Changes of nitrides characteristics in Li-N system synthesized at different pressures

Article abstract of DOI:10.1016/j.jallcom.2013.06.173

For the Li-N system samples were obtained at pressures of nitrogen from 1 to 10 atm. Energy-dispersive X-ray (EDX) spectrum of a sample of Li-N subjected to degradation shows that lithium nitride turned into carbonate as evidenced by the predominant content of carbon and oxygen. Upon synthesis of lithium nitride at a positive pressure of nitrogen, the b-modification is formed, which can be achieved at a pressure 500 times lower than that described in literature, required to create a high-pressure phase. The increase in carbon content with increasing of synthesis pressure of lithium nitride confirms the change in stoichiometry of its structure formed with high nitrogen content.

Full text of DOI:10.1016/j.jallcom.2013.06.173

Journal of Alloys and Compounds 581 (2013) 23–27
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jalcom
Changes of nitrides characteristics in Li–N system synthesized at
different pressures
Oleg V. Ignatenko a, Valery A. Komar a, Sergey V. Leonchik a, Natalia A. Shempel a, Antoaneta Ene b,
Alina Cantaragiu b, Marina V. Frontasyeva c, , Valery N. Shvetsov c
⇑
a Scientific and Practical Materials Research Centre of NAS of Belarus, P. Brovka St., 19, Minsk 220072, Belarus
b ‘‘Dunarea de Jos’’ University of Galati, Faculty of Sciences and Environment, Department of Chemistry, Physics and Environment, 47 Domneasca St., 800008 Galati, Romania
c Joint Institute for Nuclear Research, 6 Joliot-Curie St., Dubna, 141980 Moscow Region, Russia
a r t i c l e i n f o
a b s t r a c t
Article history:
For the Li–N system samples were obtained at pressures of nitrogen from 1 to 10 atm. Energy-dispersive
X-ray (EDX) spectrum of a sample of Li–N subjected to degradation shows that lithium nitride turned into
carbonate as evidenced by the predominant content of carbon and oxygen. Upon synthesis of lithium
nitride at a positive pressure of nitrogen, the b-modification is formed, which can be achieved at a pres-
sure 500 times lower than that described in literature, required to create a high-pressure phase. The
increase in carbon content with increasing of synthesis pressure of lithium nitride confirms the change
in stoichiometry of its structure formed with high nitrogen content.
Received 25 February 2013
Received in revised form 15 June 2013
Accepted 28 June 2013
Available online 5 July 2013
Keywords:
Lithium nitride
High resolution electron microscopy
Grain boundaries
Ó 2013 Elsevier B.V. All rights reserved.
Phase transition
1. Introduction
[13–15]. It should also be noted that lithium nitride serves as a cat-
alyst for the formation of the cubic modification of boron nitride,
Lithium nitride is the only nitride which is formed in interaction
of lithium with nitrogen at room temperature; the increase in pres-
sure and temperature accelerates the reaction [1–3]. The phase
diagram of the Li–N system (Fig. 1), known from the literature data
[4], indicates the presence of only one compound, namely Li3N.
significantly influencing the structure and characteristics of the
latter [16,17].
The phase diagram shown in Fig. 1 describes the behavior of the
Li–N system at atmospheric pressure [4]. To the best of our knowl-
edge, the information on the effect of pressure on the characteris-
tics of this system is missing. In this regard, the study of changes in
the characteristics of nitrides in the Li–N system at different pres-
sures of synthesis is of particular interest and represents the aim of
the current investigation.
Lithium nitride has two crystalline modifications,
a and b. The
former belongs to the space group P6/mmm and has cell parame-
ters a = 0.3655 nm and c = 0.3876 nm. The crystalline structure of
this modification consists of two types of layers: one has the
composition Li2NÀ and consists of six coordinated Li centers, and
another one consists of lithium cations [3]. At a pressure of
2. Experimental
0.5–0.6 GPa,
a-modification transforms into b-modification [5–8]
with space group P63/mmc and cell parameters a = 0.3579 nm
and c = 0.6360 nm. The solid lithium nitride is an ion conductor
and has the highest conductivity among all inorganic lithium salts.
It is studied as a solid electrolyte and anode material and can be
used in accumulator batteries [3,9–11]. The solid lithium nitride
can be formed by direct reaction of the elements as well as by indi-
rect one, for example, when nitrogen reacts with lithium dissolved
in liquid metallic sodium [12]. Due to its high hydrogen uptake
capacity, lithium nitride is used as hydrogen storage material
2.1. Materials synthesis
Lithium nitride was obtained as follows. After the LE-1 grade ingots of lithium
metal with purity higher than 99.9% (Russian State Standard GOST 8774-75) were
placed in a hermetic box, they were purged with high purity argon (Russian State
Standard GOST 10157-79). The protective paraffin film was then removed from
the surface with the Nefras C2-80 solvent (Russian State Standard GOST 443-76).
After that the samples were cut in the form of parallelepipeds with linear dimen-
sions 1 Â 1 Â 10, 2 Â 2 Â 10, and 3 Â 3 Â 10 cm. The fabricated samples were
placed in a crucible made of stainless steel, which in turn was placed into the reac-
tor. The reactor was removed from the box and joined to the gas heating system.
After being connected, the reactor was purged with high purity nitrogen (Russian
State Standard GOST 9293-74); upon completion, the reactor was sealed hermeti-
cally, and the injection of pressure was carried out to the required parameters. After
15-min waiting period, the heating was turned onto predetermined temperature.
The heating rate varied by 1–3° per minute. After being held at this temperature,
⇑
Corresponding author. Tel.: +7 49621 65609; fax: +7 49621 65085.
E-mail addresses: ignatenkoov@yahoo.com (O.V. Ignatenko), aene@ugal.ro
(A. Ene), mfrontasyeva@yahoo.com (M.V. Frontasyeva).
0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jallcom.2013.06.173

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O.V. Ignatenko et al. / Journal of Alloys and Compounds 581 (2013) 23–27
2.3. SEM–EDX analysis
The samples were mounted on carbon planchettes as pure crystals. The images
and the electron beam-induced X-ray spectra were obtained using the existing re-
search infrastructure at the Dunarea de Jos University of Galati, Romania, consisting
of a scanning electron microscope (SEM) of Quanta 200 FEI type, an energy-
dispersive X-ray microanalysis (EDX) integrated system, 5 detectors of secondary
and backscattered electrons, 1 detector for transmitted electrons, and the software
for data processing, quantification of chemical composition using ZAF (Z – atomic
number, A – absorption, F – fluorescence) correction algorithm, multipoint chemical
analysis with matrix effects corrections, and the analysis of concentration profile
along a defined line.
3. Results and discussion
Typical EDX spectra of Li–N samples obtained at different syn-
thesis pressures subjected to decomposition are shown in Fig. 2
(for 6 atm) and Fig. 1S (Supplementary Material) (for 8 and
9 atm). It follows from these spectra that lithium nitride turned
into carbonate, as evidenced by the predominant content of carbon
and oxygen. Also, EDX analysis demonstrates the presence of struc-
tures with higher nitrogen content as a function of nitrogen pres-
sure (Fig. 1S).
The SEM images of powder materials of Li–N system obtained at
pressures of 1–10 atm are shown in Fig. 3 (for 3, 8 and 10 atm) and
Fig. 2S (for 1, 2, 4, 5, 6, 7 and 9 atm) (Supplementary Material).
These images show the presence of particles of irregular shape,
indicating a high reaction rate of their formation. It should be
noted that as the pressure increases, the number of small-sized
particles also grows.
Fig. 1. Phase diagram of the system Li–N [4].
both the samples and the reactor were cooled; then the samples were removed and
placed in a box. The Li–N system samples were obtained in a nitrogen atmosphere
under pressures of 1–10 atm. After being purged with argon, the samples were
withdrawn, weighed, and ground down to grain size of less than one millimeter.
2.2. Powder X-ray diffraction
The obtained Li–N powder samples were investigated by X-ray diffraction
(XRD) analysis using a DRON-3.0 diffractometer (Cu Ka radiation) at Scientific
and Practical Materials Research Centre of National Academy of Sciences of Belarus.
Fig. 2. A typical EDX spectrum of powder material of Li–N system obtained at a synthesis pressure of 6 atm after the decomposition.

O.V. Ignatenko et al. / Journal of Alloys and Compounds 581 (2013) 23–27
25
Fig. 4. Dependence of
a
(curve 1) and
c
(curve 2) lattice parameters of b-
modification of lithium nitride on the pressure of nitrogen during synthesis at a
heating rate of 3° per minute.
the literature values of b-modification [7,8]. Details of XRD pat-
terns with typical peaks used to calculate the lithium nitride lattice
parameters obtained at a pressure of 3, 6 and 9 atm are presented
in Fig. 5a–c, respectively, showing the structure modification as a
function of nitrogen pressure during growing conditions.
As pointed out in [5–7],
a ? b transition in lithium nitride oc-
curs at pressures of 0.5–0.6 GPa, which were not achieved during
the process of synthesis (synthesis pressure in this work was of
about 1.012 MPa). Thus, one can conclude that the formation of
b-structure occurs at lower energy consumption than its transfor-
mation. Therefore, it can be assumed that in order to obtain high
pressure structures in the process of formation, it is not necessary
to use the pressure of the phase transition, but a rather certain
minimum pressure, which is 500 times lower.
Moreover, it should be noted that the change in lattice param-
eters is of linear character, which is similar to the change of the gi-
ven characteristics in solid solutions.
The increase in pressure most likely leads to the formation of
structures which are more saturated with nitrogen.
The decomposition of lithium nitride in air was carried out to
measure powder materials content present in reaction products.
Chemical composition of the air at the time of decomposition of
the samples was the same for all experiments (oxygen 19.1%;
nitrogen 77.9%; carbon dioxide 0.1%; inert gases 1%; water – mois-
ture in the air – 1.9%). Fig. 6 shows the dependence of carbon (1)
and oxygen (2) contents on the synthesis pressure of these
materials.
The standard mechanism of decomposition is as follows:
1. Absorption of air moisture.
2. Reaction Li3N + 3H2O ? 3LiOH + NH3.
3. Surface absorption of CO2.
4. Reaction LiOH + CO2 ? Li2CO3 + H2O.
The obtained results confirm sufficiently the proposed mecha-
nism, taking into consideration that part of the formed water had
evaporated, and some of it remained in the crystalline form (the ra-
tio of carbon to oxygen was 1:3.5(4)).
Thus, the increase in carbon content with increasing synthesis
pressure of lithium nitride confirms the change in stoichiometry
of the structure (the structure is formed with high nitrogen con-
tent). The oxygen content reduction may be indicative of structural
changes occurring not only in the near-surface layer.
Fig. 3. SEM images of powder materials of Li–N system synthesized at: (a) 3 atm;
(b) 8 atm; and (c) 9 atm.
The XRD analysis results are shown in Fig. 4. It should be noted
that the obtained lattice parameters are in close agreement with

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O.V. Ignatenko et al. / Journal of Alloys and Compounds 581 (2013) 23–27
Fig. 5. Typical peaks in XRD pattern used to calculate the lithium nitride lattice parameters obtained at a pressure of: (a) 3 atm; (b) 6 atm; and (c) 9 atm. 2h angles of 55.5°
and 70.7° correspond to (112) and (203) reflections, respectively.

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27
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.jallcom.2013.06.
173.
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at a pressure 500 times higher; this allows one to hypothesize
the minimum pressure required to create high-pressure phases.
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Acknowledgements
Authors acknowledge the support of the BRFBR–JINR Grant
(Contract No. F11D-001, 2011) and the project by the Plenipoten-
tiary Representative of Romania at JINR (JINR order No. 81/
06.02.2012, paragraph 49).