Sm-Sm2Se3 phase diagram and properties of phases

Article abstract of DOI:10.1134/S0036023616010095

A Sm-Sm2Se3 phase diagram has been studied from 1000 K until melting. This system forms three congruently melting compounds: SmSe (ST NaCl, a = 0.6200 nm, T _m = (2400 ± 50) K, and H = 2750 MPa), Sm₃Se₄ (ST Th₃P₄, a = 0.8925 nm, T _m = (2250 ± 30) K, and H = 3350 MPa), and Sm₂Se₃ (ST Th₃P₄, a = 0.8815 nm, T _m = (2150 ± 40) K, and H = 5300 MPa). There are eutectics between Sm and SmSe phases and between SmSe and Sm₃Se₄ phases at 2.5 at % Se, 1300 K and at 54.5 at % Se, 2100 K, respectively. Within the extent of Sm₂₊ Sm₂ ³⁺ Se4-Sm₂ ³⁺Se₃ solid solution (ST Th₃P₄), the experimentally determined percentages of Sm₂₊ ions correspond with the values calculated from the formula compositions of samples. The bandgap width for SmSe_1.45 and SmSe_1.48 phases is ΔE = (1.90 ± 0.05) eV.

Full text of DOI:10.1134/S0036023616010095

ISSN 0036-0236, Russian Journal of Inorganic Chemistry, 2016, Vol. 61, No. 1, pp. 93–98. © Pleiades Publishing, Ltd., 2016.
Original Russian Text © N.Yu. Fainberg, O.V. Andreev, V.B. Kharitontsev, A.A. Polkovnikov, 2016, published in Zhurnal Neorganicheskoi Khimii, 2016, Vol. 61, No. 1, pp. 99–104.
PHYSICOCHEMICAL ANALYSIS
OF INORGANIC SYSTEMS
Sm–Sm Se Phase Diagram and Properties of Phases
2
3
a
a
b
b
N. Yu. Fainberg , O. V. Andreev , V. B. Kharitontsev , and A. A. Polkovnikov
a
Tyumen State University of Oil and Gas, ul. Perekopskaya 15a, Tyumen, 625008 Russia
b
Tyumen State University, Tyumen, ul. Semakova 10, Tyumen, 625003 Russia
e-mail: o.v.andreev@utmn.ru
Received January 27, 2015
Abstract—A Sm–Sm Se phase diagram has been studied from 1000 K until melting. This system forms three
2
3
congruently melting compounds: SmSe (ST NaCl, a = 0.6200 nm, T = (2400 ± 50ꢀ K, and H = 2750 MPaꢀ,
m
Sm Se (ST Th P , a = 0.8925 nm, T = (2250 ± 30ꢀ K, and H = 3350 MPaꢀ, and Sm Se (ST Th P , a =
3
4
3
4
m
2
3
3 4
0
.8815 nm, T = (2150 ± 40ꢀ K, and H = 5300 MPaꢀ. There are eutectics between Sm and SmSe phases and
m
between SmSe and Sm Se phases at 2.5 at % Se, 1300 K and at 54.5 at % Se, 2100 K, respectively. Within
3
4
2+
3+
2
3+
2
the extent of Sm Sm Se –Sm Se solid solution (ST Th P ꢀ, the experimentally determined percentages
of Sm ions correspond with the values calculated from the formula compositions of samples. The bandgap
width for SmSe1 and SmSe phases is ∆E = (1.90 ± 0.05ꢀ eV.
4
3
3 4
2+
.45
1.48
DOI: 10.1134/S0036023616010095
Samarium with selenium forms the compounds
This study is targeted at constructing a SmSe–
Sm Se phase diagram and determining the valence
SmSe (ST NaCl, а = 0.6200 nmꢀ and Sm Se
3
4
2
3
(
ST Th P , a = 0.8925 nmꢀ [1]. The low-temperature states of samarium and the properties of phases in the
3 4
phase α-Sm Se has an orthorhombic Sb S -type range Sm Se –Sm Se .
3 4 2 3
2
3
2 3
structure with a = 1.127 nm, b = 0.409 nm, c = 1.095 nm
[
2–4]; the high-temperature phase γ-Sm Se has an
2
3
EXPERIMENTAL
Samples were prepared from metallic samarium
CmM-1 typeꢀ and selenium (high-purity grade 22-4ꢀ.
Samarium and selenium weights taken in set propor-
tions, the total weight being 5 g, were placed in a silica
ampoule of 10–15 mL in capacity; the ampoule was
degassed and sealed off and then heat treated in a muf-
orthorhombic structure (ST Th P , a = 0.8875 nmꢀ
3
4
[
1–5]. The polymorphic transition from α-Sm Se to
2 3
(
γ-Sm Se occurs at 1270 K; ∆H = 710 J/mol [5]. The
2
3
Sm Se melting temperature was calculated to be
2
3
1
810°С [1].
Samarium polyselenides (monoclinic Sm Se and
4
7
tetragonal SmSe ꢀ thermally dissociate at high tem- fle. Starting with 570 K, the temperature was elevated
1
.8
peratures to lose some selenium [1]. Pressure mea- by 50 K each 24 h until it reached 1270 K, and then
surement is the major tool to study rare-earth chalco- maintained at this level for 100 h [3, 4, 9]. In order to
genides [6].
prepare a Sm Se sample, up to 0.5% excessive sele-
2 3
nium was placed into an ampoule to evolve to the gas
The SmSe phase, which has conductivity ρ = 2 ×
3
15
–3
phase during heat treatment at 1270 K [3].
1
0 Ω cm and charge carrier concentration n = 10 cm ,
was used as a thermoresistor. From the value of effec-
An as-synthesized sample was pounded and fused
tive magnetic moment μ = 2.18 μ , we inferred that by high-frequency currents in a tantalum (50–54 at %
eff
B
2+
3+
Seꢀ or a graphite (56–60 at % Seꢀ crucible. The sample
was twice melted and then cooled. A 60 at % Se sample
was heat treated in selenium vapor [7, 8]. Samples were
annealed for 15 min at (1750–1770ꢀ K. The temperature
maintenance accuracy is ±10 K in muffle furnaces and
±30 K in high frequency currents setup [3–9].
there are two Sm ions and one Sm ion per Sm Se
3
4
formula unit [1].
Neither the melting temperature nor melting char-
acter has been earlier determined for SmSe, Sm Se ,
3
4
and Sm Se [1–18].
2
3
Constructing a SmS–Sm S diagram will help to
The onset melting temperatures and complete
2
3
choose methods and protocols for preparing samples melting temperatures of samples were determined by
of samarium selenides SmSe, Sm Se , and Sm Se
3
visual polythermal analysis (VPTAꢀ [7, 10]. SmSe,
Sm Se , and Sm Se samples melted at temperatures
3
4
2
which are materials of applied importance.
3
4
2
3
93

9
4
FAINBERG et al.
T, K
T, K
2500
2250
2000
2500
2400
L
Sm3Se4
1
2
3
4
Sm Se + L
3
4
2250
2250
2000
1750
1500
1250
1000
SmSe + L
100
2
5
4.5%
L
1750
SmSe + L
Th3P
4
SmSe + Sm Se4
3
1500
1
345
250 2.5%
γSm
αSm
1
α
Sm + SmSe
20
α + γ
βSm
000
1
Sm
10
30
40
SmSe 51 52 53 54 55 56 57 58 59 Sm Se
2
3
at % Se
Fig. 1. Sm–Sm Se phase diagram. The state of samples as probed by VPTA: (1ꢀ onset melting and (2ꢀ complete melting. The
2
3
state of samples as probed by X-ray powder diffraction and microstructure observation: (3ꢀ single-phase and (4ꢀ two-phase con-
stitution.
in the range 20–40 K on the average. The temperature were comprised of two phases as probed by XRD and
at which a sample was rapidly shrunk was taken to be MSA. Phase percentages are as follows: SmSe (ST
the melting temperature of the sample. Temperature NaCl, a = 0.6200 nmꢀ >95 mol %; Sm Se (ST Th P ,
3
4
3 4
determination accuracy was from ±30 to ±50 K and a = 0.8925 nmꢀ <5 mol %. Polished sections of ingots
each time is indicated in the text.
show SmSe faceted grains with angles of 120°. Sm Se
3 4
grains appear as strips in between SmSe grains (Fig. 2ꢀ.
The treatment of as-synthesized samples at 1900–
X-ray powder diffraction (XRDꢀ experiments were
performed on a Dron-7 diffractometer (Ni-filtered
2
000 K yielded single-phase SmSe (a = 0.6200 nmꢀ
CuK radiationꢀ. Unit cell parameter determination
α
cakes; average microhardness of SmSe grains is H =
accuracy was ± 0.0002 nm for cubic phases and
2
750 MPа (Fig. 3ꢀ. Samarium monoselenide, just as
±
0.001 nm for orthorhombic phases.
the SmS phase [11, 12], dissociated during heat treat-
ment by the reaction
Microstructure observation (MSAꢀ was performed
on an MeTAM LV-31 optical microscope in reflected
light. Microhardness was measured with a PTM-3M
tester with precision of 5 to 7%. The methods used to
determine samarium valence states and optical charac-
teristics of samarium selenides are described earlier [8].
SmSe → Sm Se + Sm.
(1ꢀ
3
4
Plaque formed in the cooler parts of the reactor was
identified as metallic samarium.
No extensive SmSe-base solid solutions were found
[
11]. In a 49.5 at % Se sintered sample, metallic
RESULTS AND DISCUSSION
samarium was observed in between SmSe grains, in a
5
0.5 at % Se sintered sample, a Sm Se (a = 0.8925 nmꢀ
The Sm–Sm Se system forms three congruently
3 4
2
3
phase was found in between SmSe grains.
melting compounds: SmSe, Sm Se , and Sm Se (Fig. 1ꢀ.
3
4
2
3
Eutectics are formed between Sm and SmSe and
The congruent melting temperature of the SmSe
between SmSe and Sm Se . At temperatures above phase is 2400 ± 50 K as determined by VPTA. The
3
4
1
300 K, a solid solution having Th P -type cubic congruent melting of SmSe was inferred from the liq-
3 4
structure (a γ-phaseꢀ exists over the range Sm Se – uidus position. There is a peak at the SmSe line. The
3
4
SmSe phase forms eutectics with the conjugated
phases of metallic samarium and the Sm Se phase.
Sm Se .
2
3
3
4
The phase compositions of 50 at % Se samples
SmSe phaseꢀ depends on the temperature schedule of
(
The samples of Sm Se phase prepared in sinters or
3 4
high-temperature treatment. As-synthesized samples fused cooled into single phases. X-ray diffraction pat-
were cast allows once treated at 2350–2450 K and terns of these samples feature only reflections from the
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY
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Sm–Sm Se PHASE DIAGRAM AND PROPERTIES OF PHASES
95
2
3
50 at % Se
52.5 at % Se
55.5 at % Se
3
Sm Se
3
4
2
3
1
1
100 μm
100 μm
100 μm
Fig. 2. Micrographs of SmSe–Sm Se samples. These samples were prepared by melt solidification. Phases: (1) SmSe,
2
3
(
2) Sm Se , and (3) eutectic between SmSe and Sm Se phases containing 54.5 at % Se.
3 4 3 4
H, MPa
(
а)
(b)
.8924 nm
Sm Se
0
3
Sm Se4
0.892
5000
4500
4000
3500
3000
2500
3
4
0
.890
.888
0
3350 MPa
0.886
0.884
0.882
0.880
Eutectic
2
350 MPa
Sm Se
2
3
0
.8816 nm
2
000
60
5
0
51
52
53
54
55 56
at % Se
57
58
59
Sm Se3 Sm Se
57.0 57.5 58.0 58.5 59.0 59.5 60.0
Sm Se
3
SmSe
2
3
4
2
Fig. 3. Panel (a): microhardness versus composition plot for SmSe–Sm Se samples annealed at 1770 K. Microhardness values:
2
3
for SmSe, 2350 MPa (load P = 0.05 kg); for Sm Se , 3350 MPa (P = 0.05 kg); and for Sm Se , 5300 MPa (P = 0.05 kg). Panel (b): unit
3
4
2
3
cell parameter for a Th P -type cubic structure versus composition in the region of Sm Se –Sm Se continuous solid solution.
3
4
3
4
2
3
Sm Se phase (ST Th P , а = 0.8925 nm). The grain Sm Se phase melts congruently. The phase composi-
3
4
3
4
2
3
sizes as assessed by MSA averages 50–100 μm; micro- tion of the sample remained unchanged in the melt-
hardness H = 3350 MPa. The congruent melting tem- ing–cooling process. The Sm Se melting tempera-
2
3
perature of the Sm Se phase as determined by VPTA ture was 2150 ± 40 K.
3
4
is 2250 ± 30 K.
As-synthesized samples of Sm Se phase were
A continuous solid solution having Th P type
3
4
2
3
3
+
2+
structure [(Sm
(Sm ) []
x)]Se is formed
loose cakes. These samples were single phases accord-
ing to XRD; their diffraction patterns features only between Sm Se and Sm Se phases. Ln Se –Ln Se
3
(8−2x) 3
x
(1 – 4
3
4
2
3
3
4
2
reflections from an α-Sm Se phase, a Sb S -type ort- solid solutions are formed for the following lantha-
2
3
2 3
horhombic structure (a = 1.127 nm, b = 0.409 nm, c = nides: lanthanum, cerium, praseodymium, neodym-
3
1
.095 nm, unit cell volume V = 5.041 nm ).
ium, and samarium [1, 13]. The differences between
the unit cell parameters for Ln Se and Ln Se phases
3
4
2
3
The Sm Se samples were annealed and fused
2
3
3
+
0
0
0
are: Δa = 0.0003 nm for lanthanum (rLa 4f 5d 6s =
under a selenium vapor atmosphere. A single-phase
3+
0
4
.103 nm), Δa = 0.0002 nm for cerium (rCe
γ-Sm Se sample (а = 0.8816 nm) was obtained.
2
3
1
0
0
f 5d 6s = 0.101 nm); Δa = 0.0009 nm for praseo-
Microstructure observation of an etched polished sec-
3+
2
0
0
tion (HCl 1 : 150) showed oval grains with sizes of 50– dymium (rPr 4f 5d 6s = 0.099 nm); and Δa =
3
+
3
0
0
8
0 μm having microhardness of 5300 MPa. The 0.0008 nm for neodymium (rNd 4f 5d 6s = 0.098 nm).
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 61 No. 1 2016

9
6
FAINBERG et al.
(а)
(b)
I
Sm3+
R
Sm2+
1
2
3
4
1
3
2
5
6
4
6700
6710
6720
6730
hν, eV
24
20
16
12
3
−1
ν × 10 , cm
Fig. 4. Panel (a): L absorption spectra for samarium selenides: (1) SmSe , (2) SmSe , (3) SmSe1.40, (4) SmSe , (5)
III
1.34
1.37
1.45
SmSe , and (6) SmSe . Panel (b): Reflections from samarium selenide powders in the visible: (1) SmSe , (2) SmSe ,
1.45
1.46
1.48
1.48
(
3) SmSe , and (4) SmSe
.
1.40
1.33
2
+
6
0
0
2+
In Sm Se –Sm Se solid solution (rSm 4f 5d 6s =
0
0
From experimental data, it flows that Sm con-
3
4
2
3
.114 nm [14]), the unit cell parameters change from centration steadily increases over the Sm Se –Sm Se
2
3
3
4
.8924 to 0.8816 nm, Δa = 0.0108 nm, which is one extent, corresponding (within the error bar) to the
2+
order of magnitude higher than for other cerium rare Sm percentage derived from the formula (as-batch)
earths where they are stably trivalent. Valence state composition of the sample (Table 1). The solid solu-
changes of samarium atoms in Sm Se –Sm Se solid
2+
−x
3+
2
2−
4−x
3
4
2
3
tion should be formulated as Sm1 Sm Se (x = 0–1)
and the composition of the Sm Se phase as Sm
solution were determined.
2+
3
4
X-ray absorption spectra LIII (exposure time: 10–16
s
3
2
+
Sm
Se . Presumably, 4f-electron exchange between
4
[
8]) feature two absorption peaks near 6713 and 6720 eV.
2+
3+
–14
Sm and Sm occurs with a frequency of 10 s, as
2
+
The intensity ratio of these peaks is a measure of the Sm
in Sm S .
3
4
and Sm3 ion ratio (Fig. 4a). The Sm percentage of the
+
3+
2+
3+
3+
total Sm and Sm amount in the sample was derived
Experimentally determined Sm
percentages
from the plot with accuracy of ±5% (Table 1).
steadily exceed the calculated values by 2–4% (Table 1).
Table 1. Contents of Sm2+ and Sm3+ ions; characteristics of phases in Sm Se –Sm Se solid solution
3
4
2
3
3
+
Reflection
minimum
Selenium
content, at %
Se
Unit cell
parameter,
nm
Sm /Sm , mol %
tot
Samarium
selenide
Bandgap
width, ∆E, eV
m*
m0
21
–3
N
× 10 , cm
νmin, cm–
1
calcd.
measd.
57.1
57.8
58.4
59.2
60
SmSe1.34
SmSe1.37
SmSe1.40
SmSe1.45
SmSe1.48
0.8924
0.8880
0.8875
0.8845
0.8816
68
72
77
87
95
70
76
3000
900
–
0.265
0.024
–
82
1.95–1.86
1.90–0.05
1.90–0.05
88
>98
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY
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Sm–Sm Se PHASE DIAGRAM AND PROPERTIES OF PHASES
97
2
3
Table 2. Eutectic composition in the Sm–SmSe system calculated by empirical equations [17]
Melting temperature, K, of Empirical equations; eutectic composition, at % Se
low-melting phase,
Sm
high-melting
phase, SmSe
Efimov–
Vozdvizhensky’s
eutectic
1300
Cordes’s
3.7
Vasiliev’s
2.2
1345
2400
2.2
Likely, surface samarium atoms in grains and samples from the eutectic to Sm Se using the Van Laar equa-
3
4
3
+
are in the Sm state due to, for example, the oxidation tion [16].
of Sm2 to Sm by oxygen.
+
3+
In all samples from the range 0–50 at % Se, XRD
and MSA detect only SmSe phases and metallic
samarium. In 5, 10, and 25 at % Se samples, there are
orange-colored oval SmSe grains sized 20–90 μm in
the light field of metallic samarium with infrequent
inclusions of SmSe fine grains sized 3–5 μm. In view
of the high volatility of metallic samarium, VPTA was
used to determine the eutectic temperature. The tem-
Bandgap widths for Sm Se –Sm Se solid solution
2
3
3
4
samples were derived from the reflectivity (R) versus
wavenumber (ν) plot in the visible. The fundamental
absorption edges of SmSe1 and SmSe
samples
1.45
.50
coincide to within the determination error. For a
sample, a diffused absorption edge is
SmSe1
.40
observed, making it difficult to determine the bandgap perature at which a liquid phase appeared in Sm +
width ΔE. A likely reason for this is the transitions of SmSe samples was found to be 1300 ± 10 K against the
electrons lying at localized levels inside the bandgap. melting point of metallic samarium (1345 K [4, 5])
2+
Most likely, these are Sm 4f electrons, which (Fig. 1). The eutectic composition was calculated from
account to 22.8% in this sample. For SmSe and empirical equations (Table 2) [17].
1.37
SmSe1.33 samples, the R(ν) plot in the visible has no
We have not managed to prepare a compact sample
specific features in view of containing an even greater of eutectic composition. At 1200–1250 K, a noticeable
2+
Sm percentage. Additional light absorption by free part of metallic samarium was transferred to cold por-
carries is also possible (Table 1). Samarium selenides tions of an ampoule or reactor.
from SmSe1 to SmSe are semiconductors with the
.45
1.48
We took the eutectic composition to be 2.5 ± 1 at %
Se and the eutectic melting temperature to be (1300 ±
bandgap ΔΕ = 1.90 ± 0.05 eV.
10) K.
Between SmSe and Sm Se phases, a eutectic is
3
4
formed of composition 54.5 at % Se at (2100 ± 50) K.
The eutectic morphology changes depending on the
chemical composition of the sample. Polished sam-
ples of samples having compositions in the region
ACKNOWLEDGMENTS
This study was supported by the Russian Founda-
tion for Basic Research (project no. 14-03-32062) and
by Government Agreement no. 2014/228(1-14) (proj-
ect no. 996).
where primary crystals of the Sm Se phase are
3
4
formed, clearly show primary Sm Se grains as elon-
3
4
gated ovals or polyhedra of 30 to 90 μm and the eutec-
tic formed by alternating elongated crystals of SmSe and
Sm Se phases sized 3–7 μm on the average (Fig. 2). As
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3
4
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5
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3
4
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3
4
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Translated by O. Fedorova
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY
Vol. 61
No. 1
2016