Synthesis, Structure and Optical Properties of Quaternary Seleno-gallates NaLnGa₄Se₈ (Ln = Ce, Nd) and Their Comparison with the Isostructural Thio-gallates


Three new quaternary seleno-gallates containing rare-earth metals and sodium cations, have been synthesized by a solid-state route in evacuated quartz ampoules: NaLnGa4Se8 (Ln = La(I), Ce (II) and Nd (III)). The synthesis involved the stoichiometric combination of sodium polyselenides, rare-earth metal, Ga2Se3, and Se or elemental Ga in place of Ga2Se3. Single-crystal structure analysis indicated that the compounds are isostructural to the thio-analogue, NaNdGa4S8. The structures of I−III are described in terms of layers of GaSe4 tetrahedra joined by corner- and edge-sharing; the alkali-metal cations and the trivalent rare-earth metal cations occupy square antiprismatic sites between the layers. The optical properties of the compounds have been investigated and compared with the isostructural thio-gallate. The band gap of I was located around 2.65 eV. The band gaps of II and III were 2.66 and 2.73 eV, respectively, considerably narrower than their thio-analogues (3.4 eV). The contraction of the band gap was attributed to the shift of the valence band to higher energy due to the involvement of higher energy (4p) Se orbitals. The 4f → 5d gap of II is found to be located around 2.32 eV, which is 0.26 eV narrower than the thio-analogue is due to a greater dispersion of the Ln-(5d) band caused by more covalent Ce-Se bonds as well as rising of the f level energy.



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