Ionized Nitrogen Mono-hydride Bands are Identified in the Pre-solar and Carbonado
Diamond Spectra
J. GARAI1,2*
2
1Department of Mechanical and Materials Engineering, Florida International University, Miami, FL 33199
Department of Mechanical and Structural Engineering, Ybl Miklós Faculty, Szent István University, Budapest, Hungary
E-mail: [email protected]
Abstract-None of the well established Nitrogen related IR absorption bands,
common in synthetic and terrestrial diamonds, have been identified in the presolar diamond spectra. In the carbonado diamond spectra only the single
nitrogen impurity (C centre) is identified and the assignments of the rest of the
nitrogen-related bands are still debated. It is speculated that the unidentified
bands in the Nitrogen absorption region are not induced by Nitrogen but rather by
Nitrogen-hydrides because in the interstellar environment Nitrogen reacts with
Hydrogen and forms NH+; NH; NH2; NH3. Among these Hydrides the electronic
configuration of NH+ is the closest to Carbon. Thus this ionized Nitrogen-monohydride is the best candidate to substitute Carbon in the diamond structure. The
bands of the substitutional NH+ defect are deduced by red shifting the irradiation
induced N+ bands due to the mass of the additional Hydrogen. The six bands of
the NH+ defects are identified in both the pre-solar and the carbonado diamond
spectra. The new assignments identify all of the nitrogen-related bands in the
spectra, indicating that pre-solar and carbonado diamonds contain only single
nitrogen impurities.
INTRODUCTION
Nitrogen is the most important impurity in diamonds. The content and the type of the
nitrogen defects are even used to classify the diamonds (Davis, 1977). If the nitrogen content is
significant, typically measured in ppm but can be as high as 1%, then the diamonds are Type I
while diamonds with undetectable or very small amounts of nitrogen are Type II. When a
diamond forms, nitrogen atoms can replace the carbon atoms, forming an isolated single
impurity. Thus newly formed diamonds, like synthetic diamonds, almost exclusively contain
single nitrogen impurities. Energetically, a higher level of aggregation of nitrogen impurities is
1
favored, single nitrogen impurities tend to unify and form higher levels of aggregation. The
bonds between the carbon atoms in diamonds are very strong resulting in very slow diffusion,
even on the time scale of a billion years. Elevated pressures and temperatures increase diffusion
rates resulting in higher levels of aggregation, clustering two, four, or more nitrogen atoms. The
aggregation level of nitrogen can be used to estimate the mantle residence time for terrestrial
diamonds. The lack of higher level nitrogen aggregation indicates that diamonds were not
affected by high pressures and temperatures and long annealing times after formation. The
nano-size of the pre-solar diamonds, which typically contain in the order of 103 C atoms and 10
N atoms (Jones & d’Hendecourt, 2000; Jones, 2001), might put constraints on the Nitrogen
aggregation.
About 98% of natural terrestrial diamonds contain defects with 2 and 4 aggregated nitrogen
atoms. Based on the aggregation level of the nitrogen, Type I diamonds are subdivided into two
subgroups. Type Ia diamonds (98%) contain clustered nitrogen atoms, and Type Ib (0.1%)
contain scattered or single nitrogen atoms. Type II diamonds are also subdivided into Type IIa
(1-2%), which is almost pure carbon, and to Type IIb (0.1%) containing boron atoms. The
percentages refer to the natural occurrence of different diamond types. The aggregation level of
nitrogen can be identified by infrared absorption. The single (C center), pair (A center), four (B
center) nitrogen and higher aggregations, like platelets each induce well-defined characteristic
infrared absorption bands. These nitrogen-related bands are present in terrestrial and synthetic
diamonds but lacking in the pre-solar diamond IR spectra (Lewis et al. 1989; Colangeli et al.
1994; Koike et al. 1995; Mutschke et al. 1995; Hill et al. 1997; Andersen et al. 1998; Braatz et al.
2000).
The spectra of carbonado diamond contain the bands relating to substitutional N defects at
1130 cm-1 and 1344 cm-1; however, the spectra is very different from type Ib diamonds (Fig. 1)
and contains many additional unidentified bands in the Nitrogen region. No other bands relating
to higher nitrogen aggregation are present in the spectra. The rest of the characteristics bands
identified in the carbonado diamond spectra (Garai et al. 2006; Kagi and Fukura 2008) are
similar to the pre-solar bands; therefore, these diamonds are also included in this investigation. It
should be noted that the origin of carbonado remains enigmatic. The almost identical IR spectra
are not an ultimate proof for same origin and formation. The assignments of the nitrogen-related
bands in the pre-solar and carbonado diamonds spectra are still debated and the exact
configuration of N is not known.
2
NITROGEN IN INTERSTELLAR SPACE
The nitrogen in the Hydrogen rich enviroment of interstellar space can react with Hydrogen
forming nitrogen hydrides. The nitrogen molecule N2 can disassociate through the reaction
forming N+ (Adams & Smith, 1984)
He+ + N2 → (N+2)* + He
(N+2)* → N+2 + hν
→ N+ + N
The kinetic energy of this reaction is 0.28 eV, which is expected to equally divided between N+
and N. This energy is sufficient to overcome on the endoergicity of reaction
N+ + H2 → NH+ + H.
Atomic nitrogen can also be ionized by cosmic rays in interstellar space
N + hν → N+ + e-,
or
N + p → N+ + e- + p
and form NH+ ion (Mitchell et al. 1978). NH+ can also be formed through charge transfer
reaction from NH as:
H+ + NH → NH+ + H,
or through the exothermic reaction:
H2+ +
N → NH+ + H,
or can be synhetized from interstellar ammonia as:
He+ + NH3 → NH+ + H2 + He,
or through photolytic reaction (Huebner and Giguere, 1980) as
NH3 → NH+ + H2 + e-,
or synthetized from HCO as:
N+ + HCO → NH+ + CO,
or through the ionizing reaction of
NH + hν → NH+ + e-.
In Hydrogen rich environment NH+2 and NH+3 can be formed as:
NH+ + H2 → NH+2 + H and
NH+2 + H2 → NH+3 + H
In reverse NH and NH2 can also been formed through the photochemical decomposition of NH3
(Kerns & Ducan, , 1972).
3
The exact mechanisms for formation of nitrogen hydrides in space enviroment are still debated;
however, it is widly agreed and supported by observations that significant part of the nitrogen in
space is in the form of nitrogen hydrides (NH+; NH; NH2; NH3 ). Ammonia NH3, is widely
observed in dark clouds and star-forming regions (Cheung et al. 1968). and the presence of
nitrogen hydrites (NH and NH2 ) are well known in comets (e.g. Swings et al. 1941; Meier et al.
1998; Feldman et al. 1993). Both hydites, NH and NH2, have also been observed in stellar
photospheres (e.g. Schmitt 1969; Farmer & Norton 1989). Interstellar NH was first detected in
the diffuse clouds towards the star ξPer and HD 27778 by Meyer & Roth (1991). The presence
of NH has also been detected in other regions (Goicoechea et al., 2000; Crawford & Williams,
1997; Hily-Blant et al., 2010; Persson et al., 2010). Significant concentration of NH+ ion in
interstellar molecular cloud and in comets has been predicted by Almeida and Singh (1982) and
detected by Persson et al. (2010). It can be concluded that in interstellar environment Nitrogen
reacts with Hydrogen and present in the form of Nirogen-hydrides NH+; NH; NH2; NH3.
DISCUSSION
In order to extract the pre-solar diamonds from meteorites, the samples go through severe
chemical treatments, which alter the original spectra (Braatz et al. 2000). Based on these
contaminations it has been concluded that in the pre-solar diamond IR spectra only the
absorption features relating to nitrogen atoms (Braatz et al. 1999) and to the multiple phonon
absorption of diamonds (Hill et al. 2000) can be considered as intrinsic properties of the
diamond. In this study the absorption features of Nitrogen impurities are investigated.
The complete data set of previous pre-solar and carbonado diamond IR spectra reported in the
literature was collected. The characteristic bands are listed in Table 1.
In the Nitrogen region the band at 1085 cm-1 has been proposed results from chemical
treatment (Braatz et al. 2000). The spectra of carbonado diamond contain all of the reported
bands of pre-solar diamonds with the exception of the 1085 cm-1. Since the carbonado samples
were not chemically treated (Garai et al., 2006), the lack of the 1085 cm-1 band is consistent with
the proposed chemical treatment-induced origin of this band.
The band at 1285 cm-1, characteristic of A center in terrestrial diamonds, is reported in the
Orgueil and the carbonado diamond spectra. This band is greatly diminished by further
oxidation of the nano-diamond residues of Orgueil chondrite indicating that the band is an
artifact resulting from the chemical treatments (Hill et al. 1997). This band is present in all of
the chemically treated carbonado diamond spectra (Kagi & Fukura, 2008); however, it is also
4
detected in one of the non-treated carbonado spectra. Thus the possibility that this band might be
induced by A centers can not be excluded.
The characteristic bands of C centre at 1130 cm-1 and 1344 cm-1 are present in the spectra of
carbonado diamond. In the Allende and Orgueil spectra there is a band at 1122-1125 but the
1344 cm-1 band is missing. Thus the presence of the substitutional Nitrogen impurities (C
centre) is debatable in pre-solar diamonds.
None of nitrogen related bands, common in terrestrial and synthetic diamonds, are uniquely
identifiable in any of the pre-solar diamond spectra. Based on the lack of the well established
Nitrogen related bands, it is speculated that the unidentified bands in the Nitrogen absorption
region might not induced by Nitrogen but rather by Nitrogen-hydrides because in the Hydrogen
rich interstellar environment Nitrogen reacts with Hydrogen and forms NH+; NH; NH2; NH3.
The bonds between N and C are covalent. Covalent bonds are directional and dictates the
spatial arrangements of the atoms. Carbon atoms in the diamond structure have tetrahedron
coordination based on the spatial distribution of the sp3 hybridized electrons. In order to fit into
the diamond lattice the spatial distribution of the electrons of Nitrogen has to be the same. The
electronic structure of Nitrogen is 2s2 2p3. The three bonding domains of the p electrons and the
one nonbonding domain of the loan electron par electrons forms a tetrahedron skeleton structure
which fits into the diamond lattice. Thus, the single nitrogen impurity in the diamond lattice is
bonded to three of the neighboring carbon atom. This conclusion is supported by the existence
of the NV defects. If Nitrogen would bonded to four carbon atoms then the NV defects in
diamond would not be stable and NV defects would not exist permanently. The tetrahedron
skeleton structure of the diamond lattice can also be achieved through the ionization of the
Nitrogen. If Nitrogen is ionized then the remaining four electrons are hybridized to sp3. Thus N+
fits comfortably into the diamond lattice by forming four bonds. It can be predicted that among
the available nitrogen hydrides (NH+; NH; NH2; NH3) the coordination of NH+ would
preferentially fit into the bulk diamond lattice. Thus, it can be predicted that NH+ could
substitute carbon in the diamond lattice and form an NH+ defect when nitrogen hydrides are
present at the formation of the diamonds.
The absorption features of the NH+ defects are not known. In an attempt to identify the
features NH+ defects the bands of N+ defects were redshifted by increasing the mass of Nitrogen
with one atomic units due to the mass of the additional Hydrogen.
The ionization of the substitutional N (C center) and conversion into substitutional N+ (C+
center) has been predicted (Lawson & Kanfa 1993) and later experimentally verified by the
irradiation of type Ib diamonds . The irradiation-induced C+ center exhibit a sharp peak at
5
1332 cm-1 and broader and weaker peaks at 1115 cm-1, 1046 cm-1, and 950 cm-1 in their spectra
(Lawson et al. 1998). The same bands were observed in the spectra of synthetic diamonds after
radiation damage by electrons (Collins et al. 1988) along with two additional bands at 1450
cm-1and 1502 cm-1 which have been assigned to C-N. All the bands of the C+ center are red
shifted in the carbonado-diamond spectra with the exception of the band at 1332 cm-1 (Fig. 2-a).
It has been shown experimentally that N15 isotope doping does not have an effect on the
radiation-induced band at 1332 cm-1 (Samoilovitch et al. 1975). Thus no red shift is expected for
the 1332 cm-1 band from the additional mass of Hydrogen. Using a simple force constant model
the rest of the irradiation induced N+ bands are red shifted by adding one atomic unit to the mass
of Nitrogen, due to the mass of the additional Hydrogen. The calculated absorption bands of the
NH+ defects are listed in Table 2. The employed simple force constant model can be used and
gives reasonably good results, when the bonds are uniform and spherically symmetric. The
permanently positively charged Nitrogen with tetravalent bonds satisfies these requirements.
The agreement between the red shifted substitutional N+ bands and the bands of the pre-solar
and carbonado diamond spectra is very good. Based on this good agreement it is suggested that
the ionized Nitrogen-mono-hydride complex bonded into the four neighboring carbon atoms and
that the bands observed in the Nitrogen absorption region (Table 1) can be assigned to
Substitutional NH+ defects.
The presence of NH+ defects in carbonado diamonds is consistent with Hydrogen isotopic
composition investigations, which report about 70 ±30 ppm of Hydrogen, relating to the bulk of
the diamond (Demeny et al. 2011).
The presence of NH+ defects in pre-solar and carbonado diamonds is also supported by the
observed N-H-related bands. Both pre-solar (e.g., Andersen et al. 1998) and carbonado diamond
spectra (Fig. 2-b) contain intense N-H-related stretching vibration bands in the region of
3100-3400 cm-1 and an N-H bending band at 1650 cm-1. These bands are absent or significantly
weaker in the spectra of terrestrial and CVD diamonds (e.g., McNamara et al. 1994).
The two steps formation of NH+ defect, ionizing the substitutional N0 or C center by
irradiation and then hydrogenating the ionized nitrogen, might also be possible. However, no N
+-related
bands can be identified in any of the spectra. The complete lack of this “middle stage”
defects in this two step formation process indicates that the NH+ defects were most likely formed
by one step process. Thus the ionized Nitrogen Mono-hydrides were substituted into the
diamond structure at the time of the formation.
The NH+ related bands at 1420 cm-1 and 1465 cm-1 overlap with C-H deformation bands;
therefore, both NH+ and C-H deformation is assigned to these bands.
6
Despite the similarities between the pre-solar and carbonado diamonds’ IR spectra, there are
subtle differences. The substitutional NH+ band at 1332 cm-1 is reported only in the carbonado
spectra. The nitrogen in carbonado-diamond is dominantly present in the form of NH+ defects
but substitutional N0-related bands are also detectable in the spectra indicating that both
Nitrogen-Hydrides and Nitrogen were available at the time of the diamond formation. The ratio
between N0 and NH+ defects in the diamond could be an indicative of the environment in which
the diamonds were formed.
CONCLUSIONS
It is suggested that the unidentified bands observed in the nitrogen region of pre-solar and
carbonado diamond IR spectra arise from substitutional NH+ defects.
In the carbonado spectra,
the N0 bands are also detectable, which indicates that both Nitrogen and Nitrogen mono-hydride
were present at the formation of the diamond.
The bands of the substitutional NH+ defect are deduced by red shifting the irradiation-induced
N+ bands due to the mass of the additional Hydrogen. The six bands assigned to the NH+ defects
are identified in both the pre-solar and the carbonado diamond spectra. With the new
assignments, all of the nitrogen-related bands in the spectra are identified and it is shown that
pre-solar and carbonado diamonds contain almost exclusively single nitrogen impurities;
therefore, they can be classified as Type Ib.
ACKNOWLEDGEMENTS:
The authors thank to Andriy Durigin and Subrahmanyam
Venkata Garimella for the inspiring discussions and for their thoughtful comments. We also like
thank to Mike Sukop for reading and commenting the manuscript. This paper benefited from the
careful review of Hugh Hill and Anthony P. Jones.
7
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10
Fig. 1. Mid-infrared absorption spectra of carbonado diamond from the Central African
Republic (CAR) in the C-N vibrational region (solid line) are plotted. The experimental
procedure is described in Garai et al. 2006. Among the characteristic terrestrial diamond
absorption bands only the substitutional N0 bands (C Centre) are present in the spectra. The
assignments of the rest of the bands are still debated. For comparison the characteristic
absorption spectra of Ib diamond containing substitutional N (C Centre) defects (dashed line)
(Field 1992) and the spectra of Allende DM1 microdiamond (dotted line) (Lewis et al. 1989) are
also shown. The image shows a carbonado from the Bangui Region, Central African Republic.
The patina of the surface is a characteristic feature of carbonado diamonds.
11
12
Fig. 2. Mid-infrared absorption spectra collected from thin section of Brazilian (BR) and Central
African republic (CAR) carbonado diamond are plotted (Garai et al. 2006). The spectra are an
intrinsic property of the diamond because no chemical treatment was used in the preparation.
The assigned vibrational regions are identified. a) The irradiation-induced N+ related bands are
present but red shifted in the carbonado spectra. The observed red shift is consistent with one
atomic mass unit increase of the nitrogen, which can be caused by the hydrogenation of the
ionized nitrogen. b) The presence of hydrogenated nitrogen is supported by the N-H Stretch and
Bending bands at around 3200 cm-1 (3.12 µm) and 1652 cm-1 (6.05 µm) respectively.
13
Table 1 Reported spectral bands for pre-solar and carbonado diamonds in cm-1and their
proposed assignments.
ALLENDE
MURCHISON
ORGUEIL
CARBONADO
1Lewis
2Koike
3Andersen
4Braatz
5Colangeli
6Mutschke
4Braatz
7Hill
8Garai
et al.
1989
et al.
1995
et al.
1998
et al.
2000
et al.
1994
et al.
1995
et al.
2000
et al.
1997
et al.
2006*
3210
3115
3236
1640
1634
1590
1632
1403
1361
1401
1399
3164
Int.
%
1462
1456
1402
1385
1234
TERRESTRIAL
(assignments)
10McNamara
and ref. therein 2003
11Lawson et al. 1998
Assignments to presolar and carbonado diamonds by
Previous Studies
This Paper
1, 2, 3, 5, 6, 7 N-H
(~3200)
1616
1623
1650
1399
1210
1178
1150
1122
1122
1108
1108
1109
1090
1080
1090
1028
and
Fukura
2008
Stretch
N-H Stretch
C=C/C=O stretch, 2
C=O/N-H stretching, 3 O-H
N-H Bending
bend (in H2O)
Subst. NH+;
3 C-H deformation (CH /CH )
3
2
C-H def.(CH3/CH2)
1, 3, 5 Interstitial N/
Subst. NH+;
C-H deformation (CH3)
C-H def. (CH3)
1, 5 Aromatic
1612
26
1175
4
1420
14
1438
(1332)
3
1334
1344 (Subst. N) 11
1284
1282 (A aggregates) 10
1220
1220
11
1155
(1163)
40
(1132)
34
1126
1100
100
1100
1125
1084
1084
1090
1640
1465
1289
1173
9Kagi
1088
1072
1054
-
Subst. NH+
chem. treat.7/A cent.
1220 (C-N Stretch) 10
C-O/C-N Stretch
3 C-O/C-N/C-C Stretch/
(C-NH+) Stretch
1130 (Subst. N) 10
Interstitial N
1 C-O Stretch/ Interstitial N
2, 5, 6, 7
(C-N0) Stretch
Subst. N0
4
C-O Stretch
Subst. NH+
4
C-O(H), CF2
chem. treatment4
1030
45
Subst. NH+
909
72
Subst. NH+
*The numbers represent the averages of the bands from the three spectra. The numbers in parentheses relates to bands observed in two of the spectra.
Table 2 The irradiation induced N+ bands are red shifted by adding one atomic unit to the mass
of Nitrogen, due to the mass of the additional Hydrogen. The red shifts were calculated using
simple force constant model.
Assignment
Frequency (cm-1)
C-N+
C-NH+
950 (10.53 µm)
935 (10.70 µm)
1046 (9.56 µm)
Irradiation induced
Substitutional N+ 1115 (8.97 µm)
(C+-centre)
1450 (6.90 µm)
1030 (9.71 µm)
1502 (6.66 µm)
1479 (6.76 µm)
1098 (9.11 µm)
1428 (7.00 µm)
14