Floating Zone Growth of Large and Defect-free Ca12Al14O33 Single Crystals

The growth of high-quality single crystals of Ca12Al14O33 (C12A7, mayenite) with lengths of several centimeters and diameters of up to 1.5 cm by the floating zone technique is described. In contrast to earlier reports, an atmosphere with reduced oxygen content (2% O2/98% N2) was used. The formation of large gas bubbles in the molten zone as well as the presence of tiny gas inclusions in the crystals are the main obstacles of the floating zone growth. In order to avoid both problems, a two-step crystallization was applied. In the first step, the polycrystalline feed rod was crystallized with a comparatively high speed of 5 mm/h, while in the second melting step a low growth speed of 0.2 mm/h was applied. Contaminations by silicon or zirconium were identified as an additional severe problem. Such contaminations can be avoided using grinding devices made of polyamide. The crystals obtained grow preferably parallel to the ⟨211⟩-direction of the cubic space group I4̅3d. They are completely transparent and suited for optical investigations and single-crystal neutron diffraction.