A Series of MAX Phases with MA-Triangular-Prism Bilayers and Elastic Properties

Article abstract of DOI:10.1002/anie.201814128

We report a new type of MAX phase (M=transition metals, A=main group elements, and X=C/N), Nb₃As₂C, designated as 321 phase. It differs from all the previous M_n+1AX_n phases in that it consists of an alternate stacking of one MX layer and two MA layers in its unit cell, while only one MA layer is allowed in usual MAX phases. The new 321 phase exhibits a bulk modulus of Nb₃As₂C up to 225(3) GPa as determined by high-pressure synchrotron X-ray diffraction, one of the highest values among MAX phases. Isostructural 321 phases V₃As₂C, Nb₃P₂C, and Ta₃P₂C are also found to exist. First-principles calculations reveal the outstanding elastic stiffness in 321 phases. Among all 321 phases, Nb₃P₂C is predicted to have the highest elastic properties. These 321 phases, represented by a chemical formula M_n+1A_nX, were added as new members to the MAX family and their other properties deserve future investigations.

Full text of DOI:10.1002/anie.201814128

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Accepted Article
Title: New series of MAX phases with MA-triangular-prism bilayers:
synthesis, structure, and elastic properties
Authors: Hongxiang Chen, Dongliang Yang, Qinghua Zhang, Shifeng
Jin, Liwei Guo, Jun Deng, Xiaodong Li, and Xiaolong Chen
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Angewandte Chemie International Edition
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COMMUNICATION
New series of MAX phases with MA-triangular-prism bilayers:
synthesis, structure, and elastic properties
Hongxiang Chen, Dongliang Yang, Qinghua Zhang, Shifeng Jin, Liwei Guo, Jun Deng, Xiaodong Li,
Xiaolong Chen*
Abstract: We report a new type of MAX phase, Nb
as 321 phase. It differs from all the known Mn+1AX
consists of an alternate stacking of one MX layer and two MA layers
3
As
2
C, designated
electrical and thermal properties enable them to be promising
materials for various applications , especially in the structural or
conductive components under high temperatures. Besides, MAX
[
4]
n
phases in that it
in its unit cell, while only one MA layer is allowed in usual MAX phases. phases also have drawn much attention as the precursor of
[
5]
The new 321 phase exhibits a bulk modulus of Nb
3
As
2
C up to 225(3)
MXenes . Owing to the relatively weak bonding of M-A in MAX
phases, a series of two dimensional transition metal carbides can
GPa as determined by high pressure synchrotron X-ray diffraction,
one of the highest values among the known MAX phases.
[
6]
be obtained from acid etching of selected MAX phases .
The study of MAX phases started from 1960s when the H-phases
were found by Nowotny’s group[7] . Now, over 60 MAX phases are
found. Depending on the value of n, the M AX, M AX and M AX
2 3 2 4 3
phases are usually named as 211, 312 and 413 phases,
respectively[1b]. The crystal structures of MAX phases usually
Isostructural 321 phases V
to exist. First-principles calculations reveal the outstanding elastic
stiffness in 321 phases. Among all 321 phases, Nb C is predicted
to possess the highest elastic properties. These 321 phases,
represented by a chemical formula Mn+1 X, were added as new
3 2 3 2 3 2
As C, Nb P C and Ta P C are also found
3
P
2
A
n
members to the MAX family and their other properties deserve future
investigations.
3
share a common symmetry with space group P6 /mmc. Each X
atom is coordinated by six M atoms forming an octahedron, and
each M atom by six A atoms forming a hexagonal prism as shown
in Figure 1. Mn+1AX
n MX layers consisting of the edge-shared M
one MA layer consisting of edge-shared M A triangular prisms. It
n
can be regarded as an alternate stacking of
MAX phases[1] belong to a family of non-van-der-Waals-type
6
X octahedrons and
layered compounds with a general formula of Mn+1AX
n
(n = 1, 2,
6
3
) in which M = transition metals, A = main group elements, and
should be noted that two and three MX layers can be combined
into edge-shared bilayer and trilayer as in 312[8] and 413[9] phases,
X = C/N. MAX phases are also termed as metallic ceramics[2] for
their combined characteristic properties of both ceramics and
metals. Some of them host high thermal and electrical
conductivity, good elastic performance, excellent resistance to
corrosion, oxidation, and thermal shock, easy machinability and
2 2 2 2 2 2
Ga S C, Ta Se
C[10], Ta C[11], and
respectively. Besides, Mo
Nb
C[12], named as 221 phases, also have a similar structure
with Mn+1AX phases, where two layers of A atoms exist between
2
S
2
n
the MX layers. The multi-layering of MA layers, however, has
never been found among the known MAX phases.
unusual damage-tolerance[2a, 3]
.
These combined elastic,
[*]
Dr. H. Chen, Dr. Q. Zhang, Dr. S. Jin, J. Deng, Prof. Dr. L. Guo,
Prof. Dr. X. Chen
Research & Development Center for Functional Crystals ,
Laboratory of Advanced Materials and Electron Microscopy
Beijing National Laboratory for Condensed Matter Physics, Institute
of Physics, Chinese Academy of Sciences
Beijing 100190, China
E-mail: chenx29@iphy.ac.cn
Dr. H. Chen
School of Materials Science and engineering
Fujian University of Technology
Fuzhou 350118, China
Dr. D. Yang, and Prof. Dr. X. Li
Beijing Synchrotron Radiation Facility
Institute of High Energy Physics, Chinese Academy of Science,
Beijing 100049, China
J. Deng, Prof. Dr. X. Chen
School of Physical Sciences
Figure 1. The crystal structure of Mn+1AX
n 6
phases. The schematic of the M X
octahedron and M A triangular prism in MAX phases, and the crystal structure
6
of 211, 312, and 413 phases viewed from [110] direction.
University of Chinese Academy of Sciences
Beijing 101408, China
Prof. Dr. X. Chen
Collaborative Innovation Center of Quantum Matter
Beijing 100084, China
Prof. Dr. L. Guo
Center of Materials Science and Optoelectronics Engineering
University of Chinese Academy of Sciences
Beijing 100049, China
Prof. Dr. L. Guo
Songshan Lake Materials Laboratory
Dongguan, Guangdong 523808, China
In this Communication, we report new MAX phases in As-, P-
3 2 3 2 3 2
contained systems, namely, Nb As C, V As C, Nb P C, and
Ta C. Their crystal structures are determined by X-ray
3
P
2
diffraction (XRD) and high resolution transmission electron
microscopy (HRTEM). These isostructural phases can be
regarded as formation from the alternate stacking of one MX layer
and two MA layers in their unit cell. They can be expressed as
M
n+1
A
n
X (n=2), henceforth referred to as 321 phases. In contrast
Supporting information for this article is given via a link at the end of
the document.
to the known MAX phases, here the multi-layered MA layer is
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Angewandte Chemie International Edition
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COMMUNICATION
firstly observed in 321 phases. The pure phase of Nb
3
As
2
C was
All peaks in the powder XRD pattern can be well indexed as a =
3.36056(6) Å, and c = 18.69508(35) Å with space group No. 194,
obtained by an optimized synthesis process. It is found that
Nb As C has a bulk modulus up to 225(3) GPa as revealed by
3
2
P6
Nb
suite[13]. The Rietveld refinements converge to the agreement
factors R = 4.11% and Rwp = 5.45%. The atom positions of 321
phases are listed in Table S1. For Nb As C, z = 0.05951(4), z
0.15849(5).
As shown in Figure 3(b), the crystal structure of Nb
distinct structural features from the conventional Mn+1AX
There are two Nb positions in Nb As C in the unit cell, as labeled
3
/mmc as shown in Figure 3(a). The crystal structure of
high pressure synchrotron XRD (HPXRD) study with pressure up
to 47 GPa, which is confirmed by the results from the first-
principles calculations. These 321 phases form a new class of the
MAX family.
3
2
As C was solved by direct method using EXPO2014 program
p
3
2
M
A
=
3
As
2
C exhibits
phases.
n
3
2
by Nb1 and Nb2. Each As atoms is coordinated by three Nb1
atoms and three Nb2 atoms forming a triangular prism; Each C
atom is coordinated by six Nb1 atoms forming an octahedron.
Different from Mn+1AX
here are bilayers. The crystal structure of Nb
n
phases, the MA-triangular-prism layers
As C could be seen
3
2
as an alternate stacking of MX-octahedron layer and MA-
triangular-prism bilayer. Similar to most MAX phases, the Nb, As,
and C atoms of 321 phases are all in hexagonal closed packing
positions.
The atomic arrangement in the unit cell was examined by high-
angle annular dark-field (HAADF) and annular-bright-field (ABF)
scanning transmission electron microscopy (STEM) as shown in
Figure 3(c) and 3(d). The atomic stacking of Nb and As atoms
along the [001] direction were clearly obserevd in HAADF and
ABF images, matching well with the solved structure. Due to the
low contrast of C atoms, it is hard to observe the C atoms[ in
HADDF image. But in ABF image, C atoms can be identified lying
in between the Nb atom layers. The atomically resolved HAADF
and ABF images justify the reliability of the solved structure.
14]
Figure 2 The synthesis of Nb
3 2
As C phases. (a) The synthetic process of
Nb As C. (b) The theoretical free energy of two react paths. (c) Differential
3
2
thermal analysis of 850 ℃ sintered sample (NbAs + Nb + C).
The 321 phases were synthesized by a two-step process. Take
Nb As C as an example, the process is illustrated in Figure 2(a).
3 2
High purity Nb, As and graphite powders were mixed and sealed
in an evacuated quartz tube, then heating the quartz tube to 850
C in 20 hours, and kept for 10 hours. The main by-products are
NbAs, Nb, and graphite as revealed by XRD (see Figure S1).
Secondly, the as-prepared powder was pulverized, pelletized,
sealed in a quartz tube, and heated to 1280 C in 8 hours and
3 2
kept for 15 hours. The end-product is Nb As C with grain size 2-
10 m (Figure S2). The elemental composition determined by
inductively coupled plasma-atomic emission spectrometry is Nb :
As : C = 2.9 : 1.9 : 1, very close to the nominal composition. We
found that long-time sintering around 1000 C always leads to the
formation of Nb
phase with furthur sintering at 1280 C. Thermodynamic
calculations (see Figure 2(b)) indicate the reaction path Nb AsC
NbAs = Nb As C is unfavored as the free energy (F) is
possitive. In comparison, the free energy for the reaction NbAs +
Nb + C = Nb As C is negative, meaning that this path is possible.
The differential thermal analysis (see Figure 2(c)) confirms this.
An exothermic peak at 1260 C should be an indication of the
reation temperature, in agreement with our synthetic temperature.
2
AsC (see Figure S3), and we can not obtain 321
2
+
3
2
Figure 3 Crystal structure of Nb
3
As
2
C (a) The crystal structure of Nb
As C. (c) The HADDF image
C viewed from [100] direction. (d) The ABF
3 2
As C.
3
2
(b) The Rietveld refinement of the XRD data of Nb
3
2
of the atomic arrangement of Nb
3
As
2
image.
3 2
It is noted that Nb As C will decompose under high vacuum when
the sample was heated up to ~1600 ℃ (see Figure S4).
3 2
For Nb As C, dNb1-C = 2.2365 Å is slightly shorter than the dNb-C =
2.2422 Å ; the average of dNb1-As (2.6814 Å) and dNb2-As ( 2.5867 Å)
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Angewandte Chemie International Edition
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COMMUNICATION
′
is close to the dNb-As (2.6343 Å) in Nb
2
AsC. This suggests the
As C as the case in
AsC. The strong bonding strengths in this 321 phase are
expected to exhibit high elastic performance as in As-, P-, S-
contained 211 phases[15]
HPXRD under pressures up to 47 GPa were applied to determine
the bulk modulus of Nb As C. As shown in Figure 4(a), no new
peak appears with the rising pressure, indicating the excellent
stability of Nb As C under high pressures. The strongest three
where B
The fitted B
The value of B
0
is the bulk modulus and 퐵 its first pressure derivative.
0
′
strong Nb-C and Nb-As bonding in Nb
Nb
3
2
0
and 퐵 are 225(3) GPa, and 2.6(3) GPa, respectively.
C is nearly equal to the B
AsC[ , and quite close to the highest B
0
2
0
15b]
of Nb
3
As
2
0
= 224(2) GPa
0
= 261(2) GPa for
for Nb
β-Ta AlC
2
[18]
.
4
3
. This result shows that the existence of more Nb-As
As C does not cause a decrease in bulk modulus,
bonds in Nb
3
2
3
2
which provide further evidence of that the M-A bonding and M-X
bonding are close in strength in the As- contained MAX phases.
First-principles calculations have been used to evaluate the
elastic constants cij of most MAX phases within an accuracy of
~10%[19]. We performed the first-principles calculations of the
3
2
peaks (105), (100), and (110) can be obviously seen to shift to
high angles with increasing pressures. The pressure dependence
of the normalized lattice parameters a/a
(b). Similar to the case with Zr
SC[15d], the compressibilities of
this 321 phase are quite isotropic, while are strong anisotropic
for most MAX phases. Both a and c shrink by less than 6%, and
the cell volume by about 16% at the pressure of 47 GPa. The
0
, c/c
0
are shown in Figure
elastic constants of Nb
hexagonal symmetry, there are five independent elastic constants:
11, c12, c13, c33, and c44 as shown in the Equation S1[19]. The
calculated elastic constants cij are listed in Table S2. The bulk
modulus B , Young’s modulus E , shear modulus G and Poisson
3 2
As C along with other 321 phases. For
4
2
[16]
c
V
V
V
pressure-dependent V/V
0
can be well fitted by the Birch–
ratio  as listed in Table 1 were then calculated by the Voigt
method using Equations S2-S5 . The Poisson ratio  of 321
Murnaghan[17] equation of state with a correlation coefficient R =
2
[19]
0.9987,
phases are in the range of 0.22~0.24. For Nb C, the bulk
3
As
2
3
modulus B is 198.3 GPa. The calculated B for Nb AsC by the
V
V
2
7/3
5/ 3
'
0
2/3
P  3 / 2B [(V /V )  (V /V0 ) ]{1 (B  4)(V /V ) 1}
0
0
0
4
same method is near 200 GPa, consistent with the results 204
GPa calculated by Mekour et al. [20] Accordingly, the ab initio
calculations tend to underestimate the elastic properties of As-, P-
contained MAX phases. The elastic modulus and shear modulus
of Nb
than Nb
As shown in Table 1, the bulk modulus of Nb
3
As
2
C (E
V
= 306.5 GPa, G
V
= 123.4 GPa) are slightly smaller
AsC (E
V
= 313 GPa, G
V
= 126 GPa)[21]
.
2
3
P
2
3 2
C and Ta P C are
2
23.6 GPa and 222.0 GPa, respectively. Because of the
underestimation of the bulk modulus in the case of Nb As C, the
experimental bulk modulus of Nb C and Ta C might be
higher than that for Nb As C. It can be seen that the P-contained
21 phases are better elastically performed than As-contained
21 phases. Nb C (E = 371.8 GPa, G = 152.0 GPa)
3
2
3
P
2
3 2
P
3
2
3
3
3
P
2
V
V
performed best among 321 phases, even better than the
21]
calculated results in Nb
More interestingly, the calculated results of Nb
than the experimental results in Nb AlC
values among the MAX phases (E = 365 GPa, G
parallel to basal plane, and E = 353 GPa, G = 153 GPa vertical
to basal plane)[22]
Considering the possible best elastic
performance, relatively low theoretical density and low toxicity,
Nb C is a more promising material for application among these
21 phases.
2
PC (E
V
= 333 GPa and G
V
= 134 GPa)[
.
3
2
P C also higher
4
3
, which are the best
= 149 GPa
V
V
V
V
.
3
P
2
3
Table 1: The theoretical lattice parameters a’ and c’ and elastic parameters of
321 phases.
G
GPa)
V
E
V
B
V
(GPa)
a' (Å)
c' (Å)
v
(
(GPa)
Nb
3
As
2
C
3.3857
3.3334
3.3128
3.1657
18.9088
18.2091
18.1167
17.8618
123.4
152.0
149.0
121.4
306.5
198.3
0.24
0.22
0.23
0.23
Nb
3
P
2
C
371.8
365.3
297.8
223.6
222.0
181.2
Ta
3
P
2
C
C
V
3
As
2
Until now, the known As- or P- contained conventional MAX
phases include Nb AsC, Nb PC, V AsC, and V
PC[7, 23]. The
synthesis of other As- or P- contained 321 phases Ta P C,
2
2
2
2
Figure 4. HPXRD study of Nb
patterns of Nb As C, the solid lines are the results of refinements. (b) Pressure-
dependent relative cell parameters (a/a , c/c , and V/V .). The red curve is the
fit to data by the Birch–Murnaghan equation of state.
3 2
As C. (a) The pressure dependence of the XRD
3
2
3
2
Nb
3
P
2
C and V
3
As
2
C were tried. We find these phases exist but
AX or MA are often present. For
0
0
0
the secondary phases MX, M
2
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Angewandte Chemie International Edition
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COMMUNICATION
[
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the secondary phases are NbC (5.2(1)%), Nb
NbP (14.4(2)%), see Figure S5. The Rietveld refinements were
performed for Nb C and Ta C samples. For V As C, there
are many unknown peaks in the PXRD pattern as shown in Figure
S6, so we only indexed the peaks belonging to V As C to obtain
3
P
2
C content is 57.7(6) mol.%, and
2
PC (22.7(2)%),
3
P
2
3
P
2
3
2
3
2
its lattice parameters. The lattice parameters, atomic positions,
phase content, and agreement factors of 321 phases are listed in
Table S1. The structural information including the bond lengths
and bond angles are listed in Table S3.
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In conclusion, new series of MAX phases, Nb
Nb C, and Ta C, named as 321 phases were added to the
family of MAX phases. With the optimized synthesis method, pure
phase of Nb As C can be obtained, and we suggest the reaction
path should follow NbAs + Nb + C = Nb As C. XRD and TEM
results revealed that the 321 phases share common symmetry
and constitute units with the conventional Mn+1AX phases. The
3 2 3 2
As C, V As C,
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2
[
[
3
2
n
structure of 321 phases consists of alternate stacking of MX-
octahedron layers and MA-triangular-prism bilayers. The First-
principles calculations and HPXRD reveal the outstanding elastic
performance of 321 phases. The experimental bulk modulus of
[
Nb
among MAX phases. The predicted the elastic stiffness (E
71.8 GPa, G = 152.0 GPa, B = 223.6 GPa), relative low density
and low toxicity of Nb C make it a most promising material for
future application. These 321 phases are the first series of MAX
phases that can be expressed as Mn+1 X with n > 1. Their novel
structure and outstanding elastic stiffness in 321 phases are
3 2
As C is up to 225(3) GPa, which is close to the highest values
[
V
=
[
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A
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This work is financially supported by the National Natural
Science Foundation of China under granting numbers: No.
5
1532010, No. 91422303; the Key Research Program of
[
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Angewandte Chemie International Edition
10.1002/anie.201814128
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Hongxiang Chen, Dongliang Yang,
Qinghua Zhang, Shifeng Jin, Liwei Guo,
Xiaodong Li, Xiaolong Chen*
Page No. – Page No.
New series of MAX phases with MA-
triangular-prism bilayers: synthesis,
structure, and elastic properties
Text for Table of Contents
New series of MAX phases with outstanding elastic properties, named as 321 phases, were added to MAX phases family. 321
phases share common symmetry and constitute units with the conventional Mn+1AX phases. Different from Mn+1AX phases, the MA-
triangular-prism layers here are combined into bilayers. 321 phases are the first series of MAX phases that can be expressed as
X with n > 1.
n
n
M A
n+1 n
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