Ge置換Na-テニオライトのイオン伝導に与えるditrigonal holeの大きさとNa^＋イオンの位置の影響

抄録

In this study, the influence of the size of the ditrigonal hole and location of Na^＋ ions on the ionic conductivity of Ge-substituted Na-taeniolites (NaMg₂LiSi_4−xGe_xO₁₀F₂, with x＝0, 1, 2, 3 and 4, i.e., Ge-NTA with x＝0–4) was investigated. The lattice constant b and basal spacing c·sinβ of Ge-NTA with x＝0–3 increased with increase in the Ge content and their b and c·sin β exhibited a linear relationship. However, the b and c·sin β of Ge-NTA with x＝4 deviated from a linear relationship because the structural deformation of Ge-NTA with x＝4 was considerably larger than that of Ge-NTA with x＝0–3. Moreover, with increase in the Ge content, the size of the ditrigonal hole of Ge-NTA decreased; however, the lattice constant b of Ge-NTA with x＝0–3 increased. In Ge-NTA with x＝0–3, three types of Na^＋ ions exist. They are hydrated Na^＋ ions in the interlayer, dehydrated Na^＋ ions surrounded by basal oxygens, and Na^＋ ions drawn into the ditrigonal hole. Because the size of the ditrigonal hole in Ge-NTA became smaller with increase in the Ge content, the dehydrated Na^＋ ions surrounded by the basal oxygens increased and bonding between the Na^＋ ions and basal oxygens became stronger. In Ge-NTA with x＝4, the Na^＋ ions coordinated with three basal oxygens and the F⁻ ion appeared to be considerably restricted inside the ditrigonal hole. The ionic conductivity of Ge-NTA was measured from 400 to 600°C using the alternating current four-probe method. The ionic conductivity of Ge-NTA decreased with increase in the Ge content because for Ge-NTA with x＝0–3, bonding between the Na^＋ ions and basal oxygens became stronger with increase in the Ge content and for Ge-NTA with x＝4, the Na^＋ ions that were considerably restricted inside the ditrigonal hole hardly contributed to the ionic conductivity.