Structure determination of KLaS_2, KPrS_2, KEuS_2, KGdS_2, KLuS_2, KYS_2, RbYS_2, NaLaS_2 and crystalchemical analysis of the group 1 and thallium(I) rare-earth sulfide series

摘要

One of the purposes of this work is to provide a crystallographic review of group 1 and thallium rare-earth ternary sulfides M~+Ln~(3+)S_2. We have therefore determined crystal structures of KLaS_2, KPrS_2, KEuS_2, KGdS_2, KLuS_2, KYS_2, RbYS_2, which belong to the α-NaFeO_2 structural family (R-33m), as well as NaLaS2, which is derived from the disordered NaCl structural type (Fm3m). The determined structures were compared with known members of the group 1 as well as thallium(I) rare-earth sulfides by the standard tools of crystal-chemical analysis such as comparison of bondvalences, analysis of interatomic distances and comparison of the unit-cell parameters. The results indicate why the cubic structural type is limited to Li~+ and Na~+ members of the series only. The analysis has also revealed frequent problems in the reported crystal structures, especially in the determination of the K~+ compounds, probably due to severe absorption and different accuracy and sensitivity of various instruments. Intense diffuse scattering has been discovered in NaLaS_2, which will be the subject of further investigation. The newly determined as well as already known structures are summarized, together with critical comments about possible errors in the previous structure determinations.