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A 4 H -type BaMnO 3 single crystal was prepared by combining the floating zone method with high-pressure treatment at 5 GPa and 1023 K. The crystal crystallizes to a hexagonal structure with space group P 6 3 / mmc and lattice parameters a = 5.63723(5) Å and c = 9.22355(8) Å. In this structure, face-sharing MnO 6 octahedral dimers connect with each other by corner O atoms along the c -axis direction, forming an -A-B-A-C-type 4 H arrangement. A long-range antiferromagnetic (AFM) phase transition is found to occur at T N ≈ 263 K. When the synthesis pressure increases to 20 GPa, a new polymorphic phase is obtained. This higher-pressure phase still possesses the hexagonal P 6 3 / mmc symmetry, but the lattice parameters change to be a = 5.61349(2) Å and c = 13.66690(9) Å with a unit cell volume reduction of 2.05%. In this new phase, the c -axis MnO 6 dimers are separated by MnO 6 octahedral layers in the ab plane, forming an -A-B-C-A-C-B-type 6 H structure. The 6 H phase exhibits two long-range AFM orderings at T N1 ≈ 220 K and T N2 ≈ 25 K, respectively. The different magnetic properties are discussed on the basis of the detailed structural constitutions of 4 H - and 6 H -BaMnO 3 .

A new oxide, LaMn 3 Co 2 Mn 2 O 12 , was synthesized under high-pressure (7 GPa) and high-temperature (1423 K) conditions. The compound crystallizes in an AA' 3 B 4 O 12 -type quadruple perovskite structure with space group Im 3̅. The Rietveld structural analysis combined with soft X-ray absorption spectroscopy reveals the charge combination to be LaMn 3+ 3 Co 2+ 2 Mn 4+ 2 O 12 , where the La 3+ and Mn 3+ are 1:3 ordered respectively at the A and A' sites, whereas the Co 2+ and Mn 4+ are disorderly distributed at the B site. This is in sharp contrast to R 2 Co 2+ Mn 4+ O 6 (R = La and rare earth) double perovskites, in which the Co 2+ and Mn 4+ charge states are always orderly distributed with a rocksalt-type fashion, giving rise to a long-range magnetic ordering. As a result, LaMn 3 Co 2 Mn 2 O 12 displays spin glassy magnetic properties due to the random Co 2+ and Mn 4+ distribution, as demonstrated by dc and ac magnetic susceptibility as well as specific heat measurements. Possible factors that affect the B-site degree of order in perovskite structures are discussed.