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By invoking fundamental principles associated with tuning of cation–oxygen covalency, a generic design criterion for the development of high rate-capable ‘layered’ Na-transition metal oxide cathode materials for Na-ion batteries has been laid out.

Ni-containing "layered"/cation-ordered LiT

Doping plays a vital role in tuning the carrier concentration and also influences various electronic properties in semiconductors. In addition to tuning the electronic properties of semiconductors, impurities also affect vibrational properties. We report the local vibrational modes (LVMs) in N-doped n-type 6H-SiC. New phonon modes are observed in the low-temperature range. Unlike lattice phonons, LVMs are localized in both the real space and frequency domains and give rise to a distinct signature in the Raman spectra. In the present study, LVMs in N-doped 6H-SiC are identified as N atom vibrations with the help of lattice dynamical calculations of the phonon density of states using generalized gradient approximation. Moreover, the stability of the SiC is investigated for a wide temperature range signifying its application in micro-electro-mechanical systems technology. Therefore, the present study provides insight for understanding both lattice phonons and the impurity-induced vibrational modes using Raman spectroscopic analysis in corroboration with the calculation of phonon density of states.

Improvement in sensing performance by metal oxide based materials has usually been achieved by doping, morphology tuning, particle size tailoring or porosity modification. The existing models of band bending and grain boundary depletion layer that explain chemiresistive sensing address the above issues in terms of qualitative electronic behaviour. However, for thick film sensors in particular, there is a clear demarcation between the surface and the bulk. Thus the concept of gross electronic distribution needs refinement in terms of variable behavior of electrons in the surface and in the bulk so that the roles of different layers in a sensor film are distinguishable. Using the case of improved 98.2% ethanol sensing response by paramagnetic aluminium doped tin oxide mesoporous nano-systems, we have demonstrated how Al doping promotes difference in electronic behavior in the surface and in the bulk of thick film sensors, complementing the existing charge depletion model. The 4-fold improvement in sensing responses by Sn0.947Al0.144O1.881 and Sn0.869Al0.242O1.888 as compared to tin oxide have been explained in terms of shortening of the unit cell by virtue of aluminium doping, and the 2.4-fold improvement in sensing response by Sn0.869Al0.242O1.888 as compared to Sn0.947Al0.144O1.881 has been explained by surface and bulk electron mobilization. Considering the conceptual inclusiveness of correlation studied, it is expected to be applicable in explaining sensing mechanisms based on electron mobilization in other metal oxide sensor systems also.

The Zr1-xCox(x=0, 0.25, 0.53, 0.63, 1) thin films were deposited on quartz substrate using magnetron co-sputtering of Zirconium and Cobalt targets in confocal geometry. A constant pulsed direct current (PDC) on Zirconium and radio frequency (RF) of various powers on Cobalt target were applied to vary the concentration of Co in the Zr1-xCox film. The film composition was quantified using EDX measurements. The hydrogen storage capacity of these films was studied using an in-house developed hydrogen adsorption setup, in which the electrical resistivity of the film was monitored as a function of hydrogen partial pressure and temperature. The films' surface morphology and crystal structure before and after hydrogenation were characterized using atomic force microscopy and grazing incidence X-ray diffraction techniques using synchrotron radiation, respectively. An increase in the particle size after hydrogenation was observed for all the films. An increase in resistivity was also observed due to the absorption of hydrogen in all the compositions. The near stoichiometric film Zr0.47Co0.53 showed the highest hydrogen absorption level at 200 oC at all partial pressures. However, a decrease in the response at temperatures higher than 200 oC was observed in the film containing a Co concentration. The mechanism for the increase in resistivity of the film on hydrogenation is explained. Keywords: ZrCo alloy, hydrogen getter, magnetron co-sputtering, four-probe resistivity, thin film.

A model Oxide Dispersion Strengthened (ODS) alloy powder of composition Fe – 15 wt. % Y2O3 – x wt. % Ti (x = 0, 2, 5, 10 and 15) were synthesized by high energy mechanical milling in Ar atmosphere for a prolonged duration of 60 h. Synchrotron X-ray diffraction (XRD) and transmission electron microscopy (TEM) observations suggested the amorphisation of Y2O3 nano-crystallites, irrespective of Ti content, which is further studied by Raman spectroscopy. The Raman spectroscopy analysis confirms the presence of Y O bonding in the milled powder and ruled out the possibility of elemental dissociation of Y2O3 and dissolution into the Fe matrix. Annealing of the milled powders containing different amounts of Ti led to formation of different types of oxide complexes which were also studied using synchrotron XRD and TEM studies. The role of Ti in refining the dispersoids through formation of Y1.6Ti1.8Fe0.4O6.6 is established through these studies.

The present manuscript reports high-pressure investigations on nano-crystalline Scandium Oxide (Sc2O3) through synchrotron angle dispersive x-ray diffraction technique (ADXRD) up to a pressure of ∼35.4 GPa using a diamond-anvil cell (DAC). The sample, having a cubic bixbyite structure at ambient conditions, underwent substantial changes in crystallographic parameters under the application of external pressure, while exhibiting phase stability i.e. does not undergo any structural phase transition up to 35.4 GPa. The lattice parameters and the unit cell volume decrease significantly with an increase in pressure, revealing the compressible nature of the crystal structure. The pressure-volume data is fitted using the third-order Birch-Murnaghan equation of state to calculate the bulk modulus and its first derivative, which yields zero pressure bulk modulus of 217(35) GPa, with B 0 ′ = 12(4) and the corresponding ambient volume per unit molecule comes out to be 59.18(18) Å3.

Crystallographic and electronic structures of phase pure ternary solid solutions of Ni1−xCoxO (x = 0 to 1) have been studied using XRD, EXAFS and XAS measurements. The lattice parameter of the cubic rock-salt (RS) Ni1−xCoxO solid solutions increases linearly with increasing Co content and follows Vegard's law, in the complete composition range. A linear increase in the bond lengths (Ni/Co–O, Ni–Ni and Ni–Co) with “x”, closely following the bond lengths determined from virtual crystal approximation (VCA), is observed, which implies that there is only a minimal local distortion of the lattice in the mixed crystal. The optical gap of the ternary solid solution determined from diffuse reflectivity measurements shows neither a linear variation with Co composition nor bowing, as observed in many ternary semiconductors. This trend in the variation of optical gaps is explained by probing the conduction band using XAS at the O K-edge. We have observed that the variation in the onset energy of the conduction band edge with “x” is very similar to the variation in the optical gap with “x”, thus clearly indicating the dominant role played by the conduction band position in determining the optical gap. The variation in the intensities of the pre-edge peak in the XANES spectra measured at Ni and Co K-edges, and the L1/2 peak in XAS spectra measured at Ni and Co L-edges, is found to depend on the unoccupied O 2p-metal-(Ni/Co) 3d hybridized states and the bond lengths.

We investigate the superconducting critical current density (Jc), transition temperature (Tc), and flux pinning properties under hydrostatic pressure (P) for Cr0.0009NbSe2 single crystal. The application of P enhances Tc in both electrical resistivity (∼0.38 K GPa−1: 0 ≤ P ≤ 2.5 GPa) and magnetization (∼0.98 K GPa−1: 0 ≤ P ≤ 1 GPa) measurements, which leads to a monotonic increase in Jc and flux pinning properties. The field-dependent Jc at various temperatures under P is analyzed within the collecting pinning theory and it shows that δTc pinning is the crossover to δl pinning above the critical pressure (Pc ∼0.3 GPa). Our systematic analysis of the flux pinning mechanism indicates that both the density of pinning centers and pinning forces greatly increase with the application of P, which leads to an enhancement in the vortex state. Structural studies using synchrotron X-ray diffraction under pressure illustrate a stable hexagonal phase without any significant impurity phase and lattice parameter reduction with P shows highly anisotropic nature.

The combination of in-situ synchrotron X-ray diffraction and in-situ stress measurements performed during electrochemical Li-alloying/-dealloying of amorphous Si (a-Si) films, having ∼150 nm thick ...

Effect of substitution and external applied static electric field on the structural and dielectric properties for lead-free (1 − x)(Na0.50Bi0.50)TiO3 − xKSbO3 (0 ≤ x ≤ 0.06) polycrystalline ferroelectric ceramics, fabricated via a modified sol–gel method, were investigated. Structural analysis of synchrotron radiation X-ray diffraction data confirmed the rhombohedral R3c phase for all unpoled samples. After poling, the tetragonal P4bm phase appeared with the rhombohedral phase in all the substituted samples. In poled samples, the phase fraction of the rhombohedral phase suppressed from ~ 93 (for x = 0.03) to ~ 87% (for x = 0.06), while tetragonal phase fraction increased from ~ 7 to ~ 13% as a function of substitution. The high-temperature dielectric analysis confirmed the reduction in depolarization temperature with increasing substitution. Also lattice disorder creates a plateau type dielectric anomaly, which leads to thermally stable dielectric constant ~ 2970 ± 10% (200–390 °C) and ~ 2830 ± 10% (125–400 °C) for x = 0.03 and 0.06 samples, respectively. Ferroelectric measurements showed that ambient temperature ferroelectric properties are improved for x = 0.03 composition with an observed remnant polarization (2Pr ~ 53.4 µC/cm2) and coercive field (2Ec ~ 94.7 kV/cm) as compared to parent NBT compound (2Pr ~ 44.7 µC/cm2, 2Ec ~ 124.5 kV/cm). In addition, at high-temperature, antiferroelectric like ordering enhances the recoverable energy density ~ 0.73 J/cm3 (efficiency ~ 72.3%) for x = 0.06 samples as compared to parent NBT (recoverable energy density ~ 0.05 J/cm3, efficiency ~ 2.4%). These improvements in electrical properties were correlated with structural changes as a function of composition and temperature. Obtained properties suggest that substituted samples might be a suitable candidate for high-temperature stable capacitors (operating temperature > 200 °C), ferroelectric, and energy storage applications.

Detailed temperature-dependent structural, dielectric, piezo/ferroelectric, and energy storage properties were explored for the poled (Na0.5-xKxBi0.5-xLax)TiO3 (0 ≤ x ≤ 0.12) ceramics fabricated via a modified sol-gel method. Structural analysis of synchrotron source powder XRD data revealed the rhombohedral (R3c) phase for poled x ≤ 0.03 compositions. Whereas for x ≥ 0.06 samples confirmed structural transition, a mix of rhombohedral and tetragonal (P4bm) phase exists at room temperature. As a function of composition, a rhombohedral phase is found to be suppressed and the tetragonal phase promoted. Dielectric measurements corroborate that at room temperature; dielectric constant was increased with substitution. High-temperature dielectric measurement confirmed the reduction in phase transition temperatures and an increase in the diffuseness of dielectric anomalies with increasing content of K/La. Piezo/ferroelectric measurements revealed that x = 0.03 composition exhibits excellent piezo/ferroelectric properties (piezoelectric coefficient, d33 ∼ 115 pC/N, remnant polarization, 2Pr ∼ 56 μC/cm2, and coercive field, 2Ec ∼ 100 kV/cm) at room temperature. Antiferroelectric ordering improved the energy storage density and efficiency at room temperature (∼0.05 J/cm3, ∼2.6% (for x = 0) to ∼ 0.74 J/cm3, ∼87% (for x = 0.12)) and elevated temperature. For x = 0.06 sample, excellent energy storage density and efficiency ∼1.10 J/cm3 and ∼70% respectively, are obtained at 120 °C. Superior energy storage efficiency showed by x = 0.12 (∼87–∼93%, in the temperature range 30–140 °C) with almost thermally stable energy storage density (from ∼0.74 J/cm3 to ∼ 0.71 J/cm3). These drastic improvements in properties were explained in terms of structural changes as a function of composition and temperature. Observed properties suggest that substituted materials are promising candidates for piezoelectric (for x = 0.03) and energy storage (for x ≥ 0.06) applications.

Effect of high pressure on ferroelectric diisopropylammonium bromide (DIPAB) is investigated using Raman spectroscopy and angle-dispersive x-ray diffraction (XRD) measurements using a diamond anvil cell up to ∼10 GPa. Raman spectroscopic studies indicate that the monoclinic (m-II) ferroelectric phase of DIPAB is unstable under slight compression, leading to changes in molecular conformation. We have observed new peaks appearing in the lattice mode region and CH3 stretching region giving indication of conformational change. XRD investigations reveal that the m-II phase (space group P21) is partially transformed into a centrosymmetric, non-polar triclinic (P-1) structure at 0.7 GPa. Ferroelectricity is expected to weaken under compression above 0.7 GPa and ferroelectricity may disappear at a very high pressure when the m-II phase fully converted into triclinic P-1 phase. On releasing the pressure the ambient structure is fully recovered. The role of hydrogen bonds in the phase transformation is also discussed.

Influence of thermal annealing on the ion beam sputtered FeCo(Nb)B quaternary alloy thin films having thickness ∼20 nm and 80 nm was investigated. Regardless of amorphous microstructure, the as-deposited films possess uniaxial magnetic anisotropy (UMA) in the film plane. Annealing at temperature ∼503 K results in amorphous to nanocrystalline phase transformation with precipitation of bcc-FeCo phase. With increasing annealing temperature, magnetic anisotropy gradually decreases and thermal annealing at 773 K results in complete disappearance of UMA. In-plane strain distribution in such nanocrystalline FeCo(Nb)B thin films was obtained by using laboratory and synchrotron X-ray diffraction (XRD) measurements. In-plane and out-of-plane X-ray diffraction measurements suggest, growth induced long range non-uniform tensile stress, as the possible origin of in-plane uniaxial magnetic anisotropy in such films. It is demonstrated that the stress distribution becomes uniform after annealing at 773 K and results in complete disappearance of magnetic anisotropy. The study further suggests that the presence of Nb in FeCoB film improves the thermal stability, inhibits grain growth and stabilizes bcc-FeCo phase up to elevated temperatures. This information is useful for magnetic tunnel junction application, where annealing of FeCoB magnetic electrode is essential.

The mechanical behavior of free-standing Si-membranes (diaphragms) fabricated by deep reactive ion etching (DRIE) was studied in order to emphasis the magnitude of process-induced stress and/or strain distribution in both square and circular diaphragms. For this purpose, 25 μm thick circular and square diaphragms with 4 mm diameter (or side-size) were fabricated using silicon-on-insulator wafer. The thickness and surface homogeneities of the DRIE processed Si-diaphragms were analyzed by SEM and AFM, respectively. Micro-Raman spectroscopy studies revealed a non-uniform distribution of compressive stresses throughout the diaphragm surface. Using the synchrotron based Laue diffraction measurements, the residual stress values were quantitatively estimated as 19.88 and −22.83 MPa for the unloaded circular and square diaphragms, respectively. The pressure vs. displacement experiments were conducted using the bulge testing method and these studies yielded maximum mid-point deflection values of 25 and 30 μm for the circular and square diaphragms, respectively against applied pressure of 25 kPa. Based on the deflection characteristics, the average residual stress and equivalent biaxial modulus were estimated by means of standard analytical equations. Using the finite element analysis, the mid-point deflection characteristics of the diaphragms were simulated through considering the residual stresses as input parameters and the results are compared with that of those obtained by the experimental and analytical methods. The residual stress information such as those inferred in this study would help in tuning the resonant frequency of Si-diaphragms towards sensor/actuator applications.

Na-ion battery system lacks a safe, electrochemically stable, high ‘rate-capable’ and environmental/health friendly anode material. This is because the ‘workhorse’ anode material for Li-ion batteries, i.e., graphitic carbon, cannot reversibly host Na-ions in its lattice and hard carbon which is the widely explored anode for Na-ion batteries, possesses serious safety concerns and operational hazards [1-3]. In this scenario, the present work reports bi-phase Na-titanate based anode material, which is safe and environmental/health friendly, and which has the potential to exhibit excellent rate-capability and long-term cyclic stability, even at very high current densities. ‘Bi-phase NTO’, having Na2Ti3O7 and Na2Ti6O13 as primary and secondary phases, respectively, shows contributions from both the phases towards electrochemical Na-storage, as confirmed from operando synchrotron XRD studies, and is more electrochemically stable than phase pure Na2Ti3O7. Additionally, ‘bi-phase NTO’ also has safe and favourable Na-insertion/removal potential in terms of operation as anode. The reinforcement with functionalised multi-walled carbon nanotubes (MWCNTs) further improves the electrochemical performance of ‘bi-phase NTO’, bestowing it with excellent stability, stable ‘charge-averaged’ discharge/charge voltages and negligible impedance build-up over multiple cycles. Due to the presence of O-containing functional groups (facilitating favorable surface interaction with titanate) and the high aspect ratio, the MWCNTs were observed to uniformly cover the surface of the Na-titanate particles, almost ‘wrapping’ the same. This suppresses the occurrence of deleterious reactions at the Na-titanate/electrolyte interface and leads to excellent connectivity across the active particles. Accordingly, even at a very high current density corresponding to 50C, a 1st cycle reversible Na-storage capacity of >140 mAh g-1 (viz., negligible drop from capacities at lower C-rates) and capacity retentions of ~83%, ~65%, 58% after 100, 1000, 2000 cycles, respectively (see Fig. 1) were obtained with the as-developed ‘bi-phase NTO’/MWCNT-based electrodes; indicating the feasibility for long-term cycling even at very high current densities. A part of this work has been published as Pradeep et al., Electrochim. Acta 362 (2020) 137122, i.e., ref. [4] here. Keywords: Na-titanate; composite electrode; in-operando synchrotron X-ray diffraction; high rate performance; Carbon-based reinforcements; Na-ion battery Reference s : [1] P. Senguttuvan, M. R. Palacín, G. Rousse, V. Seznec and J.-M. Tarascon, Chem. Mater., 23, 4109 (2011). [2] A. Rudola, K. Saravanan, C. W. Mason, and P. Balaya, J. Mater. Chem., A 1, 2653 (2013). [3] W. Wang, C. Yu, Y. Liu, J. Hou, H. Zhu and S. Jiao, RSC Adv., 3, 1041 (2013). [4] A. Pradeep, B.S. Kumar, A. Kumar, V. Srihari, H.K. Poswal and A. Mukhopadhyay, Electrochim. Acta., 362, 137122 (2020). Figure 1