Distribution of the Se atoms in the layered compound Nb-3(Se1-xIx)I-7 studied by scanning tunneling microscopy and electronic structure calculations

The layered compound Nb-3(Se1-xIx)I-7 is obtained when the I atoms at the f ace-capping sites of Nb3I8 are substituted with Se atoms such that each Nb- 3(Se1-xIx)I-7 layer is composed of Nb3I13 and Nb3SeI12 clusters. The amount and distribution of the Se atoms in Nb-3(Se1-xIx)I-7 were examined by carr ying out scanning tunneling microscopy (STM) experiments for Nb-3(Se1-xIx)I -7. The reason the Nb3I13 and Nb3SeI12 clusters appear with different contr asts in the STM images Nb-3(Se1-xIx)I-7 was investigated by calculating par tial density plots for ordered model layers [Nb-3(Se1/3I2/3)I-7](3) and (Nb 3I8)(3). In the STM images of Nb-3(Se1-xIx)I-7, the Nb3I13 and Nb3SeI12 clu sters are distinguished because they undergo different extents of tip-force induced depression. Our analysis shows that the distribution of the Se ato ms in Nb3Se1-xI7+x is random.