- Email: [email protected]
Phase equilibria in the gallium-rich corner of the MoGaO system at 1073 K
Journal of the Less-Common
Metals, 114 (1985)
PHASE EQUILIBRIA IN THE GALLIUM-RICH Mo-Ga-0 SYSTEM AT 1073 K
R. HORYfi
355
355 - 359
CORNER
OF THE
and R. ANDRUSZKIEWICZ
Institute forLow Temperature and Structure Research of the Polish Academy PI. Katedralny 1, 50-950 Wroctaw (Poland) (Received
August
12,1984;
in revised
form February
of Sciences,
16,1985)
Summary
The phase relations in the gallium-rich corner of the Mo-Ga-0 system at 1073 K have been studied by X-ray powder diffraction. The rhombohedral R phase MosGa4i, well known in the literature, hai been identified as an oxygen-stabilized compound MoGa,.1200.005. The basic crystal data obtained are: space group, R3, a,= 9.5308, cu, = 94.86", ah = 14.036(6) A, ch = 15.043(7) 8, U,, = 2566.6 A3, D, = 7.034, D, = 7.033(6) Mg mP3, Z = 24.
1. Introduction
In our work on the ternary gallides [l] we did not entirely explain the role of oxygen in the formation of the rhombohedral MosGa4i phase. There was only the supposition that R-phase does not belong to the Mo-Ga system contrary to the suggestion in ref. 2 and recently in ref. 3. The aim of this work is to determine phase relations in the region of the Mo-Ga-0 system under discussion in order to explain the problem of the binary R phase [2] and to complete the data on the influence of oxygengroup elements in the creation of the new crystallographic forms [ 41. The R phase is known to be superconducting below 9.7 K and is diamagnetic in character in its normal state [ 31.
2. Experimental details Molybdenum (purity, 99.98%; Koch-Light Laboratories Ltd.) and gallium (purity, 99.999%) were taken as starting materials for all the syntheses, alumina crucibles and boats sealed in quartz ampoules were used, and the samples were subjected to long annealing times at 1073 K. The final products of the syntheses were investigated for phase composition using a Guinier camera with Cu Ko radiation. The hexagonal constants 0022-5088/85/$3.30
@ Elsevier
Sequoia/Printed
in The Netherlands
356
of the unit cell of the R phase were computed using an X-ray system crystallographic program [ 51. The density of a 7.7 g MoGa~.~~O~.~~-ph~e sample of X-ray purity was measured pycnometrically at 298 K. The high volatility of Moos, as compared with GazOs [6], was utilized for chemical analysis of gallium-rich Mo-Ga alloys. The accuracy of this method was checked for the MoGa, sample. The separation of the oxides in the air at 1073 K resulted in analysed chemical compositions which were highly reproducible. The mean deviation for the concentration was estimated to be less than 0.8 wt.% Ga. The Mo/Ga ratios of monoclinic and rhombohedral Mo-Ga phases (both obtained under an excess of gallium) as determined by this method are as follows: MoGa,.,s and MoGa,.r4. These results are consistent with the pycnometric densities of the phases. The oxygen content of the R phase was determined in two runs using a gas evolution method. The mean value of total oxygen content was found to be 280 Lt:50 ppm.
3. Results and discussion The phase relations existing within the triangle MosGa-P-GazO,-Ga of the Mo-Ga-0 system in a 1073 K isothermal section are presented in Fig. 1. This part of the phase diagram is well established thanks to the large number of experiments which have been conducted the results most essential of which are listed in Table 1. The phase relationships have been found to be quite simple because of the presence of one ternary phase (R-phase), without a significant homogeneity range. Therefore, two- and three-phase fields of the isothermal section surround the MoGa,,IzOo,oos compound. The MosGaR-/3-Ga20s and /3-Ga20s-R-Ga fields dominate because of small amounts of
Fig. 1. Phase relationships
in the gallium-rich
corner
of the Mo-Ga-0
system
at 1073 K.
357 TABLE
1
Results of X-ray phase composition and Mo-Ga-0 alloys
analysis of some representative
samples of Mo-Ga
Phases present
Experiments Samples Nos. 1 - 5 (see Fig. 1) prepared from substrates: No.l.Mo+Ga+Mo03 Nos. 2,3 and 4. MO + P-GaaOs + (M)MoGa~ No. 5. Ga + (M)MoGas + @-GaZOj
MoaGa + /3-Ga203 + R MosGa + /%GazOs + R Ga + @-GaaOs + R
The degassed molybdenum (reduced in hydrogen) + gallium (in excess) substrates synthesis in a high vacuum at 1073 K
(M)MoGab.ss + Gals
Rough-vacuum reaction of powdered molybdenum excess gallium at 1073 K. The substrates were not degassed
W)MoGaS.dhooS + Gals
and
The monoclinic sample from (b) was melted several times under a reduced pressure of air (1.33 Pa) for 2 h and left at 1073 K over night
(M)MoC&.sS + R
High-vacuum synthesis of a MoGa4 sample at 1073 K (molybdenum melted in an arc furnace)
(M)MoGa4
Rough-vacuum synthesis of a MoGa4 sample at 973 K
R + (M)MoGae + MosGa
Partial evaporation of gallium from the initial sample of (R + M) under vacuum (2.5 x 10m4 Pa) at 1173 K. After homogenization of the residual material at 973 K for 8 h, the compound of chemical composition MO + 4.72Ga has been gra~metrically established
R+M
Partial evaporation of gallium from the (R + M) sample under the conditions given in (g) but more prolonged. The chemical composition of the residual material changed to MO + 3.89Ga
R + (M)MoGae + Mo3Ga
oxygen in the R phase (see experiment (a} in Table 1). The remaining phase fields of the Mo3Ga-R-Ga triangle surrounding the R phase have been made visible by intentionally shifting the position of the MoGa5.1200.005 from its actual location. The MoGa,,I,aOO.Ws phase has been identified as being isostructural with VsGael 171. This R phase is systematic~ly observed in each of the oxygencontaining Mo-Ga alloys (see experiments (a), (c) - (h) in Table 1). However, when using hydrogen-reduced molybdenum or molybdenum melted in an arc furnace and high vacuum synthesis, the monoclinic (M) phase forms in accordance with the binary Mo-Ga system [8] (see experiments (b) and (e) in Table 1). All our experience obtained by studying the Mo-Ga alloys fully confirms the phase relations reported by Bornand et al. [8] i.e. the phase relationships created by cubic MosGa [9] and monoclinic MoeGaS [lo] phases, are contrary to the reports in ref. 2.
358
The results given in ref. 3 show that MoGa,_i,Oo,oos forms, presumably by a peritectic reaction at slightly lower temperature and higher gallium concentration than MosGasi . In the light of our experiments (Table l), this conclusion is unconvincing. In general, we can synthesize both phases (M and R) under an excess of gallium as well as at different temperatures. In view of the negative results of differential thermal analysis given in ref. 3, the scepticism expressed therein is quite understandable since one of the phases (M or R) would, for instance, exhibit metastability. Our results also exclude such a possibility. We believe we have proved that both phases belong to separate systems. The stabilizing role of oxygen in the creation of the R phase is a fact which becomes more convincing because of the passive properties of molybdenum [ll] and gallium [12, p. 7481 to molecular nitrogen at temperatures up to 1273 K. The analytical determination of the oxygen content of the R phase confirms the above conclusions. The R phase characteristics of our clear Guinier diagram i.e. two strong doublets ((113) - (300) and (226) - (600)), which we find visibly sensitive (especially the second) to the electrochemical nature of the third element, creates the rhombohedral phases [l]. In the case of the Mo-Ga-0 system, the stability of these doublet positions is observable independently of the phase composition of the sample. This fact, as well as our chemical analyses and density measurements, supports the actual chemical formula of R phase as being MoGas.i200.s0s. The complete crystal structure data of the R phase are: MoG~,.i20,,as5; ah = 14.036(6) A; M = 452.99;ch = 15.043(7) A; space group, R3; uh = 2566.6 A3; a, = 9.530 A; D, = 7.034; cu, = 94.86”; D, = 7.033(6) Mg rnA3; 2 = 24. In spite of a certain similarity between the crystal structures of the M and R phases [2], the morphology of their crystals (the polycrystalline samples as well) are essentially different. The R phase polycrystalline samples usually contain spherical crystals irregular in shape whereas the monoclinic ones are characterized by unique branch-like or needle-like morphology depending on the conditions of crystallization. Usually a fibre-like morphology of the powder is observed. The monoclinic compound synthesized from a 2 g molybdenum button, melted in an arc furnace with excess gallium at 1073 K is shown in Fig. 2. The results of the work presented here indicate that (i) the R phase is an oxygen-stabilized compound; (ii) the phase relationships in Mo-Ga system published by Bornand et al. [8] are confirmed.
Acknowledgments The authors are greatly indebted to Dr. Ch. J. Raub of Forschungsinstitut fiir Edelmetalle und Metallchemie, Schwabisch Gmiind, F.R.G., for oxygen analysis of the R phase.
359
Fig. 2. The morphology
of the monoclinic
Mo-Ga
phase.
References
6 7 8 9 10 11 12
R. Horyii and R. Andruszkiewicz, J. Less-Common Met., 91 (1983) L5. K. Yvon, Acta Crystallogr., Sect. B, 31 (1975) 117. A. Bezinge, K. Yvon, M. Decroux and J. Muller, J. Less-Common Met., 99 (1984) L27. R. Horyri and R. Andruszkiewicz, J. Appl. Crystallogr., 15 (1982) 248. J. M. Stewart, F. A. Kundell and J. C. Baldwin, The X-ray 74 system, Tech. Rep., 1974 (Computer Science Center, University of Maryland, College Park, MD). E. K. Kazenas and D. M. Chizhikov, Davlenie i Sostav Para Nad Okislami Khimicheskikh Elementov, Nauka, Moscow, 1976, pp. 70, 201. K. Girgis, W. Petter and G. Pupp, Acta Crystallogr., Sect. B, 31 (1975) 113. J. D. Bornand, R. E. Siemens and L. L. Oden, J. Less-Common Met., 30 (1973) 205. E. A. Wood, V. B. Compton, B. T. Matthias and E. Corenzwit, Acta Crystallogr., 11 (1958) 604. K. Yvon, Acta Crystallogr., Sect. B, 30 (1974) 853. L. E. Toth, Transition Metal Carbides and Nitrides, Academic Press, New York, 1971, p. 14. M. Hansen, Constitution of Binary Alloys, McGraw-Hill, New York, 1958, p. 450, 748.
Metals, 114 (1985)
PHASE EQUILIBRIA IN THE GALLIUM-RICH Mo-Ga-0 SYSTEM AT 1073 K
R. HORYfi
355
355 - 359
CORNER
OF THE
and R. ANDRUSZKIEWICZ
Institute forLow Temperature and Structure Research of the Polish Academy PI. Katedralny 1, 50-950 Wroctaw (Poland) (Received
August
12,1984;
in revised
form February
of Sciences,
16,1985)
Summary
The phase relations in the gallium-rich corner of the Mo-Ga-0 system at 1073 K have been studied by X-ray powder diffraction. The rhombohedral R phase MosGa4i, well known in the literature, hai been identified as an oxygen-stabilized compound MoGa,.1200.005. The basic crystal data obtained are: space group, R3, a,= 9.5308, cu, = 94.86", ah = 14.036(6) A, ch = 15.043(7) 8, U,, = 2566.6 A3, D, = 7.034, D, = 7.033(6) Mg mP3, Z = 24.
1. Introduction
In our work on the ternary gallides [l] we did not entirely explain the role of oxygen in the formation of the rhombohedral MosGa4i phase. There was only the supposition that R-phase does not belong to the Mo-Ga system contrary to the suggestion in ref. 2 and recently in ref. 3. The aim of this work is to determine phase relations in the region of the Mo-Ga-0 system under discussion in order to explain the problem of the binary R phase [2] and to complete the data on the influence of oxygengroup elements in the creation of the new crystallographic forms [ 41. The R phase is known to be superconducting below 9.7 K and is diamagnetic in character in its normal state [ 31.
2. Experimental details Molybdenum (purity, 99.98%; Koch-Light Laboratories Ltd.) and gallium (purity, 99.999%) were taken as starting materials for all the syntheses, alumina crucibles and boats sealed in quartz ampoules were used, and the samples were subjected to long annealing times at 1073 K. The final products of the syntheses were investigated for phase composition using a Guinier camera with Cu Ko radiation. The hexagonal constants 0022-5088/85/$3.30
@ Elsevier
Sequoia/Printed
in The Netherlands
356
of the unit cell of the R phase were computed using an X-ray system crystallographic program [ 51. The density of a 7.7 g MoGa~.~~O~.~~-ph~e sample of X-ray purity was measured pycnometrically at 298 K. The high volatility of Moos, as compared with GazOs [6], was utilized for chemical analysis of gallium-rich Mo-Ga alloys. The accuracy of this method was checked for the MoGa, sample. The separation of the oxides in the air at 1073 K resulted in analysed chemical compositions which were highly reproducible. The mean deviation for the concentration was estimated to be less than 0.8 wt.% Ga. The Mo/Ga ratios of monoclinic and rhombohedral Mo-Ga phases (both obtained under an excess of gallium) as determined by this method are as follows: MoGa,.,s and MoGa,.r4. These results are consistent with the pycnometric densities of the phases. The oxygen content of the R phase was determined in two runs using a gas evolution method. The mean value of total oxygen content was found to be 280 Lt:50 ppm.
3. Results and discussion The phase relations existing within the triangle MosGa-P-GazO,-Ga of the Mo-Ga-0 system in a 1073 K isothermal section are presented in Fig. 1. This part of the phase diagram is well established thanks to the large number of experiments which have been conducted the results most essential of which are listed in Table 1. The phase relationships have been found to be quite simple because of the presence of one ternary phase (R-phase), without a significant homogeneity range. Therefore, two- and three-phase fields of the isothermal section surround the MoGa,,IzOo,oos compound. The MosGaR-/3-Ga20s and /3-Ga20s-R-Ga fields dominate because of small amounts of
Fig. 1. Phase relationships
in the gallium-rich
corner
of the Mo-Ga-0
system
at 1073 K.
357 TABLE
1
Results of X-ray phase composition and Mo-Ga-0 alloys
analysis of some representative
samples of Mo-Ga
Phases present
Experiments Samples Nos. 1 - 5 (see Fig. 1) prepared from substrates: No.l.Mo+Ga+Mo03 Nos. 2,3 and 4. MO + P-GaaOs + (M)MoGa~ No. 5. Ga + (M)MoGas + @-GaZOj
MoaGa + /3-Ga203 + R MosGa + /%GazOs + R Ga + @-GaaOs + R
The degassed molybdenum (reduced in hydrogen) + gallium (in excess) substrates synthesis in a high vacuum at 1073 K
(M)MoGab.ss + Gals
Rough-vacuum reaction of powdered molybdenum excess gallium at 1073 K. The substrates were not degassed
W)MoGaS.dhooS + Gals
and
The monoclinic sample from (b) was melted several times under a reduced pressure of air (1.33 Pa) for 2 h and left at 1073 K over night
(M)MoC&.sS + R
High-vacuum synthesis of a MoGa4 sample at 1073 K (molybdenum melted in an arc furnace)
(M)MoGa4
Rough-vacuum synthesis of a MoGa4 sample at 973 K
R + (M)MoGae + MosGa
Partial evaporation of gallium from the initial sample of (R + M) under vacuum (2.5 x 10m4 Pa) at 1173 K. After homogenization of the residual material at 973 K for 8 h, the compound of chemical composition MO + 4.72Ga has been gra~metrically established
R+M
Partial evaporation of gallium from the (R + M) sample under the conditions given in (g) but more prolonged. The chemical composition of the residual material changed to MO + 3.89Ga
R + (M)MoGae + Mo3Ga
oxygen in the R phase (see experiment (a} in Table 1). The remaining phase fields of the Mo3Ga-R-Ga triangle surrounding the R phase have been made visible by intentionally shifting the position of the MoGa5.1200.005 from its actual location. The MoGa,,I,aOO.Ws phase has been identified as being isostructural with VsGael 171. This R phase is systematic~ly observed in each of the oxygencontaining Mo-Ga alloys (see experiments (a), (c) - (h) in Table 1). However, when using hydrogen-reduced molybdenum or molybdenum melted in an arc furnace and high vacuum synthesis, the monoclinic (M) phase forms in accordance with the binary Mo-Ga system [8] (see experiments (b) and (e) in Table 1). All our experience obtained by studying the Mo-Ga alloys fully confirms the phase relations reported by Bornand et al. [8] i.e. the phase relationships created by cubic MosGa [9] and monoclinic MoeGaS [lo] phases, are contrary to the reports in ref. 2.
358
The results given in ref. 3 show that MoGa,_i,Oo,oos forms, presumably by a peritectic reaction at slightly lower temperature and higher gallium concentration than MosGasi . In the light of our experiments (Table l), this conclusion is unconvincing. In general, we can synthesize both phases (M and R) under an excess of gallium as well as at different temperatures. In view of the negative results of differential thermal analysis given in ref. 3, the scepticism expressed therein is quite understandable since one of the phases (M or R) would, for instance, exhibit metastability. Our results also exclude such a possibility. We believe we have proved that both phases belong to separate systems. The stabilizing role of oxygen in the creation of the R phase is a fact which becomes more convincing because of the passive properties of molybdenum [ll] and gallium [12, p. 7481 to molecular nitrogen at temperatures up to 1273 K. The analytical determination of the oxygen content of the R phase confirms the above conclusions. The R phase characteristics of our clear Guinier diagram i.e. two strong doublets ((113) - (300) and (226) - (600)), which we find visibly sensitive (especially the second) to the electrochemical nature of the third element, creates the rhombohedral phases [l]. In the case of the Mo-Ga-0 system, the stability of these doublet positions is observable independently of the phase composition of the sample. This fact, as well as our chemical analyses and density measurements, supports the actual chemical formula of R phase as being MoGas.i200.s0s. The complete crystal structure data of the R phase are: MoG~,.i20,,as5; ah = 14.036(6) A; M = 452.99;ch = 15.043(7) A; space group, R3; uh = 2566.6 A3; a, = 9.530 A; D, = 7.034; cu, = 94.86”; D, = 7.033(6) Mg rnA3; 2 = 24. In spite of a certain similarity between the crystal structures of the M and R phases [2], the morphology of their crystals (the polycrystalline samples as well) are essentially different. The R phase polycrystalline samples usually contain spherical crystals irregular in shape whereas the monoclinic ones are characterized by unique branch-like or needle-like morphology depending on the conditions of crystallization. Usually a fibre-like morphology of the powder is observed. The monoclinic compound synthesized from a 2 g molybdenum button, melted in an arc furnace with excess gallium at 1073 K is shown in Fig. 2. The results of the work presented here indicate that (i) the R phase is an oxygen-stabilized compound; (ii) the phase relationships in Mo-Ga system published by Bornand et al. [8] are confirmed.
Acknowledgments The authors are greatly indebted to Dr. Ch. J. Raub of Forschungsinstitut fiir Edelmetalle und Metallchemie, Schwabisch Gmiind, F.R.G., for oxygen analysis of the R phase.
359
Fig. 2. The morphology
of the monoclinic
Mo-Ga
phase.
References
6 7 8 9 10 11 12
R. Horyii and R. Andruszkiewicz, J. Less-Common Met., 91 (1983) L5. K. Yvon, Acta Crystallogr., Sect. B, 31 (1975) 117. A. Bezinge, K. Yvon, M. Decroux and J. Muller, J. Less-Common Met., 99 (1984) L27. R. Horyri and R. Andruszkiewicz, J. Appl. Crystallogr., 15 (1982) 248. J. M. Stewart, F. A. Kundell and J. C. Baldwin, The X-ray 74 system, Tech. Rep., 1974 (Computer Science Center, University of Maryland, College Park, MD). E. K. Kazenas and D. M. Chizhikov, Davlenie i Sostav Para Nad Okislami Khimicheskikh Elementov, Nauka, Moscow, 1976, pp. 70, 201. K. Girgis, W. Petter and G. Pupp, Acta Crystallogr., Sect. B, 31 (1975) 113. J. D. Bornand, R. E. Siemens and L. L. Oden, J. Less-Common Met., 30 (1973) 205. E. A. Wood, V. B. Compton, B. T. Matthias and E. Corenzwit, Acta Crystallogr., 11 (1958) 604. K. Yvon, Acta Crystallogr., Sect. B, 30 (1974) 853. L. E. Toth, Transition Metal Carbides and Nitrides, Academic Press, New York, 1971, p. 14. M. Hansen, Constitution of Binary Alloys, McGraw-Hill, New York, 1958, p. 450, 748.