Local Structure Of Cuin3se5: X-Ray Absorption Fine Structure Study And First-Principles Calculations

A detailed extended x-ray absorption fine structure (EXAFS) study of CuInSe2 and CuIn3Se5 on Cu-K, In-K, and Se-K edges was performed. It was found that CuInSe2 and CuIn3Se5 have well-defined local structure with the same average Cu-Se and In-Se bond lengths. They can be best described by structures containing weighted local tetrahedral cationic clusters around each Se: 2Cu+2In (k=8), and equal number of V-Cu+Cu+2In (k=7) and V-Cu+3In (k=9), where k denotes the nominal number of valence electrons of the cation clusters. CuInSe2 consists of 100% k=8 clusters and CuIn3Se5 consists of 20% k=8 and 40% k=7 and 40% k=9 clusters. First-principles band structure calculations of various CuInSe2, CuIn3Se5 and CuIn5Se8 compounds confirmed that the average Cu-Se and In-Se bond lengths in various ordered vacancy structures are identical to within the calculation uncertainty, in agreement with the present EXAFS measurements. The first-principles calculations also find that the formation energy of several possible crystal structures for CuIn3Se5 and CuIn5Se8 are very similar, which explains why the long-range order of CuIn3Se5 is not uniquely determined.