Comparison of different reducing agents for the synthesis of mayenite electride C12A7:2e− and a new photometric method to determine its electron density

Discovered in 2003 the mayenite electride (Ca 12 Al 14 O 33–x (2e – ) x ) is the first electride being stable under ambient conditions. Different reaction conditions are known to result in samples with strongly deviating electron contents x. Various methods for determining x have been applied but have individual disadvantages. Therefore, a new alternative approach based on the reduction of dichromate and its photometric quantification is introduced and evaluated in this paper. For this, mayenite oxide was synthesised via classical solid-state synthesis and afterwards reduced by heating pellets embedded in the respective reducing agents under dynamic vacuum. Using carbon, titanium and zirconium as oxygen getters, different degrees of reduction were achieved. The electron densities of the obtained electrides were determined using iodometric titration, conductivity measurements and the new dichromate based photometry. Our results show that the latter method is feasible, robust and leads to well-reproducible, reliable values. • With the reducing agents C, Ti, and Zr electrides with different electron densities (Ca 12 Al 14 O 33-x (2e – ) x ) were obtained. • A new dichromate-based photometry was established as a simple, fast and accurate method to determine the electron contents. • Compared to iodometric titration and conductivity measurements the photometry yields better reproducible results.