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.

新合成的三种化合物Na₁₁.₆Sn₂W₀.₇₅S₁₂（标记为I）、Na₁₁.₉Sn₂W₀.₆₆S₁₂（标记为II）以及Na₁₂Sn₂W₀.₆₆Se₁₂（标记为III）均结晶为四方相（tetragonal phase），且表现出优异的钠离子电导率（Na-ion conductivity）。上述结构中的钠离子空位是通过在NaSnWQ（Q=S、Se）体系中掺杂六价钨（W⁶⁺）引入的。与此前报道的NaSnMQ（M=P、Sb）体系不同，新型NaSnWQ结构具有显著差异：经X射线与中子粉末衍射联合精修证实，六价钨原子仅占据一处具有高空位浓度的晶体学独立位点。我们发现，采用六价钨进行异价掺杂（aliovalent doping）可提升离子电导率：一方面六价钨可稳定溶于四面体位点（tetrahedral site），另一方面其高氧化态特性能够引入钠离子空位。此前我们基于OSIRIS谱仪的准弹性中子散射（Quasielastic Neutron Scattering, QENS）测试、MARI谱仪的声子（phonon）测试以及从头算（ab-initio）计算，对Na₂Ti₃O₇、Na₂Si₂O₅和Li₂Si₂O₅中的钠/锂离子扩散行为进行了研究。本次研究我们拟对Na₁₂₋ₓWₓ’Sn₂S/Se₁₂系列化合物开展准弹性中子散射与非弹性中子散射（Inelastic Neutron Scattering, INS）测试，以探究钨掺杂对声子谱与钠离子扩散行为的影响，以及二者之间的潜在关联。我们预期，与钠离子振动相关的声子谱会随温度发生显著变化。结合上述测试与从头算分子动力学模拟（ab-initio molecular dynamics simulations），将有助于阐明钠离子的宏观扩散行为及其与声子之间的内在联系。我们预计钠离子的跳跃运动将表现为强准弹性散射信号。