Novel variable phases in the quaternary Pb-Bi-Te-Se system along the PbBi2Te4-²PbBi2Se4² isopleth section Текст научной статьи по специальности «Химические науки»

54

AZERBAIJAN CHEMICAL JOURNAL № 4 2019

ISSN 2522-1841 (Online) ISSN 0005-2531 (Print)

UDC 546:544.016/.013:546.81/87:546.23/24

NOVEL VARIABLE PHASES IN THE QUATERNARY Pb-Bi-Te-Se SYSTEM ALONG THE PbBi2Te4-"PbBÎ2Se4" ISOPLETH SECTION

Z.S.Aliev

Azerbaijan State Oil and Industry University Institute of Physics, NAS of Azerbaijan

ziyasaliev@gmail.com; ziya.aliev@asoiu.edu.az

Received 01.07.2019

The phase eqilibria in the Pb-Bi-Te-Se quaternary system along the PbBi2Te4-"PbBi2Se4" isopleth section was investigated experimentally using differential thermal analysis and X-ray diffraction analyses of the equilibrated alloys. It was shown that the system is non-quasi-binary one and featured by the formation of the tetradymite-type solid solutions based on the PbBi2Te4 in the wide range of concentration. The system has a more complex phase equilibria scheme that comes from its host reciprocal PbTe-PbSe-Bi2Se3-Bi2Te3 system. One can see that, in order to reveal the phase equilibria scheme and understand the crystallization scheme, reactions types etc., the thorough investigation of the entire pointed reciprocal PbTe-PbSe-Bi2Se3-Bi2Te3 system is very important.

Keywords: tetradymit-type solid solutions, layered materials, bismuth selenide, lead bismuth telluride, phase diagram.

doi.org/10.32737/0005-2531-2019-4-54-58 Introduction

Up to date, numerous binary or more complex compounds have been reported to have topological insulating properties [1-4]. Among them, the most studied materials are tetradymite-like layered compounds in which each atomic layer consists of only one type of atoms. Nevertheless, it has been predicted that it is possible to design the disordered alloys with statistical distribution of different types atoms [5-8]. For instance, a series of the similar heterostructured compounds like PbBi4Te7, PbBi6Te10, Sn(Bi,Sb)4Te7 etc., formed by alternation of quintuples (five-layered blocks) [AV2Te3] and septuples (seven-layered blocks) [AIVAV2Te4] (X=Bi, Sb; Y=Pb, Sn) blocks of TIs have been theoretically [4, 9-11] and experimentally investigated [3, 4, 12]. As can be seen, the mentioned heterostructured compounds are only tellurium-based compounds. However, theoretically there is no any impediment to partial substitution of the tellurium by selenium that leads to significant modication in the electronic structure. For example, it was shown in [9] that the substitution of Te by Se in septuples of the PbBi2Te4-xSex compound results by significant increase of the bulk band gap. In the aforementioned type hetero-structures, which are contain alternation of the quintuples and septuples with both Se and Te atoms, the band gaps is expected to be significant wide.

The phase diagram of the PbTe-Bi2Te3 system and its layered ternary compounds are investigated in numerous of works. According to first appeared report by Elagina [13], this system hosts only one intermediate ternary phase, PbBi4Te7 that melts incongruently at 850 K. Later, the new version of the phase diagram was appeared by Hirai [14] in which new metastable compound, Pb2Bi2Te5 was shown instead of stable PbBi4Te7. This compound was found to be melt peritectially at 851 K and to stable until 668 K. Below, it eutectoidally decomposes into PbTe+Bi2Te3 eutectoid mixture. The existence of this compound was confirmed in Refs. [15] as thin films state and its crystal lattice parameters were reported.

The latest version of the phase diagram of the PbTe-Bi2Te3 system and ternary compounds formed in this system which are belong to «(PbTe)w(Bi2Te3) homologous series, can be found in the series of publications by Shelimova and Karpinskii [16, 17]. In these works, shown that the compound PbBi4Te7 melts congruently at 858 K, whereas PbBi2Te4 has a peritectic melting point at 856 K. Furthermore, three ternary layered compounds belonging to the homologous series «(PbTe)w(Bi2Te3) were found along this section: Pb2Bi6Te11, PbBi6Te10, and PbBi8Te13.

Unlike PbTe-Bi2Te3 system, in the PbSe-Bi2Se3 compounds, the PbSe bilayers are not incorporated into Bi2Se3 quintuples to form PbBi2Se4 septuples containing Pb atoms in the central atomic plane (Se-Bi-Se-Pb-Se-Bi-Se) [17]. Instead, the composition of the PbSe-Bi2Se3 alloys can be expressed as [(PbSe)5]n[(Bi2Se3)3]m, which are the monoclinic crystal structure can be described as wBi2Se3 quintuple layers sandwiched by adjacenting of n bilayers of rocksalt structured PbSe [18], forming a natural multilayer heterostructure that consisted by topological insulator and an ordinary insulator. One can see that, starting from the PbBi2Te4 compound it would be very interesting to study solubility limit of selenium in the entire PbTe-Bi2Te3 system. According to [19], within the PbSe-PbTe-Bi2Se3-Bi2Te3 reciprocal system, the tetradymite structured layered phases extends from initial PbBi2Te4 up to PbBi2Te°.88Se312, however, at higher concentrations for selenium, alloys transform to monoclin-ic structure from hexagonal.

Therefore, in order to rational design of these type new materials strongly requires the investigation of the phase diagram of the respective element systems in order to determine solubility limit, synthesis condition for the poly- and single crystalline alloys, annealing regime, crystallization sequence from liquid phase etc.

In this paper, we present the experimental investigation of the phase relationships in the Pb-Bi-Te-Se system along the PbBi2Te4-"PbBi2Se4" section in order to search new quaternary alloys with variable compositions.

Experimental Part

Synthesis. Synthesis of the PbBi2Te4-"PbBi2Se4" alloys was performed from the respective elements of a high purity grade (not less than 99.999%) in sealed (~10-2 Pa) silica ampoules at 1200 K according to phase diagram of the PbTe-Bi2Te3 [16] and PbSe-Bi2Se3 systems [18]. The samples were stirred at this temperature by shaking the furnace and then cooled down to room temperature with the switched-off furnace. The synthesized alloys were further

annealed at 820 K for 200 h in order to get complete homogenization.

Analysis. The equilibrated alloys were analyzed by X-ray powder diffraction (XRD) and differential thermal analysis (DTA). X-ray powder diffraction data were collected at room temperature using a Bruker D8 ADVANCE powder diffractometer with Cu^a1 radiation within the range of 29=5^75°. The unit cell parameters were calculated by indexing of powder patterns using Topas V3.0 software. For the DTA measurements, the THERMOSCAN-2 device equipped with chromel-alumel thermocouples was used. The heating rate was 10 Km-1. Temperatures of thermal effects were taken from the heating curves.

Results and discussion

The phase diagram of the PbBi2Te4-"PbBi2Se4" isopleth section was plotted based on the DTA and XRD data of the equilibrated alloys. The plotted phase diagram (Figure 1) shows that this system is non-quasi-binary iso-pleth of the quaternary system Pb-Bi-Te-Se along the PbTe-PbSe-Bi2Se3-Bi2Te3 plane. As can be seen from the phase diagram, the PbSe1-xTex solid solutions (a-phase) primarily crystallizes from the liquid phase.

Obviously from literature data, the boundary PbSe-PbTe sub-system of the PbTe-PbSe-Bi2Se3-Bi2Te3 reciprocal system featured by the formation of the continuous solid solution field, whereas, other two sub-systems, namely PbTe-Bi2Te3 and PbSe-Bi2Se3 host ternary compounds PbBi2Te4 [16] and Pb5Bi6Se14 [18] those melt incongruently with formation of the L+PbTe and L+PbSe biphasic mixtures.

The XRD analysis of the alloys shows that the system hosts tetradymite-type solid-solution field based on the PbBi2Te4 (Figure 2) up to ~75 mol% "PbBi2Se4".

Within the compositional range up to 75 mol% "PbBi2Se4", the crystallization process below the liquidus continuous on the peritectic L+a^y1 reaction, where y1 is the solid solution based on PbBi2Te4. This monovariant process ends up simultaneously complete consuming of melt and a-phase, consequently the system

transform to monophasic phase of yi. This is due to fact that, all the investigated alloys having chemical composition according to PbBi2Te4-xSex stoichiometry.

Within the compositional range of ~75-100 mol% "PbBi2Se4" one can see the complex physicochemical interaction of the phases,

where only through investigation of the entire reciprocal PbTe-PbSe-Bi2Se3-Bi2Te3 system can help to understand the crystallization schemes, reactions types etc. The phase diagram illustrated in Figure 2, present an assumed scheme of the phase equilibria in this phase area.

Lin (Counts)

30000

20000

10000

1 I 10

20

j__A

30

40

50

PbBi:Te,Se3

PbBijTeijSejs

60

2th. 70

Fig. 1. XRD patterns for different alloys in the PbBi2Te4-"PbBi2Se4" isopleth section. The alloys compositions are given according to their selenium content.

Fig. 2. The phase diagram of the isopleth section PbBi2Te4-"PbBi2Se4".

According to phase diagram of the PbSe-Bi2Se3 system [19], an alloy "PbBi2Se4" contains two phases, namely Pb5Bi6Te14 and Pb5Bi12Te23 which are melt incongruently at 993 and 973 K by peritectic reactions L+PbSe-^Pb5Bi6Te14 and L+Pb5Bi6Te14^Pb5Bi12Te23 respectively. Along the title isopleth, these processes are monovari-ant ones. Taking into account high probability of formation solid solutions (51 and 52) based on these ternary compounds along the PbBi2Te4-"PbBi2Se4" isopleth, these processes can be written as L+a^51 and L+51^52. Taking into account these equilibria, one can assume the presence of the following phase areas L+a^51, L+5, L+51^52 and 51+52. In our opinion, horizontal at 935 K on the phase diagram of the title isopleth section representing an invariant L+51+ 52^y1 equilibrium. In different finishing versions of this reaction, the following mono-, bi-and triphasic areas appear in the system: y1, y1+51 (or y1+52) and y1+51+52

Conclusion

The phase diagram of the PbBi2Te4-"PbBi2Se4" isopleth section of Pb-Bi-Te-Se quaternary system was studied experimentally and wide range of tetradymite-type solid solutions based on PbBi2Te4 was revealed. The given phase diagram can shed light on the design of the PbBi2Te4-xSex disordered alloys in which tellurium atoms in the septuple blocks substituted by the selenium. As known, partially substitution of the tellurium by selenium leads to significant modication in the electronic structure of the materials, i.e., introducing Se atoms in the tetradymite-type layered materials significantly increase their bulk band gaps.

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Pb-Bi-Te-Se SiSTEMiNDЭ PbBi2Te4-"PbBi2Se4" POLITERMIK KЭSiYi UZRЭ DЭYi§ЭN

TЭRKlBLi YENi FAZALAR

^.ЭИуеу

РЪ-Б1-Те-8е dбrdlй sistemindэ faza tarazllqlaп РЬВ^Те4-"РЬВ^8е4" politermik kэsiyi uzrэ tэcrйbi olaraq differensial termiki analiz vэ rentgenfaza analizi йsuПan ilэ tэdqiq ейПт^йп. Мйэууэп edilmi§dir ki, qeyd о1ипап kэsik qeyri -kvazibinar о1иЬ, geni§ qatlllq intervalmda PbBi2Te4 эsasmda tetradimit qurulu§lu bэrk mэhlulaпn этэ1э gэlmэsi ilэ xarakterizэ olunur. РЬВ^Те4-"РЬВ^8е4" politermik kэsiyi тщ-э^зЬ faza tarazhgl тэпгэ^шэ sahibdir ki, Ьи da опип mэnsub oldugu PbTe-PbSe-Бi2Se3-Бi2Te3 qar§lllqll sisteminin xйsusiyyэtlэri ilэ baghdlr Ви kэsik йzrэ ba§ verэn Ьййп tarazllq proseslэrini mйэyyэn etmэk йдйп hэmin qar§lllqll sistemin geni§ qatlhq vэ temperatur intervalmda бyrэnilmэsi vacibdir.

Лдаг sдzlэr: tetradimitэbэnzэr Ьэ^ mэhlullar; 1аук qurulщlu materiallar; bismut selenid; qmgщun bismut tellu-ridlэri; /а1а diaqraml.

НОВЫЕ ФАЗЫ ПЕРЕМЕННОГО СОСТАВА ПО РАЗРЕЗУ PbBi2Te4-"PbBi2Se4" ЧЕТВЕРНОЙ СИСТЕМЫ Pb-Bi-Te-Se

З.С.Алиев

Методами дифференциального термического и рентгенфазового анализов исследованы фазовые равновесия по политермическому сечению РЬВ^Те4-"РЬВ^8е4" четверной системы РЬ-В^Те-8е. Установлено, что данный разрез неквазибинарный и характеризуется образованием широкой области твердых растворов с тетрадимито-подобной структурой на основе РЬВ^Те4. Данный политермический разрез имеет сложную картину фазовых равновесий, обусловленную особенностями взаимной системы РЬТе-РЬ8е-В^8е3-В^Те3. Для точного установления всех равновесных процессов необходимо изучение указанной взаимной системы в широком интервале составов и температур.

Ключевые слова: тетрадимитоподобные твердые растворы, слоистые материалы, селенид висмута, теллу-риды свинца висмута, фазовая диаграмма.