All-solid-state batteries using sodium are promising candidates for next-generation rechargeable batteries due to the limited lithium resources. A practical sodium battery requires an electrolyte with high conductivity. Cubic Na₃PS₄ exhibiting high conductivity of over 10⁻⁴ S cm⁻¹ is obtained by crystallizing amorphous Na₃PS₄ synthesized by ball milling. Amorphous Na₃PS₄ crystallizes in a cubic structure and then is transformed into a tetragonal phase upon heating. In this study, in situ observation by transmission electron microscopy demonstrates that the crystallite size drastically increases during the transition from the cubic phase to the tetragonal phase. Moreover, an electron diffraction analysis reveals that amorphous domains and nano-sized crystallites coexist in the cubic Na₃PS₄ specimen, while the tetragonal phase contains micro-sized crystallites. The nano-sized crystallites and the composite formed by crystallites and amorphous domains are most likely responsible for the increase in conductivity in the cubic Na₃PS₄ specimens.

由于锂资源有限，使用钠的全固态电池是下一代可充电电池的有希望的候选者。实用的钠电池需要具有高导电性的电解质。通过使通过球磨合成的无定形Na ₃ PS ₄结晶获得表现出超过10 ^-4  S cm ^{-1 的}高电导率的立方Na ₃ PS ₄。无定形 Na ₃ PS ₄以立方结构结晶，然后在加热时转变为四方相。在这项研究中，透射电子显微镜的原位观察表明，在从立方相到四方相的转变过程中，微晶尺寸急剧增加。此外，电子衍射分析表明，在立方 Na ₃ PS ₄样品中无定形域和纳米级微晶共存，而四方相包含微米级微晶。纳米尺寸的微晶和由微晶和无定形域形成的复合物最有可能导致立方 Na ₃ PS ₄样品中电导率的增加。