Distribution of the Se Atoms in the Layered Compound Nb3(Se1 - x I x )I7 Studied by Scanning Tunneling Microscopy and Electronic Structure Calculations

The layered compound Nb3(Se1 - x I x )I7 is obtained when the I atoms at the face-capping sites of Nb3I8 are substituted with Se atoms such that each Nb3(Se1 - x I x )I7 layer is composed of Nb3I13 and Nb3SeI12 clusters. The amount and distribution of the Se atoms in Nb3(Se1 - x I x )I7 were examined by carrying out scanning tunneling microscopy (STM) experiments for Nb3(Se1 - x I x )I7. The reason the Nb3I13 and Nb3SeI12 clusters appear with different contrasts in the STM images Nb3(Se1 - x I x )I7 was investigated by calculating partial density plots for ordered model layers [Nb3(Se1/3I2/3)I7]3 and (Nb3I8)3. In the STM images of Nb3(Se1 - x I x )I7, the Nb3I13 and Nb3SeI12 clusters are distinguished because they undergo different extents of tip-force induced depression. Our analysis shows that the distribution of the Se atoms in Nb3Se1 - x I7+ x is random.