CHINESE JOURNAL OF PHYSICS VOL. 40, NO. 2 APRIL 2002 A First-Principles Study of Physical Properties of Monatomic Structures of B, C, N, and O B. R. Wu 1;3 , C. M. Sung 2 , S.-L. Lee 3 ,and M. F. Tai 4 1 General Education Center, Tainan Woman’s College of Arts and Technology, Tainan, Taiwan 710, R.O.C. 2 Department of Materials and Mineral Resources, National Taipei University of Technology, Taipei, Taiwan 104, R.O.C. 3 Department of Chemistry, National Chung Cheng University, Chia-yi, Taiwan 621, R.O.C. 4 Department of Physics, National Chung Cheng University, Chia-yi, Taiwan 621, R.O.C. (Received December 12, 2001) We have performed a series of studies on the properties of monatomic solid boron (B), carbon (C), nitrogen (N) and oxygen (O) under high pressure with a first-principles method. Four structures with different coordination numbers, i.e., cubic diamond, simple cubic, body center cubic and face center cubic structures, are calculated to investigate the relationship of physical properties and coordination numbers. Our investigation shows that all the four elements under high pressure favor a higher coordination number structure and become more metallic like. The bulk moduli have also been investigated, showing that atomic volume is the most important factor that determines the bulk moduli. The effects of composition and coordination on bulk moduli are secondary for B, C, N, and O. The investigation also reveals that the species having a small atomic volume will have a large bulk modulus. As expected, the bulk modulus of diamond is the largest one we studied. We found that the bulk modulus of carbon in simple cubic structure (3.37 Mbar) is slightly less than that of c-BN (3.67 Mbar), the second hardest material in the world. The bulk moduli of B in fcc structure (2.8 Mbar) and N in simple cubic structure (2.57 Mbar) are larger than that of SiC (2.24 Mbar). PACS. 64.60.–i – General studies of phase transitions. PACS. 71.15.Nc – Total energy and cohesive energy calculations. I. Introduction The composition and structure of a material are both crucial factors for determining the properties of a material such as hardness, e.g., the extreme difference of hardness and conductance between graphite and diamond [1]. The bonding strength of a crystal structure is manifested as a material’s hardness and rigidity, and a convenient measure of such a bonding strength is its bulk modulus, the specific incompressibility of a material. Superhard materials have attracted much attention from industry as they have many superior properties, e.g., the highest compressive strength and thermal conductivity. In forming superhard structures or compounds, the elements boron (B), carbon (C), nitrogen (N) and oxygen (O) are unique in that they are first period elements with p-electrons. As there are no inner p-electrons in the core to push the valence electrons outward, these p-electrons can retreat further inward relative to larger atoms of the same group of elements. Moreover, due to the absence of an inner p orbital in the electron core, the atomic volumes of http://PSROC.phys.ntu.edu.tw/cjp 187 c ° 2002 THE PHYSICAL SOCIETY OF THE REPUBLIC OF CHINA