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25Condensed Matter and Interphases (Kondensirovannye sredy i mezhfaznye granitsy)
Original articles
DOI: https://doi.org/10.17308/kcmf.2020.22/2821 ISSN 1606-867X
Received 12 Mach 2020 elSSN 2687-0711
Accepted April 2020 Published online 25 June 2020
Synthesis and Properties of Synthetic Aikinite PbCuBiS3
© 2020 O. M. Aliev*, S. T. Bayramovab, D. S. Azhdarovaa, Sh. H. Mammadov® , V. M. Ragimovaa, T. F. Maksudovab
aM. Nagiyev Institute of Catalysis and Inorganic Chemistry of National Academy of Sciences of Azerbaijan, 113 G. Javid ave., Baku AZ1143, Azerbaijan
bBaku European Lyceum, 37 Rostropovich str., Baku AZ 1001, Azerbaijan Abstract
The goal of this study was the synthesis and study of the properties of synthetic aikinite, PbCuBiS3. The synthesis was carried out in evacuated quartz ampoules for 7-8 h; the maximum temperature was 1250-1325 K. Next, the samples were cooled and kept at 600 K for a week. Then the ampoules were opened, the samples were carefully ground, and after melting, annealed at 600-800 K, depending on the composition, for at least two weeks to bring the samples into equilibrium. The annealed samples were studied by differential thermal (DTA), X-ray diffraction (XRD), microstructural (MSA) analyses, as well as microhardness measurements and density determination. XRD was performed using D 2 PHASER with CuK radiation and a Ni filter.
a
CuBiS2-PbS, C^S-PbCuBiSj, Bi2S3-PbCuBiS3, PbBi2S4-PbCuBiS3, PbBi4S7-PbCuBiS3 sections of quasi-triple system Cu2S-Bi2S3-PbS were studied using the complex of physical and chemical analysis methods and their phase diagrams were plotted. It was found that in addition to the PbBi2S4-PbCuBiS3 section, all sections are quasi-binary and they were characterized by the presence of limited solubility regions based on the initial components. The study of the CuBiS2-PbS section revealed the formation of a quaternary compound PbCuBiS3 occurring in nature as the mineral aikinite, congruently melting at 980 K. We established that PbCuBiS3 crystallizes in a rhombic syngony with lattice parameters a = 1.1632, b = 1.166, c = 0.401 nm, Pnma space group, Z = 4. Using DTA and XRD methods we established that PbCuBiS3 compound is a phase of variable composition with a homogeneity range from 45 to 52 mol%/PbS. The PbCuBiS3 compound is a p-type semiconductor with a band gap energy of AE = 0.84 eV.
Keywords: aikinite, compound, single crystal, structure, thermodynamic function, band gap energy. For citation: Aliev O. M., Bayramova S. T., Azhdarova D. S., Mammadov Sh. H., Ragimova V. M., Maksudova T. F. Synthesis and Properties of Synthetic Aikinite PbCuBiS3. Kondensirovannye sredy i mezhfaznye granitsy = Condensed Matter and Interphases. 2020; 22(2): 182-189. DOI: https://doi.org/10.17308/kcmf.2020.22/2821
1. Introduction controllable and have a wide spectrum of action.
Minerals possess specific semiconductor, A2B3 № (A = Sb> Bi; B = S> Se> Te) cha^gemdes optical, and electro-optical properties, allowing p°ssess ^erm^ec^k and ph^ekc^k them to be used as semiconductors in special properties. These features of Aematomh create devices. All this determines the attention paid to fawuraMe randans for thrii: mdespread use in the synthesis of aikinite and the growth of aikinite the electromcs mdustey [1-7] single crystals. Compound PbCuBiS3 occurs in nature in the
Now one of the most promising materials of form of a min^ai and crystallizes in ^mbk modern electronics are chalcogenide phases of the syngony with a iattire p^d: a = L1632, b = U66, A,B3 type. Physicochemical and electrophysical c = °.401 nm> Pnma space §roup> Z = 4 [8-18]. The parameters of these materials are easily structure is similar to the tfmrture of anti^nite
__K^CuC^ and others [3] and similar to the structure of
Sharafat H. Mammadov, e-mail: azxim@mail.ru
®_ The content is available under Creative Commons Attribution 4.0 License.
bournonite PbCuSbS3 and zeligmannite PbCuAsS3. The exact distribution of Pb and Bi atoms in the crystal lattice has not been elucidated. In comparison with Bi2S3 in aikinite replacement of Bi+3 with Pb+2 compensated by the inclusion of a single Cu+ ion of such small size that it occupies the empty spaces of the lattice without distorting it.
The lead atoms in the aikinite structure are surrounded by five sulphur atoms with distances from 0.284 to 0.298 nm and two more sulphur atoms separated by 0.324 nm, and the coordination is close to octahedral (Fig. 1).
Extensive experimental studies of natural minerals showed that all the considered compounds are ordered, their compositions are limited to a certain stoichiometry and very narrow regions of solid solutions, as was previously assumed in [19]. According to available data, the structure of all these minerals is a derivative of the structure of bismuthine, obtained by substitution of Bi in the latter with Pb with the simultaneous addition of Cu atoms to the vacant tetrahedral sites Cu + Pb + Bi [8, 9]. There are three types of ribbons in the structures of these minerals [9], the combination of which can lead to the motives of all the minerals of this series: bismuthine Bi.S,,
4 6'
krupkaite CuPbBi3S6, and aikinite.
It should be noted that although the ratio between metal sulphides is Cu2S:PbS in the aikinite-bismuthine series is constant and equal to 1:2, in a selenium-containing mineral of a similar composition Cu2Pb3Bi8(S, Se)16 it is 1:3, which also exhibited as slightly altered arrangement of atoms. The basis of the structural motif is composed by zigzag ribbons of Pb and Bi octahedra connected along the edges, linked together by pairs of other octahedrons also with an edge bonds. In the voids of this structure, additional Pb and Bi atoms are located in seven-dimensional coordination. The environment of Cu atoms is intermediate between tetrahedral and plane triangular, the Cu-S distances are 232 nm (3 S) and 252 nm (1 S) [13, 17].
Earlier, we [20, 25] synthesized and studied the physicochemical and physical properties of complex sulfosalts based on the PbCuSbS3 bournonite mineral.
The purpose of this work was the synthesis and study of the properties of synthetic aikinite PbCuBiS .
Fig. 1. The crystal structure of the aikinite mineral PbCuBiS3 [1]
2. Experimental
Quaternary sulfosalt PbCuBiS3 was revealed while studying the CuBiS2-PbS section of the quasi-triple Cu2S-Bi2S3-PbS system. The quasi-triple system, except for CuBiS2-PbS, was studied by based on Cu3BiS3-PbCuBiS3, CuBi3S5-PbCuBiS3, Cu2S-PbCuBiS3, PbBi2S4-PbCuBiS3, PbBi4S7-PbCuBiS3 and Bi2S3-PbCuBiS3 sections. The position of the connodes in the Cu2S-Bi2S3-PbS system is shown in Fig. 2.
The quaternary alloys for the study were obtained by the vacuum-thermal method from the source alloys (CuBiS2, PbS, CuBi2S4 etc.), previously synthesized from ultra-pure elements (Cu - 99.997 %, Pb - 99.994 wt%, Bi - 99.999 wt%, S - 99.9999 wt%). The maximum temperature was 1250-1325 K. The synthesis was carried out in evacuated quartz ampoules for 7-8 h; then, the samples were cooled and kept at 600 K for a week [26]. Then the ampoules were opened, the samples were carefully ground, and after melting, annealed at 600-800 K, depending on the composition, for at least two weeks to bring the samples into equilibrium, Table 1.
Annealed samples were studied by physicochemical analysis: thermal analysis was carried out using Kurnakov pyrometer NTR-73 (heating rate 10 °/min, standard Al2O3, chromel-alumel thermocouple); XRD patterns were obtained using D 2 PHASER Brucker diffractometer (Cu^-radiation, Ni-filter); the microhardness of the samples was measured using PMT-3 microhardness tester (optimal load of 0.02 kg), the microstructure of the alloys
Fig. 2. The position of the connodes in the PbS-Cu2S-Bi2S3 system. The compositions of the synthesized samples are indicated by black dots
Table 1. Results of physicochemical analysis of alloys of the CuSbS2-PbS system
Composition, Thermal effects, K Microhardness Density, g/cm3 Phase
mol% PbS solidus liquidus (x107 Pa) composition
0.0 - 780 2200 7.40 CuBiS2 (single phase)
5.0 715 765 2250 7.38 a
10 650 730 2300 7.32 a+b
15 650 690 2300 7.30 a+b
20 650 700 eutectic 7.28 a+p
25 650 765 - 7.28 a+b
30 650 830 1980 7.26 a+p
40 650 925 1980 7.24 a+p
45 - 970 1970 7.22 b
50 - 980 1980 7.20 b
52 - 970 1990 - b
55 815 930 1990 - b + PbS
60 810 865 - 7.12 b+ PbS
65 815 - eutectic 7.06 b+ PbS
70 815 920 720 6.90 b+ PbS
80 815 1070 720 6.82 b+ PbS
90 815 1240 720 6.70 b+ PbS
100 - 1400 720 6.11 Pbs (single phase)
was studied using a MIM-7 microscope, and the density was determined by the pycnometer.
3. Results and discussion
We will discuss in details the CuSbS2-PbS system, in which the sulfosalt PbCuBiS3 was found.
As can be seen from Table 1, the microhardness value related to the PbCuBiS3 quaternary compound, increased on both sides from 50 mol% PbS, but it decreased with stoichiometric composition. This shows that a range of homogeneity exists based on the PbCuBiS3 compound. According to the results of XRD and microstructural analysis, it
was found that PbCuBiS3 is a phase of variable composition and that solubility at a eutectic temperature (650 K) was 10 mol%, while with decreasing temperature it sharply narrows, not exceeding 7 mol% PbS at 300 K.
The MSA analysis showed that with the exception of compositions 0-7 and 45-52 mol% PbS, all alloys were biphasic.
According to the physicochemical analysis, a phase diagram of the CuBiS2-PbS system was plotted and it is shown in Fig. 3. As can be seen from the figure, the system is characterized by the presence of PbCuBiS3 sulfosalt melting at 980 K congruently. Coordinates of eutectic points: 20 mol% PbS 650 K, and 65 mol% PbS 815 K. XRD demonstrated that in the range of concentrations of 0-7 mol% PbS, only reflections related to CuBiS2 were observed on diffractograms. These solutions crystallize in rhombic syngony, and with an increase in the concentration of PbS, the lattice parameters increase (a = 0.614^0.620, b = 0.391^0.395, c = 1.493^1.502 nm, Pnma space group, Z = 4).
In the range of concentration 7^45 mol% PbS a-solid solutions based on CuBiS2 and b-solid solutions based on quaternary sulfosalt PbCuBiS3 co-crystallize and in the 52^100 mol% PbS two phases (b + PbS) co-crystallize. Composition 50 mol% PbS in terms of interplanar spacing and intensity differed from the source sulphides. Calculation of XRD patterns of the quaternary compound PbCuBiS3, as well as XRD patterns of the initial sulphides for comparison are presented in Table 2.
X-ray analysis confirmed the formation of quaternary sulfosalts PbCuBiS3, found in nature in the form of the mineral aikinite in CuBiS2-PbS system. It was found that sulfosalt crystallizes in rhombic syngony with unit cell parameters a = 1.1632, b = 1.166, c = 0.4017 nm, Pnma space group, Z = 4.
The Bi2S3-PbCuBiS3 section is of eutectic type. The composition of the eutectic point determined by the plotting of the Tamman's triangle was 50 mol% Bi2S3 at 800 K. Solubility based on Bi2S3 was 5 mol%, based on PbCuBiS3 -7 mol% (Fig. 4a).
The Cu2S - PbCuBiS3 section was quasi-binary and eutectic with limited solubility based on the starting sulphides (Fig. 4b).
Fig. 3. Phase diagram of the CuBiS2-PbS system
Conclusion on the formation of solid solutions based on a-, b- and g-Cu2S was based on the results of the DTA and MSA methods. On thermograms of samples containing 7 and 8 mol % PbCuBiS3, thermal effects below the solidus temperature were revealed. These effects were associated with the formation and decomposition of a solid solution based on g-Cu2S. This was also confirmed by the MSA data. In samples containing from 2 to 5.5 mol% PbCuBiS3, the second phase was present in the form of needle insertions, and eutectic was not revealed. In the Cu2S-PbCuBiS3 system in the range of concentrations of 2.0-90 mol% PbCuBiS3 in a condensed state, two phases were in equilibrium: aCu2S-based solid solution and b-PbCuBiS3 based solid solution. They were clearly distinguishable by MSA and formed a eutectic of the composition 40 mol % Cu2S and T = 850 K. Eutectic in the indicated concentration range was present on the sections of all section samples and it was represented by the alternation of needle crystals of the PbCuBiS3 phase and oval Cu2S crystals. Based on a-Cu2S a limited solution was formed, which at 300 K reached 2 mol% PbCuBiS3.
The phase transitions a-Cu2S ^ b-Cu2S ^ g-Cu2S had a eutectic nature and occurred at 375 and 580 K, respectively. Thermal effects related to p-Cu2S^ g-Cu2S were revealed only for alloys
Table. 2. Interplanar distances and intensities of the CuBiS2, PbCuBiS, and PbS lines for comparison
CuBiS2 PbCuBiS3 PbS
d , E exp* I/Iq d , Ä exp I/Iq hkl d , Ä exp I/ iq
4.700 8 4.070 4 220 3.790 2
3.200 10 3.770 1 011 3.442 9
3.100 8 3.670 10 130 3.283 3
3.020 10 3.580 7 111,310 2.965 10
2.810 2 3.180 9 121 2.311 2
2.340 9 2.880 8 040, 221 2.693 10
2.290 4 2.740 2 410 1.780 9
2.160 9 2.680 3 131, 330 1.707 8
1.960 5 2.620 6 311 1.480 5
1.880 7 2.580 1 240 1.359 6
1.800 8 2.560 1 420 1.324 10
1.780 3 2.510 3 231 1.209 8
1.755 3 2.570 2 150 1.141 7
1.655 8 2.170 3 241 1.048 3
1.560 5 2.150 3 250, 421
1.475 2 2.020 5 440, 051
1.450 4 1.984 4 431,151
1.365 3 1.974 3 530, 112
1.320 4 1.883 1 202, 600
1.260 2 1.805 4 441
1.125 5 1.766 1 351,133
1.208 5 1.648 4 042, 170
1.190 5 1.593 4 270
1.168 7 1.514 1 370
1.112 3 1.475 2 171
1.100 3 1.406 4 740
1.380 2 561
1.354 2 612
1.330 3 003
1.278 2 661
1.158 1 770
containing 10^20 mol% PbCuBiS,, therefore, this transition in Fig. 4b was marked with a dotted line.
The PbBi2S4-PbCuBiS, section was partially quasi-binary due to the incongruent nature of the melting of sulphide PbBi2S4. The solubility based on PbCuBiS, sulfosalt was 8 mol% at 300 K, and at a eutectic temperature of 15 mol% Coordinates of the eutectic point were 40 mol% PbCuBiS, and T = 825 K.
The PbBi4S7-PbCuBiS, section was quasi-binary and eutectic with limited solid solutions. The eutectic corresponds to 55 mol% PbBi4S7 and 800 K. The solubility of PbCuBiS, in PbBi^ at eutectic temperature was 10 mol%, at ,00 K it
decreased to 5 mol% PbCuBiS, (a-solid solution), and the solubility of PbBi4S7 in quaternary sulphide was 18 mol% at eutectic temperature and decreased to 10 mol% PbBi4S7 at 500 K p-solid solution).
PbCuBiS, single crystals were obtained by targeted crystallization of a stoichiometric melt in vertical quartz ampoules. An ampoule with a cone-shaped bottom was placed in a furnace with a small temperature gradient in height. After the formation of the melt, directed cooling was carried out at a speed of 4 °/h for 48 h until the entire melt solidified, then the furnace was cooled at a speed of 60 °/h. Thus, polycrystalline ingots with a large number of cracks were obtained.
Fig. 4. Phase diagram of the BLS,-PbCuBiS, (a) and Cu2S-PbCuBiS3 (b) systems
However, it was possible to cut single crystal sections with a length of 1 mm, free of visible defects from these ingots (Fig. 5).
Significantly larger single crystals of the PbCuBiS3 compound were obtained by the similar method in quartz ampoules with a diameter of 5 mm. The single crystals reached a length of 812 mm with a diameter of 5 mm. One feature of the PbCuBiS3 compound should be noted: directional crystallization of stoichiometric composition without special additives always provided ingots characterized mainly by p-type conductivity. It is possible that this was the result of a deviation from stoichiometry due to the combination of volatile components (sulphur and bismuth). The ingot always contained an excess of copper, which in significant quantities dissolves in the compound and provides p-type conductivity. Therefore, any doping of the PbCuBiS3 compound (obtained PbCuBiS3 crystals were doped with erbium) during crystal growth by directed
cooling will occur against the background of this phenomenon.
The standard thermodynamic functions of the PbCuBiS3 compound were calculated: 5 0298 = 253.1±5 J/(mol%), AS0298 = -14.1±3 J/mol, AH 0_„= - 2 7 0.2 ±10 kJ/mol and
2 9 8
AG°98 = -266.3±10 kJ/mol
The photoconductivity spectra of pure PbCuBiS3 crystals and crystals doped with erbium grown by the directed crystallization method from the melt were studied. The spectral dependences of these sulfosalts are shown in Fig. 6. As can be seen, crystals grown by directional crystallization had approximately the same photosensitivity, which at 293 K was ISt/It = 103 when illuminated by natural light, and when the temperature decreased, it grew and reached 105 at 100 K. In
Fig. 5. Single crystals of the PbCuBiS, compound
Fig. 6. The spectral dependence of the photoconductivity of PbCuBiS3 (1) and PbCuBiS3-Er (2) grown by the directed crystallization method
crystals of both types, bands of shallow levels -traps with an activation energy of 0.25-0^5 eV, as well as trap levels with an activation energy AE = 0.50-0.60 eV, were observed. The band gap energy calculated from photoconductivity was AE = 0.84-0.91 eV.
Thus, the nature of the formation of PbCuBi, sulfosalt was revealed based on studying the quasi-triple Cu2S-Bi2S,-PbS system based on CuBiS2-PbS, Cu2S-PbCuBiS,, PbBi2S4-PbCuBiS,, PbBi4S7-PbCuBiS, and Bi2S,-PbCuBiS, sections. The single crystals of synthetic aikinite PbCuBiS, were grown and some of its properties were studied.
4. Conclusions
1. The CuBiS2-PbS, Cu2S-PbCuBiS,, Bi2S,-PbCuBiS,, PbBi2S4-PbCuBiS,, PbBi4S7-PbCuBiS, sections of quasi-triple Cu2S-Bi2S,-PbS system were studied by a set of physicochemical analysis methods and their phase diagrams were plotted. It was established that in addition to the PbBi2S4-PbCuBiS, section, all sections were quasi-binary and they were characterized by the presence of limited solubility regions based on the source components.
2. The study of CuBiS2-PbS section revealed the formation of a quaternary compound of the PbCuBiS, composition, found in nature in the form of the mineral aikinite, melting congruently at 980 K. It was established that PbCuBiS, crystallizes in rhombic syngony with lattice parameters a = 1.16S2, b = 1.166, c = 0.401 nm, Pnma space group, Z = 4.
,. PbCuBiS, single crystals were grown by directional crystallization and the spectral dependence of photoconductivity was studied. It was established that PbCuBiS, possesses photosensitivity in the visible region of the spectrum.
Conflict of interests
The authors declare that they have no known competing financial interests or personal relationships that could have influenced the work reported in this paper.
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Information about the authors
Ozbek M. Aliev, DSc in Chemistry, Professor, M. Nagiyev Institute of Catalysis and Inorganic Chemistry of National Academy of Sciences of Azerbaijan, Baku, Azerbaijan; e-mail: azxim@mail.ru.
Sabina T. Bayramova, PhD in Chemistry, Baku European Lyceum, Baku, Azerbaijan; e-mail: azxim@ mail.ru.
Dilbar S. Ajdarova, DSc in Chemistry, Chief Researcher, M. Nagiyev Institute of Catalysis and Inorganic Chemistry of National Academy of Sciences of Azerbaijan, Baku, Azerbaijan; e-mail: azxim@ mail.ru.
Valida M. Ragimova, PhD in Chemistry, Assistant Professor, Leading Researcher, M. Nagiyev Institute of Catalysis and Inorganic Chemistry of National Academy of Sciences of Azerbaijan, Baku, Azerbaijan; e-mail: azxim@mail.ru.
Sharafat H. Mammadov, PhD in Chemistry, Assistant Professor, M. Nagiyev Institute of Catalysis and Inorganic Chemistry of National Academy of Sciences of Azerbaijan, Baku, Azerbaijan; e-mail: azxim@mail.ru. ORCID ID: https // orcid.org / 00000002-1624-7345.
All authors have read and approved the final manuscript.
Translated by Valentina Mittova
Edited and proofread by Simon Cox
