Single Crystals of C12A7 (Ca12Al14O33) Substituted with 1 mol % Iron

Single crystals of Ca12Al14O33 (C12A7, mayenite) substituted with 1 mol % Fe were grown by the floating zone technique in an atmosphere of 2% O-2/98% N-2. The iron substitution leads to an increase of the unit cell parameter by roughly 0.002 angstrom. Optical spectroscopy reveals an additional broad absorption band at 310 nm for the iron-substituted sample reflecting the yellow color of the crystals. XANES measurements at the Fe-K absorption edge prove the incorporation of Fe3+ on the aluminum sites. Measurements of the magnetic susceptibility resulted in a value of 5.8 mu(B) per iron ion as expected for high-spin Fe3+. C12A7 is known to be a very good oxygen ion conductor at high temperatures. In pure oxygen, the iron substitution leads to a slightly reduced conductivity between approximately 500 and 800 degrees C, while at higher temperatures, the conductivity is very similar to the pristine material. Under reducing conditions, the slope of log(sigma) vs log(p(O-2)) of the iron-substituted sample was found to be steeper than that for pure C12A7.