S M Sosovska, I D Olekseyuk, O V Parasyuk - The quasi-ternary cdse-ga2se3-bi2se3 system - страница 1

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Chemistry of Metals and Alloys

Chem. Met. Alloys 3 (2010) 5-11 Ivan Franko National University of Lviv www.chemetal-journal.org

The quasi-ternary CdSe-Ga2Se3-Bi2Se3 system

S.M. SOSOVSKA1, I.D. OLEKSEYUK2, O.V. PARASYUK2*

1 Department of Chemistry, Lutsk National Technical University, Lvivska St. 75, 43018 Lutsk, Ukraine

2 Department of Inorganic and Physical Chemistry, Lesya Ukrainka Volyn National University,

Voli Ave 13, 43025 Lutsk, Ukraine * Corresponding author. E-mail: oleg@univer.lutsk.ua

Received January 29, 2010; accepted June 29, 2010; available on-line November 5, 2010

Differential-thermal analysis, X-ray diffraction, and metallography were used to study the interaction between the components of the quasi-ternary CdSe-Ga2Se3-Bi2Se3 system. Phase diagrams of four polythermal sections, the isothermal section at 670 K, as well as a projection of the liquidus surface have been constructed. The projection of the liquidus surface consists of six fields of primary crystallization of the phases, which are separated by 12 monovariant curves and 11 invariant points. The type of mono- and invariant processes in the system has been investigated and the coordinates of the invariant points have been determined. The system is triangulated by the quasi-binary section CdGa2Se4-Bi2Se3 into two sub-systems CdSe-CdGa2Se4-Bi2Se3 and CdGa2Se4-Ga2Se3-Bi2Se3. The present results can be employed for the growth of CdGa2Se4 single crystals from non-stoichiometric melts.

Thermal analysis / Chalcogenide / Phase diagram / Isothermal section / Liquidus surface

Introduction

Compounds Anf3in2CVI4 with defect chalcopyrite structure (CdGa2S4 structure type) are promising materials for non-linear optics. For instance, non­linear properties of HgGa2S4 [1,2] and Hg1-xCdxGa2S4 [2] have been extensively studied during recent years. Some of their parameters, like resistance against laser radiation, exceed by far those of the chalcogenide compounds that are presently used for the parametric frequency conversion of CO2 lasers [2]. Among the numerous representatives of this group, we focus on CdGa2Se4. A comprehensive review of its optical properties can be found in [3]. The ternary cadmium gallium selenide has been known for a long time [4], but its industrial application is limited due to difficulties in growing large bulk crystals. Although the CdGa2Se4 compound melts congruently, it undergoes a polymorphous transformation, which hinders the single crystal growth using directional crystallization of the stoichiometric solution. There were attempts to grow CdGa2Se4 single crystals using chemical transport reactions with iodine as a transport agent [5-9]. Furthermore, the growth of single crystals from different melts is described in several studies [10-15], since non-stoichiometric melts can offer growth temperatures lower than the polymorphous transformation temperature. In this respect, the selection of the solvent is an important issue. We have tested a series of different solvents: SnSe2 [12], SnSe [13], Sb2Se3 [14], Bi2Se3 [15], and PbSe [16]. The corresponding binary phase diagrams exhibit a primary crystallization field of the low-temperature CdGa2Se4 modification. Unfortunately, the formation of undesirable solid solutions on the basis of CdGa2Se4 occurs in all the above systems, except 3 i2Se3 and PbSe. Therefore, we conclude that the two latter solvents seem to be the most promising for successful crystal growth [15]. The part of the liquidus that belongs to the low-temperature CdGa2Se4 modification in the CdGa2Se4-Bi2Se3 system is not large though. However, there may exist a possibility to widen the primary crystallization region by using the more complex CdSe-Ga2Se3-Bi2Se3 system, where possible ternary eutectics should lower the crystallization temperature of intermediate alloys inside the system.

The binary compounds CdSe, Ga2Se3, and Bi2Se3 melt congruently at 1512 K, 1278 K [17], and 979 K [18], respectively; they possess narrow homogeneity ranges, and therefore, act as components of a quasi-ternary system. Cadmium selenide crystallizes in the wurtzite structure type, space group P63mc with lattice parameters а = 0.42999 and с = 0.70109 nm [19]. Ga2Se3 crystallizes in the zinc blende structure type, space group F43m, a = 0.5422 nm [20], whereas

Bi2Se3 exhibits a trigonal structure, space group R3m, with а = 0.41404(7), c = 2.8640(6) nm [15].

The boundary systems CdSe-Ga2Se3, CdSe-Bi2Se3, and Ga2Se3-Bi2Se3 were investigated in [21-25]. There is one congruently melting compound CdGa2Se4 in the CdSe-Ga2Se3 system. According to [21], the coordinates of the two eutectic points are 39 mol.% Ga2Se3 / 1208 K and 64 mol.% Ga2Se3 / 1206 K, which are in agreement with those given in [23]. The polymorphous transformation of CdGa2Se4 takes place at 1090 K. The solid solubility in Ga2Se3 extends to 17 mol.% CdSe at 1206 K; on the CdSe side the solid solution contains 32 mol.% Ga2Se3 at the eutectic temperature. The solid solubility in CdGa2Se4 is ~2 mol.% and is shifted to the Ga2Se3 side. Monotectoid decomposition of the solid solution at 988 K is observed for alloys in the concentration range of 5-25 mol.% Ga2Se3. According to [22], the coordinates of the eutectic points are 45 mol.% and 55 mol.% Ga2Se3 for 1203 and 1213 K, respectively. The solid solubility in the system components is 13 and 25 mol.% Ga2Se3 [23]. The low-temperature modification of CdGa2Se4 crystallizes in a defect chalcopyrite structure (space group І4) with unit cell parameters а = 0.57430 and с = 1.0752 nm [15].

The CdSe-Bi2Se3 system was investigated in [24]. In this system, a peritectic reaction results in the formation of the CdBi2Se4 compound, which is stable only in the temperature interval 877-1009 K. The coordinates of the peritectic point are 95 mol.%

Bi2Se3 / 1009 K. The solid solution based on CdSe

contains 0.25 mol.% Bi2Se3 at 723 K; when raising the temperature it extends to 1 mol.% at 1000 K. The solid solution of Bi2Se3 contains up to 3 mol.% CdSe

at 723 K.

The Ga2Se3-Bi2Se3 system is described in [18,25]. The system is quasi-binary and of the eutectic type. The liquidus consists of the primary crystallization fields of the Ga2Se3 and Bi2Se3 solid solutions. The coordinates of the eutectic point are 65 mol.% Bi2Se3 / 900 K [18] or ~61 mol.% Bi2Se3 / 893 K [25]. The solid solubility of the components does not exceed 5 mol.% [25].

The CdGa2Se4-Bi2Se3 phase diagram is of the eutectic type [15]. Its liquidus consists of three fields of primary crystallization of solid solutions based on the high- and low-temperature modifications of CdGa2Se4 and Bi2Se3. The eutectic point is located at ~86 mol.% Bi2Se3 and 929 K. A horizontal line at 1092 K corresponds to the polymorphous transformation of high-temperature CdGa2Se4 into low-temperature CdGa2Se4.

Analyzing the above phase diagrams, we expect that lowering the crystallization temperature is feasible in the CdSe-Ga2Se3-Bi2Se3 quasi-ternary system because of the possible formation of ternary eutectics. This work is focused on the investigation of phase equilibria in the CdSe-Ga2Se3-Bi2Se3 system, performed in order to find concentration regions that are suitable for the growth of single crystals of the low-temperature modification of CdGa2Se4 using the solution-melt method.

Experimental

For the investigation of the CdSe-Ga2Se3-Bi2Se3 system, 54 alloys were prepared, the compositions of which are plotted on the concentration triangle shown in Fig. 1. Their synthesis was carried out by fusing calculated amounts of the elements (purity better than 99.99%) in evacuated quartz ampoules. The alloys were held for 3 hours at the maximum temperature, which was 1320-1170 K depending on the composition. Afterwards, the ampoules were cooled slowly (10 K/h) down to 670 K. At this temperature, the alloys were annealed for 250 hours and then rapidly quenched in cold water.

The obtained alloys were investigated with differential-thermal analysis (DTA), X-ray diffraction (XRD), and metallography. DTA signals were recorded on a Paulik-Paulik-Erdey derivatograph (Pt/Pt-Rh thermocouple). XRD was performed on powders using a DRON-4-13 diffractometer (Cu&Ta-radiation), and the microstructure of polished alloys was examined on a Leica VMHT Auto microhardness tester.

Triangulation

The isothermal section of the CdSe-Ga2Se3-Bi2Se3 system at 670 K was constructed using XRD and metallography results (Fig. 2). The CdGa2Se4 compound forms a quasi-binary section with Bi2Se3. The section divides the CdSe-Ga2Se3-Bi2Se3 system into   two   sub-systems,   namely Ga2Se3-Bi2Se3-

CdGa2Se4 and CdSe-Bi2Se3-CdGa2Se4. The CdBi2Se4

compound is formed by a peritectic reaction in the CdSe-Bi2Se3-CdGa2Se4 sub-system. This compound is stable only in a limited high-temperature interval and does not exist at the annealing temperature 670 K. Within the range of existence of CdBi2Se4, the CdSe-Bi2Se3-CdGa2Se4 sub-system could be conventionally triangulated into CdGa2Se4-Bi2Se3-CdBi2Se4 and CdGa2Se4-CdBi2Se4-CdSe sub-systems.

Results

The common section at 75 mol.% Bi2Se3: 'GaBi3Se4'-'CdBi6Se10' section (Fig. 3)

The liquidus of the 'GaBi3Se4'-'CdBi6Se10' section consists of the primary crystallization fields of g[1], 5,

and p. The secondary crystallization involves two binary eutectics ([L<=>a+g (field 5), L<=>5+g (6)] in the sub-liquidus part of the Ga2Se3-CdGa2Se4-Bi2Se3 sub­system (0-50 mol.% 'CdBi6Se10' in the section). The crystallization in this part of the section is completed by a eutectic reaction at 892 K (point E1 in Fig. 7). The alloy with 50 mol.% 'CdBi6Se10' is a two-phase one (e4, Fig. 7). For the alloys from the concentration interval ~55-62 mol.% 'CdBi6Se10', the L+P<=>5+4 peritectic reaction is observed at 942 K, as well as the secondary crystallization fields, L+8+P (field 7) and L+4+P (field 8). Since the alloys of this part of the section are located in the CdGa2Se4-Bi2Se3-CdBi2Se4 sub-system, the primary crystallization field of the solid solution of the ternary compound 4 (field 10) can be found in the sub-liquidus part. The lowering of the CdBi2Se4 decomposition temperature indicates the existence of a certain homogeneity region based on CdBi2Se4, and one additional invariant solid-state process 4^P+T+5. Crystallization of these alloys completes with the L<=>5+y+4 ternary eutectic process

(922 K, E2).

The common section at 50 mol.% Bi2Se3: 'BiGaSe3'-CdBi2Se4 (Fig. 4)

The 'BiGaSe3'-CdBi2Se4 section contains three fields of primary crystallization: L+a (field 2), L+5 (3), and L+P (4). There are 5 regions of secondary crystallization involving the liquid phase (L+a+g (field 5), L+a+5 (6), L+5+g (7), L+5+P (8), L+P+4 (9)) in the sub-liquidus part. The alloys located in the Ga2Se3-CdGa2Se4-Bi2Se3 sub-system (0-50 mol.% CdBi2Se4) solidify according to the ternary eutectic reaction L<=>a+5+y at 892 K (Е1, Fig. 7). The alloy containing 50 mol.% Bi2Se3 contains two phases (е4, Fig. 7). For most samples from the CdGa2Se4-CdBi2Se4-CdSe sub-system, the crystallization continues with the invariant peritectic horizontal, which belongs to the 5-U3-4-P plane (942 K) and finishes with the ternary eutectic reaction L<=>5+g+4 at

922 K 2, Fig. 7). The horizontal line at 853 K

corresponds  to  the  eutectoid  decomposition of

CdBi2Se4.

The common section at 25 mol.% Bi2Se3: 'BiGa3Se6'-'Cd3Bi2Se6' (Fig. 5)

Although the diagram of the 'BiGa3Se6'-'Cd3Bi2Se6' section is similar to the previous one, it has several peculiarities. One difference is that the section contains a primary crystallization field of 5' (field 3). Fields 4 correspond to the simultaneous existence of the low- and high-temperature modifications of CdGa2Se4. Together with the liquidus isotherms they are used to determine geometrically the position of the two ternary peritectics U1 and U2 (Fig. 7). Another difference is that the section crosses the CdGa2Se4-CdBi2Se4-CdSe sub-system (67-100 mol.% 'Cd3Bi2Se6'), where the crystallization is completed by a peritectic reaction at 942 K. The thermal decomposition of CdBi2Se4 takes place at 853 K, however, a liquid phase is not required for the reaction.

The CdGa2Se4-CdBi2Se4 section (Fig. 6)

The CdGa2Se4-CdBi2Se4 section is not quasi-binary. Its liquidus consists of primary crystallization fields of solid solutions based on high-temperature and low-temperature CdGa2Se4, and CdSe. The horizontal line at 1078 K corresponds to the beginning of the

'GaBi,Se6'

80 'CdBifiSe„

Fig. 3 Phase diagram of the 'GaBi3Se4'-'CdBi6Sew' section: (1) L; (2) L+g (3) L+5; (4) L+P; (5) L+g+a; (6) L+g+5; (7) L+5+P; (8) L+P+4; (9) L+5+4; (10) L+4; (11) L+4+g; (12) 4+g (13) У+4+Р; (14) y+a; (15) g+a+5; (16) P+5+4; (17) g+5; (18) P+5+g (19) y+p.

Fig. 4 Phase diagram of the 'BiGaSe3'-CdBi2Se4 section: (1) L; (2) L+a; (3) L+5; (4)

L+P; (5) L+a+g; (6) L+5+a; (7) L+5+g; (8)

L+P+5; (9) L+P+4; (10) L+5+4; (11) L+4; (12) L+y+4; (13) 5+g+4; (14) y+4; (15) P+y+4; (16)

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S M Sosovska, I D Olekseyuk, O V Parasyuk - The quasi-ternary cdse-ga2se3-bi2se3 system