Synthesis, structure and optical properties of new lanthanide-based derivatives Ln(2)SbS(5)Br (Ln = La, Ce)

The first bromothioantimonates of cerium and lanthanum, Ce2SbS5Br (I), CeLa SbS5Br (II) and La2SbS5Br (III), have been synthesized and characterized. I and III crystallize in the Pnma (n degrees 62)space group while the struct ure of II was refined in the P2(1)2(1)2(1) (n degrees 19) space group proba bly due to an ordering between Ce and La. The cell parameters are: a = 8.84 7(2) Angstrom, b = 5.492(1) Angstrom, c = 17.697(6) Angstrom, V= 859.9(6) A ngstrom (3) for I; a = 8.9023(9) Angstrom, b = 5.5113(6) Angstrom, c = 17.8 09(2) Angstrom, V= 873.8(3) Angstrom (3) for II and a = 8.905(2) Angstrom, b = 5.526(1) Angstrom, c = 17.883(3) Angstrom, V= 880.0(5) Angstrom (3) for III. These three materials exhibit the same novel structural arrangement w ith lanthanides surrounded by sulfur and bromine anions in two different Ln S,Br, and LnS(8)Br units. Some sulfur atoms are engaged in S-S bonding dime rs while antimony exhibits a SbS4E coordination (E = lone pair), the charac teristics of which hint at a stereo-active 5s(2) electron pair. The charge balance in the materials is written as Ln(2)(III)Sb(III)(S-2)-S-II(3)-Br-II . The same red color of the three materials rules out the occurrence of the Ce4f(l) --> Ce-Sd(I) electronic transition usually observed in Ce containi ng sulfides. In contrast, band structure calculations (TB-LMTO-ASA) assigne d the observed absorption threshold around 2.08 eV for the three phases to the existence of a VB --> CB electronic transition, i.e. an unpaired S or B r --> Sb or paired S charge transfer. (C) 2001 Editions scientifiques et me dicales Elsevier SAS. All rights reserved.