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Funding: Faraday Institution (Grant Number(s): FIRG018), Diamond Light Source (Grant Number(s): SP14239), Engineering and Physical Sciences Research Council (Grant Number(s): EP/L017008/1, EP/R023751/1, EP/T019298/1), SPRing8 (Grant Number(s): 2021A1425). Sodium layered oxides showing oxygen redox activity are promising positive electrodes for sodium‑ion batteries (SIBs). However, structural degradation typically results in limited reversibility of the oxygen redox activity. Herein, the effect of Zn‑doping on the electrochemical properties of P3-type sodium manganese oxide, synthesised under air and oxygen is investigated for the first time. Air‑Na 0.67 Mn 0.9 Zn 0.1 O 2 and Oxy‑Na 0.67 Mn 0.9 Zn 0.1 O 2 exhibit stable cycling performance between 1.8 and 3.8 V, each maintaining 96% of their initial capacity after 30 cycles, where Mn 3+ /Mn 4+ redox dominates. Increasing the voltage range to 1.8‑4.3 V activates oxygen redox. For the material synthesised under air, oxygen redox activity is based on Zn, with limited reversibility. The additional transition metal vacancies in the material synthesised under oxygen result in enhanced oxygen redox reversibility with small voltage hysteresis. These results may assist the development of high‑capacity and structurally stable oxygen redox‑based materials for SIBs. Publisher PDF

Wurtzite AlN films with high Fe concentrations were deposited on Si(100) substrates using radio-frequency sputtering, and the growth process was investigated. X-Ray diffraction (XRD) analysis with parallel incident X-rays showed that a thick film (∼2.5 μm) had a non-polar a-axis orientation, similar to the films deposited on SiO2 glass substrates. Through powder XRD analysis, it was found that non-wurtzite fine crystals and/or amorphous materials formed during the initial stage of deposition, followed by wurtzite nucleation with various orientations of the axes, such as a, m, and r orientations. The XRD results also implied that the a-axis-oriented regions drastically increased with increasing film thickness. Scanning transmission electron microscopy (STEM) and energy-dispersive X-ray spectroscopy analyses suggested that one of the candidates for the non-wurtzite fine crystalline material was γ-Al2O3. STEM analysis also demonstrated that a-axis-oriented single-crystal grains grew from randomly oriented small wurtzite grains that increased in diameter with increasing deposition time. Al K-edge X-ray absorption near-edge structure results suggested that the electronic structure in the polar c-axis direction of wurtzite lying in the film plane developed with deposition time, thereby further supporting the findings of the XRD and STEM analyses.

L. D. gratefully acknowledges financial support from the Swedish Energy Agency (contract 2020-005246). Y.C. and D.O.S. are grateful to the Faraday Institution for funding the MICHAEL computing cluster hosted at University College London (UCL). Our membership of the UKs HEC Materials Chemistry Consortium, which is funded by EPSRC (EP/L000202, EP/R029431), this work used the ARCHER2 UK National Supercomputing Service(http://www.archer2.ac.uk). We are also grateful to the UK Materials and Molecular Modelling Hub for computational resources, which is partially funded by EPSRC (EP/P020194/1 and EP/T022213/1). We are grateful to the Engineering and Physical Sciences Research Council (EPSRC) Light Element Facility Grant (EP/T019298/1) for funding the acquisition of the Raman spectrometer. EF is grateful for an EaStCHEM studentship. This work was supported by the Faraday Institution (grant number FIRG018). Anion redox reactions offer a means of enhancing the capacity of layered sodium transition metal oxide positive electrode materials. However, oxygen redox reactions typically show limited reversibility and irreversible structural changes upon cycling, resulting in rapid capacity loss. Here, the Ti substituted Na4/7[□1/7Ti1/7Mn5/7]O2 (where □ represents a transition metal vacancy) is presented as a positive electrode material for sodium ion batteries. Na4/7[□1/7Ti1/7Mn5/7]O2 delivers a reversible capacity of 167 mAh g -1 after 25 cycles at 10 mA g -1 within the voltage range of 1.6 – 4.4 V and presents enhanced stability compared with Na4/7[□1/7Mn6/7]O2 over the voltage range 3.0 – 4.4 V. The structural and electronic structural changes of this Ti4+ substituted phase are investigated by powder X-ray diffraction, X ray absorption spectroscopy, electron paramagnetic resonance and Raman spectroscopy, supported by density functional theory calculations. These results show that the Na4/7[□1/7Mn6/7]O2 structure is maintained between 3.0 – 4.4 V, and the presence of TiO6 octahedra in Na4/7[□1/7Ti1/7Mn5/7]O2 relieves structural distortions from Jahn Teller distorted Mn3+O6 between 1.6 – 4.4 V. Furthermore, Ti4+ substitution stabilises the adjacent O 2p orbitals and raises the ionicity of the Mn O bonds, increasing the operating potential of Na4/7[□1/7Ti1/7Mn5/7]O2. Thereby providing evidence that the improved electrochemical performance of Na4/7[□1/7Ti1/7Mn5/7]O2 can be attributed to Ti4+ substitution. This work provides insight and strategies for improving the structural stability and electrochemical performance of sodium layered oxides. Publisher PDF

Funding information: This work was supported by the Faraday Institution (Grant No. FIRG018). The authors gratefully acknowledge support from the Engineering and Physical Sciences Research Council (EPSRC), Grant Nos. EP/L017008/1, EP/R023751/1 and EP/T019298/1. Sodium layered oxides which display oxygen anion redox behaviour are considered promising positive electrodes for sodium-ion batteries because they offer increased specific capacities. However, they suffer from irreversible structural changes resulting in significant capacity loss and limited oxygen redox reversibility. Here the effect of Cu substitution on the electrochemical performance of P3-type sodium manganese oxide is examined by evaluating the structural and electronic structural evolution upon cycling, supported by density functional theory (DFT) calculations. Over the voltage range 1.8–3.8 V vs. Na/Na+, where the redox reactions of the transition metal ions contribute entirely towards the charge compensation mechanism, stable cycling performance is maintained, showing a capacity retention of 90% of the initial discharge capacity of 166 mA h g−1 after 40 cycles at 10 mA g−1. Over an extended voltage range of 1.8–4.3 V vs. Na/Na+, oxygen anion redox is invoked, with a voltage hysteresis of 110 mV and a greater initial discharge capacity of 195 mA h g−1 at 10 mA g−1 is reached. Ex-situ powder x-ray diffraction patterns reveal distortion of the P3 structure to P'3 after charge to 4.3 V, and then transformation to O'3 upon discharge to 1.8 V, which contributes towards the capacity fade observed between the voltage range 1.8–4.3 V. DFT with projected density of states calculations reveal a strong covalency between the copper and oxygen atoms which facilitate both the cationic and anionic redox reactions in P3-type Na0.67Mn0.9Cu0.1O2. Publisher PDF

Wurtzite AlN film is a promising material for deep ultraviolet light-emitting diodes. However, some properties that attribute to its crystal orientation, i.e., c-axis orientation, are obstacles in realizing high efficiency devices. Constructing devices with non-c-axis oriented films is a solution to this problem; however, achieving it with conventional growth techniques is difficult. Recently, we succeeded in growing a-axis oriented wurtzite heavily Fe-doped AlN (AlFeN) films via sputtering. In this article, we report the electronic structures of AlFeN films investigated using soft X-ray spectroscopies. As-grown films were found to have conduction and valence band structures for a film with c-axis in film planes. Simultaneously, it was found that large gap states were formed via N-p and Fe-d hybridization. To remove the gap states, the films were annealed, thereby resulting in a drastic decrease of the gap states while maintaining a-axis orientation. We offer heavy Fe-doping and post annealing as a new technique to obtain non-polar AlN films.

Band structure calculations for radiofrequency-sputtered AlN-films doped with various 3d-transition-metals (TM: V, Cr, and Mn) were conducted to investigate the origin of the characteristic optical absorption structures. Experimentally evaluated crystal structures and lattice constants of the synthesized films were adopted for supercells. The model calculations showed that additional energy bands mainly consisting of 3d e and t states of TMs are formed in the band gap of AlN (6.2 eV), and that their potentials depend on the TM species. It was also shown that the Fermi levels of Cr- and Mn-doped AlN lie within the spin-up t band, while the Fermi level of V-doped AlN lies between the spin-up e and t bands. These findings imply that the materials have TM species-dependent, multiple absorption paths with lower energy than the band gap energy of AlN, resulting in optical absorption in the near-ultraviolet, visible, and infrared regions.

We investigated the band structure and photoconductivity of heavily Ti-doped (up to 11.0% of Ti) AlN films (AlTiN) with an optical absorption starting at around 2.2 eV. X-ray diffraction (XRD) and transmission electron microscopy results revealed that the films consisted of a c-axis oriented wurtzite structure. Ti K-edge extended X-ray absorption fine structure and X-ray absorption near edge structure (XANES) results suggested that most of the Ti atoms occupied Al sites having a 3+/4+ mixed valence. Ab initio calculations were conducted using lattice constants estimated from XRD data for supercells with Ti atoms occupying Al sites. It was revealed that the Ti-driven states were formed just below the bottom of the conduction band (CB) of AlN, and almost merged with it at high Ti concentrations. The calculated band structure was experimentally verified using X-ray photoemission spectroscopy of the valence band (VB) region and N K-edge XANES. At the excitation energy near the absorption edge, photoconduction was observed for films with 8.3% and 11.0% Ti, meaning that the Ti-driven states started forming a CB at these Ti concentrations. Photoelectron yield spectroscopy results implied that the potential energy of the VB was around 6.0 eV with respect to the vacuum level. Together with the absorption edge energy, it is concluded that AlTiN is one of the promising candidate materials for solar energy conversion.

We report herein on a high degree of wurtzite [11-20] orientation in AlN films heavily doped with Fe (AlFeN). The AlFeN films were deposited by RF sputtering on SiO2 glass and sapphire (0001) substrates. X-ray diffraction measurements revealed the Fe concentration dependence of the preferred orientation of these films. At a high Fe concentration (10–26.1%), the films had a [11-20] orientation, whereas films with less than 5% of Fe had a [0001] orientation. This tendency was evident on sapphire (0001) substrate as well as on the SiO2 glass substrate. The highest degree of [11-20] orientation was achieved for 19.4% of Fe. Transmission electron microscopy observations of the [11-20]-oriented AlFeN films (Fe: 19.4%) showed that randomly oriented nuclei form near the interface with the substrate, and subsequently, columnar grains selectively grow with a [11-20] orientation. These results suggest that AlFeN (11-20) films may be potentially applicable as a buffer layer to grow non-polar AlN on various substrates.

For highly efficient photoconversion devices, 3d-transition-metal-doped AlN is a candidate intermediate-band material. Here, we synthesized and investigated V-doped AlN (AlVN; V ≤ 11%) films. The optical absorption spectra of the films showed characteristic features including a peak in the infrared region and shoulders in the visible light region. These features remained essentially unchanged for the various V concentrations. X-ray diffraction (XRD), transmission electron microscopy (TEM), and V K-edge X-ray absorption fine structure (XAFS) measurements were carried out to clarify the crystallographic origin of the optical absorption features. The XRD profiles revealed that all films had a c-axis-oriented wurtzite structure. The TEM analyses supported the XRD results. The V K-edge X-ray absorption near-edge structure indicated that the V atoms in the AlN lattice were surrounded by N atoms with non-centrosymmetric conditions and had an oxidation state close to 3+. Extended XAFS (EXAFS) analyses implied tha...

We investigate the crystallographic and electronic properties of wurtzite Cr-doped AlN (AlCrN) films (Cr ≤12.0%) that absorb visible light. We confirmed that the films consist of wurtzite columnar single crystals that are densely packed, c-axis oriented, and exhibit a random rotation along the a-axis in plane by using transmission electron microscopy. The oxidation state of Cr was found to be 3+ using Cr K-edge X-ray absorption near edge structure, which implies that Cr can be a substitute for Al3+ in AlN. The first nearest neighbor distances estimated using Cr K-edge extended X-ray absorption fine structure (EXAFS) were found to be nearly isotropic for incident light with electric fields that are parallel and perpendicular to the plane. The results of ab initio lattice relaxation calculations for the model of wurtzite Al1-xCrxN supercell where Cr replaces Al support the EXAFS results. The calculations for the model showed that additional energy bands are formed in the band gap of AlN, in which the Fermi energy (EF) is present. As expected from the calculation results, the electrical conductivity increases with increase in the Cr concentration, implying that the density of states at EF increases monotonically. From these results, we can conclude that AlCrN films are an intermediate band material with respect to their crystallographic and electric properties.

The valence band (VB) structures of wurtzite AlCrN (Cr concentration: 0-17.1%), which show optical absorption in the ultraviolet-visible-infrared light region, were investigated via photoelectron yield spectroscopy (PYS), x-ray/ultraviolet photoelectron spectroscopy (XPS/UPS), and ab initio density of states (DOS) calculations. An obvious photoelectron emission threshold was observed ~5.3 eV from the vacuum level for AlCrN, whereas no emission was observed for AlN in the PYS spectra. Comparisons of XPS and UPS VB spectra and the calculated DOS imply that Cr 3d states are formed both at the top of the VB and in the AlN gap. These data suggest that Cr doping could be a viable option to produce new materials with relevant energy band structures for solar photoelectric conversion.