CC BY
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UDC 544.344.016:546.28923/24
PHASE DIAGRAMS IN DESIGN OF TOPOLOGICAL INSULATORS BASED ON COMPLEX THALLIUM CHALCOGENIDES
Z.S.Aliev, M.B.Babanly
M.Nagiev Institute of Catalysis and Inorganic Chemistry, NAS of Azerbaijan
ziyasaliyev@gmail.com Received 15.03.2016
The topic of this review is to briefly present the chemistry of the topological insulators (TIs) based on thallium chalcogenides. Numerous phase diagrams of ternary Tl-B -X (BV - Sb, Bi; X - Se, Te) systems include TlBVX2 type compounds which exhibit spin polarized topological surface state is evaluated and compiled. Published data on the phase equilibria in the quaternary Tl-Sb-Bi-X and Tl-BV-X-X' systems are also reviewed and it has been revealed that their quasiternary Tl2X-Sb2X3-Bi2X3 and reciprocal 3Tl2X+B V X3 ^3Tl2X'+B V X3 subsystems form a wide range of solid solutions based on compounds TlBVX2 and B V X3, both confirmed as potential 3D TIs materials. All phase diagrams of analyzed respective systems offer profound possibilities for growth their large single crystals from melt.
Keywords: thallium-bismuth (antimony) chalcogenides, topological insulators, phase diagrams, solid solutions, crystal growth, crystal structure.
Introduction
In last few years, binary and ternary layered chalcogenides of bismuth and antimony are attracting renewed interest because they are represent a new phase of quantum matter with innovative electronic properties. These materials, called "Topological Insulators" (TIs) [1-5], are insulators in the bulk, but they have surface states which imply metallic conduction at the surface due to the existence of well-defined topological surface states (TSS). The charge carriers of these states are spin-polarized and fully protected by the time-reversal symmetry (TRS) from the backscattering by defects, which results in nearly dissipationless current. Allowing electrons to move along their surface, but not through their inside, they emerged as an intriguing material platform for the exploration of exotic physical phenomena, somehow resembling the graphene Dirac cone physics, as well as for exciting application capabilities mainly in spintronics [6], quantum computing [7], low-power electronics [8], and optoelectronics [9].
To date, the numbers of prototype devices, exploiting unique properties of TIs, have already been proposed. For instance, TIs are believed to replace the conventional copper interconnects in integrated circuits. In the case of TIs-based interconnects, the forward and backward moving charge carriers are separated on
two different edges of the same transport medium, reducing the energy loss dramatically due to the absence of the backscattering. The prototypes of the high-speed TIs transistors have also been proposed. In analog circuit applications, the high-speed TIs transistors can realize the cut-off frequency up to terahertz (THz), much higher than that achievable in conventional transistor structures. Nowadays there is special enormous interest for TIs related their potential application in the field of plasmonics and pho-todetectors. The existence of a plasmonic excitation of surface states of TIs in the THz range has huge potential for technological use. The raising interest in THz radiation and in the related application-oriented issues in everyday life requires the progressive development of sensitive and performing systems exploiting powerful, stable and coherent sources as well as fast, sensitive and portable photodetectors. Room temperature THz detection in Field Effect Transistors (FETs) has been demonstrated in a large number of material systems as III-V semiconductors [10], Si-based complementary metal-oxide semiconductors [11], graphene [12], carbon nanotubes [13] and semiconductor nan-owires [14]. 3D TIs can be, in this perspective, a very promising alternative either pristine or with tuned topological surface states and bad gap. Among them, bismuth chalcogenides have
attracted a great interest for the promising prospect of engineering broadband and high-speed optoelectronic devices at room temperature. Bismuth chalcogenides show high mobility of TSS carriers (9000 cmW1) [15], with superb carrier density tunability (around 100%) [16]. Moreover, two-dimensional electron gas (2DEG) arising from TSS of 3D TIs supports a collective excitation in the THz range. Once integrated in FETs, TIs can allow THz detection in the over-damped plasma-wave regime or, via alternative mechanisms, exploiting inherently switchable material properties that could allow different physical regimes to dominate. Optoelectronic devices operating in the THz range can be used in applications addressed to homeland security, such as in airport security systems for detecting dangerous substances (due to that many chemical species, explosives or narcotics have characteristic spectral peaks in THz), but also in environmental applications for the control of air quality in public institutions, where traces of hazardous chemical must be identified with respect to a background spectrum. To date, THz optoelectronic devices have been used in various areas such as air pollution, climate research, industrial process control, workplace safety and medical diagnostics. FETs-based THz detectors open new possibilities of construction of real-time THz imaging systems. Therefore, further application fields of them are industrial quality inspection control, customs inspection and security screening of persons. Moreover, THz radiation has no endangering effects on human beings and enables higher contrast for "soft matter" than X-rays. The expected applications of them in the medical diagnostics, cancer cell detection and possible destruction, industrial and environmental ambient control and non-invasive security detection of explosives and drugs will have a profound impact on the quality of life and environment. Graphene has been proven not to be sufficiently efficient for this type of applications, as a consequence of the absence of a band gap [12]. However, TIs constitute a class of materials much more promising for optoelectronic and plasmonic applications, because they combine
the existence of a bulk band gap with conductive surface states.
Another one of the numerous possible applications field of TIs is design and develops the new generation of biosensor [17]. Biosensors are devices of great importance for medical diagnostics, therapy monitoring, environmental science, biotechnology etc. The discovery of the unique properties of TIs such as fast electron transportation, high spin conductivity, and good biocompatibility can give rise to a new generation of biosensors based on TIs.
In general, all promising TIs based devices will operate faster, consume less power and have much smaller size than the nowadays devices. Nowadays, among the currently known 3D TIs, the tetradymite-like layered binary Bi2Te3, Bi2Se3 and Sb2Te3 and ternary germanium, tin and lead chalcogenides most studied both theoretically and experimentally [5, 1831]. These binary and ternary compounds also serve as a good matrix for design of new materials with desired properties. They are well known for their outstanding structural and electronic properties and include a wide variety of mixed-layer materials with more complex crystal structures than their parent AiVBvi and AV2BVi 3 compounds. Their structures are derived from the tetradymite (Bi2Te2S) [32] type; however, most of them are characterized by distorted rocksalt-type slabs of varying thickness. Chemical bonding within the slabs is between ionic and polar-covalent, whereas the slabs are linked by weak van der Waals forces.
Recent studies have shown that the TlBvX2 type compounds also exhibit the TIs properties [33-35].
As we stated above, 2DEG arising from TSS of 3D TIs supports a collective excitation in the THz range, in the framework of international collaboration we have done research project in this line. We have produced the high-quality single crystalline samples, activate, probe, and exploit the collective electronic excitation of TSS in the Dirac cone. By engineering Bi2Te(3-x)Sex stoichiometry, and by gating the surface of nanoscale field-effect transistors, exploiting thin flakes of Bi2Te2.2Se0.8 or Bi2Se3,
we provide the first demonstration of room-temperature THz detection mediated by over damped plasma-wave oscillations on the "activated" TSS of a Bi2Te2.2Se0.8 flake. The reported detection performances allow a realistic exploitation of TSS for large-area, fast imaging, promising superb impacts on THz photonics [36].
Another very promising material for spintronic applications are organometallic molecules. Adding metal atoms to the organic framework can introduce new effects: magnetic impurities and organometallic complexes on the surface of a topological insulator are predicted to break the time reversal protection of the surface state. Taking into account this actuality, we have studied the electronic structure modification induced by the adsorption of different coverage of cobalt phthalocyanine on Bi2Se3 surface. The results from an angle-resolved photomission spectroscopy (ARPES) and scanning tunnelling microscopy (STM) demonstrate that that the hybrid interface can act on the topological protection of the surface and bury the Dirac cone below the first quintuple layer [37].
The study of mechanical properties is crucial for designing many technological applications as nanoelectromechanical systems (NEMS) and flexible electronic devices and, moreover, for the promising prospect of TI-based mechanical materials. However, the comprehension of mechanical properties of TIs is still unsatisfactory. Determination of the fracture toughness plays a crucial role for the potential application of topological insulators in flexible electronics and nanoelectromechanical devices. In our recent research work [38], using depth-sensing na-noindentation test we have investigated for the first time the fracture toughness of bulk single crystals of Bi2Te3 topological insulators grown by the Bridgman-Stockbarger method. Our results highlight one of the possible pitfalls of the technology based on topological insulators. By means of density functional theory (DFT) and nanoindentation tests, we have studied the mechanical properties of bismuth telluride by taking into account its anisotropy [39, 40].
In this review, the data on phase diagrams of the ternary Tl-BV-X and quaternary Tl-Sb-Bi-X, Tl-BV-X-X' (BV - Sb, Bi; X, X' - Se, Te) systems are systematically evaluated.
The role of phase diagrams in the design of TIs
The development of new multicompo-nent materials, including TIs, first of all required studying the phase equilibria in the relevant systems and the plotting the phase diagrams. The phase diagrams allow identifying new chemical compounds or phases of variable composition, as well as establishing their formation character, primary crystallization and homogeneity areas, phase transformations, etc. All these displayed data plays an important role in development or modification of methods for synthesis and crystal growth [41].
Almost all of the binary and ternary compounds with the TIs properties, and solid solutions based on them are thermodynamically stable and have been published in their equilibrium phase diagrams and thermodynamic properties were investigated in numerous studies and are routinely used for their synthesis with a given composition, growth of single crystals [42-44].
As it has been known, the melting temperature (primary crystallization) of the almost all member of the Bi2Se3 family TIs does not exceed 1000 K which the vapor pressure of more volatile components of these compounds: tellurium (Tb = 1263 K) and selenium (rb = 958 K) is quite low
[45]. The starting elementary components of those compounds and they themselves do not react with quartz up to this temperature limit. Therefore, these compounds are usually synthesized by melting elemental components in a quartz ampoule under a vacuum or inert atmosphere.
The information in the phase diagrams is especially valuable for the development or modification of techniques for direct synthesis of samples and their crystals growing from melts and solutions by directed crystallization.
The vertical Bridgman-Stockbarger and Czochralski methods as well as horizontal direct crystallization for the growth of large single crystals of TIs compounds the crystallization from the melt are routinely used techniques
[46]. The detailed description of the above methods and their different modifications can be found elsewhere [47-49].
Phase diagrams of the ternary Tl-BV-X (BV- Sb, Bi; X - Se, Te) systems
In ternary Tl-Bv-X systems thermody-namically stable phases formed on certain qua-sibinary section. Therefore, for the preparation of single crystals of many ternary compounds, the T-x diagrams of the corresponding quasi-binary systems are often used. More comprehensive information on the character of phase equilibria in the ternary systems are representing as a projection of the concentration triangle. Similar diagrams allow varying the solution-melt composition along the primary crystallization surface of grown phase and its crystallization temperature in a broader range.
Phase equilibria in the Tl-Bv-X systems have been reported in numerous research works [50-69].
The Tl-Sb-Se system. The quasibinary section Tl2Se-Sb2Se3 of this system studied in [50-55]. According to [50], three ternary compounds, namely TlSbSe2, Tl9SbSe6 and Tl5SbSe4 formed congruently at 748 and 743 and incongruently at 628 K, respectively. However, the following compounds with the compositions TlSb3Se5, TlSbSe2 and Tl9SbSe6 have been found in this system by authors of [51].
The former two compounds melt peritectically whereas the latter one is reported congruent melting. According to [52], Tl9SbSe6 and TlS-bSe2 melt congruently while TlSb3Se5 melts incongruent. The authors of [52] have also found the two immiscibility areas, covering the range of compositions 22-31 and 80-90 mol. % Sb2Se3. However, the presence of these immis-cibility areas is not confirmed in [53, 54], in which the two thallium-antimony selenide melt congruently at 750 K (TlSbSe2) and 740 K (Tl9SbSe6), and two ones melt with the decomposition on the peritectic reaction at 740 K (TlSb3Se5) and 625 K (Tl5SbSe4). An updated version of the T-x diagram of the Tl2Se-Sb2Se3 system, reported in [53] shown in Figure 1.
According to this diagram, the compound with a composition Tl3SbSe3 melts at 625 K in-congruently and has a phase transition at 575 K. In a recently published work [55] for this compound composition confirmed as Tl26Sb4Se19.
In [52, 56, 57] two versions of the complete the T-x-y-diagram of the Tl-Sb-Se system are presented. According to [52], besides the above mentioned compounds, there are also Tl2SbSe2, Tl3SbSe2, Tl4SbSe2, located on the quasi-binary section TlSbSe2-Tl, and Tl6Sb8Se12.
T,K
Fig. 1. Phase diagrams of Tl2Se-Sb2Se3 (a) [53], T№-Sb2Te3 (b) [65], Tl2Se-Bi2Se3 (c) [59], T№Bi2Te3 (d) [69] systems.
40 60 80 Bi.Te,
mol. % Bi,Te,
These compounds have not been confirmed in [57] (Figure 2), according to isopleth section TlSbSe2-Tl is not a quasi-binary and crosses quasi-binary section Tl2Se-Sb, Tl9SbSe6-Sb and stable in the subsolidus section Tl3SbSe3-Sb. Alloys with compositions Tl4SbSe2, Tl3SbSe2 and Tl2SbSe2 reported in [52] as the ternary compounds, correspond to points of intersection of the above pointed sections and are two-phase mixtures: Tl2Se+Sb, Tl9SbSe6+Sb and Tl3SbSe3+Sb. Other "compounds", found in [52] (Tl6Sb8Se12) also consists of a two-phase mixture TlSbSe2+ Se [56].
The Tl-Bi-Se system. The quasibinary section of Tl2Se-Bi2Se3 investigated in [58, 59]. According to [58], it is characterized by the formation of two congruently melting ternary compounds: TlBiSe2 (980 K) and №iSe6 (785 K). Repeated research [59] confirmed the data of [58] and showed that Tl9BiSe6 forms a continuous series of solid solutions with Tl2Se (5-phase). Authors [60] using a result of the DTA suggest the formation of the ternary phase with the composition Tl0.78Bi107Se2. A T-x-y phase diagram of the Tl-Bi-Se was constructed in [61] (Figure 2).
The Tl-Sb-Te system. The quasibinary section Tl2Te-Sb2Te3 is characterized by the formation of two ternary compounds Tl9SbTe6 and TlSbTe2 that melt congruently at 798 K and incongruently at 753 K, respectively [61]. In [62-64] this system has studied entirely. Data of [61] on a Tl2Te-Sb2Te3 section (Figure 1) are confirmed, as well as the isothermal section and the liquidus surface projection of the Tl-Sb-Te system is constructed (Figure 2).
The Tl-Bi-Te system. The phase diagram of the Tl9BiTe6-Bi2Te3 section is constructed in [65]. It is shown that the compounds Tl9BiTe6 and TlBiTe2 melt congruently at 830 and 850 K, respectively. The Tl2Te-Bi2Te3 section is studied [66], and data of [65] are confirmed. In addition it was shown that Tl9BiTe6 forms with Tl2Te-type a limited solid solutions series with morphotropic phase transition.
According to [67] the TlBiTe2 compound melts incongruently at 793 K, and the distectic maximum according to another phase with composition Tl0,83Bii,06Te2.
20 e, 40 60
at.%Sb
Tl e' D
40 e> 60 at.%B>
Fig. 2. Liquidus surface projections of Tl-Sb-Se (a) [57], Tl-Sb-Te (b) [65], Tl-Bi-Se (c) [61] and Tl-Bi-Te (d) [69] systems. The dotted line - are quasibinary sections. The primary crystallization areas of TIs phases are colored.
In [68], authors show that TlBiTe2 melt incongruently at 788 K. Besides reported ternary compounds the new phase Tl08Bi12Te2 with congruent melting at 815 K was discovered. Authors of [69] showed the complete T-x-y diagram of the Tl-Bi-Te system (Figure 2) and a new version of its isopleth section Tl2Te-Bi2Te3 (Figure1). It was found [69] that the Tl2Te-Bi2Te3 section, unlike other sections of Tl2X-B2X3 types, is no quasibinary (Figure 1). This is due to a noticeable (~5 at.%) deviation of distectic point from the stoichiometric composition TlBiTe2.
Thermodynamic properties of ternary phases in the Tl-Bv-X systems, including TlBVX2 compounds were studied in a number of research works experimentally by EMF technique [54, 57, 58, 62, 70-72].
Thus, the phase equilibria in the ternary Tl-Bv-X systems were established via consecutive studies of various research teams and the latest versions of their phase diagrams can be considered reliable enough. Compounds TlBvX2, and phases based on binary compounds B^X possess TIs properties and have vast areas of primary crystallization both on quasi-binary sections Tl2X-BV2X3 (Figure 1), and on T-x-y diagrams (Figure 2). Therefore, their single crystals can be grown from melts of various compositions both on indicated sections and on areas of primary crystallization on the T-x-y diagrams.
Quaternary systems composed by binary chalcogenides of Tl, Sb and Bi
A series of works [73-84] present the results of studies of phase equilibria in quaternary Tl-Sb-Bi-Se(Te) and Tl-Sb(Bi)-X-X' (X, X' -S, Se, Te) systems on quasiternary (Tl2X-Sb2X3-Bi2X3) and ternary reciprocal (3Tl2X+ BV X'3^3ThX'+ BV X3) planes. In these studies the primary crystallization field of nonstoichio-metric phases based on ternary TlBvX2 and binary BVX compounds were specified, that can be used for growing their single crystals.
Below, the isopleth TlBvX2-TlBvX'2 and TlSbX2-TlBiX2 (Figure 3) as well as isothermal sections at 300 K of the phase diagram (Figure 4, 5) and liquidus surfaces projection of some of these systems (Figures 4, 6) were represented.
As it can be seen from Figure 3, the sulfide-selenide systems are quasi-binary, and the system comprising the compounds TlSbTe2 and TlBiTe2 are no quasibinary due to incongruent melting character of the TlSbTe2 and significant deviation distectic point of the TlBiTe2 from the stoichiometric composition.
All eight considered systems are characterized by the formation of wide or continuous substitutional solid solutions. Should be noted that in TlBiTe2 including systems the compositions of solid solutions based on this compound might be little bit shifted from the T-x- plane.
Solid phase equilibria diagrams (Figures 3, 4) demonstrate the homogeneity region of solid solutions based on binary and ternary compounds, in particular TlBVX2 and their heterogeneous equilibria with each other. It is evident that the field of solid solutions based on TlBVX2 and BVX and possessing TI properties, located along sections TlBVX2-TlBVXf2, TlSbX2-TlBiX2 and boundary BV2X3 -BV2X3 and Sb2X3-Bi2X3 systems as narrow strips.
Liquidus surface projection of systems mentioned above (Figures 4, 6) demonstrate the field of primary crystallization of phases, in particular solid solutions based on TlBVX2 and BVX. Fields of primary crystallization of phases, which are potential TIs materials, painted green (phases based on TlBVSe2 and TlBVTe2) and yellow (phases based on Bi2Se3, Sb2Te3 and Bi2Te3) on Figures 4 and 6.
The crystal structures of thallium based TIs
The crystal structure data and features of the TlBVX2 compounds and solid solutions series based on them have been studied in [8587] and reviewed in detail by Cava et al. [88]. The TlBVX2 compounds are crystallized in rhombohedral crystal system R-3m (#166). Figure 7 shows the crystal structure of the TlBiSe2 and TlBiTe2 compounds. The metal atoms are arranged in the following positions: Tl 3a (0, 0, 0); Bi 3b (0, 0, 0.5); and anions in position with one variable coordinates: Se, Te or 6c (0, 0, 0.25). The unit cell parameters are: a = 4.24 Â (Se), a= 4.527 Â (Te), c = 22.330 Â (Se), and 23.118 Â (Te).
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o
c
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r
&
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CT\
TIShSj 20 40 60 80 TISbSe, mol. % TISbSe.
TIBiS, 20 40 60
80 TIBiSe,
T.K
L c L+P
L+r\/ L+y, L+p+y.
« T,+T; y,
T.K 1000
mol.% TIBiSe,
L d
L+y, Lty!
y, _____
, e T] +Ti I 1 1 \ */
TISbS, 20 40 60
mol. % TISbS
«0 TISbTe, TIBiS, 20
40 60 80 TlBiTe,
raol.% TlBiTe,
T.K
JL+ii L e L+P\
\\C y' [71 L^Y; __ L+P+y,
v vn V:
T'7e»—
Vi+Ti
TISbSe, 20
40 60 80 TISbTe, TIBiSe. 20 40 60
80 TlBiTe.
raol.% TISbTe,
mol.% TlBiTe.
L k
J
7+Ti /[
y \J fly,
TISbSe, 20
40 60 80 TIBiSe. TlBiTe, 20
raol.% TIBiSe,
40 60 80 TlBiTe.
mol.% TlBiTe,
Fig. 3. Phase diagrams of TlSbS2-TlSbSe2 (a) [75], TIBiS.-TlBiSe, (h) [76], TlSbS2-TlSbTe2 (c) [78], TlBiS2-TlBiTe2 (clj [83], TlSbSe.-TlSbTe. (e) |80], TlBiSe2-TlBiTe2 (/) [84]. TlSbSe2-TlBiSe2 (g) [73]. TlSbTe2-TlBiTe2 (A) [74] systems.
Tl,Se
a Tl^bSc, Zl^Tl.BiSe,
Sb.Se, 2D p, 40 60 80 Bi,Se,
mol. %Qi,Se,
N
in >
r
m <
CO
to
>
CO r-
Fig. 4. Solid-pliase equilibria diagrams and liquidus surface projections of Tl2Se-Sb2Se3-Bi2Se3 (a), (h) [73] and Tl2Te-Sb2Te3-Bi2Te3 (c), (d) [74] systems. The primary crystallization areas of potential TIs phases are colored.
VO
TISb.S. TISb.S,
S^S, 20
mol.% Sb.Sc
С
mol.% BijSc, d
f
Sb.Se,
Fig. 5. Solid-phase equilibria diagrams of reciprocal systems: (a) 3Tl2S+Sb2Se3^3Tl2Se+Sb2S3 [75]; (b) 3Tl2S+Bi2Se3^3Tl2Se+Bi2S3 [76]; (c) 3Tl2S+Sb2Te3^3Tl2Te+Sb2S [78]; (d) 3Tl2S+Bi2Te3^ 3Tl2Te+Bi2S3 [83]; (e) 3Tl2Se+Sb2Te3^3Tl2Te+Sb2Se3 [80]; f) 3Tl2Se+Bi2Te3^3Tl2Te+Bi2Se3 [84].
b
a
e
Fig. 6. Liquidus surface projections of the reciprocal systems: (a) 3Tl2S+Sb2Se3^3Tl2Se+Sb2S3 [75]; (b) 3Tl2S+Bi2Se3^3Tl2Se+Bi2S3 [76]; (c) 3Tl2S+Sb2Te3^3T№+Sb2S [78]; (d) 3Tl2S+Bi2Te3^ 3Tl2Te+Bi2S3 [83]; (e) 3Tl2Se+Sb2Te3^3T№+Sb2Se3 [80]; f) 3Tl2Se+Bi2Te3^3Tl2Te+Bi2Se3 [84]. The primary crystallization areas of potential TIs phases are colored.
Cubic close-packed (-A-B-C-A-B-C-) layers (see Figure 7) alternate in the following order: -(Se,Te)-Bi-(Se,Te)-Tl-. Despite the lack van der Waals bonds, these crystals are very well cleaved along planes (001), in which topological surface states is found. However, until recently, experimentally verified data on struc-
ture of the split surface is absent. The authors of [35] studied cleaved surface of the TlBiSe2 by scanning tunneling microscopy and photoelec-tron spectroscopy of basic levels have concluded that the thallium atoms create islets over the layers of atoms Se, covering the last half.
Conclusion
A new class of functional materials -topological insulators - due to its unusual physical properties is extremely promising for a various applications ranging from spintronics and quantum computing to medicine and security systems. Therefore, one of many interdisciplinary problems of chemistry and materials science is a significant expansion of the range of investigated materials by development of new compounds and alloys based on them using
phase diagrams of the corresponding systems.
Our literature survey has shown, that the phase equilibria in the ternary Tl-BV-Se(Te) systems, as well as in quasiternary Tl2X-Sb2X3-Bi2X3 and reciprocal 3ThX+ B^X; ~ 3ThX'+ B^X3 systems studied in detail. Constructed complete T-x-y diagram of these systems and their various isopleth and isothermal sections offer ample opportunities for growth of single crystals in ternary and more complex phases.
The work was supported by the Science Foundation of the State Oil Company of Azerbaijan Republic (The grant for the project "Preparation and study of new functional materials based on multi-nary chalcogenides for alternative sources of energy and electronics", 2014).
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FAZA DiAQRAMLARI MUROKKOB TALLiUM XALKOGENiDLORi OSASINDA TOPOLOJi iZOLYATORLARIN DlZAYNINDA
Z.S.Oliyev, M.B.Babanli
Tallium xalkogenidlari asasinda topoloji izolyatorlann (Ti) dizayninin kimyavi aspektlarina aid i§larin qisa icmali verilir. Ti xassalarina malik TlBVX2 tipli birla§malarin amala galdiyi Tl-BV-X (BV - Sb, Bi, X - Se, Te) uglu sistemlarda faza tarazliqlarina aid malumatlar sistemla§dirilmi§dir. Hamginin Tl-Sb-Bi-X va Tl-BV-X-X dord komponentli sistemlarinin Tl2X-Sb2X3-Bi2X3 kvaziuglu va 3Tl2X+BV2X3^3Tl2X+BV2X3 qar§iliqh mustavilari uzra faza tarazliqlarina aid naticalar umumila§dirilmi§ va g6starilmi§dir ki, bu sistemlar TlBVX2 va BV2X3 birla§malari asasinda, potensial 3D Ti materiallar olan geni§ bark mahlul sahalarinin amala galmasila xarakteriza olunur. Baxilan sistemlarin faza diaqramlan muvafiq Ti fazalann iri monokristallannin alinmasi ugun geni§ imkanlar agir.
Agar sozlar: tallium-bismut(stibium) xalkogenidlari, topoloji izolyatorlar, faza diaqramlari, bark mahlullar, kristal yeti§dirma, kristal qurulu§u.
ФАЗОВЫЕ ДИАГРАММЫ В РАЗРАБОТКЕ ТОПОЛОГИЧЕСКИХ ИЗОЛЯТОРОВ НА ОСНОВЕ
СЛОЖНЫХ ХАЛЬКОГЕНИДОВ ТАЛЛИЯ
З.С.Алиев, М.Б.Бабанлы
Представлен краткий обзор работ по химическим аспектам разработки топологических изоляторов (ТИ) на основе халькогенидов таллия. Систематизированы данные по фазовым равновесиям в трехкомпонентных системах Tl-BV-X (BV - Sb, Bi, X - Se, Te), в которых образуются соединения типа TlBVX2, демонстрирующие свойства ТИ. Также рассмотрены данные о фазовых равновесиях в четверных системах Tl-Sb-Bi-X и Tl-BV-X-
X и показано, что квазитройные Tl2X-Sb2X3-Bi2X3 и взаимные 3Tl2X+B f2 X 3~3Tl2X+B f2 X3 сечения этих систем характеризуются образованием широкого спектра твердых растворов на основе соединений TlBVX2 и BV2X3, являющихся потенциальными 3D ТИ-материалами. Фазовые диаграммы рассмотренных систем открывают большие возможности для получения крупных монокристаллов указанных фаз из расплава.
Ключевые слова: халькогениды таллия-висмута(сурьмы), топологические изоляторы, фазовые диаграммы, твердые растворы, выращивание кристаллов, кристаллическая структура.
