Collective Relaxation, Single Particle Motion and Short Range Order in α′-NbD x : A Quasielastic Neutron Scattering Study

Structure and Dynamics in Mg2+-Stabilized γ-Na3PO4

In parallel with advances in the synthesis of solid-state ionic conductors, there is a need to understand the underlying mechanisms behind their improved ionic conductivities. This can be achieved by obtaining an atomic level picture of the interplay between the structure of materials and the resultant ionic diffusion processes. To this end, the structure and dynamics of Mg²⁺-stabilized rotor phase material γ-Na₃PO₄, characterized by neutron scattering, are detailed in this work. The Mg²⁺-stabilized rotor phase is found to be thermally stable from 4 to 650 K. However, signatures of orientational disorder of the phosphate anions are also evident in the average structure. Long-range Na⁺ self-diffusion was probed by quasi-elastic neutron scattering and subsequently modeled via a jump diffusion matrix with consideration of the phosphate anion rotations. The resultant diffusion model points directly to coupled anion-cation dynamics. Our approach highlights the importance of considering the whole system when developing an atomic level picture of structure and dynamics, which is critical in the rational design and optimization of energy materials.

Fast Cation Conduction and Anion Rotational Disorder in the High-Temperature Phase of Lithium Sodium Sulfate

Quasielastic neutron scattering and high-frequency (10 MHz to 60 GHz) conductivity measurements have been performed on the high-temperature phase of LiNaSO4, a typical representative of a group of simple inorganic salts exhibiting both fast-cation conduction and anion rotational disorder.

The quasielastic neutron spectra are composed of two Lorentzian contributions which are consistently attributed to cation diffusion and anion rotational motion. The narrower component is dominated by incoherent sodium scattering; it shows the characteristics of jump diffusion with a jump distance of 3.8 Å and residence times between 14 ps and 22 ps.

The broader quasielastic component reflects the anion dynamics with typical reorientation times of 2 ps. The reduced Q-dependent intensities are in good agreement with the model of isotropic diffusion. The quasielastic intensity in the low Q regime (Q < 1 Å−1) indicates the involvement of cations in the reorientational motion of the translationally fixed anions.

Analysis of the conductivity data in terms of diffusivities points to an unusual cation conduction mechanism: the Haven ratio, H R = D */D σ, turns out to be considerably larger than one. This behavior, rarely observed in a typical fast ion conductor, can be traced back to a charge correlation factor which is clearly smaller than unity, indicating that charge transport is less effective than tracer transport in this material.

Influence of isotherm inflection on the loading dependence of the diffusivities of n-hexane and n-heptane in MFI zeolite. Quasi-elastic neutron scattering experiments supplemented by molecular simulations

Quasi-Elastic Neutron Scattering (QENS) experiments were carried out to determine (a) Fick diffusivity, D (b) self-diffusivity, Dself, and (c) 1/Gamma, the inverse of the thermodynamic correction factor, for n-hexane (nC6) and n-heptane (nC7) in MFI zeolite (all silica silicalite-1) at 300 K for a variety of loadings. These experimental results are compared with configurational-bias Monte Carlo (CBMC) and molecular dynamics (MD) simulations of, respectively, the adsorption isotherms and diffusivities. For n-hexane, the CBMC simulated isotherm shows a slight inflection at a loading=4 molecules per unit cell; this inflection manifests, also, in the loading dependence of 1/Gamma, obtained from QENS. The trend in the loading dependence of the Fick D and Dself of nC6 obtained from QENS matches the MD simulation results. For nC7 the CBMC simulated isotherm shows a strong inflection at a loading=4 molecules per unit cell. At this loading=4, 1/Gamma tends to zero and there is a very good match between QENS and molecular simulations for the loading dependence of 1/Gamma. Both MD simulations and QENS data on the Fick diffusivity shows a sharp maximum at a loading in the region of=4. For both nC6 and nC7 the simulated values of diffusivity are about an order of magnitude higher than those determined from QENS.