Distorted Te nets in the modulated crystal structures of RETe_1.94(1) (RE = La, Pr, Nd)

The two-dimensionally incommensurately modulated crystal structures of the compounds RETe_1.94(1) (RE = La, Pr, Nd) were investigated by single-crystal X-ray diffraction. The compounds crystallize in the tetragonal superspace group P4/n(αβ½)00(-βα½)00 (No. 85.2.58.2) with q₁ = αa*+βb*+½c* and q₂ = -βa*+αb*+½c* and share a common motif of an alternating stacking of a puckered [RETe] layer and a planar [Te] layer. This basic structural motif is observed for all reported compounds with unusually large anisotropic displacement parameters in the planar [Te] layer. Taking the modulation into account, a distortion from this perfect square planar net is noted along with vacancies in the planar [Te] layer. The distortion leads to the formation of different discrete anions, like Te^2-, Te₂^2- and Te₃^2-, similar to previously reported structures for REX_2-δ compounds (RE = trivalent rare earth metal, X = S, Se, Te). The Te-Te distances in the modulated [Te] layer are found in a narrow range as compared to those in the corresponding sulfides and selenides.