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CHEMICAL PROBLEMS 2020 no. 1 (18) ISSN 2221-8688
UDC 546.81'86'24
SYNTHESIS AND CRYSTAL STRUCTURE OFA NEW 9P-TYPE LAYERED van der
WAALSCOMPOUND SnBi4Te4
1E.N. Orujlu, 1A.E. Seidzade, 2,3Z.S. Aliev, 2I.R. Amiraslanov, 1M.B. Babanly
lAcad. M. Nagiyev Institute of Catalysis and Inorganic Chemistry ofANAS, 113, H.Javid ave., AZ1143, Baku, Azerbaijan, e-mail: elnur. oruclu@yahoo. com 2G.M. Abdullayev Institute of Physics of ANAS, 131, H. Javidave., AZ 1143, Baku, Azerbaijan 3Azerbaijan State Oil and Industry University, 20, Azadlig ave., AZ 1010, Baku, Azerbaijan
Received 03.10.2019
Abstract: Considering structural features of the already known tetradymite-like layered chalcogenide phases, we have attempt to synthesize a new mix-layered compound, SnBi4Te4. The newly synthesized alloy examined by means of differential thermal analysis, powder X-ray diffraction and scanning electron microscope techniques.The melting nature of the discovered phase is found to be incongruently at 831 K. The crystal structure of the SnBi4Te4 was elucidated from powder pattern by Rietveld method. The determined crystal structure was found to derived from tetradymite type and featured by the alternation of theseven-layered (septuple) blocks of SnBi2Te4 and bismuth bilayers. The result of this work, the existence of a new - SnBi4Te4 compound in the Sn-Bi-Te ternary systemcan shed light for the incoming research works to search for similar phases in the other releated AIV-Bi-Te (AIV = Ge, Sn, Pb ) systems. Keywords:Sn-Bi-Te ternary system, SnBi4Te4,novel layered chalcogenide, Bi-bilayers, van der Waals compounds, crystal structure, Rietveld method, topological insulator. DOI: 10.32737/2221-8688-2020-1-40-48
Introduction
The layered van der Waals (vdW) chalcogenides, in particular, tellurides of the group 15 metals have beenextensively studied during the last years as prospective thermoelectric materials and topological insulator for energy conversion and spintronic applications[1-3]. Tuning the band electronic properties of these materials are of interesting from the point of view of their applications in real optoelectronic and spintronic devices. The rational tuning of the electronic properties is possible by the doping, nanostructuring or heterostructuring by combination of various atomic blocks or layers. Thanks to the existingvdWgap in the mentioned materials, inserting the, e.g., atomic bilayers in rocksalttype septuple blocks are always simple way to modify crystal structure and electronic properties [3-5].
Recently preparation of mixed-layered compounds having thermoelectric properties are considered to be a more effective method
for material design[6-7]. The literature data shows that, the systems AwBVl-AyByi(Aw = Ge, Sn, Pb; Av=Sb, Bi; BV1=Se, Te) host tetradymite-type layered ternary compounds and exhibit promising thermoelectric properties [7-14]. On the other hand, the discovery of the topological insulating properties in these compounds made themmuch more attractive in the last few years [15-20].
The Sn-Bi-Te ternary system[21-23]has been studied by various groups of authors so far [21-23]. According to Karpinski [21], this system includes three stable ternary compounds, namely SnBi2Te4, SnBi4Te7, SnBi6Te10 which are melt by peritectic reactions at 873, 863 and 855 K, respectively. Later, Kuropatwa [22] and Chiu [23] independently have reported an additional two ternary compounds - SmBi2Te5and SnBiTe2. Literature data show that, in order to overcome metastable state, annealing at higher than 700
CHEMICAL PROBLEMS 2020 no. 1 (18)
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K is important in the compositional range of 30 and 53 mol % SnTe [21]. There by, it seems that very hard to obtain further stable homogeneous phases in the SnTe-Bi2Te3 system. May be very long annealing time could be helpful, but present reports on this system confirm that the phase diagram of this system is still contradictory and further thoroughly experimental investigations are necessary.
SnBi2Te4 is one of the main dominant phases in the SnTe-Bi2Te3system and, its septuplestructure is formed by the insertion of SnTe into rocksalt-type slabs of Bi2Te3. The crystal structure of this compound is a long-periodically stacking sequence of these septuples along the c axes. The crystal structure of the other two compounds SnBi4Te7 and SnBi6Teio built-up alternation of quintuples of the Bi2Te3 and septuples of the SnBi2Te4 thus can be considered mixed-layer blocks according to -7-5-7-5-7-5- and -75-5-7-5-5-7-5-5- sequence, respectively.
Crystal structure information of these
compounds can
be found in the Refs[21,24].
The combination of the elemental bismuth or antimony bilayers and quintuple blocks of Bi2Te3 (orSb2Te3 and Bi2Se3) and the septuple blocksof the GeBi2Te4, PbBi2Te4 ternaries are also found to be stable structures [6]. Taking into account alternation of quintuple, septupleand, mixed-layered structures exist in
the(AIVTe)„-(Bi2Te3) m(AIV = Ge, Sn, Pb) homologous series, there is a possibility to design (AIVTe) •(Bi2Te3)m-(Bi2)fe layered phases where there is an alternation of kBi bilayers, nAIVTe quintuplesand
mAIVBi2Te4septuples.
Here we present the synthesis and elucidation of a crystal structure of 9P-type heterostructured new ternary compound -SnBi4Te4, which is consists of alternating bismuth bilayers and SnBi2Te4 septuple packets. The melting nature and temperature were also presented here.
Experimental part
Elemental Tin, Bismuth, and Tellurium (99.999% purity, Alfa Aeser company) were used as starting components to synthesize polycrystalline SnBi4Te4. The stoichiometric mixture of elements was sealed in evacuated (10-2 Pa) quartz ampule and heated up to 1000 K and kept at this temperature for 5 hand thenwater quenched. In order to achieve complete homogenization, the sample annealed for ~700h at 700 K.
The sorted-outingot was examined by differential thermal analysis and powder X-ray diffraction techniques. DTA measurement was
performed using a NETZSCH 404 F1 Pegasus system at a heating rate of 10 K-min"1, while PXRD was donein a Bruker D2 PHASER diffractometer with CuKa radiation within 20 = 5-100 range at room temperature.The crystal structure refinement was performed using the EVA and Topas V4.2 softwares by Bruker.The microstructures and equilibrium composition of the title sample was determined by Tescan Vega 3 SBH scanning electron microscope equipped with ThermoScientific UltraDry Compact EDS detector.
Results and discussion
Fig. 1 shows the XRD pattern of SnBi4Te4 in comparison with initial SnBi4Te4 and Bismuth. It is clearly seen that SnBi4Te4 has a quite identical diffraction pattern with typical peaks those do not come from initial constituents. The obtained pattern fully indexed with a rhombohedral P-3m1 (#156)lattice.The crystal structure of the
obtained phase was refined by the Rietveld method and results will be discussed below.
The DTA heating thermogram for SnBi4Te4 is given in Fig. 2 with two endothermic effects at 831 K and 906 K. We assume that the first sharp endothermic thermal event corresponding to the peritectic
decomposition of SnBi4Te4 phase according toreaction.
L + X ^ SnBi4 Te
4 1 c4
Fig. 1. PXRD patterns of Bismuth, SnBi4Te4, and SnBi2Te4.
The literature data on Sn-Bi-Te ternary thermal effect confirmsthat the melting
system showed that X phase may tin telluride process starts at the end of the first effect
which is in equilibrium with liquid phase at whereas,the effect at 906 K conforms to finish
above peritectic temperature. Nevertheless, of the melting process and can be considered
further experimental results are strongly as liquidus point. necessary for the conclusion.The second wide
Time (min)
Fig. 2. DTA heating thermogram for the SnBi4Te4.
The further confirmation of the synthesized alloy comes from SEM-EDS existence and chemical composition of the measurements. Fig. 3 illustrates the
homogenous microstructure of the SnBi4Te4 alloy, where as its EDS spectrum and equilibrium composition are given in Fig. 4. The layered texture of the alloy is also clearly
seen from micrograph. Obviously, the chemical composition of the synthesized sample agrees well with the formula SnBi4Te4.
Fig. 3.SEM micrograph of the SnBi4Te4.
Fig. 4.EDS spectrum and element analysis result for the SnBi4Te4
Fig. 5. XRD pattern for the SnBi4Te4.
The crystal structure of the title the difference of intensities between the
compound was refined from powder XRD experimental and calculated by Rietveld. A 3D
pattern recorded in the range 20 = 5-100 (Fig. side view of the obtained crystal structure
5). Below the XRD pattern black curve shows illustrated in Fig. 6.
Fig. 6. A 3D side view of the refined crystal structure for the SnBi4Te4.
The refined unit cell parameters, atomic positions, and interatomic distances are shown in Tables 1, 2, and 3.
Table 1. Refined structure parameters for SnBi4Te 4.
Space group P-3m1
Unit cell parameters at298 K: a (A) c (A) 4.43306 (57) 17.7396 (57)
Cell Volume (A3) 302.01 (12)
Crystal Density (g/cm3) 8.0550 (33)
R-Bragg (%) 0.20
Table 2. Atomic positional parameters in SnBi4Te 4.
Site Np x y z Atom Occ Beq
Single positions Mixed positions
Bi1 2 1/3 2/3 0.0451(12) Bi3 1 1 1.32
Tel 2 0 0 0.1790(23) Te 1 1 1.32
Bi2 2 2/3 1/3 0.2706 (13) Bi3 Sn+2 1 0.76 0.24 1.32
Te2 2 1/3 2/3 0.3938(30) Te 1 1 1.32
Sn 1 0 0 0.5 Sn+2 Bi3 1 0.52 0.48 1.32
Table 3. Interatomic distances in Bi2-SnBi2 Te4.
Atoms Distance
Sn Te(2) 6 x 3.178(31) À
Bi(1) Bi(1) 3 x 3.019(24) À
Bi(2) Te(1) Te(2) 3 x 3.033(25) À 3 x 3.366(32) À
The values presented in Table 2 (except the "mixed positions'' column) were obtained for the condition where each crystallographic position was completely occupied by one type of atoms. However, a structural study of compounds containing similar septuple slabs shows that they are characterized by the substitution of metal atoms in atomic layers[25]. Therefore, we also
refined the occupation coefficients in this structure in case of the initial composition of SnBi4Te4 is preserved. The occupation results obtained inthis case are shown inTable 2, in the "mixed position" column. According to the results of the refinement, the Sn/Bi occupation ratio is found to be 52/48in the central layer. At the same time, in the Bi(2) site, the Bi/Sn ratio is 76/24.
Conclusion
In this report, we have synthesized a result reveals that the newly found compound
new ternary tetradymite typelayered melts peritectically at 831 K. The existence of
compound - SnBi4Te4in the Sn-Bi-Te ternary nonuple packets in the Sn-Bi-Te ternary
system.The obtained phasehas a layered system gives possibility to expect similar
structure with rocksalt-type septuple blocks of phases in the other relatedAIV-BV-Te
SnBi2Te4 and Bi-bilayers. Thermal analysis (AIV=Ge, Sn, Pb; BV= Sb, Bi) ternary systems.
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YENi9P-TiPLAYLISnBi4Te4 van der VAALSBiRLOgMOSiNiNSiNTEZi Vd
KRiSTAL QURULUÇU
1E.N. Oruclu, 1A.E. Seyidzadd, 2'3Z.S. dliyev, 2i.R dmiraslanov, M.B. Babanli
1Kataliz vd Qeyri-üzvi Kimya institutu, AMEA AZ1143, Baki, H.Cavidpr., 113; e-mail: elnur. oruclu@yahoo. com
2Fizika institutu, AMEA AZ 1143 Baki,H.Cavidpr.,131 3Azdrbaycan Dövldr Neft vd Sdnaye Universiteti AZ 1010, Baki, Azadliqpr., 20
Mövcud tetradimitdbdnzdr layli xalkogenid fazalarinin quruluç xüsusiyydtldrini araçdirmaqla SnBi4Te4tdrkibli qariçiq layli birld^mdnin istiqamdtli sintezi hdyata keçirilmiç, alinan polikristallik nümund DTA, RFA vdSEM üsullari ild tddqiq olunmuçdur. Müdyydn edilmiçdir ki, birld§md 831 K-dd peritektik reaksiya ild parçalanmaqla driyir.Nümundnin ovuntu rentgenoqraminin Rietveld metodu ild tddqiqi göstdrir ki, birld§md tetradimitdbdnzdr layli quruluçlu olub, Bi2laylari vd SnBi2Te47-layli paketldrinin tdkrarlanmasindan ibardtdir. Sn-Bi-Te ûçlû sistemindd beld birld^mdnin olmasi,AIV-Bi-Te (AIV = Ge, Sn, Pb ) sistemldrindd dd oxçar tdrkibli fazalarin mövcud ola bildcdyini ehtimal etmdyd dsas verir.
Açar sözldr: Sn-Bi-Te ûçlû sistemi,SnBi4Te4, yeni layli xalkogenid, Bi2 laylari, van der Waals birld^mdldri, kristal quruluç, Rietveld metodu, topoloji izolyator.
СИНТЕЗ И КРИСТАЛЛИЧЕСКАЯ СТРУКТУРА НОВОГО СЛОИСТОГО ВАН-ДЕР-ВААЛЬСОВОГО СОЕДИНЕНИЯSnBi4Te49 Р-ТИПА
1Э.Н. Оруджлу/А.Э. Сеидзаде, 23З.С. Алиев,2И.Р. Амирасланов, 1М.Б. Бабанлы
1Институт катализа и неорганической химии им. акад. М.Нагиева
Национальной АН Азербайджана AZ1143, Баку, пр.Г.Джавида, 113; e-mail: elnur. oruclu@yahoo. com 2Институт физики Национальной АН Азербайджана AZ1143, Баку, пр.Г.Джавида, 131 3Азербайджанский Государственный Университет Нефти и Промышленности
AZ1010, Баку, пр.Азадлыг, 20
Учитывая структурные особенности уже известных тетрадимитоподобных слоистых халькогенидных фаз, нами былпроведен направленный синтез нового смешанно-слойного соединения SnBi4Te4. Полученный полукристаллический образец был исследован методами дифференциального термического и рентгенофазового анализов, а также сканирующей электронной микроскопии. Установлено, что полученное соединение плавится инконгруэнтно при 831 К. Из порошковой рентгенограммы методом Ритвельдаопределена кристаллическая структура соединения SnBi^eM выявлено, что она относится к тетрадимитному типу и характеризуется чередованием семислойных блоков SnBi2Te4 и бислоёв висмута.Существование нового соединения SnBi4Te4 в тройной системе Sn-Bi-Teдает основание для поиска аналогичных фаз в других подобныхсистемахAIV-Bi-Te (A1V = Ge, Sn, Pb).
Ключевые слова: тройная система Sn-Bi-Te, SnBi4Te4, новый слоистый халькогенид, Bi-бислои,ван-дер-ваальсовые соединения, кристаллическая структура, метод Ритвельда, топологический изолятор.
