An ab initio modelling for pristine and defect-induced emissions from NaCe(WO${}_4$)${}_2$. A potential material for solid-state lighting application

Tetragonal NaCe(WO${}_4$)${}_2$ with oxygen vacancy has been successfully synthesised by a cetyltrimethylammonium bromide (CTAB)-assisted hydrothermal technique. X-ray diffraction, scanning electron microscopy, transmission electron microscopy, high-resolution transmission electron microscopy, etc. have been employed for detailed structural analysis, whilst oxygen vacancies, explored by Raman, FTIR and X-ray photoelectron spectroscopies, lead to distorted CeO${}_8$ dodecahedra (c.a. CeO${}_7$ and CeO${}_6$). It has been observed from photoluminescence spectra that the synthesised samples exhibit intense emission at 435 nm due to 5d-4f transition of Ce${}^{3+}$ within CeO${}_8$ dodecahedra, whilst emission at 485 nm is assigned for same transition within CeO${}_7$. CeO${}_6$ provides two emissions at 451 and 520 nm. Herein, we have identified that CTAB plays an important role in the formation of CeO${}_7$ and CeO${}_6$. In contrast to earlier ethylene-diamine-tetra-acetic acid-assisted NaCe(WO${}_4$)${}_2$ with predominant green emission, our samples exhibit strong violet emission indicating less formation of CeO${}_7$ and CeO${}_6$ by CTAB. Herein, our ab initio calculation using density functional theory reveals that valence and conduction bands comprise O 2p and O 2p-Ce 5d orbitals, respectively, whilst Ce 5d${}_z$${}^2$ and 5d${}_{yz}$, 5 d${}_{xz}$ orbitals mostly facilitate 5d-4f transition within CeO${}_7$ and CeO${}_6$. Commission Internationale de l'Elcairage coordinates are found in violet region with correlated colour temperature ∼ 6491 K indicating that CTAB-assisted hydrothermally synthesised NaCe(WO${}_4$)${}_2$ can be a potential material for cold solid-state lighting application.