The Existing States Of Hydrogen In Diamonds

Yang Zhi-jun(Mineralogy) Directed by Prof. Peng Ming-sheng and Xie Xian-deIt is revealed that natural diamonds may contain as much as 59 different trace elements. The concentration of each trace element is above 1 ppm(10"6). However, the major impurities are nitrogen, hydrogen, oxygen and boron. The study of the forms of hydrogen in diamonds not only enriched the contents of mantle mineralogy, material sciences and gemology, but also provided some important informations for dealing with some problems concerning the Earth's mantle and core. On the basis of previous researchers, this thesis has paid much attention to the crystal chemistry and mineralogical characters of diamonds and to the chemical state of hydrogen in diamonds. Some desirable forms of hydrogen (e.g. lattice hydrogen, interstitial hydrogen, bond-centered hydrogen and anti-bond-centered hydrogen) have been predicted using ab inito and MNDO, some of which were confirmed using the orientated and(or) micro-area infrared spectroscopic data quantitatively. Our results provided some theoretical and experimental evidences for the enhancement and synthesis of diamonds.The crystal chemistry and mineralogical characters of diamonds were studied using group theory analysis, SEM, X-ray diffraction, EPR, FTIR and Raman spectroscopy in this work. Diamond is Raman active and its invariable sub-group is Td and translate group. The different mico-structures between natural and synthetic diamonds, abnormal optical characteristics and the inclusions of different shapes and compositions indicate that diamonds are inhomogeneous. The inhomogeneous diamonds can produce infra-red absorptions including single phonon absorption, C-C lattice vibration absorption and local mode vibration absorption related with defects. Therefore, FTIR spectroscopy is an effective method for studying the forms of hydrogen in diamonds.The FTIR and Raman vibration spectra of diamonds show that hydrogen has different chemical states. For example, C-H bond, N-H bond, HF, H2O, -OH and H2. The theoretical analysis and study of micro-fluid inclusions confirmed the presence of paramagnetic hydrogen and H"1" in diamonds. The implantation of hydrogen and quenching experiments of diamondindicate that the hydrogen is of self-trapped origin. The self-trapped sites are lattice site, interstitial site, bond-centered site and anti-bond-centered site.Our Ab inito calculations on lattice hydrogen showed that each carbon vacancy intends to attract hydrogen atoms its surrounding. A maximum of four hydrogen atoms can be accommodated at each vacancy. If C-C sp3 hybrid bonds of the host atoms are replaced by C-H bond, the maximum number of substitute hydrogen atoms tends to be three. Thus, in the infra-red spectrum of diamonds -CH3 radical is found to be the most far-ranging lattice hydrogen radical.MNDO study on interstitial H?(paramagnetic hydrogen) showed that the tetrahedral interstitial hydrogen atom is the most stable interstitial H? But comparing to the lattice, bond-centered and anti-bonded-centered located hydrogen, we found the sequence of stabilization is: vacancy trapping H? bond-centered H?tetrahedral interstitial H? hexagonal interstitial H? anti-bond-centered H?rhombus center interstitial H? MNDO calculations on interstitial H+ also showed that interstitial H+ is unstable in diamond. It will move from one interstitial site to another. However, the hexagonal interstitial site may be the relatively more stable site for interstitial H+.On the basis of our orientated infrared spectra data, we found that there are different concentrations of C-H and C-H bond in the directions vertical to (100), (110) and (111). Analysis of the concentration difference between the C-H bond and the C-H bond in the different directions showed that hydrogen tends to replace carbon atom of C-H bonds related to the direction vertical to (111). We found that the concentrations of H20, platelets and C-H bond are different in the directions vertical to (100), (110) and (111) hi type la diamonds. H2O an...