Phase Equilibria in the Sc2S3-Ln2S3 (Ln = Dy, Er, or Tm) Systems

Abstract

Phase diagrams for the Sc 2 S 3 -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 P 2 1 / m , and melt congruently: for Dy 3 ScS 6 , a = 1.118 nm, b = 1.262 nm, c = 0.354 nm, β = 94.7°, 1800 K, H = 2600 MPa; for Er 3 ScS 6 , a = 1.113 nm, b = 1.258 nm, c = 0.353 nm, β = 94.5°, 1830 K, H = 2800 MPa; for Tm 3 ScS 6 , a = 1.112 nm, b = 1.229 nm, c = 0.352 nm, β = 94.3°, 1835 K, H = 2940 MPa. The LnScS 3 (Ln = Dy or Er) complex sulfides, with orthorhombic structures, space group Pnma , melt incongruently: for DyScS 3 , a = 0.700 nm, b = 0.637 nm, c = 0.943 nm, 1810 K, H = 3800 MPa; and for ErScS 3 , a = 0.697 nm, b = 0.633 nm, c = 0.942 nm, 1800 K, H = 3800 MPa. As the ionic radii r Ln3+ and r Sc3+ approach Ln Sc to each other in the row Dy-Er-Tm, the solubility in Sc 2 S 3 increases, at 1670 K being equal to 13 mol % Dy 2 S 3 , 30 mol % Er 2 S 3 , and 40 mol % Tm 2 S 3 . 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 % Dy 2 S 3 , 1730 K and 81 mol % Dy 2 S 3 , 1740 K and 65 mol % Er 2 S 3 , 1700 K and 83 mol % Er 2 S 3 , 1730 K and 70 mol % Tm 2 S 3 , and 1755 K and 84 mol % Tm 2 S 3 .