Abstract:
The aim of this project was to identify the mechanism involved in mediating the exceptional ionic conductivity measured in Ba3Y4O9 above 643K (370°C) by Goodenough and Zhen (1991). Ceramic samples of the compound were investigated using AC impedance methods, thermal methods, transmission electron microscopy, X-ray and neutron powder diffraction, and vibrational spectroscopy. The powder diffraction measurements showed that Ba3Y4O9 is isostructural with Ba3Yb4O9, crystallizing in R3 (C34) with lattice parameters at 293K (20°C): a=6.1100±0.0004Å, c=25.187±0.003Å. Impedance measurements on samples prepared and tested in dry O2 and ambient air conditions indicated that the highly conducting state was associated with a partial decomposition product formed on exposure to ambient air. Thermal measurements indicated a reversible transition, endothermic at 683K (410°C) on the heating cycle and exothermic at 652K (379°C) on the cooling cycle, which is very similar to the solid-melt transition in Ba(OH)2. Raman spectroscopy identified the presence of OH- in the ceramic material after it has been exposed to air through the O-H stretching vibration at ~3600cm-1.
The conclusion from the experimental evidence was that ceramic samples of Ba3Y4O9 become ionically conductive after exposure to moist air. The OH- units in the water hydrogen bond to Ba ions over the surface of the Ba3Y4O9 grains forming an amorphous partial decomposition layer. The ionic conductivity is then mediated via proton conduction across OH- units. Above ~670K there is a melt transition of the barium hydroxide decomposition products and this is associated with a significant increase in the ionic conductivity as can be seen in the experimental conductivity results.
