Phase diagrams of novel Tl4SnSe4-TlSbSe2-Tl2SnSe3 quasi-ternary system following DTA and X-ray diffraction

Article abstract of DOI:10.1016/j.jallcom.2016.02.078

Phase relation in the Tl₄SnSe₄-TlSbSe₂-Tl₂SnSe₃ quasiternary system were studied by the DTA and X-ray diffraction in combination with mathematical modeling. The phase diagrams of the Tl₄SnS

Full text of DOI:10.1016/j.jallcom.2016.02.078

Accepted Manuscript
Phase diagrams of novel Tl SnSe –TlSbSe –Tl SnSe quasi-ternary system
4
4
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following DTA and X-ray diffraction
I.E. Barchiy, A.R. Tatzkar, A.O. Fedorchuk, K. Plucinski
PII:
S0925-8388(16)30347-4
10.1016/j.jallcom.2016.02.078
JALCOM 36689
DOI:
Reference:
To appear in: Journal of Alloys and Compounds
Received Date: 12 November 2015
Revised Date: 6 February 2016
Accepted Date: 8 February 2016
Please cite this article as: I.E. Barchiy, A.R. Tatzkar, A.O. Fedorchuk, K. Plucinski, Phase diagrams of
novel Tl SnSe –TlSbSe –Tl SnSe quasi-ternary system following DTA and X-ray diffraction, Journal
4
4
2
2
3
of Alloys and Compounds (2016), doi: 10.1016/j.jallcom.2016.02.078.
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Phase diagrams of novel Tl4SnSe4–TlSbSe2–Tl2SnSe3 quasi-ternary system following
DTA and X-ray diffraction
I.E. Barchiy1, A.R. Tatzkar1, A.O. Fedorchuk2, K.Plucinski3*
1Department of chemistry, Uzhgorod national university, Pidgirna St,. 46, Uzhgorod 88000, Ukraine
2Department of Inorganic and Organic Chemistry, Lviv National University of Veterinary Medicine
and Biotechnologies, Pekarska St., 50, Lviv 79010, Ukraine
3IMUT Warsaw, Kaliskiego 2, 00-908, Warsaw, Poland
* Corresponding author. e-mail: kpluc2006@wp.pl
Abstract
Phase relation in the Tl4SnSe4–TlSbSe2–Tl2SnSe3 quasiternary system were studied by the DTA and X-ray diffraction
in combination with mathematical modeling. The phase diagrams of the Tl4SnSe4–TlSbSe2 and Tl2SnSe3–TlSbSe2
systems, the perspective views of the phase interaction in the ternary system, the liquidus surface projection, the
isothermal section at 423 K were built for the first time. The Tl4SnSe4–TlSbSe2–Tl2SnSe3 system is of the invariant
eutectic type and is characterized by the formation of limited solid solutions following initial ternary compounds. New
complex compounds are not formed.
Keywords: Thallium, Tin, Stibium, Selenide, phase diagrams, thermal analysis, X-ray diffraction, solid state reaction
1. Introduction
The one of the main tasks for the semiconductor materials science and engineering consists the development of new
functional materials. Particular interest present complex Tl chalcogenide crystals and their alloy derivatives. Among
them particular interest is attracted to the Tl2Se–SnSe2–Sb2Se3 ternary system. These ternary system are interesting for
investigation due to formation of ternary compounds which reveal unusual optical, thermoelectric and photoelectrical
behaviors. They have broad practical application in infrared optoelectronics, thermoelectricity, pirometry, optical
triggering, gratings etc. Tl-containing structures are expected to possess low thermal conductivity due to effective
scattering of phonons that are responsible for the transfer of heat in the materials. Thallium complex compounds have
recently attracted interest as promising thermoelectric materials [1,2]. Thallium is similar in crystal chemical behavior
to some of the alkali metals. It forms preferentially compounds in oxidation state +1 and its ionic radius is very close to
the ionic radius of potassium, however the electronegativity is higher. Replacement of potassium by thallium may
enhance the electrical conductivity of a compound, as a result of substantial reduction of the ionicity of the chemical
bonds. Also, thallium is a heavy element and its introduction into a semiconductor will lead to a decrease of the thermal
conductivity.
The Tl2Se–SnSe2 system is characterized by the formations of Tl4SnSe4, Tl2SnSe3 ternary compounds, which
congruently melt at 718 K and 738 K [3-6], respectively. In the Tl2Se–Sb2Se3 system there are formed two congruently
melting compounds: Tl9SbSe6 (743 K) and TlSb2Se2 (730 K) [7-9]. The SnSe2–Sb2Se3 system is of the eutectic type
[10]. The Tl2Se–SnSe2–Sb2Se3 ternary system is divided into the five separate secondary subsystems Tl2Se–
Tl4SnSe4−Tl9SbSe6,
Tl4SnSe4−TlSbSe2−Tl9SbSe6,
Tl2SnSe3−Tl4SnSe4−TlSbSe2,
Tl2SnSe3−TlSbSe2–SnSe2,
SnSe2−TlSbSe2–Sb2Se3 with the four quasi-binary sections Tl4SnSe4−Tl9SbSe6, Tl4SnSe4−TlSbSe2, Tl2SnSe3−TlSbSe2

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and SnSe2−TlSbSe2 [12]. The knowledge of the structure plays a crucial role for the design of crystals with advanced
optoelectronic features [12-17] including infrared photo-transparency, multi-photon excitation, optical modulators in the
Ir spectral range. In the present work we will present the data of the DTA and X-ray diffraction studies in combination
with mathematical modeling of the phase diagrams for the Tl4SnSe4–TlSbSe2 and Tl2SnSe3–TlSbSe2 systems.
Furthermore, the perspective views of the phase interaction in the ternary system, the liquidus surface projection, the
isothermal section will be explored.
2. Materials and methods
As educts we used thallium (I), tin (IV) and stibium (III) selenides. Synthesis of these binary compounds was carried
out with high-purity elements (better than 99.99 wt.%). T12Se, SnSe2 and Sb2Se3 binary compounds were prepared by
the single-temperature method from stoichiometric amounts of the initial elements in evacuated quartz containers. The
binary compounds were purified by zone crystallization methods. The Tl4SnSe4, Tl2SnSe3 and TlSbSe2 ternary
compounds were obtained from stoichiometric amounts of initial binary selenides. Identification of binary and ternary
compounds was done by the DTA and X-ray analysis.
The multicomponent alloys were synthesized from ternary complex selenides in quartz ampoules evacuated to a
residual pressure of 0.13 Pa. After thermal treatment at highest temperature (823 K) for 24 h the samples were slowly
cooled (25-30 K per hour) down to 423 K and homogenized at this temperature for 336 hours. Subsequently the
ampoules were quenched into cold water.
The phase equilibria in the ternary system were investigated by the differential thermal (DTA), X-ray powder
diffraction, microstructure (MSA) analyses. More details about the performed DTA, XRD diffraction and
microstructure are given in the Supplement 1. The classical methods of physico-chemical analysis were used in
combination with the simplex method of mathematical modeling of phase equilibria in multicomponent systems [18].
More details about this modeling approach are given in the Supplement 2. The differential thermal analysis was carried
out by means of a device including an x-y recorder PDA-1 and a chromelalumel thermocouple, with an accuracy of ±5
K. The samples were heated and cooled in a furnace using an RIF-101 programmer, which provided a linear
temperature variation. X-ray powder diffraction was carried out on a DRON-3-13 diffractometer (Cu Kα radiation, Ni
filter). Rietveld refinements of X-ray powder diffraction data were performed by using the WinCSD program [19]. The
microstructure analysis was carried out with a metallographic microscope Lomo Metam R1.
3. Results and discussion
3.1. Tl4SnSe4–TlSbSe2 quasibinary system
Tl4SnSe4–TlSbSe2 quasibinary system is characterized by the eutectic processes L↔Tl4SnSe4+htmTlSbSe2 (see Fig.1).
The lines of primary crystallization are intersected in the invariant point with coordinates 62 mol./% TlSbSe2, 619К.
Limited solid solutions are formed in the system: β on the base of Tl4SnSe4 and δ, δ’ following low-, high-temperature
polymorphic modification of TlSbSe2, respectively. Eutectical processes δ’↔β+δ is observed at 595 К. The solid
solution range based on TlSbSe2 do not exceed 14 mol.% at 423 K.

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Fig.1. Phase diagram of the Tl4SnSe4–TlSbSe2 system (the accuracy of the points corresponds to 5 K)
3.2. Tl2SnSe3–TlSbSe2 quasibinary system
The phase diagram of the Tl2SnSe3–TlSbSe2 system is presented in Fig.2. This system belongs to the Rozeboom type V
and is characterized by the invariant eutectic processes L↔ltmTl2SnSe3+htmTlSbSe2. The coordinates of the eutectic
point correspond to 70 mol.% TlSbSe2 at 635К. In this system are formed ε, ε’ solid solutions based on low-, high-
temperature polymorphic modifications of Tl2SnSe3 and δ, δ’ solid solutions based on low-, high-temperature
polymorphic modifications of TlSbSe2, respectively. The metatectic processes ε’↔L+ε based on the polymorphic
transformation of the ternary compound Tl2SnSe3 is observed at 693 К, eutectic processes δ’↔ε+δ based on the
polymorphic transformation of TlSbSe2 takes place at 597 К. At 423 K the existence of the solid solution range of low-
temperature polymorphic modification of TlSbSe2 is less than 11 mol.%. The lattice parameters of initial ternary
compounds and solid solutions based on TlSbSe2 are shown in Table 1.
Fig.2. Phase diagram of the Tl2SnSe3–TlSbSe2 system (the accuracy of the points corresponds to 5 K)

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Table 1. Lattice parameters of TlSbSe2, Tl4SnSe4, Tl2SnSe3 ternary compounds and solid solutions based on TlSbSe2.
Phase
Lattice parameters
SG P1211, a = 8.481, b = 8.411, c = 15.800 (Å), β = 102.39°, V = 1108.82 Å3
SG Pnam, a = 8.051, b = 8.169, c = 21.240 (Å), V = 1396.93 Å3
Tl4SnSe4 [20]
ltm-Tl2SnSe3 [21]
ht-TlSbSe2 [22]
SG Cmcm, a = 4.514, b = 11.958, c = 4.211 (Å), V = 227.3 Å3
SG P1211, a = 9.137, b = 4.097, c = 12.765 (Å), β = 111.75°, V = 443.83 Å3
SG P1211, a = 9.112(3), b = 4.100(2), c = 12.633(5) (Å), β = 110.64(1)°, V = 441.67 Å3
SG P1211, a = 9.105(3), b = 4.104(2), c = 12.645(5) (Å), β = 110.68(2)°, V = 442.06 Å3
ltm-TlSbSe2 [23]
ltm-TlSbSe2 (this work)
(TlSbSe2)0.95(Tl4SnSe4)0.05
(TlSbSe2)0.95(Tl2SnSe3)0.05 SG P1211, a = 9.114(2), b = 4.107(2), c = 12.645(4) (Å), = 110.72(1)°, V = 442.7 Å3
The solid solutions (TlSbSe2)0.95(Tl4SnSe4)0.05 and (TlSbSe2)0.95(Tl2SnSe3)0.05 have chemical compositions
Tl1.095Sb0.905Sn0.048Se2, Tl1.024Sb0.927Sn0.049Se2, respectively.
3.3. Tl2SnSe3–Tl4SnSe4–TlSbSe2 quasiternary system
A perspective view and the projection of the liquidus surface of the Tl2SnSe3–Tl4SnSe4–TlSbSe2 system are shown in
Fig.3 and Fig.4, respectively. The points A’, B’ and C’, which are located on the edges of triangular prism, represent the
melting temperature of the ternary selenides Tl2SnSe3 (735 K), Tl4SnSe4 (714 K), TlSbSe2 (728 K). The liquidus of the
ternary system consists of four primary crystallization areas: Tl4SnSe4(B’)–e1–E–e2–Tl4SnSe4(B’) (β-phase),
TlSbSe2(C’)–e2–E–e3–TlSbSe2(C’) (δ’-phase), Tl2SnSe3(A’)–m1–m2–Tl2SnSe3(A’) (ε’-phase) and m1–e1–E–e3–m2–
m1 (ε-phase). The fields of primary crystallization are divided by monovariant lines e1–E (process L↔β+ε), e2–E
(process L↔β+δ’), e3–E (process L↔β+ε), m2–m1 (process ε’↔L+ε), which cross at the ternary invariant points Е
(20 mol.% Tl2SnSe3, 32 mol.% Tl4SnSe4, 48 mol.% TlSbSe2, 614 К). In the sub-liquidus part three surfaces depict the
monovariant metatectic processes ε’↔L+ε based on the polymorphic transformation between the low– and high–
temperature modifications of the ternary compound Tl2SnSe3 (715–693 K), invariant eutectic processes L↔β+δ’+ε (at
614 K) and invariant eutectoid processes δ’↔β+δ+ε (at 587 K). At temperature 616 K all alloys possess solid state
phase.
The isothermal section at 423 K of the Tl2SnSe3–Tl4SnSe4–TlSbSe2 system is shown in Fig.5. New complex compounds
were not observed in the ternary system. The types and temperature of the processes in the Tl2SnSe3–Tl4SnSe4–TlSbSe2
quasiternary system are shown in Table 2.

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Fig.3. Perspective view of the Tl2SnSe3–Tl4SnSe4–TlSbSe2 system
Fig.4. Liquidus surface of the Tl2SnSe3–Tl4SnSe4–TlSbSe2 system
Fig.5. Isothermal section at 423 K of the Tl2SnSe3–Tl4SnSe4–TlSbSe2 system
Table 2. Types, temperature of the processes in the Tl2SnSe3–Tl4SnSe4–TlSbSe2 system

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Types
Temperature, K
735
melting of Tl2SnSe3
A’: Tl2SnSe3(sol)↔Tl2SnSe3(liq)
polymorphic transformation of Tl2SnSe3
melting of Tl4SnSe4
710
A”: Tl2SnSe3(high)↔Tl2SnSe3(low)
B’: Tl4SnSe4(sol)↔Tl4SnSe4(liq)
C’: TlSbSe2(sol)↔TlSbSe2(liq)
C”: TlSbSe2(high)↔TlSbSe2(low)
е1: L↔β+ε
714
melting of TlSbSe2
728
polymorphic transformation of TlSbSe2
binary invariant eutectic
binary invariant eutectic
binary invariant eutectic
binary invariant metatectic
binary invariant metatectic
binary invariant eutectoid
binary invariant eutectoid
monovariant eutectic
647
693
619
е2: L↔β+δ’
635
е3: L↔β+ε
715
m1: ε’↔L+ε
693
m2: ε’↔L+ε
595
δ’↔β+δ
597
δ’↔ε+δ
693–622
674–622
724–677
715–693
614
е1–E: L↔µ+η
monovariant eutectic
е2–Е: L↔σ+η
monovariant eutectic
е3–E: L↔µ’+σ
monovariant metatectic
ternary invariant eutectic
m1–m2: ε’↔L+ε
Е: L↔β+δ’+ε
4. Crystallochemistry of the compound TlSbSe2 and their solid state alloys derivatives
According to Fig. 1,2,3 and 5, the most prolonged solid solutions for the titled compounds formed by TlSbSe2. The
crystallochemistry analysis of TlSbSe2 has shown presence interatomic distances Se-Se (Fig.6a) at the border of sum of
ionic radiuses (for atomas Se–2 r=1.91 Å). This fact does not exclude a possibility to existence of the pairs Se-Se.
Localization of electrons for these pairs could favor a deficiency by cations for the such kind of materials. For the
system TlSbSe2–Tl2SnSe3, formation of solid solutions on the basis of TlSbSe2 occurs at a fixed number of cationic
atoms for the existed solid alloys. So here we have a coexistence of cations which may favor a shortening of chemical
bonds Se-Se ( see Fig. 6b). For the system TlSbSe2–Tl4SnSe4, formation of solid solutions is increasing number of
cationic atoms for the existed solid alloys. This can cause defects on anions in the materials on the basis of TlSbSe2
solid solutions. Following Fig. 6c, such substitution may lead to a further decrease of Se-Se bond lengths. So depending
on the synthesis conditions and thermal treatment of the materials for the entire existed solid solutions range one can
obtain the materials with a variable number of defects. However, the defects may cause to favorable changes of these
compounds. More details about the solution forming are given in the Supplement 3.

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Fig.6. Chemical bond distances Se-Se (Å) for structure of ltm-TlSbSe2 compounds: (a) solid state alloys
(TlSbSe2)0.95(Tl2SnSe3)0.05 (b) and (TlSbSe2)0.95(Tl4SnSe4)0.05 (c).
5. Conclusions
Differential thermal, X-ray phase, microstructure analyses and mathematical modeling of phase equilibria in the multi-
component systems were used to construct the two quasibinary sections of the systems Tl4SnSe4–TlSbSe2, Tl2SnSe3–
TlSbSe2, perspective view of the Tl2SnSe3–Tl4SnSe4–TlSbSe2 system, the liquidus surface projection and isothermal
section at 423 K. The character of the monovariant processes, the temperatures and coordinate of the invariant processes
in the ternary system were determined. In this system there exists one invariant processes: Е – L↔β+δ’+ε (20 mol.%
Tl2SnSe3, 32 mol.% Tl4SnSe4, 48 mol.% TlSbSe2, 614 К). The existence of solid solutions of the ternary compounds
Tl2SnSe3, Tl4SnSe4, TlSbSe2 was established. New complex compounds were not formed in the ternary system.

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>Two Tl4SnSe4–TlSbSe2,Tl2SnSe3–TlSbSe2 systems were
explored.
> Invariant processes in the ternary system were determined.
> New complex compounds were not observed in ternary
system.