Synthetic diamonds of various colors, some as small as approximately 2 millimeters, can be created using the high-pressure and high-temperature (HPHT) technique. The infrared absorption spectrum of a type IaB diamond reveals distinct regions: (1) the absorption due to nitrogen impurities, predominantly from B-centers, (2) the characteristic peak associated with platelets, (3) the self-absorption of the diamond lattice itself, and (4) the peaks attributed to hydrogen at 3107 and 3237 cm⁻¹.

Imperfections in the Crystal Lattice of Diamond

The crystal lattice of diamond is not always perfect; it commonly exhibits imperfections . These imperfections can arise from irregularities within the lattice structure itself or from extrinsic impurities, which may be substitutional atoms or atoms lodged in interstitial spaces, introduced either during the diamond’s formation or at a later stage. These defects profoundly influence the material properties of diamond , most notably affecting its diamond color and electrical conductivity . These effects are intrinsically linked to the diamond’s electronic band structure .

The presence and nature of these defects can be ascertained through various spectroscopy techniques. These include electron paramagnetic resonance (EPR), which probes the magnetic properties of unpaired electrons, and luminescence spectroscopy, such as photoluminescence (excited by light) or cathodoluminescence (excited by an electron beam), which analyzes the light emitted by the material. Additionally, infrared (IR), visible, and ultraviolet (UV) light absorption spectra provide crucial information. The absorption spectrum is not only vital for identifying specific defects but also for quantifying their concentration. Furthermore, analyzing these spectra can help distinguish between natural diamonds, synthetic diamonds, and those whose properties have been altered through diamond enhancement processes. [1]

Labeling of Diamond Centers

A long-standing tradition in diamond spectroscopy involves assigning a specific, often numbered, acronym to each defect-induced spectrum (e.g., GR1). While this convention has been generally followed, there are notable exceptions, such as the A, B, and C centers. The nomenclature can be a source of confusion due to several factors: [2]

• Similar Symbols: Some labels are easily confused due to their similarity, such as 3H and H3.
• Duplicate Labels: In some instances, the same labels have been inadvertently assigned to distinct centers identified through different spectroscopic methods, like EPR and optical techniques. For example, the N3 EPR center and the N3 optical center are unrelated. [3]
• Inconsistent Logic: While some acronyms possess a clear rationale, such as N3 (N for natural, indicating observation in natural diamonds) or H3 (H for heated, indicating observation after irradiation and subsequent heating), many others lack a discernible logic. This is particularly true for labels like GR (general radiation), R (radiation), and TR (type-II radiation), where the distinction between them is not clearly defined. [2]

Defect Symmetry

The symmetry of defects within crystalline structures is formally described using point groups . Unlike space groups , which characterize the overall symmetry of a crystal lattice and include translational symmetry, point groups only consider rotational, reflectional, and inversion symmetries. Consequently, there are far fewer point groups than space groups. In diamond, observed defects exhibit symmetries belonging to the following categories: tetrahedral (T_d), tetragonal (D_2d), trigonal (D_3d, C_3v), rhombic (C_2v), monoclinic (C_2h, C_1h, C₂), and triclinic (C₁ or C_S). [2][4]

Understanding the symmetry of a defect is instrumental in predicting its optical properties. For instance, in pure diamond, single-phonon infrared absorption is forbidden due to the presence of an inversion center in its lattice. However, the introduction of any defect, even a highly symmetrical one like a substitutional nitrogen-nitrogen pair, disrupts this symmetry. This disruption leads to defect-induced infrared absorption, which serves as a primary method for quantifying defect concentrations in diamond. [2]

In synthetic diamonds produced via high-pressure, high-temperature (HPHT) synthesis [5] or chemical vapor deposition , [6][7] defects with symmetry lower than tetrahedral tend to align themselves along the crystal growth direction. This phenomenon of alignment has also been observed in other materials, such as gallium arsenide , [8] indicating it is not unique to diamond.

Extrinsic Defects

Elemental analyses of diamonds reveal a diverse array of impurities. However, many of these originate from minute inclusions of foreign materials, often too small to be detected with an optical microscope . Furthermore, it is possible to introduce virtually any element into the diamond lattice through ion implantation . More fundamentally, certain elements can be incorporated directly into the diamond lattice as individual atoms or small clusters during the diamond growth process. As of 2008, these elements include nitrogen , boron , hydrogen , silicon , phosphorus , nickel , cobalt , and potentially sulfur . While manganese [9] and tungsten [10] have been definitively identified in diamond, their presence might be attributable to foreign inclusions rather than lattice incorporation. Initial reports of isolated iron in diamond [11] were later reinterpreted as micro-particles of ruby formed during the synthesis process. [12] Oxygen is presumed to be a significant impurity in diamond, [13] yet it has not been spectroscopically confirmed within the diamond lattice itself. [Citation needed] Two electron paramagnetic resonance centers, OK1 and N3, were initially attributed to nitrogen-oxygen complexes, but later studies suggested they might be related to titanium. [14] However, these assignments are indirect, and the concentrations involved are relatively low (on the order of parts per million). [15]

Nitrogen

Nitrogen stands out as the most prevalent impurity in diamond, capable of constituting up to 1% of a diamond’s mass. [13] Early investigations assumed all lattice defects were structural anomalies, but subsequent research established the widespread presence of nitrogen in various configurations. Most nitrogen atoms integrate into the diamond lattice as individual atoms, implying that nitrogen-containing molecules dissociate before incorporation. Nevertheless, molecular nitrogen can also be incorporated into the diamond lattice. [16]

The absorption of light and other material characteristics of diamond are significantly influenced by the quantity and aggregation state of nitrogen. While all aggregated nitrogen forms contribute to absorption in the infrared spectrum, diamonds with aggregated nitrogen are typically colorless, exhibiting minimal absorption in the visible spectrum . [2] The four primary forms of nitrogen are as follows:

C-nitrogen center

The C center represents electrically neutral, single substitutional nitrogen atoms within the diamond lattice. These centers are readily detectable via electron paramagnetic resonance (where they are confusingly referred to as P1 centers) [17] and impart a deep yellow to brown coloration. Diamonds containing predominantly C centers are classified as type Ib and are often referred to as “canary diamonds,” which are exceptionally rare in gem quality. The majority of synthetic diamonds produced using the HPHT method incorporate substantial amounts of nitrogen in the C form, with the nitrogen originating from the surrounding atmosphere or the graphite source material. Even a concentration of one nitrogen atom per 100,000 carbon atoms can produce a distinct yellow hue. [18] As nitrogen atoms possess five valence electrons (one more than the carbon atoms they replace), they function as “deep donors .” Each substituting nitrogen atom contributes an extra electron, creating a donor energy level within the band gap of diamond. Incident light with energy exceeding approximately 2.2 eV can excite these donor electrons into the conduction band , resulting in the observed yellow color. [19]

The C center exhibits a characteristic infrared absorption spectrum, featuring a sharp peak at 1344 cm⁻¹ and a broader feature at 1130 cm⁻¹. Absorption at these specific peaks is routinely employed to quantify the concentration of single nitrogen atoms. [20] An alternative method, utilizing UV absorption around 260 nm, was later found to be unreliable. [19]

When acceptor defects are present in diamond, they can ionize the fifth electron of the C center, transforming it into a C+ center. This C+ center displays a distinct IR absorption spectrum with a sharp peak at 1332 cm⁻¹ and broader, weaker peaks at 1115, 1046, and 950 cm⁻¹. [21]

A-nitrogen center

The A center is considered the most common defect found in natural diamonds. It is characterized by a neutral, nearest-neighbor pair of nitrogen atoms substituting for carbon atoms in the lattice. The A center exhibits UV absorption with a threshold at approximately 4 eV (corresponding to 310 nm), rendering it invisible to the naked eye and thus not contributing to coloration. Diamonds where nitrogen predominantly exists in the A form are classified as type IaA. [22]

Although the A center is diamagnetic , ionization by UV light or strong acceptors can induce an electron paramagnetic resonance spectrum, designated W24. Analysis of this spectrum provides unambiguous evidence for the N=N structure. [23]

The IR absorption spectrum of the A center lacks sharp features and differs significantly from those of the C or B centers. Its most prominent peak, located at 1282 cm⁻¹, is routinely used to estimate the concentration of nitrogen present in the A form. [24]

B-nitrogen center

There is a general consensus among researchers that the B center (sometimes referred to as B1) comprises a carbon vacancy surrounded by four nitrogen atoms substituting for carbon atoms. [1][2][25] This model aligns with various experimental observations, although direct spectroscopic confirmation is still lacking. Diamonds where nitrogen primarily forms B centers are rare and are classified as type IaB. Most gem-quality diamonds contain a mixture of A and B centers, along with N3 centers.

Similar to A centers, B centers do not induce color, and no specific UV or visible absorption bands are attributed to them. Early assignments of the N9 absorption system to the B center have since been refuted. [26] The B center is characterized by a distinct IR absorption spectrum (as depicted in the infrared absorption figure) featuring a sharp peak at 1332 cm⁻¹ and a broader feature at 1280 cm⁻¹. This latter peak is commonly used to quantify the nitrogen concentration in the B form. [27]

It is worth noting that many optical peaks observed in diamond can have similar spectral positions, leading to considerable confusion among gemologists. Spectroscopists rely on the entire spectral profile, rather than a single peak, for accurate defect identification, and also consider the diamond’s growth history and any processing it may have undergone. [1][2][25]

N3 nitrogen center

The N3 center is structurally defined as three nitrogen atoms surrounding a vacancy. Its concentration is consistently a fraction of that found for A and B centers. [28] The N3 center is paramagnetic , allowing its structure to be well-established through the analysis of the P2 EPR spectrum. [3] This defect gives rise to a characteristic absorption and luminescence line at 415 nm, meaning it does not cause coloration on its own. However, the N3 center is invariably accompanied by the N2 center, which possesses an absorption line at 478 nm and does not exhibit luminescence. [29] Consequently, diamonds with a high concentration of N3/N2 centers typically appear yellow.

Boron

Diamonds containing boron as a substitutional impurity are designated as type IIb. This type represents only about one percent of all natural diamonds and is most frequently encountered in blue to grey hues. [30] Boron acts as an acceptor in diamond: boron atoms have one fewer valence electron than the carbon atoms they replace. This deficiency creates an electron hole within the band gap, capable of accepting an electron from the valence band . This process facilitates the absorption of red light. Due to the relatively small energy (0.37 eV) [31] required for an electron to transition from the valence band, holes can be thermally liberated from boron atoms to the valence band even at room temperature. These mobile holes can move under an electric field , rendering the diamond electrically conductive and classifying it as a p-type semiconductor . Remarkably few boron atoms are needed to achieve this conductivity—typically, one boron atom per million carbon atoms suffices.

Boron-doped diamonds exhibit transparency down to approximately 250 nm and absorb certain red and infrared wavelengths, contributing to their characteristic blue color. Upon exposure to short-wavelength ultraviolet light, they may exhibit blue [phosphorescence]. [31] Beyond optical absorption, boron acceptors have also been detected using electron paramagnetic resonance. [32]

Phosphorus

Phosphorus can be intentionally introduced into diamonds grown via chemical vapor deposition (CVD) at concentrations reaching up to approximately 0.01%. [33] Within the diamond lattice, phosphorus substitutes for carbon atoms. [34] Similar to nitrogen, phosphorus possesses one more valence electron than carbon, thereby acting as a donor. However, the ionization energy of phosphorus (0.6 eV) [33] is significantly lower than that of nitrogen (1.7 eV) [35] and is small enough to allow for thermal ionization at room temperature. This favorable property of phosphorus in diamond makes it attractive for electronic applications, including the development of UV light-emitting diodes ([LEDs]) emitting at 235 nm. [36]

Hydrogen

Hydrogen is a critically important impurity in many semiconductors, including diamond. Hydrogen-related defects manifest differently in natural diamonds compared to synthetic diamond films. These films, often produced using various chemical vapor deposition (CVD) techniques, are synthesized in a hydrogen-rich atmosphere (with a hydrogen-to-carbon ratio typically exceeding 100) and are subjected to significant bombardment by plasma ions during growth. Consequently, CVD diamond invariably contains substantial amounts of hydrogen and lattice vacancies. In polycrystalline films, much of the hydrogen can be found at the grain boundaries or within non-diamond carbon inclusions. Within the diamond lattice itself, hydrogen-vacancy [37] and hydrogen-nitrogen-vacancy [38] complexes have been identified in their negative charge states through electron paramagnetic resonance . Additionally, numerous hydrogen-related IR absorption peaks have been documented. [39]

Experimental evidence confirms that hydrogen can passivate electrically active boron [40] and phosphorus [41] impurities. This passivation process is believed to result in the formation of shallow donor centers. [42]

In natural diamonds, several hydrogen-related IR absorption peaks are commonly observed, with the most intense ones appearing at 1405, 3107, and 3237 cm⁻¹ (refer to the IR absorption figure). The precise microscopic structure of the corresponding defects remains unknown, and it is even uncertain whether these defects originate within the diamond lattice or from foreign inclusions. The grey coloration observed in some diamonds from the Argyle mine in Australia is often associated with these hydrogen-related defects, although this attribution is yet to be definitively proven. [43]

Nickel, Cobalt, and Chromium

During the high-pressure, high-temperature (HPHT) synthesis of diamonds, metals such as nickel, cobalt, or chromium are typically added to the growth medium to facilitate the catalytic conversion of graphite into diamond. This process often results in the formation of metallic inclusions. However, individual nickel and cobalt atoms can also incorporate into the diamond lattice, a fact confirmed by characteristic hyperfine structure observed in electron paramagnetic resonance , optical absorption, and photoluminescence spectra. [44] The concentration of isolated nickel atoms can reach as high as 0.01%. This incorporation is particularly noteworthy given the significant size difference between carbon and these transition metal atoms and the inherent rigidity of the diamond lattice. [2][failed verification][citation needed]

A multitude of Ni-related defects have been identified using electron paramagnetic resonance , [5][46] optical absorption, and [photoluminescence], [5][46] in both synthetic and natural diamonds. [43] Three primary structural configurations are discernible: substitutional Ni, [47] nickel-vacancy complexes, [48] and nickel-vacancy complexes decorated with one or more substitutional nitrogen atoms. [46] The “nickel-vacancy” structure, also termed “semi-divacancy,” is characteristic of many large impurities in diamond and silicon (e.g., tin in silicon [49]). Its formation mechanism is generally understood as follows: a large nickel atom occupies a substitutional site, then expels a neighboring carbon atom (creating a vacancy nearby), and subsequently shifts to a position between the original substitutional site and the newly formed vacancy.

While cobalt and nickel share similar physical and chemical properties, the concentrations of isolated cobalt within diamond are substantially lower than those of nickel, typically in the parts-per-billion range. Several defects related to isolated cobalt have been detected via electron paramagnetic resonance [50] and [photoluminescence], [5][51] although their precise structures remain undetermined.

A chromium-related optical center was reported following ion implantation and subsequent annealing of Type IIA synthetic diamonds. [52][53] However, a later study, replicating the annealing conditions but omitting the chromium implantation, cast doubt on the original attribution of this defect center to chromium. [54]

Silicon, Germanium, Tin, and Lead

The semi-divacancy model, where a large impurity atom (such as Ni, Co, Si, S, etc.) replaces two carbon atoms, is considered for large impurities in diamond. The precise bonding details with the diamond lattice in this configuration are still uncertain.

Silicon is a common impurity found in diamond films grown by chemical vapor deposition (CVD), typically originating from the silicon substrate or from silica components within the CVD reactor. It has also been observed in natural diamonds in a dispersed form. [55] Isolated silicon defects within the diamond lattice have been identified through a sharp optical absorption peak at 738 nm [56] and via electron paramagnetic resonance . [57] Consistent with other large impurities, the predominant form of silicon in diamond has been identified as a silicon-vacancy (Si-V) complex, essentially a semi-divacancy site. [57] This center functions as a deep donor, with an ionization energy of 2 eV, rendering it unsuitable for most electronic applications. [58]

Si-vacancies constitute a minor fraction of the total silicon present. It is hypothesized, though not definitively proven, that much of the silicon substitutes directly for carbon atoms, making it undetectable by most spectroscopic methods due to the similar electronic configurations of silicon and carbon atoms. [59]

Germanium, tin, and lead are generally absent in natural diamonds but can be introduced during the growth process or via subsequent ion implantation. These impurities can be detected optically through the corresponding germanium-vacancy [60], tin-vacancy, and lead-vacancy centers, [61] which exhibit properties similar to those of the Si-vacancy center . [62]

Similar to nitrogen-vacancy (N-V) centers, Si-V, Ge-V, Sn-V, and Pb-V complexes are all considered to have potential applications in quantum computing. [63][61]

Sulfur

Around the year 2000, considerable effort was directed towards doping synthetic CVD diamond films with sulfur, aiming to achieve n-type conductivity with a low activation energy . While initial reports claimed success, [64] these findings were later challenged, [65] as the observed conductivity was p-type, not n-type, and was attributed to residual boron, a highly effective p-type dopant in diamond, rather than sulfur.

To date (as of 2009), the only robust evidence for isolated sulfur defects in diamond comes from hyperfine interaction structures observed in electron paramagnetic resonance . The associated center, known as W31, has been detected in natural type-Ib diamonds at low concentrations (parts per million). It has been assigned to a sulfur-vacancy complex, again, similar to nickel and silicon, occupying a semi-divacancy site. [66]

Intrinsic Defects

The most straightforward method for generating intrinsic defects in diamond involves displacing carbon atoms through irradiation with high-energy particles, such as alpha (helium), beta (electrons), gamma rays, protons, neutrons, or ions. This irradiation can occur either in a laboratory setting or naturally (see Diamond enhancement – Irradiation ). The primary defects produced are Frenkel defects , where carbon atoms are knocked from their regular lattice sites into interstitial sites , leaving behind vacancies. A crucial distinction between vacancies and interstitials in diamond is their mobility: interstitials are mobile even at liquid nitrogen temperatures during irradiation, [67] whereas vacancies only begin to migrate at temperatures around 700 °C.

Vacancies and interstitials can also be created in diamond through plastic deformation, albeit in significantly lower concentrations.

Isolated Carbon Interstitial

An isolated interstitial carbon atom has never been experimentally observed in diamond and is considered unstable. Its interaction with a regular carbon atom in the lattice results in the formation of a “split-interstitial.” This defect involves two carbon atoms sharing a lattice site and forming covalent bonds with neighboring carbon atoms. This configuration has been extensively characterized using electron paramagnetic resonance (designated the R2 center) [68] and optical absorption techniques. [69] Notably, unlike most other defects in diamond, it does not produce photoluminescence .

Interstitial Complexes

During irradiation, the isolated split-interstitial can migrate through the diamond crystal. When it encounters other interstitials, it aggregates to form larger complexes, such as di-interstitials (pairs of split-interstitials) and tri-interstitials. These complexes have been identified through electron paramagnetic resonance (R1 and O3 centers), [70][71] as well as optical absorption and photoluminescence measurements. [72]

Vacancy-Interstitial Complexes

Most high-energy particles not only displace carbon atoms from their lattice positions but also impart sufficient excess energy for rapid migration through the lattice. However, when gentler irradiation methods, such as gamma irradiation, are employed, the imparted energy is minimal. Consequently, the resulting interstitials remain in close proximity to their original vacancies, forming vacancy-interstitial pairs that can be identified through optical absorption measurements. [72][73][74]

Vacancy-di-interstitial pairs have also been produced, albeit through a different mechanism involving electron irradiation. [75] In this scenario, individual interstitials migrate during irradiation and aggregate to form di-interstitials, a process that preferentially occurs near existing lattice vacancies.

Isolated Vacancy

The isolated vacancy is one of the most extensively studied defects in diamond, both experimentally and theoretically. Its most significant practical characteristic is its optical absorption, similar to [F-centers] in other materials. This absorption is responsible for imparting a green, and sometimes even a green-blue, color to pure diamond. A defining feature of this absorption is a series of sharp spectral lines known as GR1-8, with the GR1 line at 741 nm being the most prominent and important. [73]

The vacancy acts as a deep electron donor/acceptor, with its electronic properties dependent on its charge state. The energy level for the +/0 charge states is located at 0.6 eV, and for the 0/- states, it is at 2.5 eV above the valence band . [76]

Multivacancy Complexes

Upon annealing pure diamond at temperatures around 700 °C, vacancies become mobile and aggregate to form divacancies. These divacancies are characterized by specific optical absorption signatures and can be detected by electron paramagnetic resonance . [77] Similar to single interstitials, divacancies do not produce photoluminescence. At higher annealing temperatures, around 900 °C, divacancies anneal out, leading to the formation of multivacancy chains. These are detectable by EPR [78] and are presumed to include hexavacancy rings. The latter are expected to be invisible to most spectroscopic techniques and have, indeed, not been detected to date. [78] The annealing process involving vacancies leads to a change in diamond color from green to yellow-brown. A similar mechanism, involving vacancy aggregation, is also believed to be responsible for the brown coloration observed in plastically deformed natural diamonds. [79]

Dislocations

Dislocations represent the most common structural defect found in natural diamonds. The two primary types of dislocations are the glide set and the shuffle set. In the glide set, bonds break between atomic layers that are not directly aligned. In the shuffle set, the breaks occur between atoms of the same index. The presence of dislocations results in dangling bonds , which introduce energy levels within the band gap, thereby enabling light absorption. Broadband blue photoluminescence has been reliably correlated with dislocations through direct observation in an [electron microscope]. However, it has been noted that not all dislocations exhibit luminescence, and there is no consistent correlation between dislocation type and the emission parameters. [81]

Platelets

Most natural diamonds contain extended planar defects aligned along the <100> crystallographic planes, referred to as “platelets.” These defects range in size from nanometers to many micrometers, with larger ones being readily visible under an optical microscope due to their luminescence. [83] For an extended period, platelets were tentatively associated with large nitrogen complexes, believed to be nitrogen sinks formed during the high-temperature synthesis of diamond. However, direct measurements using EELS (an analytical electron microscopy technique) revealed minimal nitrogen content within the platelets. [82] The currently accepted model posits that platelets consist of a large, regular arrangement of carbon interstitials. [84]

Platelets produce sharp absorption peaks in IR spectra at positions between 1359–1375 and 330 cm⁻¹. Notably, the position of the former peak varies with platelet size. [82][85] As observed with dislocations, broad photoluminescence centered around 1000 nm has been linked to platelets, confirmed by direct observation in an electron microscope. Studies of this luminescence suggest that platelets possess a “bandgap” of approximately 1.7 eV. [86]

Voidites

Voidites are nanometer-sized, octahedral clusters found in many natural diamonds, as revealed by electron microscopy . [87] Laboratory experiments have demonstrated that annealing type-IaB diamond at high temperatures and pressures (exceeding 2600 °C) leads to the fragmentation of platelets and the formation of dislocation loops and voidites. This suggests that voidites are a consequence of the thermal degradation of platelets. Unlike platelets, voidites have been found to contain significant amounts of nitrogen, in a molecular form. [88]

Interaction Between Intrinsic and Extrinsic Defects

Extrinsic defects (impurities) and intrinsic defects (vacancies, interstitials) can interact to form new defect complexes. Such interactions typically occur when a diamond containing extrinsic defects undergoes either plastic deformation or is subjected to irradiation followed by annealing.

The interaction of vacancies and interstitials with nitrogen is particularly significant. Carbon interstitials react with substitutional nitrogen atoms to form a bond-centered nitrogen interstitial, which exhibits strong IR absorption at 1450 cm⁻¹. [89] Vacancies are efficiently trapped by A, B, and C nitrogen centers. The trapping rate is highest for C centers, significantly lower for A centers (8 times less), and even lower for B centers (30 times less). [90] The C center (single nitrogen atom) traps a vacancy to form the well-known nitrogen-vacancy center , which can exist in neutral or negatively charged states. [91][92] The negatively charged state holds potential applications in quantum computing . When A and B centers trap a vacancy, they form the corresponding 2N-V (H3 [93] and H2 [94] centers, where H2 is essentially a negatively charged H3 center [95]) and the neutral 4N-2V (H4 center [96]) complexes. The H2, H3, and H4 centers are noteworthy due to their prevalence in many natural diamonds and their potential to alter diamond color (H3 or H4 causing yellow hues, H2 causing green hues).

Boron interacts with carbon interstitials to form a neutral boron-interstitial complex, characterized by a sharp optical absorption at 0.552 eV (2250 nm). [76] To date (as of 2009), there is no documented evidence for complexes formed between boron and vacancies. [25]

In contrast, silicon does react with vacancies, resulting in the optical absorption observed at 738 nm. [97] The proposed mechanism involves the trapping of migrating vacancies by substitutional silicon, leading to the formation of the Si-V (semi-divacancy) configuration. [98]

A similar mechanism is anticipated for nickel, for which both substitutional and semi-divacancy configurations have been reliably identified (see “Nickel, Cobalt, and Chromium” subsection). An unpublished study involving electron irradiation and subsequent annealing of diamonds rich in substitutional nickel, followed by meticulous optical measurements at each annealing step, yielded no evidence for the creation or enhancement of Ni-vacancy centers. [48]

See Also

• Chemical vapor deposition of diamond
• Crystallographic defect
• Diamond color
• Diamond enhancement
• Gemstone irradiation
• Material properties of diamond
• Nitrogen-vacancy center
• Synthetic diamond