Successive Short- and Long-Range Magnetic Ordering in Ba₂Mn₃(SeO₃)₆ with Honeycomb Layers of Mn³⁺ Ions Alternating with Triangular Layers of Mn²⁺ Ions

Mixed-valent Ba₂Mn²⁺Mn₂³⁺(SeO₃)₆ crystallizes in a monoclinic P2₁/c structure and has honeycomb layers of Mn³⁺ ions alternating with triangular layers of Mn²⁺ 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 Mn³⁺ (S = 2) ions interact with those of half-integer spin Mn²⁺ (S = 5/2) ions.