Abstract
The two-dimensionally incommensurately modulated crystal structures of the compounds RETe1.94(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 Te2-, Te22- and Te32-, similar to previously reported structures for REX2-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.
Highlights
The crystal structures observed for the rare earth metal polychalcogenides REX2– (RE = Y, La–Nd, Sm, Gd–Lu; X = S, Se, Te; 0 0.2) show a broad variety of different patterns, including conventional commensurate and incommensurately modulated superstructures based on a common basic unit cell
We report in the following on the Te-deficient compounds of the composition RETe1.94 (1) (RE = La, Pr, Nd), which highlights a significant difference to the corresponding sulfide and selenide systems where this composition is not documented
Special care has to be taken when performing the mineralization experiments, as the Te-richer RETe3 compounds can be formed if the initial ratio of the rare earth metal and tellurium is not corrected for the amount of REI3 formed due to the addition of I2 (Poddig et al, 2018)
Summary
The phase width of the REX2– compounds adds another parameter to the distortion in the planar [X] layer by introducing vacancies and forcing the layer to compensate the charge of the missing X atoms This is usually achieved by forming a more or less isolated X 2– anion for every vacancy along the X22À anions, resulting in an average charge of [X]1–À, as described for the CeSe1.9-type and the Gd8Se15–-type structures (Doert & Muller, 2016). Most of the listed compounds are described in superspace group P4/n( 12)00(À 12)00 (No 85.2.58.2), though with different values for and , resulting in their unique structural features These structures show a similar pattern of X 2À and X22À anions along with some vacancies as observed for the commensurate superstructures of the sulfides and selenides.
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