Study On The Synthesis Of Diamond In A Fe-Ni-C-N-H-O System At High Pressure And High Temperature

Diamond is an extreme functional material with many commercial and industrial applications,but sometimes forgotten is that diamond is also an important earth mineral.The impurities and inclusions in natural diamonds carry significant information about the chemistry and mineralogy of the mantle at different depths of the earth,and about the deep crystallization process.The obtained information can help researchers determine key information related to diamond,beyond that which is easily determined from surface-obtained samples:such as growth pressure,temperature,and age.It can also be used to deduce the mechanisms of mineral composition at various mantle depths and conditions.The origin and growth mechanisms of natural diamond are still controversial matters to be experimentally delineated.A thorough analysis of the crystallization process of natural diamond and its internal inclusions is of great value and significance for understanding the mantle convection and the carbon cycle process in the earth’s interior.The current level of scientific and technological achievements in earth science do not permit direct observation of diamond formation and growth environments,or collection of samples from these environments.Researchers have established a variety of different growth models based on a substantial sampling of internal inclusions from diamonds,and on related mineralogical information.For example,research has provided an applicable understanding of various transition metal（TM）systems,among them TM-C-N,TM-C-O,TM-C-H,TM-C-H-O,TM-C-N-H-O,TM-H₂O,TM-Silicate,TM-C-Carbonate,and TM-C-S.There are also good knowledge bases on some non-metallic systems,such as Na₂SO₄-C,Na₂CO₃-C/Na₂C₂O₄,and on pure natural mineral systems,such as kimberlite,peridotite,etc.The traditional geological point of view holds that natural diamonds grow in the environment containing C-H-O fluids,but this system fails to fully explain the huge difference between natural diamonds and synthetic diamonds in nitrogen impurity content and existing form.Rather,it is the C-N-H-O fluids system that is not only consistent with the C-H-O environment but also fully considers the difference in nitrogen impurity content and existing form between natural diamond and synthetic diamond,so it is a very likely growth environment for natural diamond.At present,there is little published research on synthesis of diamond in the C-N-H-O fluids system,and there is a lack of in-depth research that could lead to complete understanding of the system.Thus,building the C-N-H-O fluids system and exploring the crystallization law and crystal characteristics of diamond in this system have important value for exploring the origin of natural diamond and deducing the growth mechanism of natural diamond.In this work,Fe,Ni and Fe₆₄Ni₃₆were selected as metal solvents,niacin was selected as additive to build a system containing C-N-H-O fluids.All experiments were performed on a China-type large volume cubic high-pressure apparatus（SPD-6×1200）.Applying the temperature gradient method,high-quality,gem grade diamond single crystals with rich nitrogen content,and with hydrogen and oxygen impurities,were successfully prepared.A scanning electron microscope（SEM）and an optical microscope（OM）were used to characterize the crystal form and surface morphology of diamond,Raman spectroscopy was used to characterize the crystal quality,Fourier-transform infrared spectroscopy（FTIR）was used to analyze the content and existing form of nitrogen impurities in diamond crystal,and x-ray photoelectron spectroscopy（XPS）was used to analyze the binding forms of impurity elements in diamond crystals.The process and characteristics of spontaneous nucleation of graphite into diamond,and the mass transfer process of diamond crystal growth in the added-niacin system were analyzed.The changes of the composition of the metal solvents in the C-N-H-O fluids system were analyzed by x-ray diffraction（XRD）and XPS.The information gained is of great importance for understanding the mechanisms of metal solvent property changes in C-N-H-O fluids and for optimizing metal solvents.The main research results of this paper are as follows:1.A system containing C-N-H-O fluids was constructed using niacin as an additive,and gem grade diamonds with rich nitrogen content,and with hydrogen and oxygen impurities,were successfully prepared under high temperature and high pressure using Fe₆₄Ni₃₆,pure Fe and pure Ni as metal solvents.In the three metal solvent addition systems,with the increase of the niacin addition ratio,the crystal growth rate gradually decreased.Investigation with OM and SEM showed that the growth texture,corrosion pit,growth step,and fault of the diamond crystal surface gradually increased,showing obvious two-dimensional growth characteristics.The nitrogen impurity content in the crystal gradually increased with the increase of the niacin addition ratio.With the increasing proportion of niacin,the photoluminescence characteristics of diamond crystals also changed significantly.The photoluminescence intensity of diamond crystals synthesized in Fe₆₄Ni₃₆ metal solvent has increased significantly,the luminous intensity of NV color centers has changed significantly,and there are also several composite defect centers（NE1 defect center and NE3 defect center）related to Ni impurities.2.The natural diamond is grown in the high-temperature and high-pressure environment inside the earth,and the synthesis temperature is also a key parameter for the growth of synthetic diamond.The crystallization law and crystal characteristics of diamond crystals grown at different synthesis temperatures are obviously different.In a system using Fe₆₄Ni₃₆ as the metal solvent and niacin as the additive,the influence of the change of synthesis temperature on the crystallization law and crystal characteristics of diamond crystals was investigated,and the changes of impurity content,existing forms and defect types in diamond crystals were analyzed.The experimental results show that with the increase of synthesis temperature,the crystal form of diamond crystal changes little,but the defects such as growth texture on the surface of diamond crystal and cracks in the crystal are significantly reduced,indicating that an increase of synthesis temperature improves the crystal quality of diamond crystal.Raman spectroscopy confirmed this view.With the increase of synthesis temperature,the graphite related sp² C-C Raman scattering intensity of diamond samples decreased,the sp³ C-C Raman scattering intensity increased,the Raman half peak width（FWHM）gradually decreased,and the diamond’s crystallinity increased.The color of diamond crystal gradually deepened with the increase of niacin addition ratio,from light green to dark green,the nitrogen impurity content in diamond crystal gradually increased,and the proportion of A-center nitrogen gradually increased.This is the result of higher temperature and a greater hydrogen impurity level in diamond jointly promoting the movement and aggregation of nitrogen atoms.Some diamond crystals have characteristic infrared absorption lines related to magnesite,carbon dioxide,and Y center.At 5.8 GPa,with the increase of synthesis temperature,S3 defect centers and H3 defect centers,which are common in natural diamonds,appear in diamond crystals.Those characteristics are very similar to those of some natural yellowish brown natural Ib-Ia A diamonds containing CO₂,a finding that is of great value for exploring the origin of natural Ib-Ia A diamonds.3.In the Fe₆₄Ni₃₆ system,the influence of niacin addition on the two key processes of diamond growth was investigated,namely the process of spontaneous nucleation of graphite carbon source into diamond,and the process of mutual mass transfer between graphite carbon source and metal solvent.The results show that the process of spontaneous nucleation and conversion of graphite carbon source into diamond is severely inhibited when the proportion of niacin is increased,which in turns shows that the spontaneous nucleation and conversion rate of graphite carbon source is low in the same synthesis environment,and the spontaneously nucleated diamond in the carbon source is small and has many defects.The mutual mass transfer process between graphite carbon source and metal solvent is severely inhibited with increasing niacin,and the content of carbon diffused to metal solvent,and metal solvent diffused to carbon source in the same environment is significantly reduced.The metal solvents were comprehensively analyzed by XRD and XPS.The results showed that the addition of niacin reduced the content of zero valent metal Fe and Ni in the metal catalyst,the content of intermediate metal carbides decreased,and the proportion of metal oxides increased.When nicotinamide is added,CO and NO generated from decomposition of nicotinamide can be detected in the metal solvent,which indicates that they are adsorbed by the metal solvent,and this has an obvious impact on the growth process of the diamond surface.These factors together inhibit the spontaneous nucleation of graphite into diamond,hinder the mass transfer process and surface process of diamond growth,and so reduce the growth rate of diamond crystal and increase the occurrence surface growth defects.4.The difference between pure iron and pure nickel as metal solvent for diamond synthesis is evident in the experimental results,which show that compared with pure iron,using pure nickel as metal solvent requires higher temperature and pressure conditions to grow high-quality diamond single crystals.Under similar conditions,using pure nickel as a metal solvent to grow single diamond crystals results in more defects.The content of nitrogen impurities in diamond crystals grown in those two kind of metal solvents gradually increased with the increase of the niacin addition ratio.In the diamond crystal synthesized with pure nickel as the metal solvent,the nitrogen impurity content is always higher,and the nitrogen content shows a more obvious upward trend with the increase of niacin addition ratio,and the degree of aggregation of nitrogen impurities is higher.The rare B center nitrogen appeared in the diamond grown in pure nickel metal solvent;the infrared characteristic absorption intensity of the B center nitrogen increased significantly with the increase of niacin addition ratio.The difference between diamond crystals grown in pure iron and pure nickel metal solvent is due to the stronger interaction between pure iron and carbon than the interaction between pure nickel and carbon.In the iron and carbon interaction,iron-carbon compounds are easily formed,which lower the solid melting point and lower the reaction barrier,so the required synthesis temperature and pressure are lower.At the same time,the interaction between pure iron and nitrogen is strong,permitting more nitrogen to dissolve,reducing the occurrence of free nitrogen impurities in the synthesis system reducing the nitrogen impurities entering the diamond crystal.The iron and carbon interactions result in a relatively slow increase of nitrogen content with the increase of nicotinamide addition.This is the first experimental work to directly synthesize single diamond crystal containing B center nitrogen using pure nickel as metal solvent,which indicates that the existence of nitrogen impurities in diamond may be directly related to environmental composition,in addition to synthesis temperature,findings of great significance for exploring the material composition of the environment in which natural Ia B-type natural diamond are formed.