Structure and ionic conductivity of well-aligned polycrystalline sodium titanogallate grown by reactive diffusion

• Published in: Journal of solid state chemistry Vol. 229; pp. 252 - 259
• Main Authors: Hasegawa, Ryo, Okabe, Momoko, Asaka, Toru, Ishizawa, Nobuo, Fukuda, Koichiro
• Format: Journal Article
• Language: English
• Published: United States Elsevier Inc 01.09.2015
• Subjects: ACTIVATION ENERGY; ANNEALING; CERAMICS; Crystalline orientation; DIFFUSION; DOPED MATERIALS; ELECTRIC CONDUCTORS; ELECTRON MICROSCOPY; GALLIUM OXIDES; Grain-aligned polycrystals; IMPEDANCE; INORGANIC, ORGANIC, PHYSICAL AND ANALYTICAL CHEMISTRY; Ionic conductor; MATRIX MATERIALS; MONOCLINIC LATTICES; MONOCRYSTALS; PLATES; POLYCRYSTALS; Reactive diffusion; SODIUM COMPOUNDS; SODIUM IONS; Sodium titanogallate; TITANATES; X-RAY DIFFRACTION; Crystalline orientation; Reactive diffusion; Sodium titanogallate; Ionic conductor; Grain-aligned polycrystals
• ISSN: 0022-4596; 1095-726X
• DOI: 10.1016/j.jssc.2015.06.005

We prepared the b-axis-oriented polycrystalline Na0.85Ti0.51Ga4.37O8 (NTGO) embedded in Ga2O3-doped Na2Ti4O9 matrix using the reactive diffusion technique. When the sandwich-type Ga2TiO5/NaGaO2/Ga2TiO5 diffusion couple was heated at 1323K for 24h, the NTGO polycrystal was readily formed in the presence of a liquid phase. The resulting polycrystalline material was characterized by X-ray diffractometry, electron microscopy and impedance spectroscopy. We mechanically processed the annealed diffusion couple and obtained the thin-plate electrolyte consisting mostly of the grain-aligned NTGO polycrystal. The ionic conductivity (σ) of the electrolyte along the common b-axis direction steadily increased from 1.3×10−4 to 7.3×10−3S/cm as the temperature increased from 573 to 1073K. There was a slope change at ca. 792K for the Arrhenius plot of σ; the activation energies were 0.39eV above this temperature and 0.57eV below it. The NTGO showed the crystal structure (space group C2/m) with substantial positional disordering of one of the two Ga sites. The Na+ ions occupied ca. 43% of the Wyckoff position 4i site, the deficiency of which would contribute to the relatively high ionic conductivity along the b-axis. The reactive diffusion could be widely applicable as the novel technique to the preparation of grain-aligned ceramics of multi-component systems. We have prepared the b-axis-oriented Na0.85Ti0.51Ga4.37O8 polycrystal embedded in Ga2O3-doped Na2Ti4O9 matrix by the heat treatment of sandwich-type diffusion couple of Ga2TiO5/NaGaO2/Ga2TiO5. The resulting Na0.85Ti0.51Ga4.37O8 electrolyte showed the ionic conductivity ranging from 1.3×10−4S/cm at 573K to 7.3×10−3S/cm at 1073K. [Display omitted] •The b-axis-oriented polycrystalline Na0.85Ti0.51Ga4.37O8 is successfully prepared.•Crystal structure of Na0.85Ti0.51Ga4.37O8 is determined by single-crystal XRD.•The polycrystal shows relatively high Na+ ion conductivity along the common b-axis.•Reactive diffusion is successfully used for the preparation of grain-aligned ceramics.