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

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Polycrystalline samples of Ba 3 Cr 2 ReO 9 and Ba 2 CrReO 6 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). Ba 3 Cr 2 ReO 9 and Ba 2 CrReO 6 both crystallize in the 6H hexagonal structure with space group symmetry P 6 3 / mmc . The transition metal ions in Ba 3 Cr 2 ReO 9 are ordered with Re 6+ ions preferentially occupying the octahedra that share corners, and Cr 3+ ions preferentially occupying sites in the face-sharing bioctahedra. Though there is a small amount of antisite mixing between the two ions. Ba 2 CrReO 6 adopts a very similar crystal structure with a substantially higher fraction of Re 5+ in the face-sharing bioctahedra. Both Ba 3 Cr 2 ReO 9 and Ba 2 CrReO 6 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 Cr 3+ 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 Cr 2 O 9 bioctahedron.