CRYSTAL GROWTH & DESIGN
Czochralski Growth of 12CaO‚7Al2O3 Crystals Kazuhisa Kurashige,*,†,‡ Yoshitake Toda,† Satoru Matstuishi,† Katsuro Hayashi,† Masahiro Hirano,† and Hideo Hosono† Frontier CollaboratiVe Research Center, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama, 226-8503 Japan, and High Performance Materials R&D Center, Hitachi Chemical Co., Ltd., 1380-1, Tarazaki, Hitachinaka-city, Ibaraki, 312-0003 Japan
2006 VOL. 6, NO. 7 1602-1605
ReceiVed January 18, 2006; ReVised Manuscript ReceiVed May 9, 2006
ABSTRACT: Single crystals of nanoporous 12CaO‚7Al2O3 (C12A7) as large as 27 mm in diameter, 120 mm in length, and 207 g in weight have been successfully grown by the Czochralski method. We found that a 2.0% oxygen-containing nitrogen atmosphere and an iridium crucible are the critical factors for success. The crystal ingot exhibited an orange color resulting from the incorporation of Ir4+ ions of ca. 5 × 1017 cm-3, which presumably occupy Ca2+ sites. X-ray diffraction analysis indicated that the ingot obtatined was a single crystal with few domains. Introduction The crystal of 12CaO‚7Al2O3 (C12A7) belongs to the cubic space group I4h3d with a lattice constant of 1.199 nm.1 The unit cell consists of two molecules and has a chemical formula represented as [Ca24Al28O64]4+ + 2O2-. The former constitutes a positively charged lattice framework containing 12 cages with a free space of ca. 0.4 nm inner diameter.1,2 Figure 1 illustrates the unit cell of C12A7, where one cage is marked in deep colors and the others are in pale colors. The two oxygen ions randomly occupy 2 out of the 12 cages, and the rest of the cages are empty, compensating the positive charges of the framework. Each oxygen ion in the cage is coordinated with the six Ca2+ ions constituting the cage framework, and the distance between the Ca2+ and the O2- is 50% longer than that in CaO crystals. These oxygen ions are in a very loosely bound state and are called “free” oxygen ions; they can be replaced by monovalent anions such as OH-, F-, and Cl-.2,3 Recently, we reported that the free oxygen ions can be successfully replaced by active anion species unstable in the atmosphere, i.e., O-, O2-, H-, and electrons.4-9 Because of these exceptional characteristics, C12A7 crystals are expected to be used in many applications such as cold electron emission, active anion beam emission, optical writing of conductive wires utilizing photoconversion from insulator to conductor, and as various catalytic agents for chemical reactions.4-22 To realize some of these applications, specifically electron and ion beam emission, high-quality single crystals are indispensable. Further, C12A7 single-crystal substrates will make it possible by homo-epitaxial growth to fabricate C12A7 thin films, which are favorable for various electronic device applications. The high-quality single crystal is also essential in clarifying the intrinsic electronic and optical properties of C12A7 and its derivatives. So far, single crystals of C12A7 have been grown by two kinds of techniques: the Czochralski (CZ) and floating zone (FZ) methods. A single crystal of about 20 mm in diameter grown by the CZ method exhibited a strong Tyndal scattering due to the inclusion of small Ir particles.23 On the other hand, a transparent C12A7 single crystal of about 10 mm diameter was grown by the FZ method.24 * To whom correspondence should
[email protected]. † Tokyo Institute of Technology. ‡ Hitachi Chemical Co., Ltd.
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Figure 1. Schematic representation of C12A7 unit cell (cubic). Twelve O2- (red), six Ca2+ (blue), and eight Al3+ (green) ions drawn in dark colors form a cage. The unit cell contains 12 cages; the free oxygen ions (not shown in the figure) are trapped in 2 out of the 12 cages.
However, it is extremely difficult to eliminate micropores from the grown crystal. C12A7 melt is a very good solvent for O2 gas, but the solubility significantly decreases in the supercooled liquid or glass state.25 As a consequence, the O2 molecules dissolved in the melt in the form of O2- will precipitate upon cooling. This is the primary origin of micropores remaining in the grown crystal. Though iridium crucibles are commonly used in the CZ method to grow crystals having high melting points, it is difficult to use them in an oxidative atmosphere because iridium reacts strongly with oxygen at high temperatures. Since platinum crucibles are less reactive with oxygen, they are commonly used to grow crystals having melting points of
