Influence of alkali metal ions on the defect induced photoluminescence properties of double tungstate compounds ACe(WO4)2 (A = Li, Na, K): experimental and ab initio theoretical study†

Abstract

Defect-induced alkali-metal cerium double tungstate compounds, ACe(WO₄)₂ (where A = Li, Na, K), have been synthesized through a trisodium citrate-based hydrothermal process. The influence of alkali-metal ions on the local structure of ACe(WO₄)₂ has been explored using various methods, including the Rietveld technique for powder X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Although the ACe(WO₄)₂ compounds exhibit similar transitions, they differ in luminescent intensity. Notably, in the case of the alkali metal Na, the material displays a larger crystal compactness due to its comparable ionic radii with Ce³⁺. This proximity indicates lower distortion. Conversely, Li and K possess significantly different ionic radii from Ce³⁺, leading to pronounced crystal distortion. The ACe(WO₄)₂ materials show emissions in blue and green spectra, including blue I (439 nm), blue II (462 nm), blue III (487 nm), and green (531 nm). The blue I emission is attributed to the 5d → 4f transition within the CeO₈ polyhedra, whereas the blue III emission arises from the same transition within CeO₇ polyhedra. The blue II and green emissions result from the formation of CeO₆ polyhedra. Additionally, ab initio calculations employing density functional theory reveal that the valence and conduction bands are composed of O 2p and O 2p–Ce 5d hybridization, respectively. Notably, the 5d_xy, 5d_xz, 5d_yz, 5d_x²−y², and 5d_xz, 5d_x²−y² orbitals significantly contribute to the 5d–4f transition within CeO₇ and CeO₆ polyhedra, respectively. The resulting Commission Internationale de l'Éclairage (CIE) coordinates in the blue region, coupled with a correlated color temperature (CCT) of approximately 7800 K, suggest that ACe(WO₄)₂ materials hold promise for applications in cold solid-state lighting.