In Situ Neutron Diffraction Studies of LiCe(WO4)2 Polymorphs: Phase Transition and Structure–Property Correlation

Abstract

Polymorphs of LiCe(WO4)2 [α-LiCe(WO4)2 and β-LiCe(WO4)2] were successfully synthesized by a citric acid-assisted sol–gel method for the first time. Phase purity and crystallinity were confirmed by powder X-ray diffraction and further characterized by X-ray photoelectron spectroscopy, field emission scanning electron microscopy, Raman, and thermogravimetric–differential thermal analysis measurements. Investigation of the effect of calcination temperature and time indicated the existence of an irreversible structural phase transition from low-temperature (LT) β-phase to high-temperature (HT) α-phase, which is systematically followed by in situ HT powder neutron diffraction (NPD) studies. Rietveld refinements using NPD data revealed that α-LiCe(WO4)2 crystallizes in tetragonal Scheelite-type structure (I41/a), while β-LiCe(WO4)2 crystallizes in triclinic α-LiPr(WO4)2-type structure (P1̅). Furthermore, β-LiCe(WO4)2 undergoes a reconstructive phase transition where WO6 of the β-phase rearranges to WO4 at HT in α-phase and is supported by Raman measurements. Anodic redox activities of these polymorphs were determined by cyclic voltammetry and galvanostatic charge–discharge measurements. Interestingly, β-phase has shown promising results compared with that of α-phase, which is attributed to the easy Li-ion diffusion in between the parallel layers of WO6 and also to the greater structural stability of the β-phase. Thus, our initial understanding on the structure–property of these LiCe(WO4)2 polymorphs provides insights into the design of new insertion anode electrodes for Li-ion batteries.