Quasi-Ternary System Cu₂Se-GeSe₂-As₂Se₃

Abstract

Direct single-temperature method has been employed to synthesize 137 samples of the Cu₂Se-GeSe₂-As₂Se₃ system. The samples have been investigated by x-ray diffraction and differential thermal analysis. The isothermal section at 513 K (240  °C), two phase diagrams GeSe₂-As₂Se₃ and Cu₂GeSe₃-As₂Se₃, four vertical sections Cu₂GeSe₃-CuAsSe₂, Cu₈GeSe₆-CuAsSe₂, Cu₈GeSe₆-As₂Se₃, A-B (A = 85 mol.% GeSe₂-15 mol.% Cu₂Se; B = 85 mol.% As₂Se₃-15 mol.% Cu₂Se), and the liquidus surface projection onto the concentration triangle have been constructed. The existence of the ternary compounds Cu₂GeSe₃, Cu₈GeSe₆ and CuAsSe₂ have been confirmed. The fields of the primary crystallization of phases, character, temperatures and coordinates of nonvariant points have been determined.

1 Introduction

The binary compounds Cu₂Se, GeSe₂, As₂Se₃ melt congruently at 1421 K (1148 °C), 1013 K (740 °C), and 648 K (375 °C),[1] respectively, possess insignificant homogeneity regions and may serve as components of the quasi-ternary system Cu₂Se-GeSe₂-As₂Se₃. The study of this quasi-ternary system is of current interest because it is formed by binary compounds with important semiconducting properties.[2,3]

Phase equilibria in the quasi-binary system Cu₂Se-GeSe₂ were studied in Ref 4,5,6,7. Two ternary compounds were found in the system, Cu₂GeSe₃ and Cu₈GeSe₆.[4,5,6,7] Cu₂GeSe₃ melts congruently at 1053 K (780 °C), and Cu₈GeSe₆ is formed in the peritectic reaction at 1083 K (810 °C) and has two polymorphous transformations at 983 K (710 °C) and 333 K (60 °C).[7] The interactions of Cu₈GeSe₆ and Cu₂GeSe₃, Cu₂GeSe₃ and GeSe₂ are eutectic. The eutectic points have the following coordinates: e₁-38 mol.% GeSe₂ and 1033 K (760 °C); e₂-83 mol.% GeSe₂ and 960 K (687 °C). The Cu₂GeSe₃ compound crystallizes in space group Imm2, a = 1.1860(3) nm, b = 0.3960(1) nm, c = 0.5485(2) nm;[8] Cu₈GeSe₆ in space group P6₃cm, a = 1.2648(5) nm, c = 1.176(4) nm.[9]

The Cu₂Se-As₂Se₃ phase diagram was described in Ref 10,11,12,13. The system features one ternary compound CuAsSe₂ which melts incongruently at 725 K[10] and crystallizes in space group R3 (Cu₇As₆Se₁₃ structure type) with the cell parameters a = 1.4025(3) nm, c = 0.961(3) nm.[14]

The GeSe₂-As₂Se₃ system is a section of the ternary system Ge-As-Se that was studied in terms of its glass-forming ability.[15,16,17] According to these studies, no intermediate phases form in the GeSe₂-As₂Se₃ system. No phase diagram of this system has been reported in the literature.

2 Experimental Methods

Phase equilibria in the quasi-ternary system Cu₂Se-GeSe₂-As₂Se₃ were studied on 137 samples. The alloys were synthesized by direct single-temperature method from high-purity elements (Cu 99.99, Ge 99.99, Se 99.9997, As 99.9999 wt.%) in evacuated to the residual pressure 0.133 Pa and sealed quartz containers. The synthesis was performed in a shaft-type furnace with temperature control with an accuracy of ± 5 K (± 5 °C). The maximum synthesis temperature was 1453 K (1180 °C), the heating and cooling rate was 10 K/h (10 °C/h). Homogenizing annealing at 513 K (240 °C) was held for 600 h, after which the samples were quenched into 25% aqueous NaCl solution.

Obtained samples were investigated by x-ray diffraction (XRD) method (DRON 4-13 diffractometer, CuK_α radiation, 10° < 2θ < 80°, 0.05° scan step, 1 s exposure in each point) and differential thermal analysis (DTA) (“Thermodent H307/1” furnace with a PDA-1 XY-recorder, Pt/Pt-Rh thermocouple). The two-phase or three-phase composition of the samples was also checked by the microstructural analysis (MSA), which was performed on a PMT-3M microhardness tester.

3 Results and Discussion

3.1 The Quasi-Binary System GeSe₂-As₂Se₃

Phase diagram of the GeSe₂-As₂Se₃ system was investigated by DTA and XRD methods (Fig. 1). The liquidus consists of the curves of the primary crystallization of α-solid solutions of GeSe₂ and β-solid solutions of As₂Se₃, which at 513 K (240 °C) extend to less than 4 mol.%. The system solidus is the eutectic horizontal at 618 K (345 °C). The eutectic point was determined by the Tamman triangle according to the literature[18] as 20 mol.% GeSe₂, 80 mol.% As₂Se₃. The alloys below the eutectic horizontal are two-phase.

Fig. 1

Phase diagram of the GeSe₂-As₂Se₃ system: 1-L, 2-L + α, 3-L + β, 4-α, 5-α + β, 6-β (where α-solid solutions of GeSe₂, β-solid solutions of As₂Se₃)

3.2 The Quasi-Binary System Cu₂GeSe₃-As₂Se₃

The phase diagram of the Cu₂GeSe₃-As₂Se₃ system was constructed from XRD and DTA results (Fig. 2). The system liquidus consists of the curves of the primary crystallization of η-solid solutions of Cu₂GeSe₃ and β-solid solutions of As₂Se₃. Nonvariant eutectic process L ↔ η + β takes place at 633 K (360 °C). The eutectic point e₄ was determined by the Tamman triangle as the composition of 11.7 mol.% Cu₂GeSe₃, 88.3 mol.% As₂Se₃. The alloys below the eutectic horizontal are two-phase, β-solid solutions of As₂Se₃ and η-solid solutions of Cu₂GeSe₃. The solid solubility at 513 K (240 °C) is less than 2 mol.% Cu₂GeSe₃ and 2 mol.% As₂Se₃, respectively.

Fig. 2

Phase diagram of the Cu₂GeSe₃-As₂Se₃ system: 1-L, 2-L + η, 3-L + β, 4-η, 5-η + β, 6-β (where β-solid solutions of As₂Se₃, η-solid solutions of Cu₂GeSe₃)

3.3 The Vertical Section Cu₂GeSe₃-CuAsSe₂

The Cu₂GeSe₃-CuAsSe₂ section was investigated by DTA and XRD methods (Fig. 3). The section liquidus is represented by the curves of the primary crystallization: ab of η-solid solutions of Cu₂GeSe₃, bc of LTM-Cu₈GeSe₆, cd of γ-solid solutions of Cu₂Se. The section crosses the plane of the nonvariant peritectic process L_U1 + γ ↔ LTM-Cu₈GeSe₆ + δ at 700 K (427 °C) (δ—solid solutions of CuAsSe₂), where the volumes of monovariant peritectic processes L + γ ↔ LTM-Cu₈GeSe₆ and L + γ ↔ δ converge. The plane of the nonvariant peritectic process L_U2 + LTM-Cu₈GeSe₆ ↔ η + δ lies at 650 K (377 °C) and the volumes of monovariant eutectic processes L ↔ η + LTM-Cu₈GeSe₆ and L ↔ LTM-Cu₈GeSe₆ + δ converge to this plane. The nonvariant peritectic process at 650 K (377 °C) ends with the exhaustion of both L and the crystals of LTM-Cu₈GeSe₆, since this section is the connecting line of the plane of this peritectic process. Therefore, the section alloys below 650 K (377 °C) are two-phase as confirmed by XRD data (Fig. 3).

Fig. 3

The vertical section Cu₂GeSe₃-CuAsSe₂: 1-L, 2-L + η, 3-η, 4-η + LTM-Cu₈GeSe₆, 5-L + η + LTM-Cu₈GeSe₆, 6-L + LTM-Cu₈GeSe₆, 7-L + LTM-Cu₈GeSe₆ + γ, 8-L + γ, 9-L + γ + δ, 10-δ + γ, 11-L + LTM-Cu₈GeSe₆ + δ, 12-η + δ, 13-δ (where γ-solid solutions of Cu₂Se, δ-solid solutions of CuAsSe₂, η-solid solutions of Cu₂GeSe₃)

3.4 The Vertical Section Cu₈GeSe₆-CuAsSe₂

The Cu₈GeSe₆-CuAsSe₂ section was investigated from DTA and XRD results (Fig. 4). Liquidus of the section is the curve of primary crystallization of γ-solid solutions of Cu₂Se. The section then crosses the plane of the nonvariant process HTM-Cu₈GeSe₆ ↔ LTM-Cu₈GeSe₆ + γ + L_U3 at 970 K (697 °C) which is due to the polymorphous transition of Cu₈GeSe₆ in the Cu₂Se-GeSe₂ system. The section crosses the plane of the nonvariant peritectic process L_U1 + γ ↔ LTM-Cu₈GeSe₆ + δ at 700 K (427 °C). The process in the alloys of this section at this temperature ends with the exhaustion of both liquid and the crystals of γ-solid solutions of Cu₂Se because the section coincides with the connecting diagonal of the plane of the nonvariant peritectic process. Therefore, the alloys below 700 K (427 °C) are two-phase, LTM-Cu₈GeSe₆ and δ, which is confirmed by XRD results (Fig. 4).

Fig. 4

The vertical section Cu₈GeSe₆-CuAsSe₂: 1-L, 2-L + γ, 3-γ + HTM-Cu₈GeSe₆, 4-L + γ + HTM-Cu₈GeSe₆, 5-HTM-Cu₈GeSe₆ + LTM-Cu₈GeSe₆ + γ, 6-LTM-Cu₈GeSe₆ + γ, 7-L + LTM-Cu₈GeSe₆ + γ, 8-L + γ + δ, 9-δ + γ, 10-LTM-Cu₈GeSe₆ + δ, 11-δ (where γ-solid solutions of Cu₂Se, δ-solid solutions of CuAsSe₂)

3.5 The Vertical Section Cu₈GeSe₆-As₂Se₃

The Cu₈GeSe₆-As₂Se₃ section was investigated by DTA and XRD methods (Fig. 5). The section liquidus consists of the curves ab (of the primary crystallization of γ-solid solutions of Cu₂Se), bc LTM-Cu₈GeSe₆), cd (δ-solid solutions of CuAsSe₂), and df (β-solid solutions of As₂Se₃). The nonvariant process HTM-Cu₈GeSe₆ ↔ LTM-Cu₈GeSe₆ + γ + L_U3 at 970 K (697 °C) is caused by the polymorphous transition of Cu₈GeSe₆ at 983 K (710 °C) in the Cu₂Se-GeSe₂ system. The section then crosses another plane of the nonvariant peritectic process L_U2 + LTM-Cu₈GeSe₆ ↔ η + δ at 650 K (377 °C). The volume of the monovariant eutectic process L ↔ δ + LTM-Cu₈GeSe₆ converges to this plane. The alloy 25 mol.% Cu₈GeSe₆-75 mol.% As₂Se₃ is two-phase according to XRD (Fig. 5) because the nonvariant process L_U2 + LTM-Cu₈GeSe₆ ↔ η + δ at point g ends with the exhaustion of both liquid and the crystals of LTM-Cu₈GeSe₆, so the field 11 is two-phase. This field separates two three-phase regions of the co-existence of LTM-Cu₈GeSe₆, δ, η phases (field 10) and of the monovariant eutectic process L ↔ δ +η (field 12). The latter it ends at the plane of nonvariant eutectic process L_E1↔ δ + η + β at 565 K (292 °C). The field of another monovariant eutectic process L ↔ δ + β (field 15) also converges to this plane. Below this plane the alloys are three-phase, δ + η + β (field 18).

Fig. 5

The vertical section Cu₈GeSe₆-As₂Se₃: 1-L, 2-L + γ, 3-HTM-Cu₈GeSe₆ + γ, 4-L + γ + HTM-Cu₈GeSe₆, 5-HTM-Cu₈GeSe₆ + LTM-Cu₈GeSe₆ + γ, 6-γ + LTM-Cu₈GeSe₆, 7-L + γ + LTM-Cu₈GeSe₆, 8-L + LTM-Cu₈GeSe₆, 9-L + δ + LTM-Cu₈GeSe₆, 10-LTM-Cu₈GeSe₆ + η + δ, 11-η + δ, 12-L + η + δ, 13-L + δ, 14-L + β, 15-L + δ + β, 16-δ + β, 17-LTM-Cu₈GeSe₆ + δ, 18-δ + η + β, 19-β (where β-solid solutions of As₂Se₃, γ-solid solutions of Cu₂Se, δ-solid solutions of CuAsSe₂)

3.6 The Vertical Section A–B (A: 85 mol.% GeSe₂-15 mol.% Cu₂Se; B: 85 mol.% As₂Se₃-15 mol.% Cu₂Se)

The section A–B was investigated by DTA and XRD methods (Fig. 6). The section liquidus consists of the curves ab of the primary crystallization of α-solid solutions of GeSe₂, bc (η-solid solutions of Cu₂GeSe₃), cd (δ-solid solutions of CuAsSe₂). The vertical section crosses two subsystems of the quasi-ternary system Cu₂Se-GeSe₂-As₂Se₃. In the GeSe₂-Cu₂GeSe₃-As₂Se₃ subsystem, the section crosses the plane of nonvariant eutectic process L_E2↔ α + η + β at 540 K (267 °C). The nonvariant eutectic process L_E1↔ η + δ + β takes place in the Cu₂GeSe₃-As₂Se₃-Cu₂Se subsystem at 565 K (292 °C). The samples below the planes of these processes are three-phase (fields 11, 12). These three-phase regions are separated by two-phase field 6, the existence of which is caused by the intersection with the quasi-binary system Cu₂GeSe₃-As₂Se₃. Monovariant eutectic processes L ↔ α + η and L ↔ η + β converges onto the plane of the nonvariant process at 540 K (267 °C). Monovariant eutectic processes L ↔ η + β (field 5), L ↔ η + δ (field 7), L ↔ β + δ (field 8) converges to the plane of the nonvariant eutectic process at 565 K (292 °C). The phase composition at the annealing temperature of 513 K (240) was determined by x-ray phase analysis (Fig. 6).

Fig. 6

The vertical section A–B (A: 85 mol.% GeSe₂-15 mol.% Cu₂Se; B: 85 mol.% GeSe₂-15 mol.% Cu₂Se): 1-L, 2-L + α, 3-L + α + η, 4-L + η, 5-L + η + β, 6-η + β, 7-L + η + δ, 8-L + β + δ, 9-L + δ, 10-δ + β, 11-η + α + β, 12-η + δ + β (where α-solid solutions of GeSe₂, β-solid solutions of As₂Se₃, γ-solid solutions of Cu₂Se, δ-solid solutions of CuAsSe₂)

3.7 Isothermal Section of the Quasi-Ternary System Cu₂Se-GeSe₂-As₂Se₃ at 513 K (240 °C)

Based on the results of x-ray diffraction and MSA of the 137 samples (Fig. 7) the isothermal section of the system at 513 K (240 °C) was plotted (Fig. 8). It was investigated, that binary compounds crystallizes: Cu₂Se in space group C2/c, a = 0.7135(2) nm, b = 1.2383(1) nm, c = 2.7387(4) nm, β = 94.307(2); GeSe₂ in space group P2₁/c, a = 0.7007(2) nm, b = 1.6819(5) nm, c = 1.1806(3) nm, β = 90.74(2); As₂Se₃ in space group P2₁/c, a = 0.4267(2) nm, b = 0.9874(5) nm, c = 1.2794(7) nm, α = 109.96(4), what is in good agreement with literature data [19] for Cu₂Se, [20] for GeSe₂ and [21] for As₂Se₃. The existence of the three ternary compounds Cu₈GeSe₆, Cu₂GeSe₃ and CuAsSe₂ was confirmed. Diffractograms of the ternary compounds were indexed: Cu₂GeSe₃ in space group Imm2, a = 1.1859(1) nm, b = 0.3951(4) nm, c = 0.54879(2) nm; Cu₈GeSe₆ in space group P6₃cm, a = 1.26421(2) nm, c = 1.17567(3) nm; CuAsSe₂ in space group R3 (Cu₇As₆Se₁₃ structure type) with the cell parameters a = 1.4014(2) nm, c = 0.9583(3) nm, what is in good agreement with literature data.[8,9,14]

Fig. 7

Chemical and phase compositions of the Cu₂Se-GeSe₂-As₂Se₃ system samples at 513 K

Fig. 8

The isothermal section of the quasi-ternary system Cu₂Se-GeSe₂-As₂Se₃ at 513 K

The quasi-ternary system is divided into 4 subsystems: Cu₂Se-Cu₈GeSe₆-CuAsSe₂; Cu₈GeSe₆-CuAsSe₂-Cu₂GeSe₃; CuAsSe₂-Cu₂GeSe₃-As₂Se₃, and Cu₂GeSe₃-GeSe₂-As₂Se₃. No extensive solid solutions were found, and the solid solubility based on binary and ternary compounds is less than 2 mol.%.

3.8 Liquidus Surface Projection of the Quasi-Ternary System Cu₂Se-GeSe₂-As₂Se₃

Liquidus surface projection of the Cu₂Se-GeSe₂-As₂Se₃ system (Fig. 9 and Table 1) was plotted using the literature data,[7,10] the experimental results of the study of four vertical sections, two phase diagrams and additional samples to find out the nonvariant points (Fig. 9). The liquidus surface consists of the fields of the primary crystallization of γ-solid solutions of Cu₂Se, δ-solid solutions of CuAsSe₂, HTM-Cu₈GeSe₆, LTM-Cu₈GeSe₆, η-solid solutions of Cu₂GeSe₃ (the largest field), α-solid solutions of GeSe₂, and β-solid solutions of As₂Se₃. The fields are separated by 11 monovariant curves and 13 nonvariant points. The Cu₂GeSe₃-As₂Se₃ section is quasi-binary and divides the studied system into two subsystems, Cu₂Se-Cu₂GeSe₃-As₂Se₃ and As₂Se₃-Cu₂GeSe₃-GeSe₂. Point U₁ lies on the plane of the nonvariant peritectic process L_U1 + γ ↔ δ + LTM-Cu₈GeSe₆ that takes place at 700 K (427 °C). Point U₂ belongs to the plane of the nonvariant peritectic process L_U2 + LTM-Cu₈GeSe₆ ↔ η + δ at 650 K (377 °C). The lines of monovariant eutectic equilibria L_U2-E1 ↔ η + δ, L_e5-E1↔ δ + β, L_e4-E1↔ η + β converge to the point E₁ of the nonvariant eutectic process L_E1↔ η + δ + β at 565 K (292 °C). The points U₃ and U₄ lie on the planes of the isothermal processes HTM-Cu₈GeSe₆ ↔ LTM-Cu₈GeSe₆ + γ + L_U3 and HTM-Cu₈GeSe₆ ↔ LTM-Cu₈GeSe₆ + η + L_U4 at 970 K which exist due to polymorphic transformation of HTM-Cu₈GeSe₆ into LTM-Cu₈GeSe₆ in the Cu₂Se-GeSe₂ quasi-binary system. In the As₂Se₃-Cu₂GeSe₃-GeSe₂ subsystem one nonvariant eutectic process L_E2↔ α + η + β takes place at 540 K (267 °C). The curves of monovariant eutectic processes L_e2-E2↔ η + α, L_e3-E2 ↔ α + β, L_e4-E2 ↔ η + β converge to the point E₂.

Fig. 9

Liquidus surface projection of the quasi-ternary system Cu₂Se-GeSe₂-As₂Se₃

Table 1 Character, temperatures of nonvariant processes and coordinates of nonvariant points of the quasi-ternary system Cu₂Se-GeSe₂-As₂Se₃

4 Conclusions and Future Work

The component interaction in the Cu₂Se-GeSe₂-As₂Se₃ system has been investigated by direct synthesis, x-ray phase and differential thermal analysis methods. For the first time phase equilibria in the quasi-ternary system have been investigated, and the triangulation of the system has been performed at 513 K (240 °C). At this temperature the existence of six single-phase fields has been identified based on the components GeSe₂, As₂Se₃, Cu₂Se of the system and the ternary compounds Cu₂GeSe₃, Cu₈GeSe₆, CuAsSe₂. The liquidus surface projection of the Cu₂Se-GeSe₂-As₂Se₃ quasi-ternary system has been built based on the literary and obtained results of the investigations of the vertical sections Cu₂GeSe₃-CuAsSe₂, Cu₈GeSe₆-CuAsSe₂, Cu₈GeSe₆-As₂Se₃, A–B (A = 85 mol.% GeSe₂-15 mol.% Cu₂Se; B = 85 mol.% As₂Se₃-15 mol.% Cu₂Se) and two phase diagrams GeSe₂-As₂Se₃ and Cu₂GeSe₃-As₂Se₃. The liquidus surface projection consists of fields of primary crystallization of γ-solid solutions of Cu₂Se, δ-solid solutions of CuAsSe₂, η-solid solutions of Cu₂GeSe₃, α-solid solutions of GeSe₂ and β-solid solutions of As₂Se₃, and HTM-Cu₈GeSe₆, LTM-Cu₈GeSe₆. They are separated by 11 monovariant curves and 12 nonvariant points. Six nonvariant processes take place in the Cu₂Se-GeSe₂-As₂Se₃ system: L_U1 + γ ↔ δ + LTM-Cu₈GeSe₆ at 700 K (427 °C), L_U2 + LTM-Cu₈GeSe₆ ↔ η + δ at 650 K (377 °C), L_E1↔ η + δ + β at 565 K (292 °C), L_E2↔ α + η + β at 540 K (267 °C), the points U₃ and U₄ lie on the planes of the isothermal processes HTM-Cu₈GeSe₆ ↔ LTM-Cu₈GeSe₆ + γ + L_U3 and HTM-Cu₈GeSe₆ ↔ LTM-Cu₈GeSe₆ + η + L_U4 at 970 K which exist due to polymorphic transformation of HTM-Cu₈GeSe₆ into LTM-Cu₈GeSe₆ in the Cu₂Se-GeSe₂ quasi-binary system.

This work was performed in order to gather more data for a thermodynamic assessment of the Cu₂Se-B^IVSe₂-As₂Se₃ systems, where B^IV-Si, Ge, Sn, which is currently in progress.