Lanthanide polysulfides at high pressures

High pressure behavior of selected stoichiometric lanthanide disulfides and sesquisulfides is studied in a diamond cell by X-ray diffraction: Raman sp ectroscopy, and optical reflectivity. Upon compression to 20 GPa, their occ urs a reversible phase transition in layered alpha ' -LnS(2) (Ln: La-Nd) ra re earth disulfides (P2(1)/b, C-2h(5) Z=4) with the distorted anti-Fe2As st ructure (the. PbFCl type, P4/nmn, D-4h(7,) Z=2) to higher symmetry superstr uctures of the parent anti-Fe2As type. This transformation at about 5 GPa f or alpha ' -Las(2), alpha ' -CeS2, and alpha ' -PrS2, and at about 8 GPa fo r alpha ' -NdS2 is associated wittl changes in the scheme of direct optical transitions. gamma -Gd2S3 with the defect Th3P4 structure (I (4) over bar 3d, Z=4) remains stable to at least 30 GPa. There occurs a reversible phase transition in epsilon -Lu2S3 (Lu2S3-I) with the corundum structure (R (3) over barc) to the Th3P4 type (I (4) over bar 3d) above 5.0 GPa (Lu2S3-V or gamma -Lu2S3). There is no evidence for the presence of intermediate phases of the Tm2S3-II (Lu2S3-II, P2(1)/m) U2S, (Lu2S3-III, Pnma), and NdYbS3 (Lu 2S3-IV, Pnma) types up to 20 GPa. All the compounds remain semiconducting i n the entire pressure range covered in this study. (C) 2001 Elsevier Scienc e B.V. All rights reserved.