Diamond Crystallization from a Sulfur-Carbon System at HPHT Conditions Y. N. Palyanov,* I. N. Kupriyanov, Y. M. Borzdov, A. G. Sokol, and A. F. Khokhryakov Institute of Geology and Mineralogy, Siberian Branch of Russian Academy of Sciences, Koptuyg aVe 3, NoVosibirsk, 630090, Russia
CRYSTAL GROWTH & DESIGN 2009 VOL. 9, NO. 6 2922–2926
ReceiVed March 4, 2009; ReVised Manuscript ReceiVed March 26, 2009
ABSTRACT: In this paper, we report on diamond synthesis in the sulfur-carbon (S-C) system at temperatures in the range of 1550-2000 °C and pressures of 6.3-7.5 GPa. It is established that crystallization of carbon phases (diamond and graphite), the degree of the graphite-to-diamond conversion, and diamond crystal morphology are primarily determined by the pressure-temperature (P-T) conditions. It is suggested that the most probable medium of diamond crystallization in the S-C system is the melting of carbon disulfide CS2. Fourier transform infrared (FTIR) microspectroscopy revealed that synthesized diamond crystals contained nitrogen impurity in the form of A and C defects with a total nitrogen concentration up to 1500 ppm. For the crystals with high nitrogen content, hydrogen-related centers are typically observed. Addition of water to the S-C system results in a decrease in the diamond synthesis temperature and a change in diamond crystal morphology from cuboctahedron to octahedron. Introduction Among the nonmetallic-solvent catalysts used for diamond synthesis under high pressure-high temperature (HPHT) conditions, of particular interest are elemental substances, such as phosphorus and sulfur. An increased attention to these elements is to a greater extent stimulated by the fact that both phosphorus and sulfur are considered potential elements for diamond doping.1-4 HPHT synthesis of diamonds in simple binary systems, like phosphorus-carbon (P-C) and sulfur-carbon (S-C), is also of interest for getting a better understanding of the mechanisms of diamond nucleation and growth from nonmetallic solvents. It has been established experimentally that melts of phosphorus and sulfur are able to dissolve carbon and provide diamond synthesis at high pressures and temperatures. Diamond crystallization in the P-C system has been studied in a number of works,5-8 and it has been proved that phosphorus incorporates into the diamond structure under HPHT conditions. Studies devoted to diamond crystallization in the S-C system under HPHT conditions are scarce,9,10 requiring further investigations. It should also be noted that experimental studies on diamond crystallization processes in the S-C system are of importance for understanding the conditions of natural diamond formation. For example, Haggerty,11 considering models of diamond genesis, regarded sulfur as a possible catalyst reacting with hydrogen and carbon to produce different volatiles such as H2S, COS, or CS2. Sulfur is a relatively abundant element in the Earth’s mantle,12 forming mainly Fe and Ni sulfides, which are frequently observed as inclusions in natural diamonds13,14 and may be involved in the diamond crystallization process.11,15-17 The present work is devoted to the study of diamond crystallization in the S-C system over a wide range of temperatures from 1550 to 2000 °C and pressures from 6 to 7.5 GPa and characterization of the synthesized diamond crystals. Experimental Section The starting materials were a graphite rod (99.99% purity), sulfur powder with purity not less than 99.99 %, and cuboctahedral synthetic * To whom correspondence should be addressed. Fax: +7-(383)-3307501. E-mail:
[email protected].
diamond crystals (500 µm) as the seed crystals. Experiments were carried out using an 8-6 type multianvil “split-sphere” apparatus with eight steel outer anvils, six tungsten carbide inner anvils and a 300 mm outer diameter of the sphere.18 A high pressure cell had the shape of a tetragonal prism 19 × 19 × 22 mm with a graphite heater 9 mm in diameter and 15.8 mm in height. Temperature was measured in each experiment with a Pt-30% Rh/Pt-6% Rh thermocouple. Pressure was calibrated at room temperature by the change in resistance of standard substances and at high temperatures by the graphite-diamond equilibrium. Details of the calibration of the pressure-temperature (P-T) parameters have been presented elsewhere.19 A graphite rod was machined into a capsule 7.2 mm in diameter and 7 mm high. Sulfur powder together with the seed crystals was pressed into a cylindrical sample and placed into the graphite capsule. Some experiments were performed with addition of Ti, as a getter, or H2O. In the latter case, Pt capsules were utilized. The initial reagents (sulfur, distilled water, and graphite disk) were loaded into a Pt capsule and sealed by arc welding.19 Phase composition of samples after experiments was examined using a powder X-ray diffractometer (XRD). Spontaneously nucleated diamonds and diamonds nucleated on seed crystals were studied by optical and scanning electron (SEM) microscopy. Crystallographic indices of faces of diamond crystals were determined using a two-circle goniometer GD-1. Spontaneous diamond crystals were characterized by infrared (IR) absorption spectroscopy to reveal their impurity content. IR spectra were recorded using a Bruker Vertex 70 Fourier transform infrared (FTIR) spectrometer fitted with a Hyperion 2000 microscope. Inclusions in synthesized diamond crystals were studied by X-ray diffraction (XRD) using a Gandolfi camera and by Raman microspectroscopy. Raman spectra were measured using a Bruker Senterra dispersive Raman microscope.
Results and Discussion The results of experiments performed in the S-C system at 6.3, 7.0, and 7.5 GPa and temperatures ranging from 1550 to 2000 °C are summarized in Table 1. Duration of the experiments was from 3 to 40 h. Long run times were applied to minimize the kinetic effects and yield reliable information on the formation of carbon phases in the experiments. At 6.3 GPa, the results differed considerably depending on temperature. At 1550 °C (S-1), neither spontaneous diamond nucleation nor diamond growth on the seed crystals was established. Small amounts of regrown metastable graphite was found on the capsule walls and seed crystals. After the seed crystals were cleaned from the graphite, traces of corrosion were observed on their surfaces. In run S-2 (1650 °C), metastable graphite and slight diamond
10.1021/cg900265c CCC: $40.75 2009 American Chemical Society Published on Web 04/08/2009
Diamond Crystallization Sulfur-Carbon System
Crystal Growth & Design, Vol. 9, No. 6, 2009 2923 Table 1. Experimental Resultsa
run N
P, GPa
T, °C
time, h
diamond growth on seeds
diamond nucleation
S-1 S-2 S-3 S-4 S-5 S-6 S-7 S-8 S-9 S-10 S-11 S-12
6.3 6.3 6.3 7 7 7 7 7 7 7 7.5 7.5
1550 1650 1750 1550 1600 1700 1750 1800 1900 2000 1600 1700
12 20 9 20 40 40 7 7 3 17 40 18
+ + + + + + + + + + +
+ + + + + + +
mechanism Gr f Dm transformation
R, % Gr f Dm transformation
diamond morphology
TGG
{100}
FG, TGG FG, TGG FG, TGG FG, TGG FG
50 30 40 60
