Phase diagram of ZnAs2–MnAs system

Article abstract of DOI:10.1016/j.mencom.2018.03.038

The ZnAs₂–MnAs system, characterized by X-ray powder diffraction, differential thermal and microstructure studies, is of the eutectic type, with the coordinates of eutectic 73 mol% ZnAs₂ and 27 mol% MnAs and T_m = 716 °C. T

Full text of DOI:10.1016/j.mencom.2018.03.038

Mendeleev
Communications
Mendeleev Commun., 2018, 28, 219–221
Phase diagram of ZnAs2–MnAs system
Sergey F. Marenkin,a,b Alexey I. Ril*a and Irina V. Fedorchenkoa,b
a N. S. Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences, 119991 Moscow,
Russian Federation. E-mail: lelik101@mail.ru
b National University of Science and Technology ‘MISIS’, 119049 Moscow, Russian Federation
DOI: 10.1016/j.mencom.2018.03.038
MnAs (wt%)
20
0
10
L + ZnAs2
10
30
40
L + MnAs
50
48
800
The ZnAs2–MnAs system, characterized by X-ray powder
diffraction, differential thermal and microstructure studies, is
of the eutectic type, with the coordinates of eutectic 73 mol%
ZnAs2 and 27 mol% MnAs and Tm = 716 °C. The solubility of
MnAs in ZnAs2 is lower 1 mol%. Alloys of ZnAs2 with MnAs
are ferromagnetic with Tc ≈ 318 K, their magnetization being
increased with raising the MnAs content.
771
750
L
716
(±5°C)
e
ZnAs2 + MnAs
700
ZnAs2
0
20
30
40
60 MnAs
MnAs (mol%)
In parallel with the design of new spintronic materials based on
superlattices formed as a combination of magnetic and non-
magnetic nanolayers, in which giant magnetic resistance (GMR)1–3
and tunnel magnetic resistance (TMR)4,5 effects were discovered,
an intense research is now underway into grained structures
comprised of a nonmagnetic matrix and ferromagnetic nano-
inclusions.6 The matrix can be nonmagnetic metals7–9 or
insulators.10 Our standpoint is that semiconductor matrix shows
the greatest promise in this context.11–13 Semiconductors are
distinguished by high carrier motility, long relaxation times, and
thereby considerable free paths of carriers. This facilitates the
design of grained structures with GMR and TMR effects.
The ferromagnetic MnAs phase crystallizes in hexagonal lattice
(space group P63/mmc) with the unit cell parameters a = 3.72
and c = 5.71 Å and transforms into a paramagnetic phase at
40–45°C, which crystallizes in an orthorhombic lattice (space
group Pnma).18,19
To choose optimal compositions and synthesis parameters
for grained structures, phase equilibrium in the ZnAs2–MnAs
system was studied using a set of physicochemical methods.†
XRD pattern of precursors (Figure 1, curves 1, 4) coincided
with the ICDD PDF-2 for ZnAs2 and MnAs.11,16 The synthesized
precursors crystallized in the space group P21/c for ZnAs2 and
space group P63/mmc for MnAs. The corresponding XRD patterns
The semiconductor matrix we chose to use in this study was
ZnAs2 and ferromagnetic component was MnAs. ZnAs2 is a
hoping semiconductor with a band gap of 1 eV and considerable
anisotropic optical and electric properties. It crystallizes in the
monoclinic structure (space group P21/c) with the unit cell
parameters: a = 9.277, b = 7.691 and c = 8.010 Å.14,15 MnAs is a
ferromagnetic with the Curie point at 318 K and with saturation
magnetization of 3.4 mB per manganese atom,16 which has semi-
metal properties and 300 K conductivity of ~2×104 W cm–1.17
†
The alloys of ZnAs2 and MnAs were obtained from the compounds
synthesized by the fusion of the high-purity elements Zn, As and Mn. The
synthesis was carried out in electric furnaces using temperature regulators,
with accuracy of ±1 and computer control in double walled fused silica
ampoules evacuated to ~2–10 Pa. The inner ampoule was graphitized
to exclude the reaction of its walls with melts. The temperature and time
regimes were chosen in view of high arsenic vapor pressures. First, the
sample was heated up to 630°C and incubated at this temperature for at
least 24 h. The long time of exposure served for better homogenization.
The second step involved heating at 20 K h–1 up to 930°C and subsequent
exposure for a period of up to 24 h for homogenization of melt. The mass
of each sample was ~10 g.
ZnAs2 (P21/c)
MnAs (P63/mmc)
The samples were investigated by X-ray powder diffraction (XRD),
differential thermal analysis (DTA), optical and scanning electron micro-
scopy (SEM). XRD experiments were performed on a Bruker D8 Advance
instrument (CuKa radiation, l = 0.1540 nm, U = 40 kV, I = 40 mA).
The recording parameters were 0.005° per 2 s of exposure in the range
10° £ 2q £ 90°. The X-ray diffraction patterns were processed with
reference to the ICDD PDF-2 powder. Microstructures were analyzed
with the EPIQUANT optical metallographic microscope and the Carl
Zeiss N Vision 40 scanning electron microscope equipped with Oxford
Instruments X-Max analyzer. Quantitative composition was calculated
from the recorded energy spectrum of the emitted X-radiation. The samples
preparation for the microstructure included cutting ingots to the washers,
grinding by powder SiC with the 20 mm granularity, and polishing with
the diamond paste with the 1 mm granularity. The etching of samples
in the dilute solution of nitric acid or CP4 (HNO3–HF–AcOH, 5:3:3)
with the subsequent washing in the ultrasonic bath for optical studies
was carried out.
4
3
2
1
10 15 20 25 30 35 40 45 50 55 60 65 70
2q/deg
Figure 1 X-ray diffraction patterns: (1) ZnAs2 precursor, (2) 80 mol%
ZnAs2 and 20 mol% MnAs, (3) 60 mol% ZnAs2 and 40 mol% MnAs,
(4) MnAs precursor.
© 2018 Mendeleev Communications. Published by ELSEVIER B.V.
on behalf of the N. D. Zelinsky Institute of Organic Chemistry of the
Russian Academy of Sciences.
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Mendeleev Commun., 2018, 28, 219–221
(a)
(b)
(c)
20 µm
10 µm
20 µm
Figure 2 Microstructure of alloys (a) 90 mol% ZnAs2 and 10 mol% MnAs, (b) 73 mol% ZnAs2 and 27 mol% MnAs (eutectic), (c) 40 mol% ZnAs2 and
60 mol% MnAs.
751
718
725
700
(a) 800
400
(b)
(c)
762
751
800
600
400
200
0
716
714
690
800
400
0
688
0
–400
–800
–1200
–400
–800
–200
–400
4000
10000
12000
4000 6000 8000 10000 12000 14000
6000
10000
14000
18000
t/s
t/s
t/s
Figure 3 DTA curves of alloys (a) 90 mol% ZnAs2 and 10 mol% MnAs, (b) 73 mol% ZnAs2 and 27 mol% MnAs, (c) 40 mol% ZnAs2 and 60 mol% MnAs.
MnAs (wt%)
of alloys (Figure 1, curves 2, 3) contained the peaks which relate
0
10
L + ZnAs2
10
20
30
40
L + MnAs
50
48
only to the phases ZnAs2 and MnAs. The change in the position
of peaks ZnAs2 in the alloys was insignificant. Parameters of the
unit cell for ZnAs2 in alloys are somewhat increased (a = 9.2891,
b = 7.6890 and c = 8.091 Å) as compared with those for pure
ZnAs2 (a = 9.277, b = 7.691 and c = 8.010 Å),14 which indicated
the small solubility of MnAs.
According to the microstructure analysis, ZnAs2 and MnAs
form an eutectic system. All alloys contain two phases: phase
ZnAs2 and eutectic [Figure 2(a)], eutectic [Figure 2(b)], phase
MnAs and eutectic [Figure 2(c)]. The composition of eutectic
was determined by SEM: for MnAs phase, 49.1 at% (Mn), and
49.1 at% (As); for ZnAs2 phase, 0.9 at% (Mn), 32.1 at% (Zn)
and 66.2 at% (As). These data show the solubility of Mn in
ZnAs2 ~1 at%.
800
771
750
L
716
(±5°C)
e
700
ZnAs2 + MnAs
ZnAs2
0
20
30
40
60 MnAs
MnAs (mol%)
Figure 4 The phase diagram of ZnAs2–MnAs system. L is the liquid.
DTA data correlate with the results of X-ray and micro-
structure analysis (Figure 3). According to the DTA of the alloy
90 and 10 mol% MnAs, the heating cycle has endothermic effect
at 718 and 751°C, which appears to be the effect of melting
eutectic and the liquids from the side ZnAs2, respectively.
The DTA showed these effects in cooling cycle at 700 and
725°C [Figure 3(a)]. The significant super cooling upon the
crystallization is characteristic of the polyanion semiconductors,
ZnAs2 relates to this category. Such semiconductors have con-
nections metal–anion (zinc–arsenic) and anion–anion (arsenic–
arsenic).
According to the DTA of heating and cooling cycles, the alloy
73 mol% ZnAs2 and 27 mol% MnAs relates to eutectic com-
position [Figure 3(b)]. The DTA of heating cycle had one endo-
thermic effect at 716°C (eutectic point). Such an effect occurred
at 690°C in the DTA of cooling cycle. The DTA of heating and
cooling cycles of the alloy 40 and 60 mol% is shown in Figure 3(c).
According to microstructure this alloy appeared after the eutectic
composition. Two endothermic effects occurred at 714 and 762°C
in the DTA of heating cycle. The first effect corresponded to
the melting eutectic, the second effect related to the liquid
MnAs. These effects in the cooling cycle were observed at 688
and 751°C of DTA curve. The decrease in the value of super
cooling upon the increase in MnAs concentrations emphasizes
the determining role of the ZnAs2 phase.
The phase diagram of ZnAs2–MnAs system was constructed
based on the DTA, X-ray and SEM data (Figure 4). The coor-
dinates of eutectic are 73 mol% ZnAs2 and 27 mol% MnAs with
Tmelt = 716°C.
The alloys of ZnAs2 with MnAs were ferromagnetic, with Tc
of ~318 K. The magnetization of these alloys increased if the
MnAs content was higher.
In conclusion, ZnAs2–MnAs is a quasi-binary section of the
Zn–As–Mn system. The interaction of ZnAs2 and MnAs has
eutectic character. The alloys of zinc diarsenide with manganese
arsenide are ferromagnetic. The magnetization of these alloys
enlarges with the increase in the MnAs content.
Phase transition temperatures were obtained from DTA measurements.
DTA was performed using 90% Pt with 10% Re as the thermocouples
calibrated according to the melting points of the high-purity elements
Sn, Zn, Sb, Al, Ge. The heating rate was 5 K min–1. The accuracy of the
determination of the temperature of thermal effects was ±5 K. The
measurements were made in evacuated and sealed quartz Stepanov’s
vessels. The weight of models was 1.5 g.
This work was supported by the Russian Foundation for Basic
Research (grant no. 16-03-00796) and the Ministry of Education
and Science of the Russian Federation in the framework of Increase
Competitiveness Programme of MISIS (P02-2017-1-16).
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Mendeleev Commun., 2018, 28, 219–221
10 A. E. Berkowitz, J. R. Mitchell, M. J. Carey, A. P. Young, S. Zhang,
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