Comparing the Descriptors for Investigating the Influence of Lattice Dynamics on Ionic Transport Using the Superionic Conductor Na₃PS_4–xSe_x

Abstract

Recent work on superionic conductors has demonstrated the influence of lattice dynamics and the softness of the lattice on ionic transport. When examining either the changes in the acoustic phonon spectrum or the whole phonon density of states, both a decreasing activation barrier of migration and a decreasing entropy of migration have been observed, highlighting that the paradigm of "the softer the lattice, the better" does not always hold true. However, both approaches to monitor the changing lattice dynamics probe different frequency ranges of the phonon spectrum, and thus, it is unclear if they are complementary. In this work, we investigate the lattice dynamics of the superionic conductor Na 3 PS 4−x Se x by probing the optical phonon modes and the acoustic phonon modes, as well as the phonon density of states via inelastic neutron scattering. Notably, Raman spectroscopy shows the evolution of multiple local symmetry reduced polyhedral species, which likely affect the local diffusion pathways. Meanwhile, density functional theory and the ionic transport data are used to compare the different approaches for assessing the lattice dynamics. This work shows that, while acoustic and inelastic methods may be used to experimentally assess the overall changing lattice stiffness, calculations of the average vibrational energies between the mobile ions and the anion framework are important to assess and computationally screen for ionic conductors.

Influence of lattice dynamics on lithium-ion conductivity: A first-principles study

“…It goes without saying that the presence of negative phonon frequencies in zero-temperature VDOS automatically invalidates any estimation of ω , due to violation of the fundamental assumptions from which harmonic approaches are deduced [32,33]. Correction of the explained computational artifacts (e.g., by using finite-temperature simulation methods as done here) is very important for an improved interpretation of experiments, since partial VDOS normally cannot be resolved directly from measurements and consequently first-principles calculations are employed for that end [20,64].…”

Finding the Right Blend: Interplay Between Structure and Sodium Ion Conductivity in the System Na5AlS4–Na4SiS4

“…In its "cubic" phase, Na 3 PS 4 exhibits a room temperature ionic conductivity of up to 4.6 · 10 −4 S cm −1 Krauskopf et al, 2018a). By substitution of S with Se, values up to 1.16 · 10 −3 S cm −1 can be achieved for Na 3 PSe 4 (Zhang et al, 2015;Krauskopf et al, 2017Krauskopf et al, , 2018b. In addition to isovalent substitution, aliovalent substitution can be used to not only alter the polarizability of the lattice and influence the size of diffusion pathways, but also to tune the charge carrier concentration.…”

Ionic Conductivity of the NASICON‐Related Thiophosphate Na1+xTi2−xGax(PS4)3

“…For instance, Na 3 PS 4 exhibits an ionic conductivity of up to 4.6×10 −4 S cm −1 in the cubic phase at room temperature . Consequently, in order to broaden the diffusion pathways and potentially increase the lattice polarizability, substitutions of S with Se were performed, which form stable solid solutions and influence the ionic conductivity . In addition, substitution of P 5+ with As 5+ increases the conductivity due to a widening of the diffusion pathways and possibly altered Na−S interactions .…”

“…[13,31,32] Consequently,i no rder to broaden the diffusion pathways and potentially increaset he lattice polarizability,s ubstitutions of S with Se werep erformed, which form stable solid solutionsa nd influence the ionic conductivity. [20,24,33,34] In addition, substitution of P 5 + with As 5 + increases the conductivity due to aw idening of the diffusion pathways and possibly altered NaÀSi nteractions. [21] Continued research on Na + conducting thiophosphates has focusedo nc ompounds derived from Li 10 GeP 2 S 12 .…”