O-Cr superexchange coupling through shared octahedral corners and antiferromagnetic Cr-Cr (or Cr-Re) direct exchange interactions across shared octahedral faces. The refined Cr³⁺ magnetic moments are typical of values seen in other oxides, suggesting that overlap of Cr 3d orbitals is not sufficient to form metal-metal bonds that lead to molecular orbitals that span the Cr₂O₉ bioctahedron.