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Size-controlled stabilization of the superionic phase to room temperature in polymer-coated AgI nanoparticles


Solid-state ionic conductors are actively studied for their large application potential in batteries1 and sensors. From the view of future nanodevices2,3,4,5, nanoscaled ionic conductors are attracting much interest. Silver iodide (AgI) is a well-known ionic conductor for which the high-temperature α-phase shows a superionic conductivity greater than 1 Ω−1 cm−1 (ref. 6). Below 147 C, α-AgI undergoes a phase transition into the poorly conducting β- and γ-polymorphs, thereby limiting its applications. Here, we report the facile synthesis of variable-size AgI nanoparticles coated with poly-N-vinyl-2-pyrrolidone (PVP) and the controllable tuning of the α- to β-/γ-phase transition temperature (Tc↓). Tc↓ shifts considerably to lower temperatures with decreasing nanoparticle size, leading to a progressively enlarged thermal hysteresis. Specifically, when the size approaches 10–11 nm, the α-phase survives down to 30 C—the lowest temperature for any AgI family material. We attribute the suppression of the phase transition not only to the increase of the surface energy, but also to the presence of defects and the accompanying charge imbalance induced by PVP. Moreover, the conductivity of as-prepared 11 nm β-/γ-AgI nanoparticles at 24 C is 1.5×10−2 Ω −1 cm−1—the highest ionic conductivity for a binary solid at room temperature. The stabilized superionic phase and the remarkable transport properties at a practical temperature reported here suggest promising applications in silver-ion-based electrochemical devices.

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Figure 1: Size dependence of the phase transition characteristics in AgI nanoparticles.
Figure 2: Structural characterization of 11 nm AgI nanoparticles.
Figure 3: Transport behaviour of 11 nm AgI nanoparticles.
Figure 4: 109Ag NMR spectra of 10 nm AgI nanoparticles.

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We thank JSPS for financial support under Grants-in-Aid for Scientific Research (B) (No. 20350030) and for Challenging Exploratory Research (No. 20655030) and under the Global COE Program ‘Science for Future Molecular Systems’ and SPring-8 for access to the synchrotron X-ray facilities.

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Authors and Affiliations



H.K. was responsible for designing and coordinating this study. R.M. interpreted and discussed the results and carried out synthesis, conductivity measurements and Rietveld refinements. T. Yonemura carried out synthesis, DSC and NMR measurements. T. Yamada and M.Y. were involved in experiments and discussion. M.Y. and R.I. contributed to the NMR measurements. T. Yonemura, K.K. and M.T. carried out the synchrotron XRD measurements. All authors commented on the paper. R.M and H.K. were responsible for writing the paper.

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Correspondence to Rie Makiura or Hiroshi Kitagawa.

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Makiura, R., Yonemura, T., Yamada, T. et al. Size-controlled stabilization of the superionic phase to room temperature in polymer-coated AgI nanoparticles. Nature Mater 8, 476–480 (2009).

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