Krauskopf Thorben, Culver Sean P, Zeier Wolfgang G
Inorg Chem. 2018 Apr 16;57(8):4739-4744. doi: 10.1021/acs.inorgchem.8b00458. Epub 2018 Apr 3.
The sodium superionic conductor NaPS is known to crystallize in one of two different structural polymorphs at room temperature (i.e., cubic or tetragonal, depending on the synthetic conditions). Experimentally, the cubic structure is known to exhibit a higher ionic conductivity than the tetragonal structure, despite theoretical investigations suggesting that there should be no difference at all. Employing a combination of Rietveld and pair distribution function (PDF) analyses, as well as electrochemical impedance spectroscopy, we investigate the open question of how the crystal structure influences the ionic transport in NaPS. Despite the average structures of NaPS prepared via ball-milling and high-temperature routes being cubic and tetragonal, respectively, the structural analysis by PDF indicates that both compounds are best described by the structural motifs of the tetragonal polymorph on the local scale. Ultimately, the high ionic conductivity of NaPS prepared by the ball-milling approach is confirmed to be independent of the crystal structure. This work demonstrates that even in ionic conductors differences can be observed between the average and local crystal structures, and it reasserts that the high ionic conductivity in NaPS is not related to the crystal structure but rather differences in the defect concentration.
钠超离子导体NaPS在室温下已知会结晶成两种不同结构多晶型物中的一种（即立方晶型或四方晶型，这取决于合成条件）。实验上，已知立方结构比四方结构表现出更高的离子电导率，尽管理论研究表明两者根本不应存在差异。我们结合里特韦尔德（Rietveld）分析和对分布函数（PDF）分析以及电化学阻抗谱，研究了晶体结构如何影响NaPS中离子传输这个悬而未决的问题。尽管通过球磨法和高温法制备的NaPS的平均结构分别为立方晶型和四方晶型，但PDF的结构分析表明，在局部尺度上，这两种化合物都最好用四方多晶型的结构基序来描述。最终，证实通过球磨法制备的NaPS的高离子电导率与晶体结构无关。这项工作表明，即使在离子导体中，平均晶体结构和局部晶体结构之间也能观察到差异，并且再次强调NaPS中的高离子电导率与晶体结构无关，而是与缺陷浓度的差异有关。
