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Single crystal growth in the Ga–Pd system
Journal of Crystal Growth ∎ (∎∎∎∎) ∎∎∎–∎∎∎
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Single crystal growth in the Ga–Pd system J. Schwerin, D. Müller, S. Kiese, P. Gille n Ludwig-Maximilians-Universität München, Department of Earth and Environmental Sciences, Crystallography Section, Theresienstr. 41, D-80333 München, Germany
art ic l e i nf o
Keywords: A1. Phase diagrams A1. Segregation A2. Czochralski method A2. Growth from high-temperature solutions B1. Intermetallic compounds B2. Catalysts
a b s t r a c t Single crystal growth of the intermetallic compounds GaPd2, GaPd, and Ga7Pd3 is described for the first time. These phases have recently been explored as highly selective heterogeneous catalysts in the semihydrogenation of acetylene. Well-oriented single-crystalline surfaces are needed to perform fundamental studies on catalytic processes. The three Ga–Pd phases were grown using the Czochralski method from Ga-rich solutions. Thermodynamic properties of the Ga–Pd system determine very different growth temperatures reaching from less than 460 1C for Ga7Pd3 to about 1200 1C for GaPd2. Avoiding mother liquid inclusion formation proved to be the key problem that could be solved by pulling rates sometimes as low as 25 mm/h and forced convection by high crystal rotation rates. In the last grown part of GaPd2 crystals decomposition into GaPd2/Ga3Pd5 lamellas occurred which can be explained either by spinodal decomposition or by nucleation caused by the GaPd2 stability region that becomes narrower with decreasing temperatures. & 2013 Elsevier B.V. All rights reserved.
1. Introduction Semi-hydrogenation of acetylene to ethylene is an important step in the industrial production of polyethylene that is usually assisted by catalysts of pure Pd or diluted Ag–Pd alloys supported on α-alumina. Recently, it has been found that some Ga–Pd compounds, namely GaPd2, GaPd, and Ga7Pd3, show improved selectivity and better long-term stability compared to the presently used ones [1–3]. This can be explained by the active-site isolation concept established by Sachtler [4]. Since only a few Pd sites on the surface of the pure or alloyed Pd are catalytically active, separation of active sites by the stable crystal structure of a stoichiometric compound is considered to isolate these active atoms more effectively. It is obvious that a fixed crystal structure will ensure long-term stability of active-site isolation much better than an alloy that allows interdiffusion and cluster formation of the constituting elements. Since catalytic reactions occur only at the surface of the catalysts, high specific surfaces are needed. They can be obtained with ultrafine particles, produced e.g. by nanoparticular synthesis, as already demonstrated by Armbrüster et al. [5] for the preparation of GaPd and GaPd2. Nevertheless, single crystals are needed to study the intrinsic properties of the materials to be used as catalysts. As to investigate well defined crystallographic surfaces of the Ga–Pd compounds and once to understand the elementary
n
Corresponding author. Tel.: þ 49 89 2180 4355; fax: þ49 89 2180 4334. E-mail address: [email protected] (P. Gille).
catalytic processes, cm3-sized single crystals of GaPd2, GaPd, and Ga7Pd3 were the aim of the study presented here. In the binary Ga–Pd phase diagram [6] that is shown in Fig. 1, there are a couple of stable compounds, among them GaPd2 (SiCo2 structure type, space group Pnma [7]), GaPd (FeSi structure type, space group P213 [8]) and Ga7Pd3 (Ge7Ir3 structure type, space group Im 3 m [9]). Already from the phase diagram it can be deduced that single crystal growth of these three compounds requires very different conditions: (i) GaPd is almost a line compound and can be crystallized from an either congruent melt or Ga-rich solution with probably no consequences with respect to the phase composition. (ii) GaPd2 is characterized by a wide stability region, retrograde solubility at the Ga-rich side and complex phase transitions for Pd-rich compositions. Due to its much higher melting point growth from Ga-rich solution has been considered the easiest way to crystallize GaPd2. (iii) Ga7Pd3 which melts peritectically at a temperature as low as 460 1C can only be grown from a Ga-rich solution below this peritectic temperature. Because of quite a lot experience in Czochralski growth of various Al-based intermetallics from high-temperature solutions we have decided to grow all these Ga–Pd compounds using the Czochralski technique from Ga-rich solutions of suitable compositions.
2. Experimental From the metallic elements (6N-grade Ga, 3N5-grade Pd) oxidic surfaces were removed by heating the Ga melt under H2 atmosphere in a glassy carbon crucible and etching Pd in aqua regia
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Please cite this article as: J. Schwerin, et al., Journal of Crystal Growth (2013), http://dx.doi.org/10.1016/j.jcrysgro.2013.11.060i
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J. Schwerin et al. / Journal of Crystal Growth ∎ (∎∎∎∎) ∎∎∎–∎∎∎ Table 1 Optimized growth parameters for Czochralski experiments from Ga-rich solutions.
Starting solution Liquidus temperature Growth direction Pulling rate Crystal rotation rate Overall temperature decrease Duration of experiment
GaPd2
GaPd
Ga7Pd3
Ga39Pd61 1200 1C [0 1 0], [0 0 1] 100 to 25 mm/h 30 rpm 150 K 14 days
Ga55Pd45 1000 1C [1 0 0], [1 1 1] 150 mm/h 30 rpm 75 K 10 days
Ga87Pd13 440 1C [1 0 0] 25 mm/h 250 rpm 105 K 25 days
the crystal detached from the residual solution. Cooling down to room temperature was done at rates of 100 K/h or even faster. Fig. 1. Ga–Pd phase diagram, after Okamoto [6].
3. Results and discussion followed by rinsing in aqua dest. and acetone. Syntheses of the starting melts of appropriate compositions were done ex-situ in a RF-heated furnace under Ar atmosphere (5N-grade). The metals were hold in 10 or 15 ml alumina crucibles supported by a graphite susceptor. During slowly heating the starting charge, usually a bright flash occurred indicating a highly exothermic reaction [10]. After having reached a temperature at least 100 K higher than the specific liquidus temperature the solutions were hold for an additional hour as to homogenize. Quenching to room temperature was done simply by switching off the RF power. Solidified syntheses for the growth of GaPd and GaPd2 could easily be removed from the synthesis crucible and transferred to the 10 ml alumina crucible used for the growth experiment. Only the synthesized solutions for the growth of Ga7Pd3 have a slush-like consistence due to the high amount of elemental Ga. Therefore, for Ga7Pd3 synthesis and growth had to be done in the same crucible. For single crystal growth use was made of a fully metal-sealed Czochralski apparatus that has been described elsewhere [11]. An oxygen-free atmosphere is considered to be an important prerequisite with these metallic melts since traces of oxides at the surface would affect seeding and growth. After evacuating and baking the growth chamber over several days it was filled with Ar (5N-grade) as to yield approx. ambient pressure at growth temperatures. The re-molten starting charges were homogenized for at least 12 h before seeding could be tried. Strategies how to initiate spontaneous nucleation in the very first experiment with a new phase have been described in an earlier paper [11]. This was used for the Ga–Pd compounds as well, although it was more difficult due to bad wetting of these melts on ceramic tips. After having obtained large enough first single crystals to prepare suitable seeds, all the experiments were done with well-oriented native seeds. Optimized growth parameters for the three studied compounds are summarized in Table 1. After seeding diameter increase was achieved by carefully adjusting the set point of the temperature controller. Slightly reducing the temperature of the solution increases the diameter of the growing crystal as it is well known from the Czochralski theory. When growing a crystal from a high-temperature solution instead of from a melt, the liquidus temperature permanently decreases due to the gradual change of the composition of the solution based on the amount of material already crystallized. This has to be compensated by an appropriate temperature program, but cannot always be optimized with a few experiments of each phase only. The overall decrease of the temperature set point during an experiment has been indicated in Table 1. Along with the decreasing growth temperature materials transport conditions become worse. That is, why we usually reduce the pulling rate in the second half of the experiment. As to complete an experiment the pulling rate of the crystal was drastically increased and thus
All crystal growth experiments done with a native seed resulted in single-crystalline materials as well. But the main problem of any growth experiment from a solution is to avoid mother liquid inclusions. During growth the excess component (Ga) is only partially incorporated into the crystal structure and the rest is rejected from the growth interface. Inclusion formation results if the solution containing excess Ga is not completely removed from the interface due to insufficient materials transport and is trapped into the growing crystal matrix. This problem becomes more severe (i) if the composition difference between solid and liquid phases is large, (ii) if the absolute solubility of the component to be crystallized is low, and (iii) if the growth temperature decreases, i.e. diffusion becomes slower and viscosity higher. For this reason, the obtained single crystals were carefully investigated with respect to possible inclusion formation. 3.1. GaPd Successful crystal growth experiments with GaPd have already been reported in an earlier paper [12]. An example of a Czochralski-grown single crystal is shown in Fig. 2a. We have grown single crystals of a total mass up to 34 g which is about 55% of the starting charge. Due to the relatively small amount of excess Ga in the starting solution, conditions were not too far away from usual melt growth. These crystals were always inclusion-free. 3.2. GaPd2 Orthorhombic GaPd2 crystals (Fig. 2b) were grown parallel to two of the lowest indexed directions. While in the first part of the crystals always inclusion-free growth occurred, at the end we obtained a high density of large inclusions (see Fig. 3a). Having trapped a droplet of high-temperature solution, the full crystallization path of that solution can be found within this locally very restricted region. This was confirmed by electron microprobe (EPMA) measurements. The inwards-directed growth follows the series of different phases that well corresponds to the phase diagram: First a shell of GaPd2 continues to crystallize followed by Ga3Pd5 after having passed the incomplete peritectic reaction. The dark inner core consists of Ga3Pd5 and GaPd resulting from the final eutectoid decay. Detailed results of this study have been described elsewhere [13]. As to avoid these inclusions we have changed the starting composition to a slightly higher Pd content that increases the liquidus temperature next to the temperature limit of our Czochralski apparatus. By this we succeeded in a higher portion of inclusion-free GaPd2 single crystal and were even able to avoid any liquid inclusion formation when gradually reducing the pulling rate in the second half of the growth
Please cite this article as: J. Schwerin, et al., Journal of Crystal Growth (2013), http://dx.doi.org/10.1016/j.jcrysgro.2013.11.060i
J. Schwerin et al. / Journal of Crystal Growth ∎ (∎∎∎∎) ∎∎∎–∎∎∎
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Fig. 2. Single crystals of different Ga–Pd compounds grown by the Czochralski method from Ga-rich solutions: (a) cubic GaPd, (b) orthorhombic GaPd2, (c) and (d) cubic Ga7Pd3.
Fig. 3. BSE images of a large inclusion of a Ga-rich solution droplet trapped into a GaPd2 single crystal (a) that crystallized as Ga3Pd5 and GaPd during cooling-down, and (b) Ga3Pd5 lamellas (dark grey) that have been formed in a GaPd2 single crystal. (Arrow indicates the growth direction [0 0 1]).
experiment to 25 mm/h. Surprisingly, when looking for inclusions using EPMA we found on polished sections of the last grown part of the crystal straight bands of changed back-scattered electron (BSE) intensities perpendicular to the growth direction (Fig. 3b) that could be identified as alternating bands of GaPd2 and Ga3Pd5. According to the growth temperature of this specific part of the crystal that was well above the peritectic temperature of Ga3Pd5, primary crystallization of the Ga3Pd5 phase from the liquid solution can be excluded. Instead, some post-growth solid-state transformation due to the retrograde solubility of the GaPd2 stability region is the only explanation. Using fine-focused Laue back-scattering diffraction it was found that the two phases that are aligned as lamellas cause well oriented single-orientation diffraction images, i.e. two-phase growth occured in a coherent manner. The Ga3Pd5 crystal structure (Ge3Rh5 structure type, space group Pbam [14]) is very similar to that one of GaPd2 allowing a shared unit cell interface with (0 0 1) GaPd2 parallel to (0 1 0) Ga3Pd5 and coinciding [1 0 0] directions of both lattices. The lattice parameter misfit within the common interface is less than 1%. Changing the growth direction of the GaPd2 crystal from [0 0 1] to [0 1 0] changes the orientation of the lamellas as well thus keeping the coherent intergrowth. From these findings it
cannot yet be decided whether post-growth decomposition of the formerly single-phase GaPd2 occurred via Ga3Pd5 nucleation or by spinodal decomposition. Further studies will be done as to reveal more details of the lamella formation process. 3.3. Ga7Pd3 Slow material transport at low growth temperatures that are limited by the peritectic decomposition of Ga7Pd3 at 460 1C [6] is the main problem in single crystal growth of this phase. Therefore, extremely slow pulling was applied going down to 11 mm/h. But even under these conditions liquid inclusion formation could not be avoided in the crystal shown in Fig. 2c. Large inclusions in a (1 0 0) slice cut perpendicular to the growth direction can be seen in Fig. 4a. Their square-shaped arrangement may be a hint for a central (1 0 0) growth facet. Again, post-growth crystallization from a trapped droplet of solution can be well explained according to the phase diagram. As measured by EPMA (Fig. 4b) Ga5Pd is the next phase to crystallize from an inclusion upon cooling from growth temperature, and the crystallization path is only completed at room temperature at the eutectic point that practically coincides with pure Ga. Due to the low melting point most of the
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Fig. 4. Ga-rich inclusions trapped into a single-crystalline Ga7Pd3 matrix. (a) (1 0 0)-oriented slice cut perpendicular to the growth direction, and (b) EPMA line plot through a Ga-rich inclusion. Data scattering in the center of the inclusion is an artefact due to irregular surface conditions, e.g. holes.
Ga stays liquid or becomes liquid during cutting and gets lost when cleaning the slice. That is, why only large pits on the as-cut surface in Fig. 4a indicate the positions of former liquid inclusions. Since it makes no sense to further reduce the pulling rate and temperature gradients can hardly be increased in these metallic melts, the only way to avoid liquid inclusion formation might be to increase materials transport by some kind of forced convection. Strengthening of convective motion is mainly needed adjacent to the solid-liquid interface where rejection of the excess Ga occurs. Most effectively this can be achieved by a much higher crystal rotation rate that is unusual in Czochralski growth. With respect to the melt flow next to the rotating crystal interface the experimental condition in Czochralski growth is well described by the model of the rotating disc which is one of the best understood models in hydrodynamics [15]. From this it is known that the centrifugal flow adjacent to a rotating disc becomes highest within the thickness of the Ekman layer δE which is the solute layer thickness in the growth experiment. This thickness can be reduced by applying higher rotation rates ω according to δE (ν/ω)½ [15] where ν is the kinematic viscosity. By increasing the crystal rotation rate from approx. 30 rpm in the usual Czochralski growth experiments to a rate as high as 250 rpm the Ekman layer thickness was reduced to almost a third and thus inclusion-free growth of Ga7Pd3 has been achieved for the first time even with a pulling rate of 25 mm/h. A crystal grown at these conditions is shown in Fig. 2d. As can be seen from this photograph, a larger seed crystal had to be taken for mechanical stability reasons as to withstand the centrifugal forces. More detailed studies on the Czochralski growth from metallic solutions using very high rotation rates are subject of ongoing investigations. 4. Summary We succeeded in growing single crystals of the intermetallic compounds GaPd2, GaPd, and Ga7Pd3 using the Czochralski technique from Ga-rich solutions. According to the individual thermodynamic properties of the three phases and based on the published phase diagram, crystal growth conditions were adjusted to the specific compounds. Growth of Ga7Pd3 at the lowest temperatures of less than 460 1C proved to be most difficult due to slow materials transport which is the key factor in crystal growth from solution. Inclusion-free growth was only possible after using very high crystal rotation rates. With GaPd2 single crystal growth the formation of mother liquid inclusions could be avoided by gradually reducing the pulling rate. Decomposition into coherent GaPd2/Ga3Pd5 lamellas occurred in the very last part of the crystals during post-growth cooling.
Acknowledgments The authors are grateful to Prof. Yuri Grin and Dr. Marc Armbrüster, both from Max-Planck-Institut für Chemische Physik fester Stoffe, Dresden, for stimulating discussions, to Renate Enders for surface polishing of the crystals and to Emil Hanfstaengl for the assistance in one of the experiments. This work has been partly conducted within the European integrated Center for the development of new Metallic Alloys and Compounds (European C-MAC).
References [1] J. Osswald, R. Giedigkeit, R.E. Jentoft, M. Armbrüster, F. Girgsdies, K. Kovnir, T. Ressler, Yu. Grin, R. Schlögl, Palladium–gallium intermetallic compounds for the selective hydrogenation of acetylene, Part I: Preparation and structural investigation under reaction conditions, J. Catal. 258 (2008) 210–218. [2] J. Osswald, K. Kovnir, M. Armbrüster, R. Giedigkeit, R.E. Jentoft, U. Wild, Yu. Grin, R. Schlögl, Palladium–gallium intermetallic compounds for the selective hydrogenation of acetylene, Part II: Surface characterization and catalytic performance, J. Catal. 258 (2008) 219–227. [3] M. Armbrüster, K. Kovnir, M. Behrens, D. Teschner, Yu. Grin, R. Schlögl, Pd–Ga intermetallic compounds as highly selective semihydrogenation catalysts, J. Am. Chem. Soc. 132 (2010) 14745–14747. [4] W.M.H. Sachtler, Chemisorption complexes on alloy surfaces, Catal. Rev. Sci. Eng. 14 (1976) 193–210. [5] M. Armbrüster, G. Wowsnick, M. Friedrich, M. Heggen, R. Cardoso-Gil, Synthesis and catalytic properties of nanoparticulate intermetallic Ga–Pd compounds, J. Am. Chem. Soc. 133 (2011) 9112–9118. [6] H. Okamoto, Ga–Pd (Gallium–Palladium), J. Phase Equilib. Diff. 29 (2008) 466–467. [7] K. Kovnir, M. Schmidt, C. Waurisch, M. Armbrüster, Yu. Prots, Yu. Grin, Refinement of the crystal structure of dipalladium gallium, Pd2Ga, Z. Kristallogr.—New Cryst. Struct. 223 (2008) 7–8. [8] M. Armbrüster, H. Borrmann, Yu. Prots, R. Giedigkeit, P. Gille, Refinement of the crystal structure of palladium gallide, PdGa, Z. Kristallogr.—New Cryst. Struct. 225 (2010) 617–618. [9] K. Khalaff, K. Schubert, Kristallstruktur von Pd5Ga2, J. Less-Common Met. 37 (1974) 129–140. [10] D. El Allam, M. Gaune-Escard, J.-P. Bros, E. Hayer, Enthalpies of formation of liquid and solid (gallium and palladium) alloys, Metall. Trans. B 23 (1992) 39–44. [11] P. Gille, B. Bauer, Single crystal growth of Al13Co4 and Al13Fe4 from Al-rich solutions by the Czochralski method, Cryst. Res. Technol. 43 (2008) 1161–1167. [12] P. Gille, T. Ziemer, M. Schmidt, K. Kovnir, U. Burkhardt, M. Armbrüster, Growth of large PdGa single crystals from the melt, Intermetallics 18 (2010) 1663–1668. [13] D. Müller, J. Schwerin, P. Gille, K.T. Fehr, High-resolution EPMA X-ray images of mother liquid inclusions in a Pd2Ga single crystal. In: IOP Conference Series: Materials Science and Engineering, accepted. [14] K. Schubert, H.L. Lukas, H.G. Meissner, S. Bhan, Zum Aufbau der Systeme Kobalt-Gallium, Palladium-Gallium, Palladium-Zinn und verwandter Legierungen, Z. Metallkd. 50 (1959) 534–540. [15] D.T.J. Hurle, The dynamics and control of Czochralski growth, in: P. M. Dryburgh, B. Cockayne, K.G. Barraclough (Eds.), Advanced Crystal Growth, Prentice Hall, London, 1987, pp. 97–123.
Please cite this article as: J. Schwerin, et al., Journal of Crystal Growth (2013), http://dx.doi.org/10.1016/j.jcrysgro.2013.11.060i
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Single crystal growth in the Ga–Pd system J. Schwerin, D. Müller, S. Kiese, P. Gille n Ludwig-Maximilians-Universität München, Department of Earth and Environmental Sciences, Crystallography Section, Theresienstr. 41, D-80333 München, Germany
art ic l e i nf o
Keywords: A1. Phase diagrams A1. Segregation A2. Czochralski method A2. Growth from high-temperature solutions B1. Intermetallic compounds B2. Catalysts
a b s t r a c t Single crystal growth of the intermetallic compounds GaPd2, GaPd, and Ga7Pd3 is described for the first time. These phases have recently been explored as highly selective heterogeneous catalysts in the semihydrogenation of acetylene. Well-oriented single-crystalline surfaces are needed to perform fundamental studies on catalytic processes. The three Ga–Pd phases were grown using the Czochralski method from Ga-rich solutions. Thermodynamic properties of the Ga–Pd system determine very different growth temperatures reaching from less than 460 1C for Ga7Pd3 to about 1200 1C for GaPd2. Avoiding mother liquid inclusion formation proved to be the key problem that could be solved by pulling rates sometimes as low as 25 mm/h and forced convection by high crystal rotation rates. In the last grown part of GaPd2 crystals decomposition into GaPd2/Ga3Pd5 lamellas occurred which can be explained either by spinodal decomposition or by nucleation caused by the GaPd2 stability region that becomes narrower with decreasing temperatures. & 2013 Elsevier B.V. All rights reserved.
1. Introduction Semi-hydrogenation of acetylene to ethylene is an important step in the industrial production of polyethylene that is usually assisted by catalysts of pure Pd or diluted Ag–Pd alloys supported on α-alumina. Recently, it has been found that some Ga–Pd compounds, namely GaPd2, GaPd, and Ga7Pd3, show improved selectivity and better long-term stability compared to the presently used ones [1–3]. This can be explained by the active-site isolation concept established by Sachtler [4]. Since only a few Pd sites on the surface of the pure or alloyed Pd are catalytically active, separation of active sites by the stable crystal structure of a stoichiometric compound is considered to isolate these active atoms more effectively. It is obvious that a fixed crystal structure will ensure long-term stability of active-site isolation much better than an alloy that allows interdiffusion and cluster formation of the constituting elements. Since catalytic reactions occur only at the surface of the catalysts, high specific surfaces are needed. They can be obtained with ultrafine particles, produced e.g. by nanoparticular synthesis, as already demonstrated by Armbrüster et al. [5] for the preparation of GaPd and GaPd2. Nevertheless, single crystals are needed to study the intrinsic properties of the materials to be used as catalysts. As to investigate well defined crystallographic surfaces of the Ga–Pd compounds and once to understand the elementary
n
Corresponding author. Tel.: þ 49 89 2180 4355; fax: þ49 89 2180 4334. E-mail address: [email protected] (P. Gille).
catalytic processes, cm3-sized single crystals of GaPd2, GaPd, and Ga7Pd3 were the aim of the study presented here. In the binary Ga–Pd phase diagram [6] that is shown in Fig. 1, there are a couple of stable compounds, among them GaPd2 (SiCo2 structure type, space group Pnma [7]), GaPd (FeSi structure type, space group P213 [8]) and Ga7Pd3 (Ge7Ir3 structure type, space group Im 3 m [9]). Already from the phase diagram it can be deduced that single crystal growth of these three compounds requires very different conditions: (i) GaPd is almost a line compound and can be crystallized from an either congruent melt or Ga-rich solution with probably no consequences with respect to the phase composition. (ii) GaPd2 is characterized by a wide stability region, retrograde solubility at the Ga-rich side and complex phase transitions for Pd-rich compositions. Due to its much higher melting point growth from Ga-rich solution has been considered the easiest way to crystallize GaPd2. (iii) Ga7Pd3 which melts peritectically at a temperature as low as 460 1C can only be grown from a Ga-rich solution below this peritectic temperature. Because of quite a lot experience in Czochralski growth of various Al-based intermetallics from high-temperature solutions we have decided to grow all these Ga–Pd compounds using the Czochralski technique from Ga-rich solutions of suitable compositions.
2. Experimental From the metallic elements (6N-grade Ga, 3N5-grade Pd) oxidic surfaces were removed by heating the Ga melt under H2 atmosphere in a glassy carbon crucible and etching Pd in aqua regia
0022-0248/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jcrysgro.2013.11.060
Please cite this article as: J. Schwerin, et al., Journal of Crystal Growth (2013), http://dx.doi.org/10.1016/j.jcrysgro.2013.11.060i
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J. Schwerin et al. / Journal of Crystal Growth ∎ (∎∎∎∎) ∎∎∎–∎∎∎ Table 1 Optimized growth parameters for Czochralski experiments from Ga-rich solutions.
Starting solution Liquidus temperature Growth direction Pulling rate Crystal rotation rate Overall temperature decrease Duration of experiment
GaPd2
GaPd
Ga7Pd3
Ga39Pd61 1200 1C [0 1 0], [0 0 1] 100 to 25 mm/h 30 rpm 150 K 14 days
Ga55Pd45 1000 1C [1 0 0], [1 1 1] 150 mm/h 30 rpm 75 K 10 days
Ga87Pd13 440 1C [1 0 0] 25 mm/h 250 rpm 105 K 25 days
the crystal detached from the residual solution. Cooling down to room temperature was done at rates of 100 K/h or even faster. Fig. 1. Ga–Pd phase diagram, after Okamoto [6].
3. Results and discussion followed by rinsing in aqua dest. and acetone. Syntheses of the starting melts of appropriate compositions were done ex-situ in a RF-heated furnace under Ar atmosphere (5N-grade). The metals were hold in 10 or 15 ml alumina crucibles supported by a graphite susceptor. During slowly heating the starting charge, usually a bright flash occurred indicating a highly exothermic reaction [10]. After having reached a temperature at least 100 K higher than the specific liquidus temperature the solutions were hold for an additional hour as to homogenize. Quenching to room temperature was done simply by switching off the RF power. Solidified syntheses for the growth of GaPd and GaPd2 could easily be removed from the synthesis crucible and transferred to the 10 ml alumina crucible used for the growth experiment. Only the synthesized solutions for the growth of Ga7Pd3 have a slush-like consistence due to the high amount of elemental Ga. Therefore, for Ga7Pd3 synthesis and growth had to be done in the same crucible. For single crystal growth use was made of a fully metal-sealed Czochralski apparatus that has been described elsewhere [11]. An oxygen-free atmosphere is considered to be an important prerequisite with these metallic melts since traces of oxides at the surface would affect seeding and growth. After evacuating and baking the growth chamber over several days it was filled with Ar (5N-grade) as to yield approx. ambient pressure at growth temperatures. The re-molten starting charges were homogenized for at least 12 h before seeding could be tried. Strategies how to initiate spontaneous nucleation in the very first experiment with a new phase have been described in an earlier paper [11]. This was used for the Ga–Pd compounds as well, although it was more difficult due to bad wetting of these melts on ceramic tips. After having obtained large enough first single crystals to prepare suitable seeds, all the experiments were done with well-oriented native seeds. Optimized growth parameters for the three studied compounds are summarized in Table 1. After seeding diameter increase was achieved by carefully adjusting the set point of the temperature controller. Slightly reducing the temperature of the solution increases the diameter of the growing crystal as it is well known from the Czochralski theory. When growing a crystal from a high-temperature solution instead of from a melt, the liquidus temperature permanently decreases due to the gradual change of the composition of the solution based on the amount of material already crystallized. This has to be compensated by an appropriate temperature program, but cannot always be optimized with a few experiments of each phase only. The overall decrease of the temperature set point during an experiment has been indicated in Table 1. Along with the decreasing growth temperature materials transport conditions become worse. That is, why we usually reduce the pulling rate in the second half of the experiment. As to complete an experiment the pulling rate of the crystal was drastically increased and thus
All crystal growth experiments done with a native seed resulted in single-crystalline materials as well. But the main problem of any growth experiment from a solution is to avoid mother liquid inclusions. During growth the excess component (Ga) is only partially incorporated into the crystal structure and the rest is rejected from the growth interface. Inclusion formation results if the solution containing excess Ga is not completely removed from the interface due to insufficient materials transport and is trapped into the growing crystal matrix. This problem becomes more severe (i) if the composition difference between solid and liquid phases is large, (ii) if the absolute solubility of the component to be crystallized is low, and (iii) if the growth temperature decreases, i.e. diffusion becomes slower and viscosity higher. For this reason, the obtained single crystals were carefully investigated with respect to possible inclusion formation. 3.1. GaPd Successful crystal growth experiments with GaPd have already been reported in an earlier paper [12]. An example of a Czochralski-grown single crystal is shown in Fig. 2a. We have grown single crystals of a total mass up to 34 g which is about 55% of the starting charge. Due to the relatively small amount of excess Ga in the starting solution, conditions were not too far away from usual melt growth. These crystals were always inclusion-free. 3.2. GaPd2 Orthorhombic GaPd2 crystals (Fig. 2b) were grown parallel to two of the lowest indexed directions. While in the first part of the crystals always inclusion-free growth occurred, at the end we obtained a high density of large inclusions (see Fig. 3a). Having trapped a droplet of high-temperature solution, the full crystallization path of that solution can be found within this locally very restricted region. This was confirmed by electron microprobe (EPMA) measurements. The inwards-directed growth follows the series of different phases that well corresponds to the phase diagram: First a shell of GaPd2 continues to crystallize followed by Ga3Pd5 after having passed the incomplete peritectic reaction. The dark inner core consists of Ga3Pd5 and GaPd resulting from the final eutectoid decay. Detailed results of this study have been described elsewhere [13]. As to avoid these inclusions we have changed the starting composition to a slightly higher Pd content that increases the liquidus temperature next to the temperature limit of our Czochralski apparatus. By this we succeeded in a higher portion of inclusion-free GaPd2 single crystal and were even able to avoid any liquid inclusion formation when gradually reducing the pulling rate in the second half of the growth
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Fig. 2. Single crystals of different Ga–Pd compounds grown by the Czochralski method from Ga-rich solutions: (a) cubic GaPd, (b) orthorhombic GaPd2, (c) and (d) cubic Ga7Pd3.
Fig. 3. BSE images of a large inclusion of a Ga-rich solution droplet trapped into a GaPd2 single crystal (a) that crystallized as Ga3Pd5 and GaPd during cooling-down, and (b) Ga3Pd5 lamellas (dark grey) that have been formed in a GaPd2 single crystal. (Arrow indicates the growth direction [0 0 1]).
experiment to 25 mm/h. Surprisingly, when looking for inclusions using EPMA we found on polished sections of the last grown part of the crystal straight bands of changed back-scattered electron (BSE) intensities perpendicular to the growth direction (Fig. 3b) that could be identified as alternating bands of GaPd2 and Ga3Pd5. According to the growth temperature of this specific part of the crystal that was well above the peritectic temperature of Ga3Pd5, primary crystallization of the Ga3Pd5 phase from the liquid solution can be excluded. Instead, some post-growth solid-state transformation due to the retrograde solubility of the GaPd2 stability region is the only explanation. Using fine-focused Laue back-scattering diffraction it was found that the two phases that are aligned as lamellas cause well oriented single-orientation diffraction images, i.e. two-phase growth occured in a coherent manner. The Ga3Pd5 crystal structure (Ge3Rh5 structure type, space group Pbam [14]) is very similar to that one of GaPd2 allowing a shared unit cell interface with (0 0 1) GaPd2 parallel to (0 1 0) Ga3Pd5 and coinciding [1 0 0] directions of both lattices. The lattice parameter misfit within the common interface is less than 1%. Changing the growth direction of the GaPd2 crystal from [0 0 1] to [0 1 0] changes the orientation of the lamellas as well thus keeping the coherent intergrowth. From these findings it
cannot yet be decided whether post-growth decomposition of the formerly single-phase GaPd2 occurred via Ga3Pd5 nucleation or by spinodal decomposition. Further studies will be done as to reveal more details of the lamella formation process. 3.3. Ga7Pd3 Slow material transport at low growth temperatures that are limited by the peritectic decomposition of Ga7Pd3 at 460 1C [6] is the main problem in single crystal growth of this phase. Therefore, extremely slow pulling was applied going down to 11 mm/h. But even under these conditions liquid inclusion formation could not be avoided in the crystal shown in Fig. 2c. Large inclusions in a (1 0 0) slice cut perpendicular to the growth direction can be seen in Fig. 4a. Their square-shaped arrangement may be a hint for a central (1 0 0) growth facet. Again, post-growth crystallization from a trapped droplet of solution can be well explained according to the phase diagram. As measured by EPMA (Fig. 4b) Ga5Pd is the next phase to crystallize from an inclusion upon cooling from growth temperature, and the crystallization path is only completed at room temperature at the eutectic point that practically coincides with pure Ga. Due to the low melting point most of the
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Fig. 4. Ga-rich inclusions trapped into a single-crystalline Ga7Pd3 matrix. (a) (1 0 0)-oriented slice cut perpendicular to the growth direction, and (b) EPMA line plot through a Ga-rich inclusion. Data scattering in the center of the inclusion is an artefact due to irregular surface conditions, e.g. holes.
Ga stays liquid or becomes liquid during cutting and gets lost when cleaning the slice. That is, why only large pits on the as-cut surface in Fig. 4a indicate the positions of former liquid inclusions. Since it makes no sense to further reduce the pulling rate and temperature gradients can hardly be increased in these metallic melts, the only way to avoid liquid inclusion formation might be to increase materials transport by some kind of forced convection. Strengthening of convective motion is mainly needed adjacent to the solid-liquid interface where rejection of the excess Ga occurs. Most effectively this can be achieved by a much higher crystal rotation rate that is unusual in Czochralski growth. With respect to the melt flow next to the rotating crystal interface the experimental condition in Czochralski growth is well described by the model of the rotating disc which is one of the best understood models in hydrodynamics [15]. From this it is known that the centrifugal flow adjacent to a rotating disc becomes highest within the thickness of the Ekman layer δE which is the solute layer thickness in the growth experiment. This thickness can be reduced by applying higher rotation rates ω according to δE (ν/ω)½ [15] where ν is the kinematic viscosity. By increasing the crystal rotation rate from approx. 30 rpm in the usual Czochralski growth experiments to a rate as high as 250 rpm the Ekman layer thickness was reduced to almost a third and thus inclusion-free growth of Ga7Pd3 has been achieved for the first time even with a pulling rate of 25 mm/h. A crystal grown at these conditions is shown in Fig. 2d. As can be seen from this photograph, a larger seed crystal had to be taken for mechanical stability reasons as to withstand the centrifugal forces. More detailed studies on the Czochralski growth from metallic solutions using very high rotation rates are subject of ongoing investigations. 4. Summary We succeeded in growing single crystals of the intermetallic compounds GaPd2, GaPd, and Ga7Pd3 using the Czochralski technique from Ga-rich solutions. According to the individual thermodynamic properties of the three phases and based on the published phase diagram, crystal growth conditions were adjusted to the specific compounds. Growth of Ga7Pd3 at the lowest temperatures of less than 460 1C proved to be most difficult due to slow materials transport which is the key factor in crystal growth from solution. Inclusion-free growth was only possible after using very high crystal rotation rates. With GaPd2 single crystal growth the formation of mother liquid inclusions could be avoided by gradually reducing the pulling rate. Decomposition into coherent GaPd2/Ga3Pd5 lamellas occurred in the very last part of the crystals during post-growth cooling.
Acknowledgments The authors are grateful to Prof. Yuri Grin and Dr. Marc Armbrüster, both from Max-Planck-Institut für Chemische Physik fester Stoffe, Dresden, for stimulating discussions, to Renate Enders for surface polishing of the crystals and to Emil Hanfstaengl for the assistance in one of the experiments. This work has been partly conducted within the European integrated Center for the development of new Metallic Alloys and Compounds (European C-MAC).
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Please cite this article as: J. Schwerin, et al., Journal of Crystal Growth (2013), http://dx.doi.org/10.1016/j.jcrysgro.2013.11.060i