Inspired by the ongoing debate about the ion dynamics in the lithium superionic conductor Li 10 GeP 2 S 12 (LGPS), we present neutron powder diffraction data in combination with analyses of differential bond valence and nuclear density maps to elucidate the underlying diffusion pathways in Li 10 GeP 2 S 12 . LGPS exhibits quasi-isotropic three-dimensional lithium diffusion pathways, which is a combination of one-dimensional diffusion channels crossing two diffusion planes. Furthermore, ultrasonic speeds of sound measurements are used to understand the lattice dynamics and obtain the Debye temperature of LGPS. Temperature dependent X-ray diffraction is performed in order to understand the local temperature-dependent behavior of the prevalent structural backbone, as well as the thermal stability of the material. At elevated temperatures, the superionic conducting Li 10 GeP 2 S 12 phase partially decomposes into Li 4 P 2 S 6 , explaining the deterioration of the ionic conductivity upon heating.

Building on the good thermoelectric performances of binary superionic compounds like Cu2Se, Ag2Se and Cu2S, a better and more detailed understanding of phonon-liquid electron-crystal (PLEC) thermoelectric materials is desirable. In this work we present the thermoelectric transport properties of the compound Cu7PSe6 as the first representative of the class of argyrodite-type ion conducting thermoelectrics. With a huge variety of possible compositions and high ionic conductivity even at room temperature, the argyrodites represent a very good model system to study structure-property relationships for PLEC thermoelectric materials. We particularly highlight the extraordinary low thermal conductivity of Cu7PSe6 below the glass limit, which can be associated with the molten copper sublattice leading to a softening of phonon modes.

Glassy, glass–ceramic, and crystalline lithium thiophosphates have attracted interest in their use as solid electrolytes in all-solid-state batteries. Despite similar structural motifs, including PS4 3–, P2S6 4–, and P2S7 4– polyhedra, these materials exhibit a wide range of possible compositions, crystal structures, and ionic conductivities. Here, we present a combined approach of Bragg diffraction, pair distribution function analysis, Raman spectroscopy, and 31P magic angle spinning nuclear magnetic resonance spectroscopy to study the underlying crystal structure of Li4P2S6. In this work, we show that the material crystallizes in a planar structural arrangement as a glass ceramic composite, explaining the observed relatively low ionic conductivity, depending on the fraction of glass content. Calculations based on density functional theory provide an understanding of occurring diffusion pathways and ionic conductivity of this Li+ ionic conductor.

X-ray diffraction data modeling. Powder diffraction patterns were modeled using a small, anorthic unit cell derived from a LP search (TOPAS Academic V4.1). Figure S1 shows the x-ray powder diffraction data of the synthesized Cu2-δSe with the corresponding Pawley fit. A triclinic unit cell with a = 7.082(8) Å, b = 4.121(3) Å, c = 7.121(3) Å, α = 106.1(0), β = 99.1(0)° and γ = 90.1(4)° has been utilized for the fits in order to determine lattice parameters for each composition.

Superionic chalcopyrites have recently attracted interest in their use as potential thermoelectric materials because of extraordinary low thermal conductivities. To overcome long-term stability issues in thermoelectric generators using superionic materials at evaluated temperatures, materials need to be found that show good thermoelectric performance at moderate temperatures. Here, we present the structural and thermoelectric properties of the argyrodite Ag8SiSe6, which exhibits promising thermoelectric performance close to room temperature.