Crystal-chemical characterization of mixed-valent indium chalcogenides by X-ray absorption spectroscopy (EXAFS)

The mixed-valent indium chalcogenides InTe, In4Se3, In4Te3, In6S7 and In6Se 7 were studied together with beta-In2S3 and T1InS(2) by In and Te K-edge EX AFS spectroscopy. These well-characterized crystalline solids were chosen a s model systems to assess the general applicability of EXAFS for structural studies of first order mixed-valent solids containing indium. The question was whether the interatomic distances and coordination numbers derived fro m EXAFS data could be used for a basic structural characterization in the s ense of "structural fingerprints" to identify the indium species present in a given sample. Indium species in the samples comprised In+, In3+, (In-2)( 4+) and (In-3)(5+), with up to 3 of them being simultaneously present. Whil e the analysis of compounds with only one indium species was possible with high accuracy, it was impossible to interpret the EXAFS data of complex mix ed-valent solids containing three different indium species (e.g, In+, In3+, (In-2)(4+)) without prior knowledge of the crystal structures. Intermediat e cases are those compounds in which two indium species are present, e.g. I n3+ in tetrahedral and octahedral coordination or In+ in a highly regular c oordination accompanied by In3+. In conclusion, EXAFS measurements alone ca n be a reliable basis for structural discussions beyond the first coordinat ion sphere only fur compounds of moderate complexity. In most cases prior i nformation of structural data from single crystals or other sources is nece ssary. This is particularly relevant for future experiments with amorphous or polycrystalline samples that cannot he studied with standard diffraction techniques.