Crystal structure of Pb2SbS2I3, and re-examination of the crystal chemistry within the group of (Pb/Sn/Sb) chalcogeno-iodides

The crystal structure of Pb2SbS2I3 was solved at room temperature and 100K. At 293K it crystallizes in the orthorhombic system, space group Cmcm (No. 63), with unit cell parameters a=4.3262(9), b=14.181(3), c=16.556(3)Å, V=1017.7(4)Å3, Z=4. The structure is disordered, and combines a split Pb site (s.o.f.=0.50) with one mixed (Pb,Sb) site with Pb and Sb in two distinct sub-positions. At 100K, it is monoclinic, space group P21/c, with unit cell parameters a=7.3629(6), b=16.466(3), c=8.5939(7)Å, β=107.14(2)°, V=995.6(2)Å3, Z=4. The structure is now fully ordered, without mixed sites. On the basis of bond valence calculations, new cation distributions are proposed for published structures of the Sn isotypes, Sn2SbS2I3 and Sn2SbSe2I3. A re-examination of the crystal structures of various (Pb/Sn/Sb) chalcogeno-iodides is presented according to modular analysis. All these structures can be described according to three types of 1D modules, (Pb/Sn)I4, (Sn)2I4 and (Pb/Sn/Sb)4(S/Se)2I4. Generally each type of 1D module gives one type of slab, and the final structure corresponds to a specific stacking of two or three among these slabs. A new structural model is proposed for “α-Sn2SI2”, which would have the non-stoichiometric composition (Sn5.42□0.58)S2(I6.87□0.12), ideally Sn27S10I34, with probably a narrow solid solution field on the SnS–SnI2 joint.