Alloying of anions is a promising engineering strategy for tuning ionic conductivity in halide-based inorganic solid electrolytes. We explain the alloying effects in Li3InBr6−xClx, in terms of strain, chemistry, and microstructure, using first-principles molecular dynamics simulations and electronic structure analysis. We find that strain and bond chemistry can be tuned through alloying and affect the activation energy and maximum diffusivity coefficient. The similar conductivities of the x = 3 and x = 6 compositions can be understood by assuming that the alloy separates into Br-rich and Cl-rich regions. Phase-separation increases diffusivity at the interface and in the expanded Cl-region, suggesting microstructure effects are critical. Similarities with other halide superionic conductors are highlighted.

The nitrile vibrational modes of the 2,3,5,6-tetrafluoro-7,7,8,8tetracyanoquinodimethane (F 4 TCNQ) anion occur across a range of different frequencies in doped conjugated polymers. We show that these shifts can be understood as resulting from the vibrational Stark effect. F 4 TCNQ − serves as a sensitive vibrational Stark probe, and its nitrile stretches provide a direct readout of the locally experienced electric fields from the nearby polaron on the doped polymer backbone. The shift of the F 4 TCNQ − B 1u mode and broadening of the B 2u mode in doped poly(3-hexylthiophene-2,5-diyl) (P3HT) confirm that the dopant anion's long axis is oriented perpendicular to the P3HT backbone. The magnitude of the vibrational shifts prove that the anion−polaron distance in F 4 TCNQ − -doped P3HT films is ∼6 Å. We also show that the intrachain polaron coherence can be varied over a range spanning 6 to about 7.5 P3HT monomer units by controlling the local polymer order and crystallinity. At the highest degree of local order, polaron delocalization is limited by the minimum intrinsic strength of the anion−polaron Coulombic interaction. This work provides the first direct experimental measurement of polaron delocalization in doped conjugated polymer films and verifies theoretical models relating polaron coherence to the shape of the doped polymer's mid-IR electronic absorption spectrum.

WB 4.2 is one of the hardest metals known. Though not harder than diamond and cubic boron nitride, it surpasses these established hard materials in being cheaper, easier to produce and process, and also more functional. Metal impurities have been shown to a?ct and in some cases further improve the intrinsic hardness of WB 4.2 , but the mechanism of hardening remained elusive. In this work we ?first theoretically elucidate the preferred placements of Ti, V, Cr, Mn, Zr, Nb, Mo, Hf, Ta in the WB 4.2 structure, and show these metals to preferentially replace W in two competing positions with respect to the partially occupied B 3 cluster site. The impurities avoid the void position in the structure. Next, we analyze the chemical bonding within these identifi?ed doped structures, and propose two different mechanisms of strengthening the material, afforded by these impurities, and dependent on their nature. Smaller impurity atoms (Ti, V, Cr, Mn) with deeply lying valence atomic orbitals cause the inter-layer compression of WB 4.2 , which strengthens the B hex -B cluster bonding slightly. Larger impurities (Zr, Nb, Mo, Hf, Ta) with higher-energy valence orbitals, while expanding the structure and negatively impacting the B hex -B cluster bonding, also form strong B cluster -M bonds. The latter effect is an order of magnitude more substantial than the effect on the B hex -B cluster bonding. We conclude that the e effect of the impurities on the boride hardness does not simply reduce to structure interlocking due to the size difference between M and W, but instead, has a significant electronic origin. File list (2) download file view on ChemRxiv WB4_with_M.pdf (3.81 MiB) download file view on ChemRxiv WB4_with_M_SI.pdf (155.96 KiB)

Lithium-rich oxychloride antiperovskites are promising solid electrolytes for enabling next-generation batteries. Here, we report a comprehensive study varying Li + concentrations in Li 3 OCl using ab initio molecular dynamics simulations. The simulations accurately capture the complex interactions between Li + vacancies ( V Li ′ ), the dominant mobile species in Li 3 OCl . The V Li ′ polarize and distort the host lattice, inducing additional non-vacancy-mediated diffusion mechanisms and correlated diffusion events that reduce the activation energy barrier at concentrations as low as 1.5% V Li ′ . Our analyses of discretized diffusion events in both space and time illustrate the critical interplay between correlated dynamics, polarization and local distortion in promoting ionic conductivity in Li 3 OCl . This article is part of the Theo Murphy meeting issue ‘Understanding fast-ion conduction in solid electrolytes’.