Photoluminescence and Magnetism in the New Magnetic\nSemiconductors:  K₂Cd₃₍₁_-<i>_x</i>₎Mn₃<i>_x</i>S₄

Abstract

A new family of magnetic semiconductors, K₂Cd₃₍₁_-<i>_x</i>₎Mn₃<i>_x</i>S₄, prepared from molten K₂S<i>_x</i>\nis reported. For <i>x</i> ≤ 0.6, the compounds crystallize in the K₂Cd₃S₄ structure type. The cell\nvolumes of the series obey Vegard's Law for solid solutions. The compounds range in color\nfrom yellow through orange brown to red and possess platelike morphologies. A structural\ntransformation occurs between <i>x</i> = 0.7 and <i>x</i> = 0.9. Red plates of K₂Cd_0.4Mn_2.6S₄ (<i>x</i> = 0.87)\ncrystallize in the space group <i>Ccca</i> with <i>a </i>= 5.9217(8) Å, <i>b </i>= 13.531(1) Å, <i>c</i> = 11.0696(8) Å,\nand <i>V</i> = 887.0(1) ų. The end member, K₂Mn₃S₄, crystallizes in the space group <i>Ccca</i> with\n<i>a</i> = 11.0637(7) Å, <i>b </i>= 26.830(2) Å <i>c </i>= 5.8180(4) Å, and <i>V</i> = 1727 ų. For <i>x</i> ≤ 0.6, the\ncompounds form as (M₃S₄)<i>_n</i>²<i>ⁿ</i>^- layers, interspersed by K⁺ cations. These layers are composed\nonly of M₃S₄^2- units shaped like truncated cubes. When <i>x</i> = 0.87, a new structure related to\nthat of Cs₂Mn₃S₄ is observed. The end member of the series (i.e., <i>x</i> = 1) K₂Mn₃S₄ is revealed\nto have a related yet surprising structure. The compounds display room-temperature band\ngaps ranging from 2.79 to 2.92 eV. Absorptions in the mid-gap region, due to Mn²⁺-based\nd−d transitions, are also observed. The compounds are strongly emissive in the visible\nspectrum at room temperature, with the light emission shifted to the red immediately upon\nthe introduction of small amounts of manganese. The magnetic properties of these compounds\nevolve from classical paramagnetic to spin glass as a function of <i>x</i>. For <i>x</i> ≥ 0.05, the\ncompounds deviate from Curie−Weiss behavior. Antiferromagnetic coupling is observed in\nall cases. With very high Mn content transitions to magnetically complex phases are observed\nwith possible spin-glass-like behavior.