Phase relationships of the ternary chromium sulfides CrII1−xMxCrIII2S4 (M = Mn,Fe, Co) with Cr3S4 and spinel structure

The phase diagrams of the chromium sulfide systems Cr_1?xM_xCr₂S₄, Cr_1?xFe_xCr₂S₄, andCr_1?xCo_xCr₂S₄ were studied with the help of X-ray powder Guinier photographs of quenched samples. The phase widths of the monoclinic Fe₃Se₄ (Cr₃S₄) type solid solutions increase with increasing temperature from x < 0.1 below 500°C to x = 0.23, 0.45, and 0.44 for M = Mn, Fe, and Co, respectively, at 900°C. The observed alterations of the unit cell parameters, e.g. increase of the volume V, but decrease of the lattice constant a with increasing × for M = Mn and Fe, are discussed in terms of the ionic radii and metal-semiconductor transitions. The phase widths of the spinel type chromium sulfides MCr₂S₄ are very small for all temperatures studied. The relative stability of the Cr₃S₄ and the spinel structure can be interpreted using site preference and covalent bonding arguments.