ALn_1±xBi_4±xS ₈(A=K,Rb;Ln=La,Ce,Pr,Nd): New Semiconducting Quaternary Bismuth Sulfides

The isostructural compounds KLa_1.28Bi_3.72S₈(I), RbLa_1±xBi_4±xS₈(II), RbCe_0.84Bi_4.16S₈(III), KCe_1±xBi_4±xS₈(IV), KPr_1±xB_4±xS₈(V) and KNd_1±xBi_4±xS₈(VI) were synthesized. The structures of compounds (I) and (III) were determined by single crystal X-ray diffraction. They crystallize in the orthorhombicPnmaspace group (#62),a=16.6524(9) Å ,b=4.0712(2) Å,c=21.589(1) Å,R1=0.0397, wR2=0.0743 (all data) for (I) anda=16.7469(2) Å ,b=4.0528(1) Å,c=21.753(1) Å,R1=0.0345, wR2=0.0613 (all data) for (III). The structure type found in these compounds is made up of NaCl-type and Gd₂S₃-type fragments which connect to form tunnels filled with alkali metal ions. The structure has three types of metal positions, a six-coordinate type and two with higher coordination numbers, seven and eight. The NaCl-type fragment contains the six-coordinate positions which are exclusively occupied by Bi atoms, while the Gd₂S₃-type fragment contains the seven- and eight-coordinate positions which display mixed bismuth/lanthanide atom occupancy. The compounds are semiconductors with band gaps around 1.0 eV and they melt incongruently between 770 and 800°C. The electrical conductivities of the La analogs and magnetic properties of the Ce analogs are reported.