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, NaNdGa 4S8. The structures of I-III are described in terms of layers of GaSe4 tetrahedra joined by comer- 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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© 2008 American Chemical Society (ACS), All rights reserved.

Publication Date

01 May 2008