Effect of Synthetic Method and Annealing Temperature on the Structure of Hollandite-Type Oxides

Hollandite is a class of metal oxide material with the general formula A₂B₈O₁₆. Several methods have been used in the synthesis of this type of metal oxide, and the synthetic methods reported have typically employed high annealing temperatures between 1200 and 1300 °C. Appropriate synthetic methods must be employed to successfully synthesize these hollandite-type oxides at lower annealing temperatures. Hollandite compounds have been synthesized using ceramic (high annealing temperature only) and coprecipitation (high and low annealing temperatures) methods. Annealing temperatures ranging from 1200 to 700 °C have been employed in the thermal treatment process. Powder X-ray diffraction and X-ray absorption near-edge spectroscopy (XANES) were conducted on hollandite-type oxides (Ba _xAl_{2 x}Ti_{8-2 x}O_16-δ; x = 1.2; and Ba _xAl _xFe _xTi_{8-2 x}O_16-δ, Ba _xFe_{2 x}Ti_{8-2 x}O_16-δ; x = 1.16). Structural comparisons between materials annealed in the temperature range from 1200 to 800 °C were made, and an examination of the XANES spectra and powder X-ray diffraction patterns has provided confirmation of the absence of significant structural changes in these hollandite materials. This study has shown that hollandite-type materials can be formed using annealing temperatures as low as 700-800 °C when a coprecipitation method is used, with little change to the local and long-range structures being detected.