Phase relations in the CuI-SbSI-SbI3 composition range of the Cu–Sb–S–I quaternary system

The phase equilibria in the Cu-Sb-S-I quaternary system were studied by differential thermal analysis and X-ray phase analysis methods in the CuI-SbSI-SbI3 concentration intervals. The boundary quasi-binary section CuI-SbSI, 2 internal polythermal sections of the phase diagram, as well as, the projection of the liquidus surface were constructed. Primary crystallisation areas of phases, types, and coordinates of non- and monovariant equilibria were determined. Limited areas of solid solutions based on the SbSI (b-phase) and high-temperature modifications of the CuI (α1- and α2- phases) were revealed in the system. The formation of the α1 and α2 phases is accompanied by a decrease in the temperatures of the polymorphic transitions of CuI and the establishment of metatectic (3750C) and eutectoid (2800C) reactions. It was also shown, that the system is characterised by the presence of a wide immiscibility region that covers a significant part of theliquidus surface of the CuI and SbSI based phases 


Introduction
Copper-antimony chalcogenides and phases based on them are considered to be potential candidates for the preparation of environmentally friendly, low-cost functional materials possessing novel desired characteristics [1][2][3]. The majority of ternary Cu-Sb-sulphides are naturally occurring minerals that have been widely explored as valuable electronic materials displaying high photoelectric, photovoltaic, radiation detector, thermoelectric, etc. properties. Earth abundance and environmental compatibility of these substances highlight the recent advances of investigations on these materials [4][5][6][7].
As it is known, one of the ways to increase the efficiency of thermoelectric materials is to complicate their composition and crystal structure [8]. In this regard, Cu-Sb chalcohalides could be considered promising research objects in terms of the search and design of new ecofriendly functional materials. However, we could not find literary information about the phase equilibria of the Cu-Sb-S-I quaternary system. There is a literary report about the formation, crystal structure, and conductivity of the Cu 5 SbS 3 I 2 compound [9]. Cu 5 SbS 3 I 2 crystallises in the orthorhombic system, space group Pnnm with the following lattice parameters a = 10.488(2), b = 12.619 (2), c =7.316(1) Å, and Z = 4 [9]. Electric conductivity and dielectric parameters of the Cu-Sb-S-I glasses have been investigated in order to evaluate their practical importance in memory switching, electrical threshold, optical switching devices, and so forth [10].
The search and design of new complex functional materials require investigation of the respective phase diagrams. The information accumulated in phase diagrams of the corresponding systems is always helpful in materials science for the development of advanced materials [11][12][13].
Considering above mentioned facts, in terms of the search for new multicomponent phases, the concentration plane Cu 2 S-CuI-SbI 3 -Sb 2 S 3 of the Cu-Sb-S-I quaternary system is of great interest. The present contribution is dedicated to the study of physicochemical interaction in the CuI-SbSI-SbI 3 (A) concentration area of the above-mentioned concentration plane.
Primary compounds of the system (A) possessing interesting functional properties have been studied in detail. Copper (I) iodide CuI is a non-poisonous, wide-gap semiconductor possessing stable p-type electrical conductivity at room temperature, fast-ionic conductivity at high temperatures, an unusually large temperature dependency, negative spin-orbit splitting, etc [14][15][16]. It has wide application in lightemitting diodes, solid-state dye-sensitised solar cells, high-performance thermoelectric elements, etc [17,18]. Antimony triiodide SbI 3 has been intensively studied as a dopant in thermoelectric materials, as a potential material for radiation detectors, as cathodes in solid-state batteries, in high-resolution image microrecording, information storage, etc. [19][20][21]. SbSI exhibits important ferroelectricity, piezoelectricity, photoconduction, dielectric polarisation properties and is widely used in the fabrication of nanogenerators and nanosensors [22][23][24][25].
Crystallographic parameters of the constituent compounds of the system A are represented in Table 1.
C u I -S b I 3 a n d S b S I -S b I 3 b o u n d a r y quasi-binary sections of the quasi-ternary CuI-SbSI-SbI 3 system have been investigated by [35][36][37], respectively. CuI-SbI 3 system forms a monotectic phase diagram. At the monotectic equilibrium temperature (~ 220 °C) the immiscibility region ranges within ~15-93 mol% SbI 3 concentration interval [35]. SbSI-SbI 3 quasi-binary section is characterised by a eutectic equilibrium at 160 °C [12,30].

Experimental part
A CuI binary compound, as well as, antimony and iodine elementary components of the Alfa Aesar German brand (99.999 % purity) were used in the course of experimental studies.
Binary SbI 3 and ternary SbSI compounds were synthesised from the elemental components in evacuated (~10 -2 Pa) silica ampoules followed by a specially designed method taking into account the high volatility of iodine and sulphur. The synthesis was performed in an inclined two-zone furnace, with the hot zone kept at a temperature 20-30 °C higher than the corresponding melting point of the synthesised compound, whereas the temperature of the cold zone was kept at about 130 °C. After the main portion of iodine and sulphur had reacted, the ampoules were relocated such that the products could melt at 230 °C (SbI 3 ) and 450 °C (SbSI). After stirring the homogeneous liquid at this temperature, the furnace gradually cooled. The purity and individuality of the obtained products were monitored using DTA and PXRD methods.
Two sets of samples (0.5 g by mass each) were prepared by сo-melting of different proportions of the preliminarily synthesised compounds and CuI of the Alfa Aesar company. After melting, most of the alloys were annealed at about ~20-30 °C below the solidus temperature for ~1000 hours in order to achieve complete homogenisation.
The DTA and PXRD methods were used to monitor the purity and individuality of the synthesised compounds and to conduct experimental studies. DTA of the samples was carried out in evacuated quartz ampoules on a differential scanning calorimeter of the 404 F1 Pegasus System (NETZSCH). Results of measurements were processed using the NETZSCH Proteus Software. The accuracy of the temperature measurements was within ±2 °C. X-ray analysis of the annealed alloys was carried out at room temperature on the Bruker D2 PHASER diffractometer with CuKa 1 radiation. The diffraction patterns were indexed using the Topas 4.2 Software (Bruker).

Results and discussion
A co-analysis of experimental results together with the literature data regarding boundary binary systems helped us to obtain the full description of phase equilibria in the CuI-SbSI-SbI 3 concentration triangle.

CuI-SbSI boundary quasi-binary system
The powder X-ray diffraction patterns of the thermally treated CuI-SbSI alloys are given in Fig. 1. As can be seen, diffraction patterns of samples in the full composition range consist of the diffraction peaks of the SbSI and lowtemperature modification of CuI.
The T-x phase diagram of the system (Fig. 2) was constructed using DTA results ( Table 2). Note that, a 1 and a 2 are solid solutions based on the HT1 -CuI and HT2 -CuI respectively, and b -is a solid solution based on SbSI.
The system is quasi-binary and forms a eutectic phase diagram. Eutectics has a ~ 45 mol% SbSI composition and crystallises at 327 °C by the reaction: The formation of a 1 and a 2 solid solution areas based on the high-temperature modifications of CuI is accompanied by a decrease in temperature of its' both phase transformations and these phase transitions occur by metatectic and eutectoid reactions.
Isotherms corresponding to the 375 and 280 °C temperatures on the phase diagram, reflect metatectic  equilibriums, respectively. The homogeneity region of the b-phase based on SbSI is maximum (~15 mol%) at the eutectic temperature (Fig. 2). Moreover, reflection angles belonging to LT-CuI and SbSI phases on powder diffractograms are fully compatible with appropriate pure compounds. It shows that the mutual solubility of these compounds is negligible at room temperature. Therefore, in Fig. 2, the decomposition curve of the b-phase is extrapolated to the SbSI compound. (Fig. 3) Fig. 3 represents a projection of the Т-х-у diagram of the CuI-SbSI-SbI 3 system, where liquidus isotherms are given in blue. The liquidus surface consists of three fields describing the primary crystallisation of the a 1 (a 2 ), b-phases, and SbI 3 . The latter occupies a small region near the appropriate corner of the concentration triangle.

Projection of the liquidus surface
Primary crystallisation surfaces of phases are limited by a number of monovariant equilibrium curves and non-variant equilibrium points ( Table 3).
The L 1 +L 2 immiscibility region in the CuI-SbI 3 boundary system sharply penetrates into the concentration triangle and covers part of the liquidus area of the b phase by crossing the eutectic curve from the point e 1 . Consequently, the L ↔ a 2 + b monovariant eutectic equilibrium shifts to the L 1 ↔ L 2 + a 2 + b nonvariant monotectic equilibrium (Fig. 3, Table 1 -MM¢ conjugate pair). K is the critical point of stratification and has a temperature of ~350 °C.
Crystallisation across the whole system ends at 165 °C by nonvariant eutectic (E) reaction.

Polythermal sections
The CuI-[B] (Fig. 4) and [A]-SbSI (Fig. 5) polythermal sections of the phase diagram of the CuI-SbSI-SbI 3 ternary system are given below and analysed in context with the projection of the liquidus surface of the system. Here, [A] and [B] are 1:1 mix ratios of the constituent compounds of the CuI-SbI 3 and SbSI-SbI 3 side binary systems, consequently.
The system CuI-[B] (Fig. 4). This section passes through the initial crystallisation areas of the a 1 (a 2 ) and b-phases and the immiscibility area in the ~ 30-70 mol% CuI concentration range. Crystallisation in the compositions rich in CuI initially continues by the monovariant monotectic L 1 ↔ L 2 + a 1 reaction and leads to the formation of the L 1 + L 2 + a 1 three-phase area. At 377 °C this phase field is replaced by the L 1 + L 2 + a 1 three-phase area as a result of the a 1 ↔ a 2 phase transition. Crystallisation in the 20-40 mol% CuI composition range continues by the L 1 ↔ L 2 + b monotectic scheme and forms the L 1 + L 2 + b phase area. Horizontal line at 318 °C belongs to the L 1 ↔ L 2 + a 2 + b nonvariant monotectic reaction ( Table 2). After this reaction, the L 2 + a 2 + b threephase area forms in the system. At 280 °C, the a 2 ↔ LT-CuI phase transitions occur and the latter phase area passes to the L 2 + b + LT-CuI. Crystallisation of all samples along the system ends at 165 o C by the nonvariant eutectic reaction (E) and the b + LT-CuI + SbI 3 three-phase mixture forms.
The system [A]-SbSI (Fig. 5). This polythermal section is situated in the L 1 + L 2 immiscibility area at the 0-40 mol% SbSI composition range and crystallisation processes occur by monotectic reactions (Fig. 3, mMK and m / M / K / conjugate curves). In the course of those processes the L 1 + L 2 + a 1 , L 1 + L 2 + a 2 , L 1 + L 2 + LT-CuI and L 1 + L 2 + b three-phase areas are formed. In the alloys rich in SbSI, crystallisation of this compound initially occurs from the liquid solution, then continues by the L 1 ↔ L 2 + SbSI monotectic scheme. All alloys are exposed to the nonvariant monotectic  reaction (m) at 318 0 C and fully crystallise by the nonvariant eutectic reaction at 165 °C. Fig. 6 shows the DTA heating curves of selected annealed samples along the boundary quasi-binary system CuI-SbSI and the abovementioned internal sections. Comparison of these curves with the corresponding T-x diagrams (Fig. 2, 4, 5), the projection of the liquidus surface (Fig. 3) and the table shows that they accurately reflect the character and temperatures of the processes occurring in the system.

Conclusion
The phase equilibria in the CuI-SbSI-SbI 3 composition range of the Cu-Sb-S-I quaternary system have been studied for the first time. Several polythermal sections of the phase diagram including the CuI-SbSI boundary system and T-x-y projection of the liquidus surface of the system was obtained by coanalysis of experimental results along with the literature data on boundary binary systems. It was determined that there are limited solid solutions based on SbSI (b-phases) and HT-CuI (a 1 -and a 2 -phases) and the system is characterised by the formation of a large immiscibility area. The types and coordinates of non-and monovariant equilibria, as well as, primary crystallisation areas of phases were determined.

Contribution of the authors
P. R. Mammadli -experimental investigations, writing original draft, making conclusions. V. A. Gasymov -powder X-ray analysis. G. B. Dashdiyeva -research concept, methodology development. D. M. Babanly -scientific management, review and editing.

Conflict of interests
The author declares that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.