The stages of nitrogen aggregation in diamond

Characterization of the nitrogen state in HPHT diamonds grown in an Fe–C melt with a low sulfur addition

This paper reports the results of high-pressure high-temperature (HPHT) diamonds growing in an Fe–C melt with introduction of 1 wt% sulfur.

The CRYSTAL code, 1976-2020 and beyond, a long story

CRYSTAL is a periodic ab initio code that uses a Gaussian-type basis set to express crystalline orbitals (i.e., Bloch functions). The use of atom-centered basis functions allows treating 3D (crystals), 2D (slabs), 1D (polymers), and 0D (molecules) systems on the same grounds. In turn, all-electron calculations are inherently permitted along with pseudopotential strategies. A variety of density functionals are implemented, including global and range-separated hybrids of various natures and, as an extreme case, Hartree-Fock (HF). The cost for HF or hybrids is only about 3-5 times higher than when using the local density approximation or the generalized gradient approximation. Symmetry is fully exploited at all steps of the calculation. Many tools are available to modify the structure as given in input and simplify the construction of complicated objects, such as slabs, nanotubes, molecules, and clusters. Many tensorial properties can be evaluated by using a single input keyword: elastic, piezoelectric, photoelastic, dielectric, first and second hyperpolarizabilities, etc. The calculation of infrared and Raman spectra is available, and the intensities are computed analytically. Automated tools are available for the generation of the relevant configurations of solid solutions and/or disordered systems. Three versions of the code exist: serial, parallel, and massive-parallel. In the second one, the most relevant matrices are duplicated on each core, whereas in the third one, the Fock matrix is distributed for diagonalization. All the relevant vectors are dynamically allocated and deallocated after use, making the code very agile. CRYSTAL can be used efficiently on high performance computing machines up to thousands of cores.

Mixed-Habit Type Ib-IaA Diamond from an Udachnaya Eclogite

The variety of morphology and properties of natural diamonds reflects variations in the conditions of their formation in different mantle environments. This study presents new data on the distribution of impurity centers in diamond type Ib-IaA from xenolith of bimineral eclogite from the Udachnaya kimberlite pipe. The high content of non-aggregated nitrogen C defects in the studied diamonds indicates their formation shortly before the stage of transportation to the surface by the kimberlite melt. The observed sectorial heterogeneity of the distribution of C- and A-defects indicates that aggregation of nitrogen in the octahedral sectors occurs faster than in the cuboid sectors.

A theoretical study of substitutional boron-nitrogen clusters in diamond

Substitutional clusters of multiple light element dopants are a promising route to the elusive shallow donor in diamond. To understand the behaviour of co-dopants, this report presents an extensive first principles study of possible clusters of boron and nitrogen. We use periodic hybrid density functional calculations to predict the geometry, stability and electronic excitation energies of a range of clusters containing up to five N and/or B atoms. Excitation energies from hybrid calculations are compared to those from the empirical marker method, and are in good agreement. When a boron-rich or nitrogen-rich cluster consists of three to five atoms, the minority dopant element-a nitrogen or boron atom respectively-can be in either a central or peripheral position. We find B-rich clusters are most stable when N sits centrally, whereas N-rich clusters are most stable with B in a peripheral position. In the former case, excitation energies mimic those of the single boron acceptor, while the latter produce deep levels in the band-gap. Implications for probable clusters that would arise in high-pressure high-temperature co-doped diamond and their properties are discussed.

Preparation of “natural” diamonds by HPHT annealing of synthetic diamonds

In this paper, IIa, Ib, N-doped and N–H co-doped diamonds were studied, and the interaction mechanisms between hydrogen and nitrogen in the diamonds were investigated in detail.

Shallow donor state due to nitrogen-hydrogen complex in diamond

Based on an ab initio calculation, we propose a possible shallowing of a nitrogen (N) donor in diamond, in contrast to the traditional thinking that it is very deep. A complex defect of N and hydrogen (H), N-H-N, should be much shallower than an isolated N donor. A qualitative scenario for formation of the N-H-N defects is presented. The existence of this complex is strongly suggested by a recent discovery of a new muonium center in N-rich diamond.

The structure and mechanism of formation of platelets in natural type Ia diamond

Observations have been made on medium- and large-sized platelets in a natural type Ia diamond. Attempts have been made to detect any regularity in the structure of the platelets by means of electron diffraction, but none was found. The displacement associated with the platelets has been determined to be extrinsic, normal to the plane of the platelet and of magnitude 0.4 ‹010›. This value has led to the proposal of a model for the platelet structure in which the platelet consists of an interstitial layer of carbon atoms pentagonally bonded to the surrounding diamond matrix. The way in which the detailed bonding may be varied, while complete tetrahedral bonding is maintained, can lead to irregular structures consistent with the diffraction evidence. Possible mechanisms for producing interstitial carbon atoms are considered and the role of nitrogen in this context and in providing possible nuclei for platelet growth is discussed.

Conversion of platelets into dislocation loops and voidite formation in type IaB diamonds

Type la natural diamonds have been heated in the temperature range of 2400-2700°C under stabilizing pressures. The specimens studied are mainly regular type IaB diamonds. Transmission electron microscopy studies of treated speci­mens show that platelets are converted to interstitial ½ a 0 <011> dislocation loops; voidites are also formed. When all the platelets have been converted, the ex­perimental features associated with them also disappear, i. e. the X-ray extra reflections (spikes), the B' local-mode absorption and the lattice absorption in the one-phonon region termed the D spectrum. It is discovered that when diamonds are heated under graphite-stable rather than diamond-stable conditions, the rate of conversion is considerably enhanced; for instance, at 2650°C there is an increase in the rate of about three orders of magnitude. This enhancement is considered to be due to the instability of the diamond structure itself and a reason for this enhancement is suggested.

The kinetics of the aggregation of nitrogen atoms in diamond

Most natural diamonds contain nitrogen as the main impurity. In some rare diamonds (termed type lb) the nitrogen is mainly present as single substitutional atoms. However, the large majority of diamonds (termed type Ia) contain the nitrogen atoms in various forms of aggregate. The different types of aggregate are the A centre (two nitrogen atoms), the N3 centre (three nitrogen atoms) and the B centre (a larger number of nitrogen atoms). These defects give characteristic absorption spectra in the infrared except for the N3 centre which gives a characteristic absorption in the visible region. In this type of diamond there are usually platelets present in the cube planes and these defects can be examined by transmission electron microscopy and infrared absorption techniques. The paper reports work in which synthetic diamonds containing a high concentration of single nitrogen atoms have been heated in a temperature range of 1500 to 2500°C under various pressures. These heat treatments have resulted in the formation of all the types of aggregate that are found in natural type la diamonds. Also some natural diamonds have been heated up to 2700°C under pressure and the ratio of the concentration of the A centres to that of the B centres has been changed. Information has been obtained on the kinetics of the aggregation process. This information has been used to give an approximate estimate of the length of time that natural diamonds spent in the Upper Mantle prior to being ejected to the surface of the Earth. It is suggested that the type I a diamonds spent between about 200 and 2000 Ma in the Upper Mantle at temperatures of between 1000 and 1400°C. Type l b diamonds either spent a comparable time in the Upper Mantle at about 800°C or a considerably shorter period if they encountered temperatures in the same range as the type la diamonds.