A reactivity study in the Mg-B system reaching for an improved synthesis of pure MgB2

Article abstract of DOI:10.1016/j.physc.2006.01.004

The binary Mg-B system was studied with respect to the existing phases MgB₂, MgB₄, MgB₇, and MgB₁₂ and their transformations into each other. As a result of these studies, a new synthesis route is reported for MgB₂ by reacting MgB₄ and Mg with each other. The obtained MgB₂ was characterized by Rietveld refinement of the powder XRD pattern (R_Bragg = 3.56%, R_wp = 10.6%), magnetic measurements, and by electron probe microanalysis (EPMA). With this new synthesis route, a better phase homogeneity is obtained when compared with MgB₂ samples prepared from the elements.

Full text of DOI:10.1016/j.physc.2006.01.004

Physica C 436 (2006) 38–42
www.elsevier.com/locate/physc
A reactivity study in the Mg–B system reaching
for an improved synthesis of pure MgB2
a
a
b
b
a,
*
Ruth Schmitt , Jochen Glaser , Thomas Wenzel , Klaus G. Nickel , H.-Jurgen Meyer
¨
a
Institut fu¨r Anorganische Chemie, Abteilung fu¨r Festko¨rperchemie und Theoretische Anorganische Chemie, Auf der Morgenstelle 18,
72076 Tu¨bingen, Germany
b
Institut fu¨r Geowissenschaften, Arbeitsbereich: Mineralogie und Geodynamik, Wilhelmstraße 56, 72074 Tu¨bingen, Germany
Received 22 September 2005; accepted 12 January 2006
Available online 17 February 2006
Abstract
The binary Mg–B system was studied with respect to the existing phases MgB2, MgB4, MgB7, and MgB12 and their transformations
into each other. As a result of these studies, a new synthesis route is reported for MgB2 by reacting MgB4 and Mg with each other. The
obtained MgB2 was characterized by Rietveld refinement of the powder XRD pattern (RBragg = 3.56%, Rwp = 10.6%), magnetic measure-
ments, and by electron probe microanalysis (EPMA). With this new synthesis route, a better phase homogeneity is obtained when com-
pared with MgB2 samples prepared from the elements.
Ó 2006 Elsevier B.V. All rights reserved.
Keywords: MgB2; Synthesis; Phase analysis; Mg borides
1. Introduction
to an increase of Hc2(0) from 16 to 32 T but to a lower
of the transition temperature of 36.2 K [15].
The recent discovery of superconductivity in MgB2, a
compound known for decades, with the remarkably high
transition temperature of 39 K [1] has attracted great inter-
est, because it introduces a new, simple binary supercon-
ductor with record high superconducting transition
temperature for a non-oxide and non-C60-based com-
pound. MgB2 appears to have many advantages in applica-
tion compared to Nb3Ge, which are on the one hand the
nearly twice as high transition temperature and on the
other the clearly lower weight. For this reason, there were
made numerous attempts concerning the synthesis of MgB2
[2–5], and theoretical calculations [6–8]. Variations in the
chemical composition of MgB2 [9], like the doping with
carbon in the boron network [10,11] or the Al substitution
[12–14], did not lead to an improvement of the supercon-
ducting properties. Carbon doping by CVD process leads
The synthesis of the ordinary compound MgB2 has
turned out to be more difficult than expected, especially
when a high yield of a high-purity sample is desired. These
difficulties are related to the chemical and thermodynamic
features of the Mg–B system. If the elements boron and
magnesium are used for preparation, a higher Mg-boride,
MgB12, is always formed [16]. Once MgB2 is annealed at
temperatures higher than the melting temperature of mag-
nesium (650 °C), which is inevitable in the synthesis pro-
cess, the material turns to become more and more Mg
deficient. In addition, traces of MgO are discovered as
the most common impurity in nearly all preparations.
For this reason, new reaction channels must be found to
improve the methods of synthesis for MgB2, and on this
basis to allow a specific introduction of pinning centres.
The phase diagram for the Mg–B system is not yet com-
pletely available. At present five phases, MgB2, MgB4,
MgB7, MgB12, and MgB20, were reported in this binary sys-
tem. The characterisation of MgB2 was first published by
Jones and March [17] in 1953. MgB2 crystallizes in the
*
Corresponding author. Tel.: +49 7071 29 76226; fax: +49 7071 29
5702.
E-mail address: juergen.meyer@uni-tuebingen.de (H.-J. Meyer).
0921-4534/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.physc.2006.01.004

R. Schmitt et al. / Physica C 436 (2006) 38–42
39
hexagonal AlB2 type in the space group P6/mmm with the
MgB4 (sample 2) was synthesized from stoichiometric
(1:4) quantities of Mg and B. The sample was first heated
to 650 °C for one day and then heated to 1200 °C and
remained at this temperature for 5 days.
A typical sample of MgB2 (sample 3) was prepared by
the common way from the elements with the nominal com-
position Mg:B = 1:3 at 850 °C for 3 days. To our experi-
ence, this composition leads to higher yields of MgB2
(90%) than the 1:2 composition (70%). Reaction products
contained MgB2 besides MgB12 and unreacted magnesium.
The remained metal can be edulcorated with a solution of
EtOH/I2 from the reaction product.
In a different set of reactions MgB2 (sample 4) was syn-
thesized from MgB4. For this reaction the previously pre-
pared MgB4 was sealed with an equal molar quantity of
magnesium into a niobium container, fused into an evacu-
ated silica tube, and heated at 750 °C for 5 days. After
cooling down slowly, a grey micro-crystalline powder of
MgB2 (sample 4) was obtained as the only product detect-
able in the X-ray powder pattern.
˚
˚
lattice parameters a = 3.0834(3) A, c = 3.522(2) A. The
crystal structure of MgB4 was refined by Naslain et al.
[18] in the orthorhombic space group Pnam with the
˚
˚
lattice parameters a = 5.464(3) A, b = 7.472(3) A, and c =
˚
4.428(3) A. Guette et al. [19] reported the determination of
the crystal structure of MgB7, which crystallizes in the space
˚
˚
group Imam with a = 5.597(3) A, b = 8.125(3) A, and c =
˚
10.480(5) A. The compositions MgB12 and MgB20 can be
considered as stuffed derivatives of the structure of b-rhom-
ꢀ
˚
˚
bohedral boron (R3m, a = 10.932(2) A, c = 23.819(5) A
[20]). MgB12 [21,16] and MgB20 [22] also crystallize rhombo-
˚
hedral with their lattice parameters of a = 11.014(7) A,
˚
˚
˚
c = 24.170(2) A and a = 10.9830(4) A, c = 24.1561(2) A
being closely related to those of b-rhombohedral boron.
In this paper, we report a new synthesis route for MgB2
and we present a possible reaction scheme that correlates
the different Mg–B phases MgB2, MgB4, MgB7, and
MgB12 and supplements the calculated binary Mg–B phase
diagram [23].
2. Experimental
2.2. X-ray diffraction studies
2.1. Syntheses
The reaction products were inspected with an X-ray
powder transmission diffractometer (STOE, StadIP) using
monochromatic Cu-Ka1 radiation. The XRD pattern of
samples 1–4 were indexed in accord with the literature
data. Samples were obtained X-ray pure. Only the powder
pattern of sample 3 showed some MgB12 as a side phase.
An additional XRD powder pattern was recorded for
MgB2 (sample 4) in a reflection set-up with a Philips PW
1830 diffractometer. This powder pattern was indexed
and the crystal structure was refined with the aid of the
program FullProf [24] (Fig. 1). Results of the structure
refinement are given in Table 1. Atom positions and isotro-
pic displacement parameters are provided in Table 2. The
refined pattern is shown in Fig. 1.
All manipulations for the synthesis of the magnesium
borides were performed in an Ar filled glove-box (MBraun)
with magnesium (chips 99+, Strem) and b-rhombohedral
boron (crystal powder 99.7%, ABCR) as starting materials.
Appropriate quantities of Mg and B were filled into nio-
bium containers, which were sealed under Ar with an elec-
tric arc and then placed into a vacuum-sealed silica
ampoule. Four types of samples are referenced here (1–4)
with respect to their syntheses and compositions, as exam-
ined by means of powder XRD and EPMA.
MgB12 (sample 1) was synthesized from stoichiometric
(1:12) quantities of Mg and B, heated at 1000 °C for 3 weeks.
Fig. 1. Observed (solid line) and calculated (discrete points) XRD-powder pattern of MgB2 (sample 4) with Bragg reflections, and the difference curve.

40
R. Schmitt et al. / Physica C 436 (2006) 38–42
Table 1
2.4. EPMA analysis
Crystal data and parameters for data collection and structure refinement
of MgB2
The samples for EPMA examinations were prepared as
reported in [16]. EPMA-WDX analyses were performed
with a JEOL 8900 RL Superprobe operated at 15 kV accel-
eration voltage and a probe current of 15 nA. The electron
beam was generally focused (<1 lm diameter). LDEB,
TAP and LDE1H crystals were used as analyzing crystals
for B, Mg, and O Ka X-rays, respectively. Synthetic
MgO and metallic b-B (99.994 wt.% B) were used as stan-
dards for the calibration. Whereas Mg and O were
Sum formula
Formula weight
Temperature
Wave length
Crystal system
Space group
Lattice parameters
Measured range
Independent reflections
Refined parameters
RBragg
MgB2
45.927 g/mol
298 K
154.051 pm CuKa1, 154.433 pm CuKa2
Hexagonal
P6/mmm
a = b = 308.468(8) pm, c = 352.44(1) pm
20° 6 2h 6 130°
22
19
3.56%
10.6%
9.59%
3.61
Rwp
Rp
v2
Table 3
Results of WDX analysis of powder samples 1–4
Sample
B in
wt.%
Mg in
wt.%
O in
wt.%
Totals in
wt.%
Composition
1 (MgB12
)
84.89
85.14
84.64
84.41
83.7
15.81
14.46
14.95
15.65
16.15
0a
100.7
99.6
99.64
99.86
99.86
MgB12.07
MgB13.24
MgB12.73
MgB12.13
MgB11.65
0a
Table 2
0.05
0a
0a
Atomic coordinates and isotropic displacement parameters
2
˚
Atom
Wyckoff-
Position
x/a
y/b
z/c
Ueq/A
Occupation
2 (MgB4)
3 (MgB2)
65.34
64.23
65.96
64.70
65.46
34.28
34.49
34.04
34.09
34.11
0.74
1.05
0.43
1.10
0.62
100.36
99.77
100.43
99.89
MgB4.29
MgB4.19
MgB4.36
MgB4.27
MgB4.31
Mg
B
1a
2d
0
1/3
0
2/3
0
1/2
0.0197(3)
0.0254(6)
0.9360(1)
1
The correlation between Ueq and the occupation of Mg is 0.08.
100.19
43.31
44.51
82.50
82.54
82.38
82.52
55.53
54.46
17.46
17.15
17.32
17.46
0.57
0.34
0.06
0a
0.1
0.05
99.41
99.31
100.02
99.7
99.8
100.03
MgB1.75
MgB1.84
MgB10.63
MgB10.82
MgB10.70
MgB10.63
2.3. Magnetic studies
The magnetic susceptibility of MgB2 (sample 4) was
studied with a SQUID magnetometer (Quantum Design,
MPMS) in the temperature region between 5 and 50 K
using a static field (H = 20 G). A sample of MgB2 was used
for the measurement in a gelatine capsule. The compound
showed superconducting behavior with a transition tem-
perature of 39 K (Fig. 2).
4 (MgB2)
48.45
49.59
46.21
48.13
53.88
53.97
54.3
0.16
0.2
0.21
0.22
102.49
103.76
100.73
102.35
MgB2.02
MgB2.07
MgB1.91
MgB2.00
54.0
a
Below detection limit of 160 ppm O.
Fig. 2. Temperature dependence of the magnetic susceptibility of MgB2 (sample 4).

R. Schmitt et al. / Physica C 436 (2006) 38–42
41
analyzed by means of the fixed time method of counting at
the peak position, the area intensity measurement mode
was used for the analysis of B to overcome problems of
peak shifts and peak shape alterations. Further details of
the analytical procedure were reported in [16]. Typical
results of EPMA analysis of some samples are shown in
Table 3.
3. Results and discussion
As it is well known, MgB2 can be formed in a straight
forward reaction of appropriate (1:2 or 1:3 molar) stoichi-
ometry of Mg and B at temperatures lower than 865 °C.
However traces of MgB12 are detected by EPMA in all of
our MgB2 samples synthesized from the elements. MgB12
was also detected during the early stages of reactions with
varying Mg and B compositions (from 1:2 to 1:7) by pow-
der XRD. These findings open grounds for the supposition
that this boron-rich compound is always produced first in
reactions between magnesium and boron through diffusion
of magnesium into the b-rhombohedral boron modifica-
tion. MgB12 is obtained as the main phase at temperatures
between 800 and 1000 °C from the reaction of a 1:12 molar
ratio of Mg and B. During the past, intercalation com-
pounds of b-rhombohedral boron were reported as
MxB12 with M = Li, Cu [25], having similar lattice para-
meters as MgB12, although structure solution and refine-
ment of this compound have not yet been successfully
performed because of the poor peak to background ratio
in the XRD powder pattern. A structure refinement based
on synchrotron data has been reported for the composition
MgB20 or rather Mg0.6B12, with Mg ions being partially
distributed over three distinct crystallographic sites.
DTA examinations have already shown that MgB2 can
be transformed by increase of temperature (610 < T <
862 °C) [26] to yield MgB4, the next more boron-rich com-
pound in this binary system. When MgB4 is heated up, it is
gradually (655 < T < 881 °C) [26] converted into MgB7 by
further evaporation of magnesium. Finally MgB7 is con-
verted into MgB12 by arc melting.
Fig. 3. Backscattered electron image of MgB2 made from MgB4 and Mg
(above) and of the sample prepared directly from the elements (below).
MgB2 (bright phase) forms rims around and veins through MgB12 (dark
phase).
Scheme 1.
these transformations obtained from reactions in the bin-
ary Mg–B system are summarized in Scheme 1.
All these reactions, involving the temperature-depen-
dent release of magnesium may be regarded as a disadvan-
tage for the straight forward synthesis of MgB2 from the
elements, where elevated temperatures are desired to over-
come the slow diffusion in solid state. During our work, we
have raised the question whether the reverse reaction, the
diffusion of Mg into a higher Mg boride, is possible or
not. In course of our reactions, we could show that it is
possible to synthesize the metal-rich boride MgB2 out of
MgB4 or MgB7. MgB2 is formed in high yield and good
homogeneity from reactions of MgB4 and Mg at 750 °C,
as represented in the Figs. 1 and 3 and corresponding data
in Table 3. The magnetic properties of our MgB2 material
is represented in Fig. 2.
Electron probe microanalyses (EPMA) were used to
examine the compositions of our prepared samples (1–4)
for a number of randomly selected spots. Results from
these analyses are summarized in Table 3. They show that
our MgB2 sample (4) prepared via MgB4 are more homo-
geneous than sample (3) prepared directly from the ele-
ments, which are composed of two different phases.
These two phases, MgB2 and Mg deficient MgB12 are rep-
resented by the bright and dark areas in the backscattered
electron image shown in Fig. 3 (below). The bright area,
analyzed as MgB2, forms rims around and veins through
the dark area, which has the composition near MgB12
(Table 3, Fig. 3).
A backscattered electron image of a MgB2 sample (4)
obtained from the reaction of MgB4 and Mg is shown on
top in Fig. 3. The image shows a quite homogeneous
(bright) domain of MgB2 embedded into epoxy resin. The
In attempt to synthesize MgB4 by reacting MgB7 with
Mg at 900 °C, this boride is not formed and the reaction
product is a mixture of MgB2 and unreacted MgB7. All

42
R. Schmitt et al. / Physica C 436 (2006) 38–42
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diboride. With our experiments we were able to demon-
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a more homogeneous material than the synthesis from the
elements as proven by EPMA and X-ray diffraction. From
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a first step in reactions. If there is enough magnesium pres-
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