Successive Short- and Long-Range Magnetic Ordering in BaMn(SeO) with Honeycomb Layers of Mn Ions Alternating with Triangular Layers of Mn Ions.

Mixed-valent BaMnMn(SeO) crystallizes in a monoclinic structure and has honeycomb layers of Mn ions alternating with triangular layers of Mn ions. We established the key parameters governing its magnetic structure by magnetization and specific heat measurements. The title compound exhibits a close succession of a short-range correlation order at = 10.1 ± 0.1 K and a long-range Néel order at = 5.7 ± 0.1 K, and exhibits a metamagnetic phase transition at with hysteresis most pronounced at low temperatures. The causes for these observations were found using the spin exchange parameters evaluated by density functional theory calculations. The title compound represents a unique case in which uniform chains of integer spin Mn ( = 2) ions interact with those of half-integer spin Mn ( = 5/2) ions.