Synthesis of carbon nitride crystals at high pressures and temperatures
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Synthesis of carbon nitride crystals at high pressures and temperatures D. W. He, F. X. Zhang, X. Y. Zhang, Z. C. Qin, and M. Zhang Institute of Physics, Chinese Academy of Science, P.O. Box 603 (34), Beijing 100080, People’s Republic of China
R. P. Liu Institute of Physics, Chinese Academy of Science, P.O. Box 603 (34), Beijing 100080, People’s Republic of China and Yan Shan University, Qinhuandao 006604, People’s Republic of China
Y. F. Xu Institute of Physics, Chinese Academy of Science, P.O. Box 603 (34), Beijing 100080, People’s Republic of China
W. K. Wang Institute of Physics, Chinese Academy of Science, P.O. Box 603 (34), Beijing 100080, People’s Republic of China and Yan Shan University, Qinhuandao 006604, People’s Republic of China (Received 11 June 1997; accepted 27 February 1998)
Carbon nitride crystals have been synthesized from C3 N4 H4 in the presence of a nickel-based alloy or cobalt as a catalyst at high pressure of 7 GPa and temperature of about 1400 ±C. Scanning electron microscopy showed rod-like, well-faceted crystals with size of several micrometers, and the N content in these carbon nitride crystals was 47–62%. X-ray diffraction indicated the crystals were composed of a –C3 N4 and ˚ c 4.715 A) ˚ b –C3 N4 . The experimental lattice constants of a –C3 N4 (a 6.425 A, ˚ ˚ and b –C3 N4 (a 6.419 A, c 2.425 A) agree with ab initio calculations very well. I. INTRODUCTION
In 1985, a particular phase of carbon nitride was predicted by Cohen1 to have a bulk modulus as high as that of diamond, the hardest known material. After that, several ab initio calculations have supported Cohen’s conjecture, predicting C3 N4 to be (meta)stable in equivalent structures as known for a – and b –Si3 N4 and also in a zincblende-like structure with C vacancies. In all three cases the bulk modulus is comparable to that of diamond.2–5 Up to now, more than ten structures of carbon nitride have been predicted6 and the shortness of the C-N bonds and the high degree of covalence in these compounds lead to expectations of a new class of superhard structural materials. Motivated by these predictions, considerable effort has been directed toward synthesizing carbon nitride compounds. The goal is not only to obtain a new superhard material, but also to evaluate the effectiveness of first-principles theories in materials science. This presents a challenge for the development of growth or synthesis techniques. Many groups have attempted to synthesize C-N compounds in the lab by employing various physical and chemical vapor thin film deposition methods, such as rf sputtering, pulsed-laser ablation, and arc-plasma jet. Unfortunately, almost all the C-N films displayed a featureless surface morphology, the composition of those C-N materials varied over a very wide range, and no 3458
http://journals.cambridge.org
J. Mater. Res., Vol. 13, No. 12, Dec 1998
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single stable phase of carbon nitride compounds could be identified.7,8 So the most important thing
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