Cd(OH)₂-NiAs related phases in the Co-Se system

The reported high temperature, wide range non-stoichiometric Co_2-xSe₂, 0 ≤ x ≤ 0.50, solid solution field has been carefully re-investigated using electron and X-ray powder diffraction. Close investigation shows that the 'solid solution' should be separated into four distinct phase regions (i)-(iv), even though the underlying average structure unit cell appears to distort smoothly and continuously as a function of composition. For values of x between 0 and 0.06 (structure type (i)), the structure is of 'defect' NiAs-type as only NiAs type parent reflections are observed. For 0.06 < x < 0.25 (structure type (ii)), sharp incommensurate satellite reflections are observed in addition to Cd(OH)₂-type parent reflections leading to a previously unreported (3 + 2)-dimensional incommensurately modulated superstructure phase. For 0.25 < x < 0.40 (structure type (iii)), a commensurate superstructure phase is observed which appears to be of Cr₇Se₈ type, space group C2/m (a = 12.4236(8) Å, b = 7.1794(13) Å, c = 8.1835(6) Å, β = 139.926(7)° for x = 0.30). Finally, for 0.40 < x < 0.50 (structure type (iv)), a further commensurate superstructure phase is observed which appears to be of Cr₃S₄ type, space group C2/m (a = 11.9439(2) Å, b = 3.5753(4) Å, c = 6.1429(7) Å, β = 119.384(8)° for x = 0.50).