Photoluminescence and magnetism in the new magnetic semiconductors: K₂Cd_3(1-x)Mn_3xS₄

A new family of magnetic semiconductors, K₂Cd_3(1-x)Mn_3xS₄, prepared from molten K₂S_x is reported. For x ≤ 0.6, the compounds crystallize in the K₂Cd₃S₄ structure type. The cell volumes of the series obey Vegard's Law for solid solutions. The compounds range in color from yellow through orange brown to red and possess platelike morphologies. A structural transformation occurs between x = 0.7 and x = 0.9. Red plates of K₂Cd_0.4Mn_2.6S₄ (x = 0.87) crystallize in the space group Ccca with a = 5.9217(8) Å, b = 13.531(1) Å, c = 11.0696(8) Å, and V = 887.0(1) ų. The end member, K₂Mn₃S₄, crystallizes in the space group Ccca with a = 11.0637(7) Å, b = 26.830(2) Å, c = 5.8180(4) Å, and V = 1727 ų. For x ≤ 0.6, the compounds form as (M₃S₄)_n^2n- layers, interspersed by K⁺ cations. These layers are composed only of M₃S₄^2- units shaped like truncated cubes. When x = 0.87, a new structure related to that of Cs₂Mn₃S₄ is observed. The end member of the series (i.e., x = 1) K₂Mn₃S₄ is revealed to have a related yet surprising structure. The compounds display room-temperature band gaps ranging from 2.79 to 2.92 eV. Absorptions in the mid-gap region, due to Mn²⁺-based d-d transitions, are also observed. The compounds are strongly emissive in the visible spectrum at room temperature, with the light emission shifted to the red immediately upon the introduction of small amounts of manganese. The magnetic properties of these compounds evolve from classical paramagnetic to spin glass as a function of x. For x ≥ 0.05, the compounds deviate from Curie-Weiss behavior. Antiferromagnetic coupling is observed in all cases. With very high Mn content transitions to magnetically complex phases are observed with possible spin-glass-like behavior.