Insights into the rich polymorphism of the Na+ ion conductor Na3PS4 from the perspective of variable-temperature diffraction and spectroscopy

Abstract

Solid electrolytes are crucial for next generation solid state batteries and Na<sub>3</sub>PS<sub>4</sub> is one of the most promising Na<sup>+</sup> conductors for such applications. In this contribution, we present a detailed investigation of the evolution in structure and dynamics of Na<sub>3</sub>PS<sub>4</sub> under the effect of temperature in the range 30 < T < 600 °C through combined experimental-computational analysis. Although x ray Bragg diffraction experiments indicate a second order phase transition from the tetragonal ground state (α, P-42<sub>1</sub>c) to the cubic polymorph (β, I-43m), pair distribution function analysis in real space and Raman spectroscopy indicate remnants of tetragonal character in the range 250 < T < 500 °C which we attribute to dynamic local tetragonal distortions. The first order phase transition to the mesophasic high temperature polymorph (γ, Fddd) is associated with a sharp volume increase and the onset of liquid like diffusive dynamics for sodium-cations (translative) and thiophosphate-polyanions (rotational) evident by inelastic neutron- and Raman- spectroscopies, as well as pair-distribution function and molecular dynamics. These results shed light on the rich polymorphism in Na<sub>3</sub>PS<sub>4</sub> and are relevant for a host of high performance materials deriving from the Na<sub>3</sub>PS<sub>4</sub> structural archetype.<br>

Anharmonic Lattice Dynamics in Sodium Ion Conductors

“…Interestingly, we find that there is coupling of mobile ion (Na + ) and host lattice dynamics because the soft mode eigenvectors in both compounds exhibit a combination of tetrahedral tilting and Na + translation with A 1 (high-frequency mode) and E (low-frequency mode) vibrational symmetry ( Table S1 ). For Na 3 PS 4 , this is in agreement with the results of Famprikis et al and Gupta et al 7 , 31 …”

“…Stoichiometric Na 3 PS 4 and Na 3 PSe 4 are known to take on tetragonal and cubic phases depending on the temperature (see Figure 1 ), with Na 3 PS 4 having recently also been discovered to possess a plastic polymorph. 5 , 29 , 31 − 33 The cubic structure (space group I 4̅3 m , T d 3 , #217) is composed of PCh 4 3– (Ch = chalcogen) tetrahedra arranged on a BCC lattice with P atoms at BCC lattice sites and Na + ions located at face centers and edges of the BCC cube. In the tetragonal structure (space group P 4̅2 1 c , D 2 d 4 , #114) the PCh 4 3– tetrahedra are tilted about the crystallographic c -axis while a subset of the Na + ions are offset along the c -axis above and below their positions in the cubic phase.…”

“…This occurred between 260 and 280 K and between 520 and 560 K in Na 3 PSe 4 and Na 3 PS 4 , respectively, both in agreement with X-ray diffraction measurements. 5 , 29 , 31 , 32 , 34…”

“…28 , 41 This assessment is supported by the strongly anharmonic thermal ellipsoids refined from synchrotron X-ray scattering experiments in the cubic phase 29 and by refinements of pair distribution function (PDF) measurements. 31 , 42 Thus, the cubic phase dynamically samples different tetrahedral tilting configurations. The disorder resulting from this dynamic symmetry breaking causes a violation of the Raman selection rules for the soft mode.…”

Computational Auxiliary for the Progress of Sodium-Ion Solid-State Electrolytes

“…These results shed light on the rich polymorphism of Na 3 PS 4 , which is relevant to its potential application in sodium-based SEs. 147 Furthermore, a series of Cl-doped Na 3 PS 4 solid-state electrolytes have been explored through a combined experimental and computational approach, and the best ionic conductivity of 1.96 mS/cm is achieved in the composition of Na 3 PS 3.8 Cl 0.2 . The AIMD simulations shed light on the merit of maximizing Cl doping in enhanced sodium-ion conduction.…”