Efeito da adição do nitrogênio como impureza no diamante

1. pubs.acs.org/crystalPublished on Web 06/02/2010r 2010 American Chemical Society
DOI: 10.1021/cg100322p
2010, Vol. 10
3169–3175
Effect of Nitrogen Impurity on Diamond Crystal Growth Processes
Yuri N. Palyanov,*,†,‡
Yuri M. Borzdov,†
Alexander F. Khokhryakov,†
Igor N. Kupriyanov,†
and Alexander G. Sokol†
†
Sobolev Institute of Geology and Mineralogy, Siberian Branch of Russian Academy of Sciences,
Koptuyg ave 3, Novosibirsk 630090, Russia, and ‡
Novosibirsk State University, Novosibirsk 630090, Russia
Received March 11, 2010; Revised Manuscript Received May 11, 2010
ABSTRACT: In this paper, we report on the influence of nitrogen concentration in metal melts on the growth processes,
morphology, and defect-and-impurity structure of diamond crystals. In two series of experiments, the concentration of nitrogen
in the growth system was varied by adding Fe3N and CaCN2 to the charge; the other parameters and conditions of the growth
were constant: FeNiC system, P = 5.5 GPa, T = 1400 °C, and duration of 65 h. It has been found that, with increasing nitrogen
concentration (CN) in the metal melt from 0.005 to 0.6 atom %, the growth of single crystal diamond is followed by formation of
aggregates of block twinned crystals and then by crystallization of metastable graphite. At the stage of single crystal growth,
an increase in CN results in an increase in nitrogen impurity concentration in diamond crystals from about 200 ppm to
approximately 1100 ppm, an increase in density of dislocations, twin lamellae, and internal strains, and a change in crystal
morphology. Further increases in CN result in formation of aggregates of block crystals with nitrogen concentration around
120-300 ppm. At nitrogen concentration in the melt higher than a certain critical value, nucleation and growth of diamond are
terminated and graphite crystallizes in the diamond stability field.
Introduction
Nitrogen is the dominant impurity in both natural and
synthetic diamonds. Most of the physical properties of dia-
mond are significantly influenced by nitrogen defects. In the
diamond lattice, nitrogen may be present as single substitu-
tional atoms (C centers) or in the form of aggregates of sub-
stitutional atoms (A and B centers). Combination of the major
nitrogen centers with the intrinsic point defects, i.e. vacancies
and interstitials, gives rise to a diverse variety of optical and
paramagnetic centers.1
The importance of nitrogen in diamond
is well illustrated by the fact that the modern classification of
diamond, initially suggested by Robertson, Fox, and Martin2
in 1934, relies on the concentration and structural form of the
nitrogen impurity. Depending on nitrogen concentration, dia-
monds are divided into type I, nitrogen-containing, and type II,
containing less than ca. 1 ppm of nitrogen. Type I diamonds are
further subdivided into type Ib, containing nitrogen impurity in
the form of single substitutional atoms (C-centers), and type Ia,
containing aggregated nitrogen forms (A- and B-centers). An
overwhelming majority of natural diamonds corresponds to
Ia type with maximum nitrogen concentrations up to 3000-
5000 ppm.3,4
Exceptionally high nitrogen concentrations up to
11000 ppm were reported for Kokchetav diamonds.5
Synthetic diamonds produced in metal-carbon systems typi-
cally contain 200-300 ppm of nitrogen. Addition of nitrogen-
containing compounds to the metal solvent-catalysts enabled
production of diamond crystals with nitrogen concentrations of
about 800-900 ppm.6,7
Recently, diamonds with the nitrogen
content ranging from 1000 to 2400 ppm were synthesized in
metal-carbon systems with NaN3 and Ba(N3)2 additives.8-10
Relatively high concentrations of nitrogen have also been found
for diamonds crystallized in nonmetallic systems. Kanda et al.11
synthesized diamonds containing 1200-1900 ppm of nitrogen
using a Na2SO4 solvent-catalyst and a BN container. Later, it
was shown that diamond synthesized from nonmetallic solvents
may contain high nitrogen concentrations even if no nitrogen
was deliberately added to the growth system. For instance, dia-
mondsproducedusing sulfur,12,13
sulfides,14
andcarbonates15
as
the solvents or in complex carbonate-oxide-sulfide systems16
were found to contain 1000-1500 ppm of nitrogen. The maxi-
mum nitrogen concentration measured for diamonds synthe-
sizedintheFe3N-Csystemwasapproximately3300ppm,which
is the highest value reported so far for synthetic diamonds.17
Taking into consideration the important role of nitrogen
in the crystallization processes of natural and synthetic dia-
monds, as well as its determining influence upon diamond
properties,wehave undertaken anexperimental study into the
effect of nitrogen concentration in the crystallization medium
(Fe-Ni-C system) on the growthprocesses and real structure
of diamond crystals.
Experimental Section
Experiments on diamond crystal growth via the temperature gra-
dient method were performed using a pressless high-pressure appara-
tus of the split-sphere type. Equipment of this type has been applied for
diamond crystal growth.18-20
The modernized high-pressure techni-
que(Figure1) andmethodsusedinthisstudyallowproductionoflarge
high quality diamond crystals weighing up to 6 carats in growth cycles
up to 300 h long.21,22
A Ni0.7Fe0.3 alloy was used as a solvent-catalyst.
Graphite (99.99% purity) and powders of Ni and Fe (99.96% purity)
were used as starting reagents. The concentration of nitrogen in the
starting materials, as provided by the producers’ specifications, was
less than 0.001 atom %. To vary the nitrogen concentration in the
growth system, nitrogen-containing compounds, Fe3N, and CaCN2
were added to the charge. Iron nitride was synthesized from carbonyl
iron with a purity of 99.999% by nitriding in a stream of ammonia
(NH3) inarunningquartzreactorat400-500°C.AnX-raydiffraction
analysis made after the nitridation revealed only the Fe3N phase.
CaCN2 was 99.9% pure. In the experiments, only the concentration of
nitrogen-containing additives was varied and the other growth condi-
tions were kept constant: pressure (P) of 5.5 GPa, temperature (T) of
1400 °C, duration of 65 h, identical sample assembly, (111) orienta-
tion of the seed crystals (Figure 2).
*To whom correspondence should be addressed. E-mail: palyanov@
uiggm.nsc.ru. Fax: þ7-(383)-3307501.

2. 3170 Crystal Growth & Design, Vol. 10, No. 7, 2010 Palyanov et al.
Produced diamond crystals were studied using optical and scan-
ning electron microscopy. Infrared spectra were recorded using a
Bruker Vertex 70 Fourier transform infrared (FTIR) spectrometer
fitted with a Hyperion 2000 microscope. Concentrations of nitrogen
in the form of A and C centers were derived from the infrared spectra
using standard procedures.23
Absorption spectra in the visible range
were measured using a Shimadzu UV-3100 spectrophotometer. Ex-
tended defects, such as dislocations, stacking faults, and microtwins,
were examined using selective etching. Details for this technique and
its application to study extended defects in diamond have been
presented in our previous paper.24
Results and Discussion
To study the effect of nitrogen concentration on the
diamond crystallization processes and diamond crystal real
structure, two series of experiments with the addition of the
nitrogen-containing compounds Fe3N and CaCN2 were per-
formed. The parameters and results of the experiments are
summarized in Table 1. The concentration of nitrogen (CN)
introduced in the form of Fe3N or CaCN2 is given in atomic
percents relative to the weight of the metal solvent. Four
different types of growth were established in the experiments,
which will be referred to hereinafter as single crystal diamond
(SCD) growth, block crystal diamond (BCD) growth, aggre-
gate diamond crystal (ADC) growth, and graphite (GR)
growth.Areferenceexperiment(run128/2),performedwithout
nitrogen additives, yielded a diamond crystal weighing 240.5 mg
(1.2 ct). The crystal has a cuboctahedral habit and exhibits
{111}, {100}, {311}, and {110} faces, given in decreasing order
of development. The total nitrogen concentration measured
for the base {111} growth sector is about 225 ppm, caused
by nitrogen impurity in the starting reagents. The crystal had
a brownish-yellow color typical for type Ib diamonds.
Diamond crystals grown under similar conditions contain
dislocations and stacking faults with an average density of
about 2 Â 103
cm-2
and (2-7.5) Â 102
cm-2
, respectively.
Figure 3 shows the concentration of nitrogen impurity in
diamond crystals and the rate of carbon mass transfer as
functions of the nitrogen additive concentration (CN). Note,
the CN value for the reference experiment (128/2) performed
without nitrogen additives is taken to be 0.001 atom %, as
provided by specifications for the starting reagents. It follows
that the general regularities of the crystal growth are common
for the Fe3N and CaCN2 additives. As CN increases from
0.005 to 0.3-0.4 atom %, two main stages of diamond
crystallization can be distinguished: growth of single crystal
diamond (SCD) and formation of aggregates of diamond
crystals (ADC). At nitrogen concentrations higher than a
certain critical value (0.4 atom %), nucleation and growth of
diamond are completely terminated. Under these conditions
in the field of thermodynamic stability of diamond, only
graphite crystallizes. Since the experimental series with Fe3N
and CaCN2 additives shows common regularities of crystal
growth, it is reasonable to consider the results obtained for
each growth stage.
The stage of single crystal diamond (SCD) growth occurs at
CN in the range of 0.005-0.02 atom % for Fe3N additive and
0.05-0.2 atom % for CaCN2 additive. At this stage with
increasing nitrogen content in the melt, the concentration of
nitrogen impurity in diamond crystals increases. As follows
from the infrared absorption measurements (Figure 4), nitro-
gen predominantly incorporates in the form of single sub-
stitutional atoms, C-centers (an IR band peaking at 1130 cm-1
and a sharp line at 1344cm-1
). A smaller but noticeable part of
the nitrogen impurity is present in the form of nitrogen pairs,
Figure 1. High pressure apparatus of the split-sphere type: (a) general view, (b) split-sphere multianvil block. 1, clamps; 2, assembly with
semisphere cavities; 3, multianvil block (diameter 300 mm); 4, steel anvils; 5, tungsten carbide anvils; 6, high-pressure cell.
Figure 2. High-pressure cell used for diamond growth: 1, ZrO2 con-
tainer; 2, thermocouple; 3, graphite heater; 4, MgO sleeve; 5, gra-
phite (source of carbon); 6, metal melt; 7, single crystal of diamond;
8, talk ceramic.

3. Article Crystal Growth & Design, Vol. 10, No. 7, 2010 3171
A-centers (an IR band peaking at 1280 cm-1
). The highest
nitrogen impurity concentrations achieved for diamonds crys-
tallized at this stage are approximately 1100 and 850 ppm for
the NiFeC þ Fe3N and NiFeC þ CaCN2 systems, respectively.
As nitrogen concentration increases, the color of the diamond
crystals changes from brownish-yellow to dark greenish-
brown, and eventually, at the highest nitrogen concentrations,
the crystals are virtually black and opaque. Optical absorption
spectra measured for diamond crystals with different nitrogen
concentrations are shown in Figure 5. In addition to the
absorption continuum caused by photoionization of nitrogen
donors (C-centers), the spectra exhibit a specific broad ab-
sorption band with a maximum at around 660 nm. The
strengthofthebandincreaseswithnitrogenconcentration.Ob-
viously, it is this absorption band which, in combination with
the absorption related to C-centers, gives rise to the observed
greenish component in the color of diamonds produced in this
study. The nature of the 660-nm absorption band is not clear at
the moment. Note that, in a number of previous works on
HPHT synthesis of diamonds with high nitrogen concentra-
tions, it was also found that such diamonds had typically green
ordarkgreencolor.8-10
Althoughnovisibleabsorptionspectra
were reported in these studies, it is possible that a similar
absorption band may be responsible for the observed colors.
As shown in Figure 6a-c, at the SCD stage an increase in
nitrogen concentration results in significant enhancement
of anomalous birefringence in diamond crystals. As CN inc-
reases, the crystal morphology progressively changes. The
{311} and {110} faces become less developed up to complete
disappearance, and the development of the {100} faces de-
creases substantially. It is found that, with increasing nitrogen
concentration in diamond crystals, the density of dislocations
Table 1. Experimental Resultsa
nitrogen content, ppm
run N additive CN, atom % product product wt, mg C-form A-form total
128/2 SCD 240.5 210 15 225
166/1 Fe3N 0.005 SCD 260 293 32 325
169/2 Fe3N 0.02 SCD 126.1 907 170 1077
164/1 Fe3N 0.025 BCD 286 127 0 127
161/1 Fe3N 0.1 ADC 165.7 223 0 223
138/1 Fe3N 0.2 ADC 271.7 240 0 240
182/2 Fe3N 0.3 ADC þ GR 327.6 300 15 315
134/1 Fe3N 0.4 GR 260
139/1 Fe3N 1.3 GR 338
132/1 Fe3N 3 GR 442
134/2 Fe3N 4.5 GR 351
159/1 CaCN2 0.05 SCD 241.8 343 55 398
154/1 CaCN2 0.1 SCD 161.8 440 80 520
180/1 CaCN2 0.13 SCD 135.8 513 184 697
156/2 CaCN2 0.2 SCD 89.7 620 230 850
181/2 CaCN2 0.25 ADC 312 124 0 124
179/1 CaCN2 0.3 ADC 299 146 0 146
176/1 CaCN2 0.4 ADC þ GR 279.5 173 0 173
184/2 CaCN2 0.5 GR 318.5
187/2 CaCN2 0.6 GR 331.5
a
SCD, single crystal diamond; BCD, block crystal diamond; ADC, aggregate of diamond crystals; GR, graphite.
Figure 3. Concentration of nitrogen impurity in diamond crystals (N, ppm) and the rate of carbon mass transfer (dm/dτ, mg/h) as functions of
nitrogen concentration in the melt (lg CN). (a, b) Fe3N additive; (c, d) CaCN2 additive; circles, total nitrogen concentration in diamond; triangles,
concentration of nitrogen in the form of C-centers; squares, concentration of nitrogen in the form of A-centers. For further explanations, see the text.

4. 3172 Crystal Growth & Design, Vol. 10, No. 7, 2010 Palyanov et al.
and stacking faults increases. Planar defects spread mainly
from theseedalongthree {111} planes, forming a tetrahedron.
Within thistetrahedron,thedensityofthedislocation etchpits
reaches a value of 6.5 Â 105
cm-2
, that is more than 2 orders of
magnitude higher than that in the reference crystal. Taking
into account data from ref 24, most abundant are partial
dislocationsandcompleteedgedislocations. Thus, atthe stage
of single crystal growth, an increase in nitrogen concentration
in the melt leads to an increase in nitrogen impurity concen-
tration in diamond crystals with an accompanying increase in
lattice strains and the density of linear and planar defects, as
well as significant enhancement of the {111} faces in the
crystal morphology. Note that, under these conditions, nitro-
gen enters the diamond structure predominantly in the form
of single substitutional atoms (C-centers), which are known to
considerably distort the diamond lattice.25
It is likely that the
most probable cause for the changes in the real structure and
properties of diamond crystals at this growth stage is the
structural nitrogen impurity, whose concentration in the
crystals correlates with the intensity of strains and the density
of dislocation. Previously it has been shown that nitrogen
impurity affects the integral perfection of the diamond lattice,
giving rise to defect-induced broadening of the diamond
Raman line.7
Withfurtherincreases in nitrogen concentration in themelt,
there is a drastic change in diamond crystallization processes
and the stage of single crystal growth is superseded by the
stage of formation of an aggregate of diamond crystals
(ADC). It is found that crystallization of aggregates occurs
at CN ranging from 0.025 to 0.3 atom % and from 0.25 to 0.4
atom % for Fe3N and CaCN2 additives, respectively. At this
stage, the concentration of nitrogen impurity in diamond
crystals sharply decreases to values lower than those in the
reference crystal. The transition from single crystal growth
(Figure 7 a) to aggregate growth (Figure 7 d and e) proceeds
via the formation of block crystals (Figure 7 b and c). Typical
of these crystals are octahedral habit and numerous twin
Figure 4. Infrared absorption spectra of diamond crystals grown in
the FeNiC þ CaCN2 system with different nitrogen concentrations:
(a) no additive; (b) 0.1 atom % N; (c) 0.2 atom % N. The spectra are
displaced vertically for clarity.
Figure 6. Anomalous birefringence of diamond crystals grown in
the FeNiC þ CaCN2 system with different nitrogen concentrations:
(a) no additive; (b) 0.05 atom % N; (c) 0.1 atom % N; (d) 0.3 atom %
N (a fragment of an aggregate of diamond crystals).
Figure 5. Optical absorption spectra of diamond crystals grown in
the FeNiC þ CaCN2 system. The spectra are displaced vertically for
clarity.
Figure 7. Diamond crystals grown from the FeNiC system with
addition of nitrogen-containing compounds: (a) single crystal;
(b and c) block crystals; (d and e) aggregates of block and twinned
crystals; (f) inclusions in the aggregate.

5. Article Crystal Growth & Design, Vol. 10, No. 7, 2010 3173
lamellae with thickness up to 100 μm, which are manifested as
steps on the {111} faces. The central areas of the {111} faces
exhibit clear microblock structure (Figure 7 b). Misorientation
of the blocks relative to the base crystal is up to 7°. The {111}
faces show a coarse-layered vicinal structure. The inclination
angles of the vicinals relative to the {111} face are up to 5°. As a
result, the {111} faces become convex, whereas the {100} faces
remain flat and smooth. Diamonds crystallized at the ADC
stage show even greater strains, giving rise to anomalous
birefringence with high interference colors (Figure 6 d).
The strains typically occur as a system of bands parallel to the
{111} faces, frequently forming a birefringence pattern of the
“tatami” type. Typical of these crystals are also numerous
metallic inclusions (Figure 7 f), which have rectilinear sharp-
cornered shape, corresponding to the contours of numerous
depressions between the microblocks on the {111} faces
(Figure 8 a). On the whole, characteristic of the aggregate
crystals is intense polysynthetic microtwinning (Figure 8 b).
The internal structure of the blocks, revealed on the polished
surfaces, typically demonstrate intense microtwinning, fre-
quently formingmicrotwinsystems withdifferentorientations
(Figure 8 c and d). The crystals also exhibit self-deformation
patterns, which are clearly visible on the polished surfaces.
Figure 8 (c and d) shows how twin lamellae and microtwins
bend following the deviations of the blocks during the growth.
Individual crystals in the aggregate have convex stepped faces
{111} and very small flat faces {100} (Figure 8 e). Growth
deformations are also frequently observed on the crystals
(Figure 8 e and f).
At a nitrogen concentration of 0.3 atom % in the NiFeC-
Fe3N system (run 182/2) and 0.4 atom % in the NiFeC-
CaCN2 system (run 176/1), growth of diamond crystal aggre-
gates and formation of metastable graphite were established.
At higher nitrogen concentrations, crystallization of diamond
is terminated and only aggregates of graphite crystals were
found in the run products (Table 1).
The experimentally established regularities allow us to dis-
cuss some aspects of the diamond crystallization process under
conditions of nitrogen impurity action, related to nitrogen
incorporation in diamond crystals during growth and impurity
induced changes in the crystal morphology and real structure.
It is known that in the metal melts the rate of diamond
growthismainly controlledby therateof carbondiffusion26,27
and crystals grow in the diffusion-limited regime.28
This is
testified by an increase in quantity of metal inclusions with in-
creasing growth rate and, finally, formation of skeletal crys-
tals.28-30
As found in this study, addition of nitrogen-containing
compounds at the stage of single crystal growth (SCD) results in
a decrease in the mass growth rate (Figure 3). From the theory
and practice of crystal growth, it is known that slowing down of
the growth rate may result from diminishing mass transfer or
slowing down of the surface processes.31,32
The features of
diamond crystallization found in this study at the SCD stage
in the presence of nitrogen-containing additives allow us to
suppose that the decrease of the growth rate with increasing
CN results from the inhibition by the surface-active impurity
and, correspondingly, the increase in relative contribution of the
kinetic resistance.31
Thus, with increasing CN in the metal melt,
the regime of diamond growth at constant parameters (P-T)
and supersaturation gradually changes from predominantly
diffusion-limitedtopredominantlykineticallycontrolled.Making
use of the developed approach for diamond crystallization
from the metal melts with increased nitrogen concentra-
tions, we have grown diamond crystals weighting up to 4 ct,
showing no metallic inclusions and containing 400-800 ppm
of nitrogen (Figure 9).
The existing concepts of crystal growth31
suggest that
shifting the diamond growth regime from diffusion-limited
to kinetically controlled leads to an increase in carbon con-
centration at the growing crystal surface. Apparently, this
promotes activation of numerous microtwins and micro-
blocks as the growth centers, and as a result, single crystal
growth (SCD) changes to the formation of a crystal aggregate
(ADC). An increase in the growth rate at the ADC stage is
probably caused by the presence of intense growth centers and
an enlarged surface of the aggregate relative to a single crystal.
The transition to the third stage, crystallization of graphite
(GR), represents a particular question requiring further in-
vestigations. In fact, crystallization of metastable graphite in
Figure 8. Morphology and internal structure of aggregate diamond
crystals: (a) microblock pattern on the {111} faces; (b) a (111)
polysynthetic twin; (c and d) polished sections of aggregates; (e) a
crystal with convex stepped {111} faces from aggregate; (f) a crystal
fragment showing growth deformations.
Figure 9. Diamond crystals weighing 2.5-4 ct with increased ni-
trogen concentrations. Scale grid 5 Â 5 mm.

6. 3174 Crystal Growth & Design, Vol. 10, No. 7, 2010 Palyanov et al.
the field of thermodynamic stability of diamond has been
observed in a number of previous studies on diamond
synthesis,33,34
but no clear explanation for this phenomenon
has emerged yet.
Let us now consider the effect of nitrogen impurity on the
diamond crystal morphology. We found that with increasing
nitrogen concentration in the metal melt, along with re-
duction of the growth rate, the morphology of the crystals
changes in the following sequence: {111}>{100}, {311},
{110} f {111} > {100}, {311} f {111} . {100}. It is known
that, for diamonds grown in the metal melts without nitrogen
additives, the concentrations of nitrogen in the growth sectors
{111}/{100}/{311}/{110} are related as approximately 100/
50/10/1.35
The results of this study show that, with increas-
ing CN, the first to disappear are the {110} growth sectors,
containing the lowest concentrations of nitrogen, followed by
the {311} growth sectors. The {100} growth sectors, contain-
ing nitrogen impurity in concentrations typically two times
lower than the {111} sectors, disappear almost completely at
the maximal CN values. On the other hand, it is known that
when nitrogen activity in the melt is reduced by adding
nitrogen getters, crystallized diamonds show an enhanced
development of the low-nitrogen growth sectors {100},
{311}, and {110}.27,28,35,36
Thus, we can infer that nitrogen
can be considered as an impurity stabilizing octahedral
growth form of diamond.
Experimental data obtained in this study are of interest not
only for diamond synthesis but also for understanding com-
plicated questions of natural diamond formation. Growth of
morphologically different diamond crystals with different
degrees of crystalline perfections has been traditionally con-
sidered from the point of view of changing supersatura-
tion.37,38
In this work we showed experimentally that one of
the important factors, which may govern not only the mor-
phology and defect content of diamond crystals but also
carbon phase formation, is the nitrogen concentration in the
crystallization environment. As follows from the established
regularities, highnitrogen concentrationsinthecrystallization
environment may be responsible for the formation of dia-
monds with polycentric structure of the faces, which is typical
for many natural diamonds. The action of the impurity may
lead to intense twinning, formation of block crystals, and
polycrystalline aggregates, whose formation under natural
conditions is debatable.38,39
Another important finding is the
experimental evidence for the formation of considerable
deformations in diamond crystals arising directly during the
growth process (self-deformation) and their impurity-related
character.
Summary
(1) With an increase in nitrogen concentration in metal
melts, the crystallization of diamond proceeds through
the following stages: single crystal f block crystal with
microtwins f aggregate of block crystals and twins. At
nitrogen concentrations higher than a certain critical
value (0.4 atom %), the nucleation and growth of
diamond are terminated and graphite crystallizes in
the field of thermodynamic stability of diamond.
(2) At the stage of single crystal growth, an increase in
nitrogen concentration in the crystallization medium
results in regular changes in the impurity content,
morphology, and crystalline perfection of diamond
crystals:
• The concentration of nitrogen impurity in dia-
monds increases from about 200 to approximately
1100 ppm; this is accompanied by growth in the
intensity of the absorption band with a peak near
660 nm, which is responsible for the observed
green coloration of nitrogen-doped crystals.
• The growth form of diamond crystals changes in
the following sequence: {111} > {100}, {311},
{110} f {111}>{100}, {311} f {111}.{100},
which allows us to consider nitrogen as an im-
purity stabilizing octahedral growth form.
• The density of dislocations and twin lamellae, as
well as anomalous birefringence, caused by inter-
nal strains, increases with nitrogen concentration.
(3) The stage of growth of aggregates of block crystals
and twins occurs at high concentrations of nitrogen in
the metal melt and is characterized by the appearance
of strong strains, intense microtwinning, entrapment
of inclusions, and development of considerable self-
deformation phenomena. Despite the fact that the
nitrogen concentration in the growth environment
is high, aggregate crystals contain 120-300 ppm of
nitrogen.
Acknowledgment. The authors thank A. Mogileva and
T. Molyavina for their assistance in the course of this study.
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