Phonon Dynamics and Na diffusion in Na12-xWx’Sn2S/Se12

The newly synthesized compounds Na11.6Sn2W0.75S12(I), Na11.9Sn2W0.66S12(II), and Na12Sn2W0.66Se12(III) crystallize in the tetragonal phase and exhibit significant Na-ion conductivity. The Na vacancies in these structures have been created by doping W6+ in NaSnWQ (Q = S, Se). Unlike previously reported NaSnMQ (M = P, Sb) systems, the new NaSnWQ structures are distinctly different in which W6+ atoms exclusively reside on a crystallographically distinct site with high vacancies as confirmed by joint refinements from X-ray and neutron powder diffraction. We found that aliovalent doping with W6+ boosts the ionic conductivity because of its solubility in the tetrahedral site and being available in high oxidation state, it creates Na-ion vacancies. Our earlier QENS measurements on OSIRIS as well as phonon measurements on MARI along with ab-initio calculations were used to understand the Na/Li-ion diffusion in Na2Ti3O7, Na2Si2O5 and Li2Si2O5. Now we propose to perform the QENS and INS measurements in Na12-xWx’Sn2S/Se12 compounds to investigate the effect of W-doping on the phonon spectrum and Na diffusion, and their possible interplay. We expect significant changes in the phonon spectrum with temperature associated with Na vibrations. These measurements along with ab-initio molecular dynamics simulations would help to understand the large diffusion behavior and their relation with phonons. We expect the hopping of Na-ion would manifest as a strong quasielastic signal.