Large Local Internal Stress in an Elastically Bent Molecular Crystal Revealed by Raman Shifts.

The structural dynamics involved in the mechanical flexibility of molecular crystals and the internal stress in such flexible materials remain obscure. Here, the study reports an elastically bending lipidated molecular crystal that shows systematic shifts in characteristic vibrational frequencies across the bent crystal region - revealing the nature of structural changes during bending and the local internal stress distribution. The blueshifts in the bond stretching modes (such as C═O and C-H modes) in the inner arc region and redshifts in the outer arc region of the bent crystals observed via micro-Raman mapping are counterintuitive to the bending models based on intermolecular hydrogen bonds. Correlating these shifts with the trends observed from high-pressure Raman studies on the crystal reveals the local stress difference between the inner arc and outer arc regions of the bent crystal to be ≈2 GPa, more than an order of magnitude higher than the previously proposed value in elastically bending crystals. High local internal stress can have direct ramifications on the properties of molecular piezoelectric energy harvesters, actuators, semiconductors, and flexible optoelectronic materials.

Development of a high-rate-capable O3-structured 'layered' Na transition metal oxide by tuning the cation-oxygen bond covalency.

By developing a high-rate-capable O3-structured Na(Li0.05Ni0.3Si0.05Ti0.45Cu0.1Mg0.05)O2-based cathode material for Na-ion batteries, wherein partial substitution of more electronegative Si4+ for Ti4+ in the transition metal layer has weakened-cum-lengthened the Na-O bond, enlarged the 'inter-slab' spacing and, thus, enhanced the Na-transport kinetics, a design criterion has been laid-out in the above context.

Understanding the Effects of Tetrahedral Site Occupancy by the Zn Dopant in Li-NMCs toward High-Voltage Compositional-Structural-Mechanical Stability via Operando and 3D Atom Probe Tomography Studies.

Ni-containing "layered"/cation-ordered LiTMO2s (TM = transition metal) suffer from Ni-migration to the Li-layer at the unit cell level, concomitant transformation to a spinel/rock salt structure, hindrance toward Li-transport, and, thus, fading in Li-storage capacity during electrochemical cycling (i.e., repeated delithiation/lithiation), especially upon deep delithiation (i.e., going to high states-of-charge). Against this backdrop, our previously reported work [ACS Appl. Mater. Interfaces 2021, 13, 25836-25849] revealed a new concept toward blocking the Ni-migration pathway by placing Zn2+ (which lacks octahedral site preference) in the tetrahedral site of the Li-layer, which, otherwise, serves as an intermediate site for the Ni-migration to the Li-layer. This, nearly completely, suppressed the Ni-migration, despite being deep delithiated up to a potential of 4.7 V (vs Li/Li+) and, thus, resulted in significant improvement in the high-voltage cyclic stability. In this regard, by way of conducting operando synchrotron X-ray diffraction, operando stress measurements, and 3D atom probe tomography, the present work throws deeper insights into the effects of such Zn-doping toward enhancing the structural-mechanical-compositional integrity of Li-NMCs upon being subjected to deep delithiation. These studies, as reported here, have provided direct lines of evidence toward notable suppression of the variations of lattice parameters of Li-NMCs, including complete prevention of the detrimental "c-axis collapse" at high states-of-charges and concomitant slower-cum-lower electrode stress development, in the presence of the Zn-dopant. Furthermore, the Zn-dopant has been found to also prevent the formation of Ni-enriched regions at the nanoscaled levels in Li-NMCs (i.e., Li/Ni-segregation or "structural densification") even upon being subjected to 100 charge/discharge cycles involving deep delithiation (i.e., up to 4.7 V). Such detailed insights based on direct/real-time lines of evidence, which reveal important correlations between the suppression of Ni-migration and high-voltage compositional-structural-mechanical stability, hold immense significance toward the development of high capacity and stable "layered" Li-TM-oxide based cathode materials for the next-generation Li-ion batteries.

Facile and Scalable Development of High-Performance Carbon-Free Tin-Based Anodes for Sodium-Ion Batteries.

Tin (Sn)-based anodes for sodium (Na)-ion batteries possess higher Na-storage capacity and better safety aspects compared to hard carbon -based anodes but suffer from poor cyclic stability due to volume expansion/contraction and concomitant loss in mechanical integrity during sodiation/desodiation. To address this, the usage of nanoscaled electrode-active particles and nanoscaled-carbon-based buffers has been explored, but with compromises with the tap density, accrued irreversible surface reactions, overall capacity (for "inactive" carbon), and adoption of non-scalable/complex preparation routes. Against this backdrop, anode-active "layered" bismuth (Bi) has been incorporated with Sn via a facile-cum-scalable mechanical-milling approach, leading to individual electrode-active particles being composed of well-dispersed Sn and Bi phases. The optimized carbon-free Sn-Bi compositions, benefiting from the combined effects of "buffering" action and faster Na transport of Bi, to go with the greater Na-storage capacity and lower operating potential of Sn, exhibit excellent cyclic stability (viz., ∼83-92% capacity retention after 200 cycles at 1C) and rate capability (viz., no capacity drop from C/5 to 2C, with only ∼25% drop at 5C), despite having fairly coarse particles (∼5-10 µm). As proven by operando synchrotron X-ray diffraction and stress measurements, the sequential sodiation/desodiation of the components and, concomitantly, stress build-ups at different potentials provide "buffering" action even for such "active-active" Sn-Bi compositions. Furthermore, the overall stress development upon sodiation of Bi has been found to be significantly lower than that of Sn (by a factor of ∼3.8), which renders Bi promising as a "buffer" material, in general. Dissemination of such complex interplay between electrode-active components during electrochemical cycling also paves the way for the development of high-performance, safe, and scalable "alloying-reaction"-based anode materials for Na-ion batteries and beyond, sans the need for ultrafine/nanoscaled electrode particles or "inactive" nanoscaled-carbon-based "buffer" materials.

Pressure driven structural phase transition in EuTaO4: experimental and first principles investigations.

In this article we report the synthesis, characterization and high pressure (HP) investigation on technologically important, rare earth orthotantalate, EuTaO4. Single phase polycrystalline sample of EuTaO4has been synthesized by solid state reaction method adopting monoclinic M'-type fergusonite phase with space groupP2/c. Structural and vibrational properties of as synthesized compound are investigated using synchrotron based x-ray powder diffraction, and Raman spectroscopic techniques respectively. Both the techniques show presence of an isostructural, first order, reversible phase transition near 17 GPa. Bulk modulus obtained by fitting the experimental pressure volume data for low pressure and HP phase is 136.0(3) GPa and 162.8(21) GPa. HP phase is accompanied by an increase in coordination number around Ta atom from 6 to 8. First principles calculations under the frame work of density functional theory also predicts the isostructural phase transition and change in coordination around Ta atom, corroborating the experimental findings.

Growth of sillenite Bi12FeO20 single crystals: structural, thermal, optical, photocatalytic features and first principle calculations.

Ideal sillenite type Bi12FeO20 (BFO) micron sized single crystals have been successfully grown via inexpensive hydrothermal method. The refined single crystal X-ray diffraction data reveals cubic Bi12FeO20 structure with single crystal parameters. Occurrence of rare Fe4+ state is identified via X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS). The lattice parameter (a) and corresponding molar volume (Vm) of Bi12FeO20 have been measured in the temperature range of 30-700 °C by the X-ray diffraction method. The thermal expansion coefficient (α) 3.93 × 10-5 K-1 was calculated from the measured values of the parameters. Electronic structure and density of states are investigated by first principle calculations. Photoelectrochemical measurements on single crystals with bandgap of 2 eV reveal significant photo response. The photoactivity of as grown crystals were further investigated by degrading organic effluents such as Methylene blue (MB) and Congo red (CR) under natural sunlight. BFO showed photodegradation efficiency about 74.23% and 32.10% for degrading MB and CR respectively. Interesting morphology and microstructure of pointed spearhead like BFO crystals provide a new insight in designing and synthesizing multifunctional single crystals.

Anomalous high pressure behaviour in nanosized rare earth sesquioxides.

We report Raman spectroscopic studies of the nanosized rare earth sesquioxides, namely yttrium sesquioxide (Y(2)O(3)), gadolinium sesquioxide (Gd(2)O(3)) and samarium sesquioxide (Sm(2)O(3)), under high pressure. The samples were characterized using x-ray diffraction, Raman spectroscopy and atomic force microscopy at atmospheric pressures. Y(2)O(3) and Gd(2)O(3) were found to be cubic at ambient, while Sm(2)O(3) was found to be predominantly cubic with a small fraction of monoclinic phase. The strongest Raman peaks are observed at 379, 344 and 363 cm(-1), respectively, for Y(2)O(3), Sm(2)O(3) and Gd(2)O(3). All the samples were found to be nanosized with 50-90 nm particle sizes. The high pressures were generated using a Mao-Bell type diamond anvil cell and a conventional laser Raman spectrometer is used to monitor the pressure-induced changes. Y(2)O(3) seems to undergo a crystalline to partial amorphous transition when pressurized up to about 19 GPa, with traces of hexagonal phase. However, on release of pressure, the hexagonal phase develops into the dominant phase. Gd(2)O(3) is also seen to develop into a mixture of amorphous and hexagonal phases on pressurizing. However, on release of pressure Gd(2)O(3) does not show any change and the transformation is found to be irreversible. On the other hand, Sm(2)O(3) shows a weakening of cubic phase peaks while monoclinic phase peaks gain intensity up to about a pressure of 6.79 GPa. However, thereafter the monoclinic phase peaks also reduce in intensity and mostly disordering sets in which does not show significant reversal as the pressure is released. The results obtained are discussed in detail.