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.