Cesium and bromine doping into hexagonal boron nitride
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I. INTRODUCTION Hexagonal boron nitride is a layered hexagonal compound isoelectronic to graphite. Since the early attempts to intercalate BN by Croft,1 there have been several reports regarding various intercalation compounds based on hexagonal boron nitride (BNIC). 2 " 4 Except for layer stacking, the structures of graphite and hexagonal boron nitride are similar, as indicated in Table I. For this reason, intercalation compounds based on BN are expected to be similar to graphite intercalation compounds (GIC's), the most widely studied of the intercalation compounds. The in-plane bond length of B-N and the interlayer distance are the same as graphite to within a few percent.5 The bonding energies are also similar. These similarities would suggest that intercalation of a dopant into the interlayer space of boron nitride could occur. There is, however, a striking physical difference between these materials. While graphite is a semimetal with a small band overlap, boron nitride is a transparent insulator with a band gap of ~ 5 eV.6"10 The wide band gap character of BN greatly reduces the intercalation reaction because the electronic system is very stable and inactive due to the wide band gap. Activation of electrons across this wide gap is required for a chemical reaction to occur. The simple physicochemical considerations listed above have all been confirmed experimentally, while both positive and negative results on the intercalation of BN have been reported.2-3 Using black lustrous BN powder, Croft1 reported a weight increase ranging from 2%-13% by the reaction with FeCl3, A1C13, and NH 3 . He also observed an exfoliation, one of the characteristics of the intercalation a)
Present address: Toshiba R & D Center, Komukai Saiwai-ku, Kawasaki 210, Japan.
J. Mater. Res. 1 (5), Sep/Oct 1986 http://journals.cambridge.org
process, of a resultant complex of the fine powder. Riidorff and Stumpp,2 however, found no change in weight and color of their white BN powder by reactions similar to those employed by Croft.1 Freeman and Larkindale 1112 examined both acceptor (metal halides) and donor (Li, Na, K) doping into a white BN powder. They detected an expansion of the interlayer distance in FeCl3-doped BN and for the donors they observed a weight increase corresponding to the formula (BN) 13K together with an expansion of the interlayer distance to 4.4 A. Mugiya et al.13 also examined alkali metal doping of BN and reported a deep blue complex of BN with Cs. Ohhashi and Shinjo14 intensively studied FeCl3-doped BN and found no appreciable absorption in the Mossbauer spectrum, which indicated that the Fe content of the FeCl3-doped BN powder was extremely small. They inspected the reaction conditions and concluded that the pink color and x-ray diffraction peaks reported 1112 as characteristics of a BN intercalation compound with FeCl3 should be attributed to FeOCl formed on the surface of the BN and associated with a slight addition of H 2 O in the reaction tubes. Bartlett et al.15 reported a very unstable, deep blue
TABLE I.
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