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The commensurate superstructure nature of tetragonal tungsten bronze Pb2K0.5Li0.5Nb5O15(PKLN) was found,which is associated with the ordering of the atoms in A sites (especially 15-coordinated sites).The replacing of 0.5K+ with 0.5Li+ in PKN do not only increases the axis ratio (b/a) but also induces the tilting of NbO6 octahedrons.It is interesting that the former enhances negative thermal expansion along polar b axis but the latter caused extra positive thermal expansion along layered c axis.The bulk thermal expansion behavior will finally be determined by the combined effect of the two interactions.This study gives further insight into the structural and thermal expansion features of TTB oxides at atomic level.The mechanism will be helpful to understand the structure-activity relationships of complex polyhedral framework materials and can be used to design the thermal-mechanical properties of functional materials.