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Adoption of renewable energy is essential to address the challenge of climate change, but that necessitates energy storage technologies. Lithium-ion batteries, the most ubiquitous solution, are insufficient for large-scale applications, so sodium-ion batteries (SIBs), an alternative, are of great current interest. To design and synthesise a commercially viable SIB with required performance, a fundamental understanding of the materials is imperative. Neutron diffraction and spectroscopy provide a unique insight of atomistic understanding of structure and dynamics in materials, therefore in this perspective we have explored how these techniques have been used in SIB research. As neutrons have high penetrability in materials and neutron-matter interaction probabilities are independent of the atomic number, they can provide unique information about motion of particles in bulk materials in the pico- to nanosecond time scales. This makes neutron scattering techniques important tools in battery research. Sodium has a low neutron cross section, which makes computational simulations essential for analysing neutron scattering data of SIB materials. With the availability of high flux neutron sources, high resolution instruments and high performance computers and simulations tools, neutron spectroscopy has been an emerging technique in the last decade for SIB research. In this perspective, we have shown that neutron diffraction is the most popular, while neutron spectroscopies, are just emerging. Computational simulation methods, both force field based and from first principles, are common but still are mostly used independent of neutron experiments. We have identified that suitable improvements of instrumentation, sample environments and simulations methodology will allow these techniques to be more accessible in future., Comment: 35 pages, 4 figures, 3 tables

We present the investigations on atomic dynamics and Li+ diffusion in crystalline and amorphous Li2Si2O5 using quasielastic (QENS) and inelastic neutron scattering (INS) studies supplemented by ab-initio molecular dynamics simulations (AIMD). The QENS measurements in the amorphous phase of Li2Si2O5 show a narrow temperature window (700 < T < 775 K), exhibiting significant quasielastic broadening corresponding to the fast Li+ diffusion and relaxation of SiO4 units to the crystalline phase. Our INS measurements clearly show the presence of large phonon density of states (PDOS) at low energy (low-E) in the superionic amorphous phase, which disappear in the non-superionic crystalline phase, corroborating the role of low-E modes in Li+ diffusion. The frustrated energy landscape and host flexibility (due to random orientation and vibrational motion of SiO4 polyhedral units) play an essential role in diffusing the Li+. We used AIMD simulations to identify that these low-E modes involve a large amplitude of Li vibrations coupled with SiO4 vibrations in the amorphous phase. At elevated temperatures, these vibrational dynamics accelerate the Li+ diffusion via a paddle-wheel like coupling mechanism. Above 775 K, these SiO4 vibrational dynamics drive the system into the crystalline phase by locking SiO4 and Li+ into deeper minima of the free energy landscape and disappear in the crystalline phase. Both experiments and simulations provide valuable information about the atomic level stochastic and vibrational dynamics in Li2Si2O5 and their role in Li+ diffusion and vitrification., Comment: 31 pages, 7 Figures, Supplementary Materials

We have performed quasielastic and inelastic neutron scattering (QENS and INS) measurements from 300 K to 1173 K to investigate the Na-diffusion and underlying host dynamics in Na2Ti3O7. The QENS data show that the Na atoms undergo localized jumps up to 1173 K. The ab-initio molecular dynamics (AIMD) simulations supplement the measurements and show 1-d long-ranged diffusion along the a-axis above 1500 K. The simulations indicate that the occupancy of the interstitial site is critical for long-range diffusion. The nudged-elastic-band (NEB) calculation confirmed that the activation energy barrier is lowest for diffusion along the a-axis. In the experimental phonon spectra the peaks at 10 and 14 meV are dominated by Na dynamics that disappear on warming, suggesting low-energy phonons significantly contribute to large Na vibrational amplitude at elevated temperatures that enhances the Na hopping probability. We have also calculated the mode Gr\"uneisen parameters of the phonons and thereby calculated the volume thermal expansion coefficient, which is found to be in excellent agreement with available experimental data., Comment: 22 Pages, 10 Figures

We have investigated the dynamics of Na ions in amorphous Na2Si2O5, a potential solid electrolyte material for Na-battery. We have employed quasielastic neutron scattering (QENS) technique in the amorphous Na2Si2O5 from 300 to 748 K to understand the diffusion pathways and relaxation timescales of Na atom dynamics. The microscopic analysis of the QENS data has been performed using ab-initio and classical molecular dynamics simulations (MD) to understand the Na-ion diffusion in the amorphous phase. Our experimental studies show that the traditional model, such as the Hall and Ross (H-R) model, fairly well describe the diffusion in the amorphous phase giving a mean jump length of ~3 {\AA} and residence time about 9.1 picoseconds. Our MD simulations have indicated that the diffusion of Na+ ions occurs in the amorphous phase of Na2Si2O5 while that is not observed in the crystalline orthorhombic phase even up to 1100 K. The MD simulations have revealed that in the amorphous phase, due to different orientations of silicon polyhedral units, accessible pathways are opened up for Na+ diffusions. These pathways are not available in the crystalline phase of Na2Si2O5 due to rigid spatial arrangement of silicon polyhedral units., Comment: 24 Pages, 12 figures

Both structural glasses and disordered crystals are known to exhibit anomalous thermal, vibrational, and acoustic properties at low temperatures or low energies, what is still a matter of lively debate. To shed light on this issue, we studied the halomethane family C-Br_n-Cl_4-n (n = 0,1,2) at low temperature where, despite being perfectly translationally ordered stable monoclinic crystals, glassy dynamical features had been reported from experiments and molecular dynamics simulations. For n = 1,2 dynamic disorder originates by the random occupancy of the same lattice sites by either Cl or Br atoms, but not for the ideal reference case of CCl4. Measurements of the low-temperature specific heat (Cp) for all these materials are here reported, which provide evidence of the presence of a broad peak in Debye-reduced Cp/T^3 and in the reduced density of states g(w)/w^2 determined by means of neutron spectroscopy, as well as a linear term in Cp usually ascribed in glasses to two-level systems in addition to the cubic term expected for a fully ordered crystal. Being CCl4 a fully ordered crystal, we also performed density functional theory (DFT) calculations, which provide unprecedented detailed information about the microscopic nature of vibrations responsible for that broad peak, much alike the boson peak of glasses, finding it to essentially arise from a piling up (at around 3 - 4 meV) of low-energy optical modes together with acoustic modes near the Brillouin-zone limits., Comment: 10 pages, 5 figures

We show clear experimental evidence of co-operative terahertz (THz) dynamics observed below 3 THz (~100 cm-1), for a low-symmetry Zr-based metal-organic framework (MOF) structure, termed MIL-140A [ZrO(O2C-C6H4-CO2)]. Utilizing a combination of high-resolution inelastic neutron scattering and synchrotron radiation far-infrared spectroscopy, we measured low-energy vibrations originating from the hindered rotations of organic linkers, whose energy barriers and detailed dynamics have been elucidated via ab initio density functional theory (DFT) calculations. For completeness, we obtained Raman spectra and characterized the alterations to the complex pore architecture caused by the THz rotations. We discovered an array of soft modes with trampoline-like motions, which could potentially be the source of anomalous mechanical phenomena, such as negative linear compressibility and negative thermal expansion. Our results also demonstrate coordinated shear dynamics (~2.5 THz), a mechanism which we have shown to destabilize MOF crystals, in the exact crystallographic direction of the minimum shear modulus (Gmin)., Comment: 10 pages, 6 figures

We have calculated the contribution of intersubband transitions to the third order optical nonlinear susceptibility, $\chi^{(3)}(\omega,\omega,\omega)$ for nonresonant as well as resonant third harmonic generation and $\chi^{(3)}(\omega,-\omega,\omega)$ for nonlinear refraction and absorption. As examples, we consider InAs/AlSb and GaAs/GaAlAs quantum wells. The effects of finite barrier height, energy band nonparabolicity, and high carrier concentrations are included. It is shown that quantum confinement, rather than the band nonparabolicity, is responsible for high values of nonresonant $\chi^{(3)}_{zzzz}$. Very high values of $\chi^{(3)}_{zzzz}$ are obtained for third harmonic generation and two photon absorption for incident wavelength near 10.6 $\mu$m. Intensity dependence of refractive index and of absorption co-efficient is also discussed for intensity well above the saturation intensity. Effective medium theory is used to incorporate the collective effects., Comment: 9 pages, 7 figures