Article pubs.acs.org/crystal
Floating Zone Growth of Large and Defect-free Ca12Al14O33 Single Crystals Stefan G. Ebbinghaus,*,† Holger Krause,† and Frank Syrowatka‡ †
Institut für Chemie, Martin-Luther-Universität Halle-Wittenberg, Kurt-Mothes-Straße 2, D-06120 Halle/Saale, Germany Interdisziplinäres Zentrum für Materialwissenschaften, Martin-Luther-Universität Halle-Wittenberg, Heinrich-Damerow-Straße 4, D-06120 Halle/Saale, Germany
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S Supporting Information *
ABSTRACT: 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.
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INTRODUCTION Mayenite (Ca12Al14O33, C12A7) is an excellent oxygen conductor at high temperatures.1,2 The crystal structure of mayenite can be described as an “antizeolite” built of a positively charged (C12Al14O32)2+-framework, in which 1/6 of the cages are occupied by oxygen ions. Its high ionic conductivity makes mayenite a promising material as electrolyte in solid oxide fuel cells, as an oxygen gas sensor or oxidation catalyst. The cage oxygen ions can be replaced by various ions such as F−, Cl−, or OH−.3,4 Besides these classical anions, “exotic” ones such as O−, O2−, H−, and e− can occupy the cage centers.5−7 Lately, it has furthermore been shown that nitrogencontaining species can be incorporated as well.8,9 Large and defect-free crystals of mayenite are required for accurate measurements of the transport properties because the experimental results may strongly be affected by grain boundaries or defects comprising macroscopic inclusions, twin domains, or cracks. In addition, high-quality crystals are mandatory for single crystal neutron diffraction experiments. Mayenite can be grown by the Czochralski method, but the crystals are contaminated by the crucible material iridium.10−12 Therefore, crucible-free approaches like the floating zone technique are required. To the best of our knowledge, details about the floating zone growth of C12A7 have only been reported in three studies. The formation of cracks and/or bubbles was recognized as the main difficulty,13 and different approaches have been suggested to avoid them. Watauchi et al.13 incorporated an additional alumina tube in the furnace as a © XXXX American Chemical Society
heat reservoir and used slow growth rates of
