Synthesis of Ln nitrides (Ln = Ce, Pr, Gd, Lu) in high pressure and temperature

Article abstract of DOI:10.1016/j.jallcom.2008.10.064

We have synthesized Ln nitrides (Ln = Ce, Pr, Gd, Lu) in a supercritical nitrogen fluid at high pressure (about 30 GPa) and high temperature (about 2000 K) using the diamond anvil cell and YAG laser heating system. New Ce and Pr nitrides have been synthes

Full text of DOI:10.1016/j.jallcom.2008.10.064

Journal of Alloys and Compounds 477 (2009) 493–497
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Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
Synthesis of Ln nitrides (Ln = Ce, Pr, Gd, Lu) in high pressure and temperature
Ken Niwaa, Masashi Hasegawaa,∗, Takehiko Yagib
a Dept. Mater. Sci., Nagoya University, Nagoya, Aichi 464-8603, Japan
b Institute for Solid State Physics, University of Tokyo, Kashiwa, Chiba 277-8581, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 10 July 2008
Received in revised form 8 October 2008
Accepted 14 October 2008
Available online 3 December 2008
We have synthesized Ln nitrides (Ln = Ce, Pr, Gd, Lu) in a supercritical nitrogen fluid at high pressure (about
30 GPa) and high temperature (about 2000 K) using the diamond anvil cell and YAG laser heating system.
New Ce and Pr nitrides have been synthesized, while the well-known NaCl-type GdN and LuN have been
obtained. It is found that the structures of the new Ce and Pr nitrides are different from those of the new
La nitrides reported recently.
© 2008 Elsevier B.V. All rights reserved.
PACS:
81.05.Zx
81.05.Je
81.40.Vw
62.50.+P
61.66.Fn
Keywords:
Lanthanide nitride
high pressure and high temperature
synthesis
Diamond anvil cell
1. Introduction
Pt nitride, PtN (ZnS-type) synthesized at 45 GPa and ∼2000 K
[6].
The combination of the diamond anvil cell and YAG laser heat-
ing (LASER–DAC) is a useful technique for the synthesis of metal
nitrides in a supercritical nitrogen fluid. The synthesis and crys-
tal growth of metal nitrides of simple metal, transition metal and
rare earth metal using the LASER–DAC system [1–7] have been
reported. For example, we have grown colorless transparent fine
crystals of GaN in the wurtzite-type structure from molten gallium
and fluid nitrogen at about 10 GPa and high temperature [1]. We
have also reported the systematic study of the syntheses and sta-
bility of transition-metal nitrides at high pressures (about 10 GPa)
and high temperatures (about 1800 K) using LASER–DAC [2–4].
Nitrides such as TiN, VN, CrN, Mn3N2, Fe2N, Co2N and Ni3N have
been easily and quickly synthesized by a simple direct nitriding
reaction between metal and fluid nitrogen. There are also sev-
eral reports on the synthesis of transition-metal nitrides using
LASER–DAC by other groups. For example, Zerr et al. reported
the synthesis of the new Th3P4-type, Zr3N4 and Hf3N4 nitrides
at 18 GPa and ∼3000 K [5]. Gregoryans et al. reported the first
Recently, we have also succeeded in synthesizing two new lan-
thanum nitrides in a supercritical nitrogen fluid at high pressure
(about 30 GPa) and high temperature (about 2000 K) using the
LASER–DAC heating system [7]. These nitrides were found to be
stable down to 5 GPa and ∼300 K in a nitrogen atmosphere. One of
the new lanthanum nitrides is a cubic P lattice-type phase. The cal-
culated lattice parameter is a = 11.979 Å at 5 GPa, 300 K. The other
nitride is a trigonal P lattice-type. The calculated lattice parame-
ters are a = 3.889 Å and c = 6.691 Å at 5 GPa, 300 K. The most likely
¯
phase of the former new La nitride is Pa3 La2N3+ı, the structure of
¯
which may be similar to the Ia3 Mn2O3-type (Ia80). The phase of the
¯
latter nitride is P3m1 La2N3+ı, the structure of which is the same
¯
as the P3m1 La2O3-type (hP5). Since La nitrides are usually syn-
thesized as the NaCl-type structure under ambient pressure, they
are novel La nitrides which are different from NaCl-type nitrides.
Other Lanthanide nitrides are also usually synthesized as the NaCl-
type structure under ambient pressure. Accordingly, it is expected
that novel Ln ones can be obtained by the LASER–DAC as well
as the La nitrides. This manuscript reports the synthesis of other
Ln nitrides (Ln = Ce, Pr, Gd, Lu) in a supercritical nitrogen fluid at
about 30 GPa by the LASER–DAC in comparison with the novel La
nitrides.
∗
Corresponding author. Fax: +81 52 789 3370.
E-mail address: hasegawa@numse.nagoya-u.ac.jp (M. Hasegawa).
0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2008.10.064

494
K. Niwa et al. / Journal of Alloys and Compounds 477 (2009) 493–497
2. Syntheses and experimental procedure
The LASER–DAC systems installed at Photon Factory, the
National Laboratory for High Energy Physics (KEK) in Tsukuba and
at the SPring8, was used for synthesizing samples. The YAG laser
(100 W CW multimode) was used for sample heating. Samples in a
lever-type DAC were irradiated by the laser beam from one side of
the DAC. The sample was observed during heating using a CCD cam-
era. The culet size of the diamond anvil was 450 ␮m in diameter. A
rhenium gasket was used and the gasket hole was about 200 ␮m in
diameter. The LASER–DAC system used in this study was described
in detail elsewhere [8].
Synthesis pressure was about 30 GPa, which was measured
before and after laser heating using the ruby fluorescence technique
at room temperature. Starting materials were solid Ln metal (about
100 × 100 × 10 ␮m3) and gaseous nitrogen. To prevent the starting
Ln metal from coming into contact with the diamond anvil, which
worked as a big heat sink, a thin NaCl plate was placed between
the Ln metal and the anvil. The nitrogen was loaded at room tem-
perature using an apparatus modified from that described before
[9]. The assembled diamond anvil cell was enclosed in the casing
and attached to the gear mechanism and then placed at the bottom
of the high-pressure vessel. The vessel was then purged several
times (usually five times) with the appropriate high-pressure pure
nitrogen gas (99.99995%) from the gas bomb by using valves. The
piston was set to the bottom position to minimize the volume of
the vessel. The pressure in the bomb was usually about 13 MPa,
and so the dilution factor by this purge process was more than
about 106. After purging, the vessel was filled with a large amount
of pure nitrogen gas again by loading into the sample chamber at
about 150 MPa. It was then compressed further by the DAC up to
about 30 GPa. During this pressure increase, the nitrogen solidi-
fies at about 3 GPa. The sample was heated by the laser for about
30 min and then quenched to room temperature by decreasing the
laser power quickly. The sample temperature during the laser heat-
ing was monitored spectroradiometrically and the temperature at
the heating spot was maintained at about 2000 50 K.
Fig. 1. X-ray diffraction patterns at room temperature (RT) when synthesizing Ce
nitride. (a) At about 30 GPa before laser-heating; (b) at about 30 GPa after laser-
heating at about 30 GPa, 2000 K; (c) after recovered to 0.1 MPa (ambient pressure).
High-resolution angle dispersive X-ray diffraction experiments
were carried out using synchrotron radiation at the Photon Fac-
tory in Tsukuba and at the SPring-8. A monochromatized X-ray was
collimated to a thin beam (30 ␮m × 30 ␮m) and used to irradiate
the sample. The diffracted X-ray was detected by an imaging plate
(IP). In order to obtain high resolution and accuracy, the IP detector
was placed about 310 mm from the sample. The X-ray intensities
recorded on the IP were measured by a laser-scanning reader and
converted into two-dimensional digital intensity data. These data
were then integrated along the polar axis which coincided with
the Debye rings observed on the IP, and the powder diffraction
intensities were obtained as a function of the 2ꢀ angle. Typical
exposure times were about 20 min. Details of the data analysis were
described elsewhere [8].
observed through the monitor of the LASER–DAC system that the
solid nitrogen melted during the laser heating. Fig. 2 shows the
photograph of the sample chamber before and after laser-heating
at about 30 GPa. The boundary between the melted and unmelted
solid nitrogen is observed in the figure (b). Therefore, the nitrid-
ing reaction occurred between Ce metal and supercritical nitrogen
fluid. The X-ray diffraction pattern just after the decompression was
also measured at 0.1 MPa and room temperature in an atmosphere
of the nitrogen gas, as shown in Fig. 1(c). The Ce nitride(s) pattern
remains almost the same as that at 30 GPa although the diffraction
intensity was changed because of the sample movement during the
decompression. After measurement at 0.1 MPa and 300 K, the X-ray
diffraction pattern was measured again in air. The pattern was a
broad halo, indicating that the synthesized nitrides got glassy due
to the decompression or oxidation. This is the same result of the
new La nitrides reported before [7].
3. Results and discussion
The well-known reported Ce nitride is of the simple NaCl-type
structure [11]. No X-ray diffractions of the NaCl-type structure can-
not be found in the pattern of the recovered sample (Fig. 1(c)).
There is another reported Ce nitride which crystallizes the La2O3-
type structure [12]. This structure is the same as that of one of the
new La nitrides recently reported by us [7]. Fig. 3 shows the com-
parison of the X-ray diffraction pattern at about 30 GPa between
the Ce nitride(s) in this study and the reported La nitrides. These
patterns are different from each other. Besides, the results of the
TEM-EDS analysis indicated that the Ce metal was nitrided. These
results indicate that one (or some) new Ce nitride(s) has (have)
Fig. 1 shows the results of the X-ray diffraction pattern when
synthesizing Ce nitride. The pattern at about 30 GPa and room
temperature before heating is identified by the combination of
tetragonal (tI2) Ce metal [10] and solid nitrogen since the start-
ing materials, Ce metal and nitrogen, do not react at this stage
and the NaCl thermal insulator was not used in this experiment.
Fig. 1(b) shows the X-ray diffraction pattern at about 30 GPa and
room temperature after heating at 30 GPa and 2000 K. It is clear
that the pattern is completely different from that in Fig. 1(a), indi-
cating that the Ce metal has reacted with the nitrogen. It was clearly

K. Niwa et al. / Journal of Alloys and Compounds 477 (2009) 493–497
495
Fig. 2. Photograph of the sample chamber at room temperature. The boundary
between the melted and unmelted solid nitrogen is indicated by an arrow in the
figure (b). (a) Before laser-heating at about 30 GPa and (b) after laser-heating at
about 30 GPa, 2000 K.
Fig. 3. Comparison of X-ray diffraction patterns of Ce nitride(s) in this study and
La nitrides in the previous study [7] at about 30 GPa and room temperature after
laser-heating at about 30 GPa, 2000 K.
been synthesized in this study. Although we have tried to analyze
the X-ray diffraction pattern, it cannot be identified so far. Since
we have not solved the structures and even index patterns, only 2ꢀ
angle, observed d-spacing and intensities are shown in Table 1.
Fig. 4 shows the X-ray diffraction pattern at about 30 GPa and
room temperature after heating at 30 GPa and 2000 K when synthe-
sizing Pr nitride. Many diffraction lines are observed in the pattern,
accompanying with the diffractions of ␣-U type Pr (unreacted Pr)
and NaCl (thermal insulator). This indicates that Pr nitride(s) has
Table 1
Observed 2ꢀ angle, d-spacing and intensity of the diffraction lines of Ce nitride(s).
Here, the intensity “ss”, “s” and “w” which indicate “very strong”, “strong” and
“weak”, respectively.
2ꢀ
7.454
7.800
8.255
8.358
8.468
8.616
8.902
9.382
9.698
9.952
10.140
10.349
10.465
10.581
10.696
11.096
11.452
11.970
d-Spacing (Å)
Intensity
2ꢀ
d-Spacing (Å)
Intensity
3.281
3.135
2.963
2.926
2.888
2.839
2.748
2.608
2.523
2.459
2.413
2.365
2.338
2.313
2.288
2.206
2.137
2.045
w
w
s
s
ss
s
12.345
13.337
13.658
14.193
14.439
14.537
14.836
15.369
15.619
16.014
16.528
16.754
16.961
17.085
18.682
18.856
19.608
1.983
1.836
1.793
1.726
1.697
1.686
1.652
1.595
1.569
1.531
1.484
1.464
1.446
1.436
1.314
1.302
1.252
s
s
w
w
s
w
s
w
w
w
w
w
s
s
w
w
w
s
s
s
w
w
w
w
s
s
w
w
w
Fig. 4. X-ray diffraction pattern at room temperature after laser-heating at about
30 GPa, 2000 K when synthesizing Pr nitride.

496
K. Niwa et al. / Journal of Alloys and Compounds 477 (2009) 493–497
Table 2
4.542(2) Å, respectively. Compared to the reported cell parameters
at ambient pressure (4.969 Å [13] and 4.7599 Å [14]), it is found
that they are compressed into about 86.75 and 86.89% in volume at
30 GPa, respectively. This suggests that they have almost the same
compressibility. Kieffer et al. reported the Mn2O3-type Gd and Lu
nitrides of which crystal structure is different from the La2O3-type
of the Ce and Pr nitrides [12]. These results are different from ours.
They were reported to be synthesized in nitrogen gas at about 3 MPa
and 1700–1800 K, accompanying with their NaCl-type phases. Since
their X-ray diffraction patterns were not shown in the manuscript,
the volume fraction of the Mn2O3-type phase has not been clear.
The different result is attributable to the different conditions of our
syntheses.
It is concluded from our present and previous studies, La, Ce
and Pr nitrides in a supercritical nitrogen fluid at about 30 GPa and
2000 K crystallize to the new phases different from the well-known
NaCl-type one. Besides, the crystal structures of the new La nitride
are different from those of the Ce and Pr ones. On the other hand, Gd
and Lu nitrides crystallize to the NaCl-type phase. La, Ce and Pr ions
are larger than Gd and Lu ones. For example, La trivalent ionic radius
is much larger than others, and Ce and Pr ones have almost the same
radius (La: 1.032 Å, Ce: 1.01 Å, Pr: 0.99 Å, Gd: 0.938 Å and Lu: 0.861 Å
[15]). These suggest that the phase formation of the Lanthanide
nitride in a supercritical fluid at high pressure and temperature is
relevant to its ionic size.
Observed 2ꢀ angle, d-spacing and intensity of the diffraction lines of Pr nitride(s).
Here, the intensity “ss”, “s” and “w” which indicate “very strong”, “strong” and
“weak”, respectively.
2ꢀ
7.307
d-Spacing (Å)
Intensity
2ꢀ
d-Spacing (Å)
Intensity
3.231
3.023
2.948
2.913
2.835
2.796
2.657
2.523
2.474
2.411
2.370
2.299
2.186
w
w
s
ss
s
ss
w
w
w
s
ss
w
w
11.050
11.353
11.577
11.960
12.804
13.131
13.441
13.912
14.132
16.292
18.925
19.201
19.898
2.138
2.082
2.041
1.976
1.847
1.801
1.760
1.700
1.674
1.453
1.252
1.235
1.192
s
7.811
8.009
8.106
8.330
8.446
8.890
9.361
9.550
9.799
9.969
10.275
10.810
w
w
w
s
w
w
s
s
ss
w
s
s
(have) been synthesized by laser heating. The observed 2ꢀ angle,
d-spacing and intensities are described in Table 2. The well-known
reported Pr nitride is of the simple NaCl-type structure [13]. There
is another reported Pr nitride which crystallizes the La2O3-type
structure as well as Ce nitride [12]. The diffraction pattern cannot
be identified by these reported Pr nitrides. This indicates that one
(or some) new Pr nitride(s) has (have) been synthesized in this
study.
Fig. 5 shows the X-ray diffraction patterns just after laser-
heating Gd and Lu metals in a supercritical nitrogen fluid at 30 GPa,
2000 K. Both patterns are simple and can be identified by the well-
known NaCl-type nitrides [14,15] and unreacted pure metal phases.
The cell parameters of GdN and LuN at 30 GPa were 4.739(1) Å and
4. Conclusions
Ln nitrides (Ln = Ce, Pr, Gd, Lu) have been synthesized in a super-
critical nitrogen fluid at high pressure (about 30 GPa) and high
temperature (about 2000 K) using the diamond anvil cell and YAG
laser heating system. New Ce and Pr nitrides have been synthe-
sized, while the well-known NaCl-type GdN and LuN have been
obtained. Although the new phases are not identified at this stage,
it is found that their structures are different from that of the new La
nitrides reported recently, i.e. La2O3-type, pseudo-Mn2O3-type [7].
Our summarized conclusion is that La, Ce and Pr nitrides crystallize
to the new phases different from the well-known NaCl-type one in
a supercritical nitrogen fluid at about 30 GPa and 2000 K. On the
other hand, Gd and Lu ones crystallize to the NaCl-type structure
phase in the same condition.
Acknowledgements
The authors are grateful to T. Ohsuna for his useful discussion.
They are also grateful to N. Sata and Y. Ohishi for their experimental
supports at the SPring-8. Experiments at the SPring-8 were per-
formed with the approval of the JASRI (PROPOSAL No. 2006A1364).
A part of this research was supported by Grant-in-Aid for Scien-
tific Research on Priority Areas “Nano Materials Science for Atomic
Scale Modification 474ꢀꢀ from the Ministry of Education, Culture,
Sports, Science and Technology of Japan and by the JFE 21st Century
Foundation.
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