Successive Short- and Long-Range Magnetic Ordering in Ba 2 Mn 3 (SeO 3 ) 6 with Honeycomb Layers of Mn 3+ Ions Alternating with Triangular Layers of Mn 2+ Ions.

Mixed-valent Ba 2 Mn 2+ Mn 2 3+ (SeO 3 ) 6 crystallizes in a monoclinic P2 1 /c structure and has honeycomb layers of Mn 3+ ions alternating with triangular layers of Mn 2+ ions. We established the key parameters governing its magnetic structure by magnetization M and specific heat C p measurements. The title compound exhibits a close succession of a short-range correlation order at T corr = 10.1 ± 0.1 K and a long-range Néel order at T N = 5.7 ± 0.1 K, and exhibits a metamagnetic phase transition at T < T N 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 3+ ( S = 2) ions interact with those of half-integer spin Mn 2+ ( S = 5/2) ions.