Structural diversity and optical properties of ternary and quaternary chalcogenides

  • Author / Creator
    Iyer, Abishek K
  • New ternary and quaternary chalcogenides were synthesized by reactions of elements or binary starting materials at high temperatures, with an emphasis on identifying structures that are noncentrosymmetric which may lead to useful properties such as nonlinear optical behaviour, magnetic ordering, or ionic conductivity. Within the quaternary RE–M–M′–Ch systems (RE = rare-earth elements; M, M′ = d-block metals and p-block metalloids; Ch = S, Se), about 30 new compounds were discovered. Most of them belong to a versatile family of noncentrosymmetric hexagonal chalcogenides RE3M1–xM′Ch7, for which the limits of formation were tested. They include RE3CuGaCh7 and La3Ag0.6GaCh7, which violate the condition of charge balance normally seen in this family; La3MGaSe7 (M = Zn, Cd), which illustrate that M atoms can occupy more than one site; RE3M0.5GeS7 (M = Co, Ni, Pd), which extend the number of examples of nonstoichiometric derivatives of this family; and La3Sn0.5InCh7, which present an exception to the rule that higher-valent substituents prefer the tetrahedral site. Attempts were made to understand the site preferences of different M and M′ atoms through bond valence sum and band structure calculations. Other quaternary phases identified in this system are RE4InSbSe9, which is isostructural to a previously known sulfide NLO material, and RE7MInS13 (M = Co, Fe), which adopts a new structure type. Ternary and quaternary Ba-containing chalcogenides were also investigated. They include Ba4Ga2Se8 and Ba12In4Se20 exhibiting diselenide units Se22–, which are less commonly found than disulfide units S22–. The quaternary chalcogenide BaRE2In2Ch7 exhibits five-coordinate In in trigonal bipyramidal geometry. Several quaternary chalcogenides in the Ba–M–M′–Ch system were found that exhibit noncentrosymmetric structures built up of M- and M′-centred tetrahedra: Ba4Ga4GeSe12, BaZnM′Se4 (M′ = Si, Ge), and Ba5CdGa6Se15. Measurement of optical properties showed that BaZnM′Se4 and Ba5CdGa6Se15 are promising candidates for NLO materials with second harmonic generation responses comparable to those of benchmark materials.

  • Subjects / Keywords
  • Graduation date
    Fall 2018
  • Type of Item
  • Degree
    Doctor of Philosophy
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