Composition-dependent optical, dielectric and d-orbital electron characteristic of high-throughput horizontal composition gradient Li1-xMgxTi2O4 combinatorial film

Abstract

The unique physical properties of LiTi2O4 and MgTi2O4 attracted extensive research interest in various fields, including new type batteries and high-temperature superconductors. However, the intricate mechanism governing the d-orbital electron within this system still awaits further excavation. Herein, we employed the combinatorial laser molecular beam epitaxy technique to fabricate a high-quality composition-spread Li1-xMgxTi2O4 film and evaluated its structure, composition, surface morphology and phonon modes by X-ray diffraction, X-ray photoelectron spectroscopy, atomic force microscopy, and Raman scattering spectroscopy. The optical constants and dielectric properties of diverse microregions with varying Li/Mg content over a wide spectral range (189–1658 nm) were measured nondestructively using spectroscopic ellipsometry, revealing the composition-dependent optical properties of Li1-xMgxTi2O4. By fitting the second derivative spectra (d2ε/dE2) using the standard critical point model, five transitions within the detected spectral range were observed. Combining previous analyses and first-principles calculation, we attribute the critical points to crystal field (CF) transition 4A2 →4T1 originating from the spin state of tetrahedral Li/Mg ions, the binding of Ti-related pairs and electron transitions between O-2p to Ti-3d orbits. This exploration of the precise structure of the d-metal band provides a positive reference for the regulation and application of relevant devices.