1. Introduction
Numerous studies on high-transition temperature (
Similar to cuprates [6, 33] and pnictides [9, 24], Ti-based superconductors (titanate) present the possibility of a mixed-valence state (with electronic configuration of 3
The atomic and electronic structures of the novel spinel LTO superconductors have been investigated. X-ray spectroscopy, X-ray absorption near-edge structure (XANES) spectroscopy, and resonant inelastic soft X-ray scattering (RIXS) spectroscopy are powerful tools for obtaining information on the local orbital character of a specific element. These approaches are also used to probe occupied or unoccupied electronic states near the Fermi level (
XANES spectroscopy is a sensitive tool that can be employed to probe unoccupied electronic states above
RIXS spectroscopic technique is used to explore the electronic structure of materials and to associate the structure with the XANES spectrum. RIXS is a process that describes the de-excitation of the final state of X-ray absorption, providing information on ground state via the excited intermediate states. Consequently, by tuning various excitation energies, certain RIXS features can be enhanced, allowing the separation of different electronic configurations in the mixed ground states [11, 30, 42]. The above conditions indicate that RIXS is a complementary tool compared with XANES. Unlike XANES, RIXS can be used to investigate a forbidden electronic transition (e.g.,
In this research, XANES and RIXS spectra were applied to understand the influence of V doping on the atomic/electronic properties of the LTVO system and to discuss the basis of the quick suppression of superconductivity. The occupied or unoccupied states near
2. Experiments
All the LTVO samples were synthesized through conventional two-step solid-state reactions with highly pure oxides of TiO2 (99%), Li2CO3 (99.99%), and V2O3 (99%) [19]. The samples were dried at 150 °C for at least 2 h. Additional Li2CO3 (5 mol%) was added into the samples to compensate for lithium evaporation at high temperature. These processes were performed in a dynamic vacuum environment. In the first step, a mixture of Li2CO3 and TiO2 was pulverized and calcined at 750 °C for 8 h in ambient air. The mixture was pulverized and calcined again at 800 °C for 12 h to form the intermediate compound Li2Ti2O5. In the second step, the stoichiometric powders of Li2Ti2O5, Ti2O3, and V2O3 were mixed and ground and then cold-pressed into pellets. The pellets were wrapped with gold foil and sintered in an alumina crucible at 880 °C for 24 h in a dynamic vacuum environment at less than 10−5 torr. The final products were stored in an Ar-filled glove box or vacuum desiccator to prevent aging. Powder X-ray diffraction (XRD) patterns [19] with X-ray (Cu, Kα =1.5418 Ǻ) radiation in a diffractometer (Philips PW3040/60) confirmed the phase purity of the product and showed the variation of the lattice parameter as a function of V dopant. Different proportions of the dopant up to 2% were selected to understand the effect of dilute doping. Oxygen stoichiometry was expected to remain constant for all small concentrations given that the preparations were all the same.
XANES spectra were obtained from the National Synchrotron Radiation Research Center (Taiwan) and were operated at 1.5 GeV with a current of 360 mA. The K-edge of Ti and V K-edge of LTVO were measured with a wiggler beamline BL17C1 equipped with Si (111) crystal monochromators. The absorption spectra were recorded in the fluorescence-yield mode with a Lytle detector [27]. These hard X-ray absorption spectra, which could provide information on the unoccupied states with transition metal
The experiments for X-ray emission spectroscopy (XES) were conducted at beamline 7.0.1 at the Advanced Light Source, Lawrence Berkeley National Laboratory. The beamline is armed with a spherical grating monochromator and an undulator (period 5 cm in 99 poles) [3]. The RIXS spectra were recorded with a high-resolution grating spectrometer at grazing incidence with a 2D detector [17]. The resolution for Ti L emission spectra was about 0.4 eV. The monochromator resolution was set up similar to that of the emission measurements.
X-ray absorption at the Ti L-edge determined that the electron in the Ti 2
3. Results and discussion
3.1. Structure of LTVO
The crystal structure of cubic spinel LTO, which belongs to the Fd3m space group, is illustrated in Fig. 1(a). The structure demonstrates that the lattice parameter of LTO is 8.404 Å (a = b = c) and contains eight AB2O4 units per unit cell. In general, a spinel structure has a total of 56 ions, 32 anions, and 24 cations per unit cell. The Li, Ti, and oxide ions are located at tetrahedral (T
3.2. Resistive properties
The transport properties of the samples are specified in Fig. 3(a) with V doping of less than 0.025. The findings indicate that the superconducting
The resistivity near 300 K increases as
3.3. Electronic structure results based on x-ray spectra
With the aim to investigate the effects of V dilute doping on LTO, this research measures XANES and RIXS at Ti L-edges. As shown in the inset in Fig. 4(a), the Ti L-edge XANES spectra exhibit several well-resolved features because of the excitations of a 2
When excitation energy is tuned to the
Complementary information is also acquired for this research by thoroughly analyzing the XANES Ti L3,2-edge spectra. The spectra of LiTi2-
The pre-edge features of Ti K-edge XANES spectra are displayed in Fig. 6(a). These spectral features at K-edge are due to the transitions from the Ti 1
The O K-edge XANES spectra [Fig. 6(c)] provide useful information about the unoccupied density of states in TMO because of the covalent mixing between O and TM ions. The two pre-edge features are at about 530 eV to 536 eV because of the strong hybridizations in Ti 3
Previous studies have indicated that LTO is a spinel superconductor, whereas spinel LVO reveals a heavy fermionic behavior with a Curie-Weiss spin susceptibility and a large electronic specific heat. Conduction arises on a Ti sublattice via the
4. Conclusion
The results on the effect of V doping on the atomic and electronic structures of LTVO, XAS, and RIXS spectra reveal that the valence of V ions remains constant, but the hybridization of Ti-O considerably varies. In particular, the results show the mixed valency nature of Ti ions and a significant variation in the hybridization of Ti 3d-O 2p states. The sharp decrease in the superconductivity of V-doped samples is ascribed to the electron–orbital interaction arising from the hybridization of TM and O orbitals. Low-energy excitation because of d-d excitation indicates that electron correlation and Ti oxidation number are enhanced, which support the XAS observation. Meanwhile, the observed properties of superconductivity are attributed to an altered density of the states of Ti 3d electrons and Ti-O hybridization. The Ti electronic configuration and lattice distortion induce the rapidly suppressed superconductivity and are attributed to hybridization rather than to the magnetic nature of the substituted ion. This research also demonstrates that RIXS is a powerful tool for investigating the electronic states and electron correlations of Ti compounds, in which the XANES spectral features are subdued.
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