We describe the preparation of SeMO3 (M = Ni, Mn) under high pressure conditions (3.5 GPa), starting from reactive H2SeO3 and MO mixtures, contained in sealed gold capsules under the reaction conditions 850 °C for 1 h. The polycrystalline samples have been studied by neutron powder diffraction (NPD) data and magnetization measurements. SeMO3 (M = Ni, Mn) are orthorhombically distorted perovskites (space group Pnma). Below TN
≈ 104 K (M = Ni) and TN
≈ 53.5 K (M = Mn) these oxides experience an antiferromagnetic ordering, as demonstrated by susceptibility and NPD measurements. The magnetic reflections observed in the neutron patterns can be indexed with a propagation vector k = 0. Both compounds present the same magnetic structure, which is given by the basis vector (0, 0, Az). It can be described as antiferromagnetic (010) layers of magnetic moments lying along the c direction, which are antiferromagnetically coupled along the b direction. For the Ni2+ ions, the ordered magnetic moment at T = 2.3 K is 2.11(3) µB, whereas for Mn2+ at T = 2.6 K, |m| = 4.64(2) µB, consistent with the electronic configurations t62ge2g (Ni2+) and t32ge2g (Mn2+).
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