Nonstoichiometry in Er–Te Binary and Er–Te–I Ternary Systems

Abstract

By X-ray single-crystal structure analysis of Er0.85Te and Er17.3Te24I8, the structures of formerly unknown nonstoichiometric lanthanide tellurides could be solved and refined. Both structures may be derived from the cubic rock-salt type. As a result of vacancy ordering, the unit cell of the basic rock-salt type is doubled in all three directions [space groupFd3m, a=1202.0(1) pm anda=1212.0(1) pm for Er0.85Te (Z=32,Rw=4.64%) and Er17.3Te24I8(Rw=3.38%), respectively]. In both compounds, one of the two symmetrically equivalent cation sites is fully occupied by Er; the other is only partially occupied. Topologically, the cations of each partial structure are forming in the rock-salt type a framework of Er4tetrahedra sharing all corners. The diffraction pattern of the ternary compound can be interpreted only if a substitution of cations by tellurium ions is assumed giving a compound with polycationic and polyanionic structural fragments. The occurrence of these structural elements may be explained by assuming a partial overlap of Er-5d-and (Te, I)-5p-levels in the bandstructure.