Synthesis, Structure, and Optical Properties of the Quaternary Seleno-gallates Na<i>Ln</i>Ga₄Se₈ (<i>Ln</i> = La, 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.