Phase equilibria in the Sc2S3-Ln2S3 (Ln = Dy, Er, or Tm) systems

Phase diagrams for the Sc2S3-Ln(2)S(3) (Ln = Dy, Er, or Tm) systems were designed in the range from 1000 K to melting temperatures. The Ln(3)ScS(6) compounds that are formed in these systems crystallize in monoclinic space group P2(1)/m, and melt congruently: for Dy3ScS6, a = 1.118 nm, b = 1.262 nm, c = 0.354 nm, beta = 94.7, 1800 K, H = 2600 MPa; for Er3ScS6, a = 1.113 nm, b = 1.258 nm, c = 0.353 nm, beta = 94.5dg, 1830 K, H = 2800 MPa; for Tm3ScS6, a = 1.112 nm, b = 1.229 nm, c = 0.352 nm, beta = 94.3dg, 1835 K, H = 2940 MPa. The LnScS(3) (Ln = Dy or Er) complex sulfides, with orthorhombic structures, space group Pnma, melt incongruently: for DyScS3, a = 0.700 nm, b = 0.637 nm, c = 0.943 nm, 1810 K, H = 3800 MPa; and for ErScS3, a = 0.697 nm, b = 0.633 nm, c = 0.942 nm, 1800 K, H = 3800 MPa. As the ionic radii r(Ln)(3+) and r(Sc)(3+) approach to each other in the row Dy-Er-Tm, the solubility in Sc2S3 increases, at 1670 K being equal to 13 mol % Dy2S3, 30 mol % Er2S3, and 40 mol % Tm2S3. LnScS(3) (Ln = Dy or Er) forms a two-sided homogeneity region, at 1670 K lying in ranges of 43-56 mol % Ln(2)S(3). The eutectic temperatures and compositions are as follows: 1700 K and 66 mol % Dy2S3, 1730 K and 81 mol % Dy2S3, 1740 K and 65 mol % Er2S3, 1700 K and 83 mol % Er2S3, 1730 K and 70 mol % Tm2S3, and 1755 K and 84 mol % Tm2S3.