Nano-scale structural inhomogeneities of CuI–Cu 2 MoO 4 superionic conducting glass observed by high resolution transmission electron microscopy Takao Tsurui a, * , Junichi Kawamura b , Kenji Suzuki c a Institute for Materials Research, Tohoku University, Katahira 2-1-1, Aoba-ku, Sendai 980-8577, Japan b Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Katahira 2-1-1, Aoba-ku, Sendai 980-8577, Japan c Advanced Institute of Materials Science, Moniwadai 2-6-8, Taihaku-ku, Sendai 982-0252, Japan Received 28 June 2005; received in revised form 15 August 2006 Available online 17 January 2007 Abstract Nano-meter sized structural inhomogeneities of (CuI) 0.52 –(Cu 2 MoO 4 ) 0.48 superionic conducting glass were investigated by high res- olution transmission electron microscopy. The as-quenched sample of CuI–Cu 2 MoO 4 is a homogeneous glass, in which the CuI compo- nent is finely and uniformly dispersed among the oxyanions of Cu 2 MoO 4 glassy matrix. A two-step crystallization, starting at 440 and 495 K, was observed in the glass. After the first step crystallization, precipitating nano-crystalline cubic CuI 2–3 nm in diameter, the elec- trical conductivity increases by about 50%. On the other hand, the electrical conductivity decreases with the second crystallization event forming crystalline phases of CuI, Cu 2 O and others 20–30 nm in diameter. Ó 2006 Elsevier B.V. All rights reserved. PACS: 61.43.Fs; 66.30.Hs Keywords: Fast ion conduction; TEM/STEM; Oxide glasses 1. Introduction Superionic conducting glasses, such as silver or copper halide based oxide glasses, exhibit an extraordinarily high ionic conductivity [1–3]. Superionic conducting glasses have attracted much attention because of the technological needs for solid electrolytes and the academic interests of understanding the ionic transport mechanism. In particu- lar, the glasses composed of AgI or CuI and some kinds of oxyanionic species are known to show a very high ionic conductivity up to 10 À2 S/m at room temperature. A close relation between the glass structure and the anomalous high ionic conductivity has been identified. The dependence of the conductivity on the local atomic structure corresponding to the coordination environment was studied by various techniques, such as infrared spec- troscopy [2,3], Raman scattering [4–6], Brillouin scattering [7], neutron scattering [8,9] and NMR [10]. On the other hand, it has been confirmed that the ionic conductivity depends on the microscopic phase structure. For example, the ionic conductivity generally decreases accompanied with crystallization [11], while the dispersion of nano-crys- talline a-AgI in a glass matrix enhances the conductivity [12]. It has been recognized for a long time that identification of the medium-range structure is indispensable for under- standing the ionic conduction mechanism of superionic conducting glasses. Actually some structural models are proposed based on the existence of ‘clusters’ or ‘microdo- mains’ of AgI or CuI with tetrahedral coordination, which is dispersed in the host matrix constructed with oxyanions [13,14]. Numerous structural studies were performed so far to clarify the medium-range structure by various 0022-3093/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2006.11.008 * Corresponding author. Tel.: +81 22 215 2832; fax: +81 22 215 2834. E-mail address: tsurui@imr.tohoku.ac.jp (T. Tsurui). www.elsevier.com/locate/jnoncrysol Journal of Non-Crystalline Solids 353 (2007) 302–307