Sm3+ in ZnS on lattice sites and wurtzite - type interstitial sites?

The dye-laser site-selective excitation technique is used to investigate 4f-4f optical spectra of three Sm3+ centers in mixed-polytype ZnS bulk crystals at 4.2 K. Measurements are made on crystals grown by the Bridgman method and by iodine transport, the latter containing Li as a codopant. One center (C), which obviously has cubic (Td) symmetry, is assigned, on the basis of a crystal-field analysis, to Sm on (AN-type) Zn sites. For the other two species (A and D), each of which is the dominant center in one of the two types of crystals, polarisation data indicate trigonal (C3v) symmetry, with symmetry axes parallel to [111]w. For these centers models are suggested involving 'wurtzite-type' interstitial site occupation, with local charge compensation by intrinsic defects and Li impurities, respectively. (author)