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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 performed quasielastic neutron scattering (QENS) experiments up to 1243 K and ab-initio molecular dynamics (AIMD) simulations to investigate the Na diffusion in various phases of NaAlSiO4 (NASO), namely, low-carnegieite (L-NASO; trigonal), high-carnegieite (H-NASO; cubic) and nepheline (N-NASO; hexagonal) phases. The QENS measurements reveal Na ions localized diffusion behavior in L-NASO and N-NASO, but long-range diffusion behavior in H-NASO. The AIMD simulation supplemented the QENS measurements and showed that excess Na ions in H-NASO enhance the host network flexibility and activate the AlO4/SiO4 tetrahedra rotational modes. These framework modes enable the long-range diffusion of Na across a pathway of interstitial sites. The simulations also show Na diffusion in Na-deficient N-NASO through vacant Na sites along the hexagonal c-axis., Comment: 14 Pages, 5 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

Yttria stabilized Zirconia (YSZ) pellets with different crystallite sizes were irradiated with 80 MeV Ag$^{6+}$ ions at room temperature and 1000 K to understand the role of crystallite size/material microstructure and irradiation temperature on the radiation tolerance against high electronic energy loss (S$_e$). X-ray diffraction and Raman spectroscopy measurements reveal that, irrespective of the irradiation temperature, the nano-crystalline samples suffered more damage as compared to the bulk-like sample. A reduction in the irradiation damage i.e. improvement in the radiation tolerance, was observed for all the samples irradiated at 1000 K. The reduction in the damage, however, was remarkably higher for the two nano-crystalline samples compared to the bulk-like sample, and hence the difference in the damage between the bulk-like and nano-crystalline samples was also significantly lower at 1000 K than that at room temperature. The irradiation damage, against S$_e$, was thus found to be critically dependent on the interplay between the irradiation temperature and crystallite size. These results are explained with the help of detailed theoretical calculations/simulations based on the 'in-elastic thermal spike' model by taking into consideration the combined effect of crystallite size and environmental (irradiation) temperature on the electron-phonon coupling factor and lattice thermal conductivity (and hence on the resulting thermal spike). Our results are crucial from the fundamental perspective of comprehending the size and temperature dependent radiation damage against S$_e$ ; and also for a number of applications, in various radiation environments, where nano-materials are being envisioned for use., Comment: This version has been communicated (for publication)

The effects of high pressure on the crystal structure of orthorhombic (Pnma) perovskite type cerium scandate have been studied in situ under high pressure by means of synchrotron x-ray powder diffraction, using a diamond anvil cell. We have found that the perovskite type crystal structure remains stable up to 40 GPa, the highest pressure reached in the experiments. The evolution of unit-cell parameters with pressure has indicated an anisotropic compression. The room-temperature pressure-volume equation of state is obtained from the experiments. From the evolution of microscopic structural parameters like bond distances and coordination polyhedra of cerium and scandium, the macroscopic behavior of CeScO3 under compression has been explained and reasoned for its large pressure stability. The reported results are discussed in comparison with high-pressure results from other perovskites., Comment: 39 pages, 1 table, 9 figures