D. I. Bletskan "Phase Equilibrium in the Binary Systems a IV

Journal of Ovonic Research Vol. 1, No. 5, October 2005, p. 53 - 60

PHASE EQUILIBRIUM IN THE SYSTEMS AIV – BVI

D. I. Bletskan*

Uzhgorod University, Uzhgorod, Ukraine

The phases in the system germanium – chalcogen are reviewed. The phase transitions are discussed.

1.4 System Ge-S

The phase diagram T-x of the binary system Ge-S has been firstly built by A. V. Novoselova et al. [39] using the data of thermal and X-ray analysis. Thereafter, the regions of the phase diagrams of the system Ge-S in the neighborhood of GeS and GeS2 have been carefully determined in [40-44] and are represented in Fig. 1.4. In this system there were found two stable chemical compounds, whose stoichiometric compositions correspond to GeS and GeSe2. The temperature conditions of GeS and GeS2 synthesis, the melting character and melting temperature, given in the literature, are different. Thus, the melting temperature of GeS, after the data of various authors are 938 K [39, 42], 940 K [40], 931 K [41, 46]. Many research reports [39, 42-44] show that GeS compound melts congruently. Nevertheless, in the papers [40, 41, 46] is demonstrated that, during very slow heating, GeS melts incongruently, according to the peritectic reaction:

GeS(solid) = Ge(solid) + Liq(53 at.% S) (1.4)

In the system Ge-S, on the germanium side, there is a large domain (3÷45 at.% S) of layer separation in the liquid state (stratification) with one phase rich in germanium and a phase with the composition approaching GeS. The monotectic horizontal line, responsible for layering in the melted phase, passes through 1193 K ± 2 K [39, 41, 42]. The line of the eutectic Ge-GeS is situated at 931 K after [41] and 938 K after [42].

In between GeS and GeS2 it is formed an eutectic with the parameters 870 ± 3 K and 57.3 at.% S according to [41], or 883 K and 60 at.% S according to [46]. In the papers [39, 45] has been established by DTA the polymorphous transformation in GeS at 863 K. The transformation was observed also by other authors at the temperatures: 868 K [42], 858 K [47] and 853 K [48, 49]. The effect corresponding to the polymorphous transformation of GeS is well expressed on the heating curves after annealing the samples at 773 – 823 K for 60 h. On the cooling curves the effect is weaker, but it is stronger for the compositions close to the stoichiometric GeS. Germanium disulphide melts congruently at 1123 K [41, 46]. There are known two low pressure polymorphous modifications of GeS2, and their transformation one into another occurs at 793 K for germanium excess and 770 K for sulphur excess. Between GeS2 and S is formed the degenerated eutectic. In the concentration interval 77 ÷ 93 at. % it is supposed the existence of the second layer-type domain in the liquid state. The temperature of its monotectic is 973 ± 5 K. The solid dissolution in the system is completely absent, but the dissolution of GeS2 in liquid sulphur starts only above 573 K and significantly increases with temperature (up to 15 mol. % Ge at 973 K).

* Corresponding author: [email protected] 54

Liq. Liq.+GeS Т, К 1193 ± 2 2 Ge+ Liq. 1123 β-GeS + Liq. 1100 2 ? Ge+ Liq. Liq. 973 931 ± 5

870 900 GeS+ Liq. β-GeS2+ Liq.

793 770 ± 3 GeS+β-GeS2

700 2 -GeS

Ge+GeS α α-GeS2+ Liq.

500 GeS+

0 20 40 60 80 100 Ge аt. % S S Fig. 1.4. Phase diagram of the system Ge-S [41].

In the paper [50] was reported the presence of the third chemical compound: Ge2S3. Nevertheless, further researches [39-44] have not confirmed the stability of this compound. According to the suppositions made in [51], Ge2S3 and its analogous compound Ge2Se3 are examples of exotic inorganic compounds, whose existence is possible only in not-periodical atom networks, i.e. in the glassy state. It is true that the system Ge-S, even in the conditions of quenching, demonstrates the tendency toward stable glass formation (see section 6.1.4).

1.5 System Ge - Se

The results of the study of T-x projection of the p-T-x phase diagram of the system Ge-Se are reported in [54-61]. The first phase diagram of this system [54] was different compared to the phase diagram of the Ge-S system. With further research, refining and changes were introduced on the phase diagram [55-61]. Thus, the operated modifications essentially changed the character of the considered diagram. The final phase diagram of the system Ge-Se is presented in Fig. 1.5. In the system Ge-Se were observed two stable chemical compounds: GeSe and GeSe2. After the data reported by many authors [55-57, 59-61], GeSe melts incongruently at 948 ± 2 K and only in one paper [58] is indicated the congruency of melting at Tmelt = 943 K. For GeSe it is characteristic the presence of two polymorphous modifications: a stable one at normal conditions of the low temperature α-phase, which crystallizes in the rhombic lattice, and a high temperature β- phase [61, 62]. The high temperature β-phase of GeSe decomposes peritectically in Ge + Liq2 at 948 K, and at 920 K decomposes in α-GeSe + Liq2. At 939 K (in the composition domain 0÷50 at. %) takes place the peritectical reaction:

Ge(solid) + β-GeSe(solid) → α-GeSe(solid) (1.5)

According to [62] at 924 K occurs the transition from the rhombic structure to the ideal structure of NaCl type with the lattice parameter of the new cubic phase: a = 5.730 Å (at 929 K). After the data of microstructure and hardness [55], the homogeneity domain of GeSe consists of several tens of at. % and is distributed probably on the right hand side of the composition with 50.4 at. % Se.

Т, K 1000 1210 Liq.1 + Liq.2 1200 Т, оС 1177 Ge + Liq. 2 Liq.2 800 1015 Ge + β-GeSe 1000 948

856 939 600 851 800

Ge + α-GeSe 400

GeSe2 + Liq.2 600 2

485

GeSe 200 -GeSe

α GeSe2 +Se 400 Ge 20 40 60 80 Se аt. % Se

Fig. 1.5. Phase diagram of the system Ge-Se [61].

Paper [60] shows deviation from stoichiometry of ±1 at. % Se at high temperature. This deviation stimulated the authors of [61] to study the phase micro-diagram in the neighborhood of the compound GeSe, which is shown in Fig. 1.6. For temperatures higher than 900 K α-GeSe is formed in the range 49.5÷50 at. % Se and β-GeSe in the range 50÷50.6 at. % Se. In Fig. 1.6 the homogeneity domain is separated by the lines situated at 49.75 and 50.25 at. % Se. 900 Т, К Т, оС

1100 Liq.2 800 Ge + Liq.2 α-GeSe + β-GeSe 1000 Ge + β-GeSe β-GeSe α-GeSe + Liq.2 700 948 β-GeSe + Liq.2 939 Liq.2 + GeSe2 900 856 600 Ge + α-GeSe 851 800 α-GeSe + GeSe 2 500 -GeSe -GeSe α 700 42 46 50 54 58 аt. % Se Fig. 1.6. The domain of the phase diagram in the system Ge-Se close to the compound GeSe [61].

On the side of the diagram corresponding to germanium rich domain is situated the layered domain of the liquid (the boundaries of the domain with 17±2 up to 40±2 at. % Se [55] and from 11÷12 to 40÷42 at. % Se according to [59]). The monotectic horizontal line corresponds to the temperature 1177 ± 3 K [55,61].

GeSe2 melts congruently at 1015 ±2 K. The compound dissolves easily in alkali solutions (caustic soda, ammonia). The solution of GeSe in alkali shows orange colour. The hydrogen peroxide, especially in alkali media, oxidizes the GeSe with the release of selenium. The nitric acid dissolves GeSe very slowly with the formation of GeO2 and selenium acid. GeSe2 is practically insoluble in salts and sulphur acids, but is dissolved in royal water [53]. The section GeSe – GeSe2 corresponds to the type of diagram of a simple eutectic, although various authors report not identical parameters of the eutectic: according to [61] the melting temperature of the eutectic is 856 ± 2 K and the composition is 56.0 ± 0.5 at. % Se; according to [55] it is 860 K and 56.5 at.% Se; according to [58] is 853 K and 62 at.% Se; according to [59] is 851 K and 57÷58 at.% Se. On the side of the diagram corresponding to selenium-rich domain, is observed the eutectic reaction: Liquid → GeSe2 + Se. The coordinates of the eutectic point are: 455 ± 1 K and 94.5 ± 0.5 at.% Se [61]. The author of the paper [60] observes that in this case the eutectic is not degenerated, as it is the case in other binary systems containing selenium. The alloys of the system Ge-Se show a remarkable tendency to undercooling and glass formation in conditions of moderate quenching (see Section 6.1.4).

1.6 System Ge-Te

Firstly, the T-x phase diagram for the system Ge-Te has been built and reported in [63] using the thermal analysis data. In the system exists only one chemical compound, GeTe, which is formed by peritectic reaction at 998 ± 3 K. Between GeTe and Te takes place an eutectic interaction with the coordinates: composition 85 at.% Te and melting temperature 648 K (Fig. 1.7). Thereafter, the system Ge-Te has been investigated in details in the composition domain 40 ÷ 60 at. % Te [64]. As opposite to the previous paper [64], it has been shown that GeTe melts congruently at 997 K. The maximum on the liquidus curve is shifted toward tellurium excess and corresponds to 50.61 at. % Te. The eutectic GeTe + Te exhibits the melting temperature of 996 K and composition 49.85 at. % Te. In order to clarify if the compound GeTe is formed as a result of a peritectic reaction, according to the paper [63], or melts congruently, according to the data from [64], there were performed many supplementary researches of the system Ge-Te in the vicinity of the compound Ge-Te. The analysis of the data related to the character of GeTe melting, shows that the major part of the experimental results [65, 67-71] agrees with the conclusions on congruent character of GeTe melting, according to [64]. Germanium telluride is a typical member of the bertolide phases of variable composition [64 – 73]. The homogeneity domain of GeTe is fully shifted relative to the stoichiometric composition on the tellurium side. In the limit of the homogeneity domain of Ge1-xTex there were identified three polymorphous modifications of GeTe: the high temperature cubic NaCl phase (β) (spatial group Fm3m), which is stable at temperatures above Tc ~ 640 ÷ 700 K, and two low temperature phases: rhombohedral (α) (spatial group R3m, a binary analogous of the A7 structure of grey α-As) and rhombic (γ) phase (structure type: SnS) [67, 68]. The homogeneity domain based on high temperature cubical modification of germanium telluride is situated in the range 50.3 ÷ 51.5 at. % Te (703 K). The high temperature β-phase of GeTe cannot be quenched and, when the temperature decreases under Tc, it transforms in the low temperature rhombohedral α phase or rhombic γ-phase as a function of the degree of deviation from stoichiometry and thermal processing regime [67, 69, 73-76]. Low temperature rhombohedral α modification is stable at small deviations from stoichiometry (0.503 < x < 0.505). A special place in the group of AIVBVI compounds is occupied by the second low temperature modification γ-Ge1-xTex, which is stable at significant deviation from stoichiometry (0.509 < x <0.512) for temperatures lower than 640 ÷ 675 K. This modification is isostructural with GeS and GeSe and has no analogs in the considered group of compounds: there is no compound in the AIVBVI group, with the exception of germanium telluride, where the decrease of symmetry is observed during the increase of the deviation from the stoichiometry.

Т, К

1200 GeTe Liq. Liq. + Ge 1000 996

Ge + β-GeTe Liq. + γ-GeTe 800 Liq.+ β-GeTe 700 675

600 648 Ge + α-GeTe Те + γ-GeTe

Ge 20 40 60 80 Te аt. % Te

Fig. 1.7. Phase diagram of the system Ge-Te [14, 67].

The state of the samples in the composition range 50.6 ÷ 50.8 at. % Te depends on the velocity of passing across Tc. α-GeTe is obtained for rapid cooling of the melt from the temperature ≥Tcub (temperature of transformation in the cubical symmetry), and in the interval of homogeneity the phase α is metastable. With the increase of the tellurium content the instability of α-GeTe increases [76]. For slow cooling of the sample or for annealing in the low temperature phase, close to Tcub is formed the γ-phase [74, 76]. The temperature of the phase transformation γ→β in germanium telluride increases with the growth of hole concentration and with the degree of rhombical distortion as opposite to phase transition α → γ, whose temperature is reduced when the hole concentration increases. It is also possible to appear transition structures of various type for different conditions of low temperature (T

49.85 50.61

Т, K Liq.+ β 1000

Liq.+ β 900 Ge + β 1 2 3 800 β

700±32 Liq. + γ γ + β 700 α + β 675±3 655±3 Ge + α γ + Те α 630 γ 600 α + γ 50 51 52 53 аt. % Те

Fig. 1.8. The homogeneity domain of the germanium telluride [65, 69]: 1.Dilatometry, 2. Points obtained by electro-physical measurements, 3. DTA.

Other versions of the phase diagram of Ge-Te described in [7, 46, 66, 68, 81], in the neighborhood of the GeTe, and, also, the differences in the phase transformation temperature (Tc) and the phase composition according to [6] must be related to the difficulty of reaching the equilibrium in the system Ge-Te at temperatures under Tc, to the different thermal history of the samples and to their state. In the paper [82] it is shown that during the measurement of the saturated vapor pressure above the melt of different composition in the Ge-Te system, it was revealed the compound GeTe2, which decomposes at ~650 K by a peritectic reaction. It was also supposed that GeTe2 exists in both crystalline and amorphous forms [12]. Nevertheless, the investigation of the phase diagram of the system Ge-Te in the concentration range 45÷100 at. % Te [67] does not confirm the existence of the compound GeTe2. We must agree with the conclusions of the paper [83], where, by XRD has been shown that after annealing at 473 K in the amorphous films is formed the metastable crystalline phase GeTe2 with cubical symmetry, isomorphous with β-crystobalite. The relative amount of this phase decreases when the film thickness increases, and at d > 6 µm does not appear anyway. When the film is annealed at 523 K the metastable GeTe2 decomposes in a mixture of GeTe and Te crystals.

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