ISSN 0036-0236, Russian Journal of Inorganic Chemistry, 2007, Vol. 52, No. 5, pp. 753–760. © Pleiades Publishing, Inc., 2007.
Original Russian Text © D.M. Babanly, Yu.A. Yusibov, M.B. Babanly, 2007, published in Zhurnal Neorganicheskoi Khimii, 2007, Vol. 52, No. 5, pp. 819–826.
PHYSICOCHEMICAL ANALYSIS
OF INORGANIC SYSTEMS
Phase Equilibria and Thermodynamic Properties
of the System Tl–TlCl–Se
D. M. Babanly, Yu. A. Yusibov, and M. B. Babanly
Rasul-zade State University, Khalilov av. 23, Baku, 370148 Azerbaijan
Received January 28, 2006
Abstract—Phase equilibria in the system Tl–Se–Cl were studied in the region Tl–TlCl–Se by DTA, X-ray
powder diffraction, emf measurements concentration circuits (–) Tl/glycerol + KCl + TlCl/(Tl–Se–Ce) (+), and
microhardness measurements. Several vertical sections, an isothermal section at 400 K, and the liquidus-surface
projection were constructed. The primary crystallization regions and homogeneity regions of phases were
determined, including the α phase of variable composition based on Tl5Se2Cl, the only ternary compound of
the system. The types and coordinates of invariant and monovariant equilibria on the í–ı–Û diagram were deter-
mined. From the results of emf measurements, the standard thermodynamic functions of formation and the stan-
dard entropy were calculated for the compound Tl5Se2Cl.
DOI: 10.1134/S0036023607050166
Previously, the ternary system Tl–Se–Cl was stud- phases of variable composition; the homogeneity
ied only along the quasi-binary sections TlCl–Tl2Se [1]
and TlCl–TlSe [2]. According to work [1], the system
TlCl–Tl2Se forms a single ternary compound, Tl5Se2Cl,
which melts incongruently by a syntectic reaction at
725 K. At the syntectic temperature, the immiscibility
region extends from 15 to 80 mol % Tl2Se. The com-
pound Tl5Se2Cl forms a eutectic with TlCl (5 mol %
Tl2Se, 689 K) and a peritectic with Tl2Se (~93 mol %
regions of the ternary compounds extend far beyond the
quasi-binary joins Tl2X– BIVX and Tl2X–B2V X3, form-
ing extensive fields along the isothermal sections of
í−ı–Û diagrams. The same situation was observed in
the systems Tl–TlI–Se [7] and Tl−TlI−Te [8]. This
proves the pertinence of the investigation of phase equi-
libria over wide ranges of compositions in other
Tl−X−Hal systems. Such data are also necessary for the
thermodynamic investigation of ternary systems by the
emf method [14, 15, 20].
Tl2Se, 684 K). The peritectic reaction L + Tl5Se2Cl
α
produces the Tl2Se-base solid solution, which extends
to 18 mol % TlCl [1].
It was determined in work [2] that the system TlCl–
TlSe has a simple eutectic phase diagram with the
eutectic degenerate at TlSe.
In view of the above, we studied the system Tl–Se–Cl
over a wide range of compositions. The results of the
restudy of the quasi-binary section TlCl–TlSe
described in work [21] differ qualitatively from the data
in above-cited work [2]. According to work [21], this
section has eutectic equilibrium (85 mol % TlSe, 593 K)
and monotectic equilibrium (685 K). At the monotectic
temperature, the immiscibility region extends from 15
to 70 mol % TlSe.
The compound Tl5Se2Cl crystallizes in a Cr5B3-type
tetragonal structure with the following unit cell param-
eters: a = 8.565 Å, c = 12.741 Å, Z = 4, space group
P4/ncc [3]. The lower thallium selenide Tl5Se3 − x
(0.06 < ı < 0.5) has the same structure [4]. Thallium tel-
luride Tl5Te3 [5, 6] and other compounds of the general
formula Tl5X2Hal (where X = Se, Te; Hal = Cl, Br, I),
which also crystallize in the Cr5B3-type structure, have
higher symmetry (space group I4/mcm) [1, 3, 7–9].
Thus, the Tl5X2Hal compounds are ternary structural
analogues of Tl5Se3 – x and Tl5Te3; they are produced by
The sections Tl5Se2Cl–TlSe and Tl5Se2Cl–Tl [22]
are non-quasi-binary by virtue of the syntectic melting
character of Tl5Se2Cl, but they are stable below the soli-
dus. An extensive Tl5Se2Cl-base homogeneity region
was found along the section Tl5Se2Cl–TlSe and an
extensive three-phase immiscibility region was found
along the section Tl5Se2Cl–Tl [22].
the substitution of anions Hal– for anions ï2–. Ternary
analogues of Tl5Se3 – x and Tl5Te3 obtained by cation
substitutions are also known: Tl4BIVX3 and Tl4BVX6
(BIV = Sn, Pb; BV = Sb, Bi; X = Se, Te) [10–17].
Here, we report the results of our complex investiga-
tion of phase equilibria and thermodynamic properties
of the system Tl–Se–Cl in the region Tl–TlCl–Se.
Analysis of phase diagrams for the systems Tl–X
[18–20] and Tl–BIV(BV)–X [10–15] shows that both
Tl5Se3 – x and Tl5Te3 and their ternary analogues are
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