Study on growth characteristics of Ib-type diamond in an Fe–Ni–C–S system

Shuai Fang a, Hongan Ma *a, Zhanke Wang a, Zhiqiang Yang a, Zheng-hao Cai a, Luyao Ding a, Xinyuan Miao a, Liangchao Chen b and XiaoPeng Jia *a
aState Key Laboratory of Superhard Materials, College of Physics, Jilin University, Changchun 130012, China. E-mail: maha@jlu.edu.cn; jiaxp@jlu.edu.cn
bKey Laboratory of Material Physics of Ministry of Education, and School of Physical and Engineering Zhengzhou University, Zhengzhou 450052, Henan, China

Received 31st July 2019 , Accepted 27th August 2019

First published on 28th August 2019


FeS is the main sulfur-containing compound in natural diamond inclusions. The research on FeS-doped diamond crystals plays an extremely important role in exploring the growth environment of natural diamond and the chemical composition of the earth. In our work, the characteristics of FeS doped diamond crystals were studied by using a China-type large volume cubic high-pressure apparatus with an iron–nickel alloy as a catalyst at 6.0 GPa and 1300–1350 °C. The crystals were characterized by OM, SEM, XPS, FTIR and Raman. Scanning electron microscopy of the crystal shows that the {100} plane of the crystal is rougher than {111}, which is related to the number of dangling bonds on different crystal faces of the crystal. The residue on the surface of the crystal was detected by mapping, and the test result showed that it was a sulfur iron and a sulfur nickel compound. FeS will decompose in the synthesis system, and the decomposition products are Fe and S2.The XPS spectrum of the crystal shows that S was successfully incorporated into the diamond lattice by forming C–S–O and C–S–SO2–C bonds, and the peak intensity was strong. The infrared spectrum shows that the N content in the crystal gradually increases as the doping ratio increases. Raman spectroscopy analysis shows that the S-doped Ib-type diamond single crystal has a high-quality sp3 structure, and the full width at half maximum (FWHM) of the crystal gradually broadens as the doping ratio increases.


1. Introduction

Diamond is one of the most famous earth minerals, with excellent physical and chemical properties, including corrosion resistance, highest heat transfer performance, high hardness and excellent optical properties.1–3 Diamonds, mainly found in kimberlites, are minerals from the upper mantle. According to the literature, the pressure and temperature of natural diamond growth are 4.5–6.5 GPa and 900–1250 °C, respectively.4–6 Recent studies of natural diamonds have found that some IIb diamonds may come from the lower mantle, about 620 km underground.7 During the growth of diamond, some materials in the diamond growth environment may be wrapped inside the diamond to form an inclusion. These inclusions inside the diamond are protected by diamonds and come to the surface with diamonds along with a series of geological activities such as magma flow and volcanic eruptions.4,7,8 The inclusions inside the diamond can visually reveal the chemical composition of materials in the diamond growth zone, which is the most direct evidence that we can explore the material composition of the mantle.8,9 Inclusions in diamonds are the main basis for the establishment of diamond growth models, and they are also the main information sources of mantle mineralogy.9,10

Among the many natural diamond inclusions found, FeS is the main sulfur-containing compound.8,10 This phenomenon indicates that FeS participates in the growth of diamond, and it should have a certain effect on the nucleation and growth of diamond. In some of the past studies, researchers have focused on the nucleation of diamond in a S–C system.11–19 The possibility of extensive diamond crystallization from natural sulfide melts was experimentally demonstrated by the spontaneous nucleation and crystallization of diamond in a series of multicomponent compositions of a pentlandite–pyrrhotite–chalcopyrite carbon system at high pressures and temperatures.12–15,17 Fe, Ni, and S are all elements abundant in the earth, so natural diamonds may grow in an Fe–Ni–S–C system. At present, most of the synthetic diamonds are mainly iron-based catalysts. Iron-based catalysts can provide stable carbon transport channels for crystal growth, and at the same time, effectively reduce the pressure of synthetic crystals. This ability makes it possible to become one of the environments in which natural diamonds are generated. Pyrite and iron–nickel components, which are common in natural diamonds, lay the foundation for the Fe–Ni–S–C system theory.8,10 Industrial diamond was grown by film growth using graphite as a carbon source and FeS as a catalyst solvent. Most of the diamonds grown under this system are mainly microcrystals, and no large-sized diamond crystals are grown.14–16 To a certain extent, this method can explain the FeS nucleation pressure and nucleation quantity of FeS, but attention has not been paid to the growth of crystals and crystal characteristics. The study of growing large-diameter diamond crystals by a temperature gradient method in an Fe–Ni–S–C system has not received much attention.

As a potential, high-quality semiconductor material, a S-doped diamond crystal has attracted the attention of many researchers. It is extremely difficult to incorporate a S atom into the diamond lattice because the atomic radius of the S atom is much larger than a C atom.15,20–23 Though diamond crystals were synthesized in S–C, (Fe, Ni)9S8–C, Fe–Ni(Co)–S–C, Ni70Mn25Co5–C systems, but there is still insufficient evidence to prove that S is incorporated into the diamond lattice.11,13,18,19,24,25 Most of the S atom in natural diamond exists in the form of FeS, and the addition of FeS as a source of sulfur in the system is more similar to the growth environment of natural diamond. Studying the growth mechanism and crystal characteristics of diamond in an Fe–Ni–S–C system is of great significance for exploring the growth environment of natural diamond.

An iron–nickel alloy was used as the catalyst, graphite was used as the carbon source, and FeS was used as the additive. The S-doped diamond single crystal was synthesized by a temperature gradient method under high temperature and high pressure conditions. The Fe–Ni–S–C system may be a natural diamond growth environment. The research on this system is beneficial to researchers to understand the growth mechanism of natural diamond profoundly. This paper provides data support for us to confirm the composition of the material composition of the mantle.

2. Experimental details

High temperature and high pressure (HPHT) testing is carried out using a China-type large volume cubic high-pressure apparatus (CHPA) (SPD-6 × 1200) in a pressure range of 5.5–6.5 GPa and a temperature range of 1300–1350 °C. Fig. 1 shows the assembly drawing for diamond synthesis. High purity graphite (99.9% purity) was used as the carbon source, an Fe64Ni36 (64/36, weight ratio) alloy was used as the catalyst, and FeS was used as the additive. FeS was purchased from Aladdin and its initial state is blocky. In order to facilitate doping, we put the bulk FeS into an agate mortar and grind it into powder, and then add it to the carbon source according to different weight ratios. In this paper, the {111} and {100} crystal faces with a size of 1 mm are selected as initial crystal face synthetic diamonds, respectively. The experimental synthesis pressure is calibrated based on the high-pressure phase transition points of Bi, Ba, and Tl, and the temperature in the chamber is calibrated based on the relationship between the input power and the temperature measured using a Pt-30% RH/Pt-6% thermocouple. Measurement error over the entire temperature and pressure range is less than 5%.25–29
image file: c9ce01194c-f1.tif
Fig. 1 Sample assembly drawing of diamond synthesis at HPHT: 1. steel cap; 2. pyrophyllite; 3. sheet graphite; 4. sheet Cu; 5. ZrO2 + MgO sleeve; 6. graphite heater; 7. metal catalyst; 8. carbon source; 9. ZrO2 + MgO pillar; 10. NaCl + ZrO2 sleeve; 11. dolomite pillar; 12. seed crystal; 13. dolomite sleeve.

After the end of the HPHT experiment, the sample was placed in a hot mixed solution of H2SO4 and HNO3 (volume ratio, 3[thin space (1/6-em)]:[thin space (1/6-em)]1) to remove the catalyst and graphite on the crystal surface. Prior to all crystal detection, it was ultrasonically cleaned in alcohol for 30 minutes to remove some fine deposits on the crystal surface. The synthesized diamond was characterized by optical microscopy (OM), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR) and Raman spectroscopy (Raman). The infrared absorption spectrum was measured using a VERTEX 70 V vacuum Fourier micro-infrared spectrometer with a spectral range of 800 to 4000 cm−1 and a spectral resolution of more than 0.4 cm−1. Raman spectroscopy of diamond was carried out using a Renishaw inVia Raman spectrometer at a room temperature excitation line wavelength of 532 nm. The spectral range is 300–4000 cm−1 with a spectral resolution of 1 cm−1.

3. Results and discussion

3.1. Optical photographs of crystals in the Fe–Ni–C–S system

The crystal in the Fig. 2 optical photograph is the crystal synthesized in the Fe–Ni–C–S fluid. The crystals in Fig. 2 all show the morphological characteristics of a cubic octahedron. With the increase of FeS in the system, the crystal form of the crystal does not change much. Fig. 2(a)–(d) correspond to D-1, D-2, D-3, and D-4 in Table 1, respectively. In the synthetic system, the proportion of FeS in the carbon source increased from 0.0 wt% to 6 wt%, and the color and crystal form of the crystal did not change significantly.
image file: c9ce01194c-f2.tif
Fig. 2 Optical images of the diamond crystals synthesized from the Fe–Ni–C–S system with the FeS additive: (a) D-1; (b) D-2; (c) D-3; (d) D-4.
Table 1 Experimental results of the diamond crystallization from the Fe–Ni–C–S system with the FeS additive
Run FeS/(wt) Pressure/(GPa) Temperature/(°C) Growth face Growth rate (mg h−1) Growth time (h) Color
D-1 0 6.0 1300 {111} 1.47 23.3 Yellow
D-2 2 6.0 1300 {111} 1.52 23.3 Yellow
D-3 4 6.0 1320 {111} 1.74 23.4 Yellow
D-4 6 6.0 1340 {111} 1.71 23.4 Yellow
D-5 2 6.0 1300 {100} 1.58 23.3 Yellow
D-6 2 6.0 1300 {111} 1.55 23.5 Yellow


Table 1 lists the growth conditions of the crystals at different doping concentrations. The pressure of the synthesized crystals is 6.0 GPa, and the temperature required to synthesize crystals of the same crystal form with increasing doping concentration is also gradually increased. In most diamond synthesis systems,26,39,40 the growth rate of diamond gradually decreases as the doping ratio increases. The relationship between the crystal growth rate and doping ratio is listed in Table 1. From Table 1, we can conclude that as the doping ratio increases, the growth rate of the crystal increases first and then decreases. When a small amount of S enters the synthesis system, the temperature required to synthesize crystals of the same crystal form increases, and an increase in temperature lowers the viscosity of the catalyst solvent. At the same time, the addition of S also reduces the viscosity of the catalyst solvent, thereby enhancing the ability to transport carbon and thereby increasing the growth rate of the crystal. However, when too much S enters the synthesis system, the additive poisons the catalyst solvent. The ability of the poisoned catalyst solvent to dissolve carbon is reduced, resulting in weakening of the driving force for crystal growth, and the final result is slowing of the crystal growth rate.13,24,25

3.2. Microscopic morphology of crystals in the Fe–Ni–C–S system

The {100} plane of industrial diamond synthesized by the film growth method in the Fe–Ni–C–S system is rougher than the {111} plane, and a similar phenomenon is also found in this paper. Fig. 3(a) shows the crystal optical photograph of D-6, and (b) and (c) shows the scanning electron micrographs of the {100} plane and the {111} plane of the crystal, respectively. As can be seen from Fig. 3, the {111} plane of the crystal is particularly flat, with only extremely fine growth texture. The {100} side of the crystal becomes a bit rough and the surface is uneven. The difference in the surface of this crystal is due to the introduction of S. This phenomenon occurs in both FeS doping and elemental S doping systems.18,24,25
image file: c9ce01194c-f3.tif
Fig. 3 (a) Crystal optical photo of D-6; SEM images of the (b) {111} and (c) {100} planes of crystals doped with 2 wt% FeS in the Fe–Ni–C–S system.

Diamond is a typical atomic crystal with different surface atoms and internal atoms. The atoms inside the crystal combine to form a covalent bond by the sp3 hybrid orbital, and there is no excess free electron inside the crystal. Since the surface of the crystal has no adjacent atoms, coordination unsaturation occurs on the surface of the crystal to form a dangling bond.26,28 Since the dangling bonds are unsaturated, the dangling bonds will adsorb the impurity elements in the system, and the adsorption force will increase as the dangling bonds increase. The atoms of the {111} plane and the {100} plane of the diamond crystal are arranged differently, and thus the number of dangling bonds is also different.

As shown in Fig. 4, since the dangling bonds on the {100} face are more than the {111} surface dangling bonds, the {100} face is more capable of adsorbing impurity elements than the {111} face. In the Fe–Ni–C–S system, FeS is transported to the surface of the crystal with the carbon source. When the experiment is finished, the impurity element combines with the dangling bonds on the crystal surface to form a compound. Since the {100} surface has a strong ability to adsorb impurity elements, a large amount of impurity elements are aggregated on the {100} plane and the dangling bonds combine to form a compound. After the end of the experiment, the experimental sample was placed in a hot acid, and the compound accumulated on the {100} surface was corroded by the acid, causing the surface of the crystal to become rough and leave pits. The growth rate of the crystal grown by the temperature gradient method is slower than that of the film growth method, so that the {100} plane of the large-sized crystal is flatter than that of the S-doped industrial diamond.14,19–21


image file: c9ce01194c-f4.tif
Fig. 4 Schematic diagram of the dangling bonds on the two crystal faces of diamond (a) {100} face; (b) {111} face.

3.3. The existing form of S in the Fe–Ni–C–S system

Fig. 5(a) shows a crystal grown with the {100} plane as a seed crystal. Fig. 5(c) shows the {111} plane of the crystal, and it can be seen that the {111} plane of the crystal is particularly smooth. Fig. 5(b) shows the {100} plane of the crystal, and a large amount of recrystallized matter exists on the crystal plane. This difference should be related to the dangling bond. Fig. 5(d) shows the enlargement of Fig. 5(b). Fig. 5(d) shows the appearance of a precipitate, and there is also a square plane. This precipitation occurs because the four elements in the Fe–Ni–C–S fluid are combined with each other after the experiment ended. When the crystal is treated with a mixed solution of concentrated sulfuric acid and concentrated nitric acid in the later stage, the substance remaining on the surface of the crystal is preserved due to the passivation due to the very high concentration of the acid.
image file: c9ce01194c-f5.tif
Fig. 5 (a) The photograph of a crystal grown with the {100} plane as a seed crystal doped with 2 wt% FeS; the microscopic morphology of the crystal: (b) {100} plane, (c) {111} plane and (d) amplification of (b); and the mapping photographs of the recrystallized material on the crystal face: (e) C, (f) S, (g) Ni and (h) Fe.

A mapping test was performed as shown in Fig. 5(d) to determine the composition of the recrystallized material. Fig. 5(e)–(h) show the elemental distribution planes of C, S, Ni, and Fe, respectively. It can be seen from Fig. 5 that there was an enrichment phenomenon of S, Ni, and Fe elements at the square crystal plane position at the melt recrystallization position. In order to determine that there is no S element in the enrichment position of S (the elemental sulfur does not react with dilute sulfuric acid), the crystal is subjected to secondary treatment with dilute sulfuric acid. After the secondary treatment is completed, the recrystallized matter on the {100} plane of the crystal completely disappears. From this, it can be confirmed that these recrystallized materials should be iron sulfide or sulfur nickel compounds.

The cavity of the synthetic diamond is sealed during the experiment and is not exchanged with foreign substances, so it can be considered that the high temperature and high pressure chamber is in a vacuum state. According to Professor Yang Bin's work, the decomposition temperature of FeS under vacuum conditions is 1175 °C.30 As shown in formula (1), FeS decomposes into Fe and S2 under vacuum conditions. Jin-ding Yan studied the decomposition reaction of FeS in H2 and He gas atmospheres. It was found that FeS decomposes at 850 °C.31,32 The Fe/Ni alloy is present in the synthesis cavity, and the melted Fe/Ni alloy reduces the decomposition temperature of FeS due to the interaction between atoms. In this paper, the temperature of the synthetic diamond cavity is above 1300 °C, at which temperature FeS will decompose.

 
FeS → Fe + 1/2S2(1)

S2 produced by the decomposition is melted in the Fe–Ni fluid to form an Fe–Ni–C–S fluid. When the experimental temperature is lowered, the sulfur dissolved in the iron–nickel fluid reacts with iron–nickel and then precipitates as a ferro–iron compound and a nickel–iron compound. In the Fe–Ni–C–S fluid, S should be in the form of S2. The S, C, and O bonds detected in the XPS spectrum confirmed the decomposition of FeS. In the natural diamond growth environment, S is likely to exist in the form of S2 or other polymeric sulfur. This is extremely important for us to study the internal environment of the earth and the environment in which diamonds grow.

3.4. Effect of the Fe–Ni–C–S system on XPS characterization of diamonds

To determine the presence of impurity elements in the diamond crystal, we performed XPS testing on the crystal. In order to eliminate the influence of H2SO4 on the test results during acid treatment, the crystals in Fig. 2(d) were broken and the broken segments of the crystals were tested. Fig. 6 shows the XPS spectrum of a 6 wt% FeS doped crystal. The peak resolution in Fig. 6(a) demonstrates the chemical state of C in the crystal, and the bond energy peaks are 283.7 eV (C–S), 284.6 eV (sp2 C–C), 285.3 eV (sp3 C[double bond, length as m-dash]C), and 288 eV (C–O).16,21,31 The peak results in Fig. 6(b) demonstrate that there are three bond energy peaks for N 1s, 398.5 eV, 399.3 eV and 400.0 eV, respectively.33–35 The peak value of 398.5 eV is related to N–C sp3 type bonding; the peak value of 399.3 eV is related to the nitrile type sp1 bonding configuration; the peak value of 400.0 eV is related and the peak at 400.0 eV was related to the long N–C bond.
image file: c9ce01194c-f6.tif
Fig. 6 XPS spectra of the diamond crystals synthesized from the Fe–Ni–C–S fluid with 6 wt% FeS additive concentrations. (a)–(c) were the XPS spectra for C, N 1s and S 2p core levels, respectively.

The XPS spectrum exhibits a particularly sharp S-related peak that is not available in other S-doped systems. It can be concluded from Fig. 6(c) that S is successfully incorporated into the diamond crystal. The S 2p spectrum in Fig. 6(c) shows three peaks centered at 167.5 eV, 168.4 eV and 168.8 eV. The peak at 167.5 eV and the peak associated with S are not specifically described in the literature. The peak at 168.4 eV is attributed to C–S–O and the peak at 168.8 eV is related to C–S–SO2–C (polymerized sulfur).36,37 This result also proves that S has successfully entered the diamond lattice and also confirmed that FeS will decompose during the synthesis. The S produced by the decomposition of FeS enters the crystal lattice of the crystal during the growth of the diamond.

3.5. Effect of the Fe–Ni–C–S system on infrared characterization of diamonds

Attention has been paid to the analysis of the internal impurity elements of natural diamond or synthetic diamond crystals. The use of infrared spectroscopy to measure the nitrogen content inside diamond crystals and the presence of N is a very effective means. The infrared spectrum of the synthetic diamond in the Fe–Ni–C–S system is shown in Fig. 7. Fig. 7(a)–(d) correspond to D-1, D-2, D-3, and D-4 in Table 1, respectively, and the doping ratio of FeS in the synthetic system is gradually increased. All of the samples in Fig. 7 showed the absorption of the defect-induced monophonic region (1400–800 cm−1) caused by nitrogen impurities. The N impurities in the crystal are all formed by the C-center, that is, the isolated N atom is substituted for the C atom, and the absorption peak positions in the spectrum are 1130 cm−1 and 1344 cm−1. Fig. 7(a) shows the infrared spectrum of D-1 (0 wt% FeS doping), and the N content in the crystal is calculated to be 141 ppm according to formula (2).38–40 This is consistent with the N content inside the crystal synthesized by the ordinary Fe/Ni alloy.
 
image file: c9ce01194c-t1.tif(2)

image file: c9ce01194c-f7.tif
Fig. 7 FTIR spectra of the diamond crystals synthesized from the Fe–Ni–C–S fluid with various FeS additive concentrations: (a) D-1; (b) D-2; (c) D-3; (d) D-4.

Table 2 lists the changes in the nitrogen content in the crystal at different doping concentrations. As the FeS doping ratio increases, the N content in the crystal gradually increases. When S enters the synthesis system, it promotes the formation of C–N bonds, which facilitates the retention of N in the crystal. Different catalyst alloys have different N contents in the crystals synthesized under the same temperature and pressure conditions. Therefore, when S enters the Fe–Ni contact media system, the nitrogen dissolving ability of the catalyst and the ability to form C–N bonds changed. This issue still requires further research.

Table 2 Concentrations of nitrogen impurities in diamonds synthesized in the Fe–Ni–C–S fluid
Run Additive (wt%) FeS Nitrogen content (ppm) C-form
D-1 0 141
D-2 2 157
D-3 4 165
D-4 6 173


3.6. Effect of the Fe–Ni–C–S system on Raman characterization of diamonds

Raman spectroscopy is a method for efficiently identifying crystals which distinguishes graphite bonded with sp3 bonds and graphite with an sp2 bond and other inclusions.38 The Raman peak and full width at half maximum (FWHM) indicate the relative mass of these crystals. Fig. 8 shows the Raman spectra of the crystal synthesized in the Fe–Ni–C–S fluid. The peak positions in the Raman spectrum of diamond are all located near 1331.9 cm−1, and there is no obvious regular peak shift. In Fig. 8, there is only one peak in the Raman spectrum of diamond (near 1331.9 cm−1) and there is no peak associated with graphite (1580 cm−1).40–42 This phenomenon indicates that the crystals synthesized in the Fe–Ni–C–S fluid are of good quality and are in contrast to other industrial diamonds synthesized in Fe–Ni–C–S fluids.
image file: c9ce01194c-f8.tif
Fig. 8 Raman spectra of the diamond crystals synthesized from the Fe–Ni–C–S fluid with various FeS additive concentrations: (a) D-1; (b) D-2; (c) D-3; (d) D-4.

It can be seen from Fig. 8 that as the doping ratio is increased, the full width at half maximum (FWHM) of the crystal gradually becomes wider. The Raman half-width of the D-1 to D-4 experiments is shown in Fig. 7. The Raman FWHM values of the D-1 to D-4 experiments were 4.05 cm−1, 4.08 cm−1, 4.10 cm−1, and 4.20 cm−1, respectively. When the S element is introduced into the Fe–Ni–C fluid, the crystal quality of the crystal is affected. FeS is an additive different from S powder. Under the conditions of 7.0 GPa at 1350 °C,10,11 FeS is used as a catalyst and graphite is used as a carbon source to directly synthesize industrial diamond. Therefore, when FeS in the synthetic system is 6 wt%, gem-quality diamond can still be synthesized, which indicates that the poisoning effect of FeS on the catalyst is small. Therefore, there is no obvious regular movement of the Raman peak because the amount of doping in the system is small.

4. Conclusions

In this paper, experiments were carried out with an iron–nickel alloy as a catalyst and FeS as an additive at 6.0 GPa and 1300–1350 °C. The color of the crystal gradually increases as the doping ratio increases. Scanning electron microscopy was used to detect the microscopic morphology of the crystal. The {100} plane of the crystal was rougher than the {111} plane of the crystal. The appearance of this phenomenon is related to the number of dangling bonds on different crystal faces of the crystal. The use of mapping to detect residues on the surface of the crystal revealed that these residues were pyrite and sulfur nickel compounds. During the experiment, FeS will decompose in the synthesis cavity, and the decomposition products are S2 and Fe. When the temperature in the synthesis chamber is lowered after the end of the experiment, S2 reacts with the iron–nickel metal in the cavity to form iron–nickel and sulfur–nickel compounds and remains on the surface of the crystal. The XPS results confirmed the presence of S in the interior of the crystal, which also confirmed the decomposition of FeS. This form of existence of S has great guiding significance for us to explore the growth environment of sulfur-containing natural diamond. The infrared spectrum of the crystal shows that the N content in the crystal increases as the doping concentration increases. As the amount of doping increases, the FWHM of the crystal gradually increases. This shows that the crystallinity of the crystal deteriorates. The introduction of S causes the crystallinity of the crystal to deteriorate, which also suggests that there is no excess S in the growth system of the gem-grade sulfur-containing natural diamond that has been found. This is extremely important for us to explore the growth system of natural sulfur-containing diamond.

Conflicts of interest

There are no conflicts of interest to declare.

Acknowledgements

This project was supported by the National Natural Science Foundation of China (11604246, 51772120, 51872112 and 11804305), the China Postdoctoral Science Foundation (2017M622360), and the Project of Jilin Science and Technology Development Plan (Grant No. 20180201079GX).

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