Crystal structures and magnetic properties of R_2PbSi_2S_8 (R=Y, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho), R_2PbSi_2Se_8 (R=La, Ce, Pr, Nd, Sm, Gd) and R_2PbGe_2S_8 (R=Ce, Pr) compounds

摘要

The crystal structures of several novel quaternary compounds R_2PbSi_2S_8 (R=Y, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho), R_2PbSi_2S_8 (R = La, Ce, Pr, Nd, Sm, Gd) and R_2PbSi_2S_8 (R=Ce, Pr) have been determined by means of single-crystal and powder X-ray diffraction. All these phases crystallize with hexagonal unit cells of the R_2PbSi_2S_8 type (space group R-3c, Pearson symbol h R78), in which rare-earth and lead atoms are statistically distributed over centers of bi-capped trigonal prisms made by chalcogen atoms. The magnetic properties of R_2PbSi_2S_8 and R_2PbSi_2S_8 with R=La, Ce, Pr and Sm have been studied in wide temperature and magnetic field ranges. The La-based materials are weakly diamagnetic, whereas all the other compounds exhibit strongly temperature dependent paramagnetic behaviour due to the presence of well localized magnetic moments of trivalent rare-earth ions.