Ammonothermal Crystal Growth of Germanium and Its Alloys: Synthesis of a Hollow Metallic Crystal Andrew P. Purdy,* Sean Case, and Clifford George Chemistry Division, Naval Research Laboratory, Washington, DC 20375 Received September 27, 2002;
CRYSTAL GROWTH & DESIGN 2003 VOL. 3, NO. 2 121-124
Revised Manuscript Received December 7, 2002
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ABSTRACT: Elemental germanium and other metals, including Cu, In, Ag, Bi, Sn, and In, are transported under acidic ammonothermal conditions from the hot zone of a sealed quartz tube to the cool zone, where both crystalline and amorphous materials can deposit. Red amorphous Ge deposits when finely divided Ge that is covered with surface oxide is used along with NH4Br mineralizer. When the Ge feedstock is mixed with other metals or metal salts prior to heating, crystals of intermetallic compounds can be grown. Hexagonal crystals of a Cu-Sn-Ge alloy, Cu3Ge, [Sn6O8Ge(NH3)]n, and unusual hollow crystals of a Ag-Sn-Ge alloy were obtained from these reactions. As part of our studies on the ammonothermal crystal growth of GaN,1,2 we attempted to prepare ternary nitrides by adding other metals and salts to the ammonothermal3 reaction systems.4 No ternary nitrides were obtained, but we observed that several of the less reactive metallic elements (Ge, Cu, Ag, Au, Bi, Sn, In) transported from the hot to the cool zones. A check of the literature revealed that many of the same elements transport in hydrothermal systems,5 and extensive studies have been reported on the hydrothermal transport of gold and silver.6 Thus, we investigated the ammonothermal system further. Our primary interest was to determine whether ammonothermal methods with acidic mineralizers (NH4X; X ) Cl, Br, I) have any promise for the synthesis of new nitrides or intermetallic compounds. All reactants were loaded into 3 mm ID/5 mm OD quartz tubes inside a Vacuum Atmospheres dri-lab, and anhydrous NH3 (Air Products) was condensed into the tubes on a vacuum line. The tubes were sealed and heated in a pressurized autoclave in a vertical orientation with the hot zone at the bottom, at approximately 30 000 psi, using an apparatus and procedures that were described previously.1,2 The approximate thermal gradient inside the tubes was found to be -10 °C/cm.1 A listing of selected experiments and their results is given in Table 1. When powdered germanium (-100 mesh, Aldrich) was used with NH4X mineralizer, the hot zone temperature required to transport Ge to the upper portion of the tubes increased in the order X ) Cl ≈ Br < I. The onset of chemical transport with bromide and chloride mineralizer is between 300 and 350 °C, while with iodide it is above 400 °C. In most experiments, the Ge deposited as a thin, shiny, foil-like silver colored film on the surface of the quartz. As shown by the SEM micrographs in Figure 1, small ∼1 µm cubic crystals and globular formations of crystallites also deposit. X-ray powder diffraction patterns show ordinary cubic Ge to be the only crystalline phase.7 When NH4Br was used as the mineralizer, a red, transparent, amorphous film deposited in the upper portion of the tube at hot zone temperatures of ∼380 °C. The red material only deposited when the finely divided Ge obtained from Aldrich was used as the feedstock, and only when it was used in small quantities. Attempts at scaleup were unsuccessful, as were attempts to produce the red material when large chunks of Ge were used instead of * To whom correspondence should be addressed. Andrew Purdy, Chemistry Division, Code 6125, Naval Research Lab, 4555 Overlook Ave, SE, Washington, DC 20375. E-mail:
[email protected]. Phone: 202-4047444. FAX: 202-767-0594.
10.1021/cg025590m
Figure 1. SEM Images of Ge deposit from exp 0 showing (A) a piece of Ge film and (B) globular formations.
finely divided powder. The much larger surface area of the finely divided powder could account for this phenomenon if oxide from the Ge surface is a component in the red material. Indeed, energy dispersive spectroscopy (EDS) spectra show a large peak for oxygen, although oxygen could have been introduced during a product isolation procedure that used an aqueous wash. The red material had an IR absorption (3230 cm-1) for N-H species as well. Red deposits were also produced with finely divided Ge and Sb using a NH4Cl mineralizer (exp 6), and EDS showed that small (∼3%) amounts of Sb were incorporated into the material. SEM images of the red materials were quite featureless. A photograph where the different colored deposits from transport of Ge are clearly visible is displayed in Figure 2. Red or orange amorphous Ge or hydrogenated Ge materials have been reported before. A report detailed the synthesis of amorphous orange Ge from halide reduction,8 and red thin films of amorphous Ge have been described. Amorphous hydrogenated Ge was readily converted to crystalline Ge around 400 °C.9 When Ge chunks were used as the nutrient, small colorless crystals that were shown by EDS analysis to contain Ge and N were sometimes obtained. Unfortunately, the crystals were too small for X-ray structural analysis. Experiments in which Ge was combined with several metals including Sn, In, and Bi did not appear to produce crystals of Ge alloys. Typically, these low melting metals would deposit in the cool zone as droplets which solidified on cooling to small balls. However, when Cu or Cu salts were added to the mix, Cu-Ge compounds were identified in every case. Other metals such as Sn can alloy with the Ge-Cu compounds. Single crystals of an Cu-Sn-Ge intermetallic (1) were isolated from exp 3. Compound 1 has the Mg structure (P63/mmc), with only one unique site in the unit cell over which the Cu, Sn, and Ge atoms are
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Table 1. Details of Selected Experiments
exp 0
1 2 3 4 5 6 a
reactants (mg)
mmol fill temp, NH3 (%) °C (17h)a
Ge(101), NH4Cl(54) above tube
19.5
Ge (28), NH4Br (30) Ge(25), NH4Br(27)
bottom of tube
66
295
no transport blk pdr, Ge
22.2
55
reheat 360 383
19.5
59
460
complete transport
Ge(15), NH4I(30), 19.5 Sn(26), Cu(14) Ge(10), NH4Br(20), 19.5 Se(16), Cu(16), Bi(20) Ge(27), NH4Br(21), 19.5 Ag(25), Sn(26)
62
426
55
495
blk pdr; Ge, unk phases blk pdr w/ silver balls
53
501
complete transport
Ge(12), NH4Cl (9), Sb(20), Te(22)
55
355
gray pdr
19.5
blk pdr, Ge
products: appearance, powder X-ray analysis, and isolated yield mg middle of tube mg top of tube
62.2 tiny amt silver dep 12.0 brownish and silvery foil-like deposits
13.8 5.3 lt gray dep; mostly Cu3Ge; unk phases
silver dep (foil like), also grayish dep; Ge 2.3 drk-light red deposit
mg
7.6 6.1
8.2 silver/drk silver dep, drk red dep, and gray dep all mixed; Ge silver/copper dep; 21.8 mostly 1 9.1 drk gray dep; mostly 3.0 Cu3Ge. Cu3Ge crystal 2
silver dep w/ needlelike 13 crystals; Ge, 4, 4x 46.8 drk gray & silver/ metallic bands; Ge
yel/goldish dep mixed 22.5 w/ gray dep; Ge, Sn, Ag, 3, 4, 4x, unk phases 1.1 drk red band; amorphous 1.3
Temperature measured in hot zone of autoclave 3 h after start of heating.
Figure 2. A photograph of the tube in exp 1 after heating.
randomly distributed.27 With only 42 data and all atoms occupying a single site, it was not possible to accurately determine the proportions of the constituents from X-ray data alone. The least-squares analysis was unstable, the site occupancy for Ge was fixed at 2%, and the remaining 98% refined to Cu 74(3): Sn 26(3). An EDS spectrum taken at 25 KV showed a composition of Cu0.68Sn0.30Ge0.02 and the proportion of Cu varied from 68 to 69% over three different measurements. Standardless EDS is typically accurate to about 3-5% when samples are flat and polished, but this sample was neither, so the magnitude of error is slightly higher. Also note that EDS is a surface technique that does not necessarily reflect the bulk material, and the surface oxide had to be scraped off the crystal to get reliable data. An examination of the calculated powder pattern for 1 showed it to be a major constituent of the overall deposit from exp 3. A twinned crystal of another Cu-Ge intermetallic (2) with a hexagonal unit cell (a ) 2.648(1), c ) 4.217(2)), very similar to the dimensions reported for Cu5Ge (Mg structure (P63/mmc), a ) 2.614, c ) 4.237),10 was isolated from exp 4 as a minor constituent of the overall cool-zone deposit. However, EDS spectroscopy showed an exact Cu3Ge composition on three different spots of the crystal. Crude electrical tests showed the crystals of 1 and 2 to be metallic, and SEM photographs of both crystals are displayed in Figure 3. The single phase regions of the known Cu-Ge-Sn phase diagram includes small amounts of (10, small yellow translucent crystals which were too small for electrical tests, and several highly conducting hollow hexagonal crystals. The yellow crystals were shown to contain N, and to have an approximate 6:1 Sn/Ge ratio by EDS, and were identified as [Sn6O8Ge(NH3)]n (3) by single-crystal X-ray analysis. Compound 3 crystallizes in the noncentrosymmetric space group F23, and consists of Sn6O84- clusters which are coordinated through their four, tetrahedrally arranged, O1 atoms to tetrahedral Ge1 atoms, which link the cages into an extended zinc blende type network (Figure 4). The NH3 molecules occupy interstices in this lattice and must be disordered since they occupy a site requiring tetrahedral symmetry. For those molecules to be NH4+ instead of NH3 would either require unusual oxidation states or violate charge neutrality. The H-atoms are within hydrogen bonding distance of O2 (1.85(2) Å), and this interaction may play a role in the assembly of 3. Sn6O8 cages with similar bond angles and distances are extremely common in Sn(II) oxo compounds.14 However, materials that crystallize in noncentrosymmetric space groups are noteworthy as possible candidates for nonlinear optical
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Figure 4. Atomic arrangement in 3. Thermal ellipsoids are at the 30% probability level. Coordinates: Ge1 (1, 1/2, 1), Sn1 (3/4, 0.4872(1), 3/4), O1 (0.9020(9), 0.4020(9), 0.9020(9)), O2 (0.6481(9), 0.3519(9), 0.8519(9)), N1 (1/2, 1/2, 1).
Figure 5. SEM images of a hollow crystals of Ag-Sn-Ge (4). (a) End view of crystal. The crystal is crimped at the rear end where it was cut from a cluster containing other crystals. (b) End view under higher magnification. (c) An assemblage containing two hollow crystals. (d) Nested, incomplete hollow crystals in metal glob.
studies.15 The oxygen to make 3 probably came from the oxidized surface of the fine Ge powder used in exp 5. A single-crystal X-ray structure determination was done on one of the hollow crystals (4, Figure 5a), and it also had the Mg structure.27 This hollow tubular single crystal was about 0.190 mm long, 75 µm wide and had a wall thickness of 4-5 µm. Some of the other hollow crystals were observed to be multiple, or incomplete with a side or corner missing. The site occupancies of 4 refined to Ag 70(9); Sn 30(9), and as with 1, the esds were likely understated. EDS spectroscopy (standardless) showed this hollow crystal to have the composition Ag 72%, Sn 26%, Ge 2%, and the other hollow crystals had a nearly identical composition (Ag 72-75%, Sn 23-25%, Ge 1-4%). Ignoring the small amount of Ge, this composition was almost within the range (78-87% Ag) of binary Ag-Sn alloys that are known to crystallize in P63/ mmc.11 The Ag-Sn-Ge ternary phase diagram is unknown. In all cases, the hollow crystals were attached to a metallic glob or cluster of crystals that was the most highly enriched in Ge at the point farthest from the hollow crystal, which for the crystals in Figure 5c,d, was 16-19% Ag, 6-9% Sn, and 72-78% Ge. It appeared that the growth of
Crystal Growth & Design, Vol. 3, No. 2, 2003 123 4 depleted Ag and Sn from the metallic mass to which it was attached. The powder patterns of the overall deposits from the middle and the top of the tube show 4 to be a minor component of each, but peaks for several similar materials (4x) of the same structure and slightly different unit cell parameters are evident in all of the cool-zone deposits of exp 5. Spontaneous formation of hollow crystals is unusual, but hardly unknown, and several different mechanisms have been described. The flat basal face and regular hexagonal morphology of 4 excludes mechanisms that involve a coalescence of whiskers or sheets as these tend to produce crystals with jagged edges and a microstructure which makes the mechanism obvious.16-19 Likewise, several other reported mechanisms are excluded.20-22 Most likely, the hollow crystals grew out of a liquid drop that was attached to the quartz surface, and the presence of a glob of metal at the end opposite to the flat face(s) of every hollow crystal provided evidence for this. Perhaps a hexagon nucleated on a growing liquid drop, and was preferentially fed by the droplet at the edges until the droplet was depleted in Ag and Sn. Such a mechanism is similar to the traditional vapor-liquid-solid (VLS) mechanism except that in the VLS mechanism a whisker is attached to a surface at one end and grows at a vapor-fed liquid droplet at the other end.23 Clearly, the flat hexagonal faces of 4 were never attached to a surface. Hollow hexagonal crystals of AgHg amalgam were grown previously on a surface of liquid mercury and also had a relatively flat basal face. However, those crystals were filled with Hg, and the capillary action of Hg rising up the crystal was an important part of the growth mechanism.24 There was no evidence than any of the hollow crystals of 4 were filled with Sn. Zinc crystals with a central cavity and a very flat basal face were also prepared using an apparatus that involved vapor-fed growth onto a droplet of molten metal,25 and hollow tubules of Ga2O3 were prepared by reaction of Ga droplets with O2/H2 plasma.26 Whatever the precise mechanism for the formation of crystals of 4, it seems likely something similar to the VLS mechanism was involved. Acknowledgment. We thank the ONR for financial support. Supporting Information Available: Additional experimental details for all reactions, calculated powder patterns for 1, 3, and 4, and X-ray crystallographic information (CIF) files for the crystals of 1, 3, and 4 are available. This material is free of charge via the Internet at http://pubs.acs.org.
References (1) Purdy, A. P. Chem. Mater. 1999, 11, 1648. (2) Purdy, A. P.; Jouet, R. J.; George, C. F.. Cryst. Growth Des. 2002, 2, 141. (3) Jacobs, H.; Schmidt, D. Curr. Top. Mater. Sci. 1982, 8, 382. (4) Purdy, A. P.; Hwang, A. Abstracts of the 219th ACS National Meeting; San Francisco, CA, March 28, 2000, INOR 597; American Chemical Society, Washington, DC. (5) (a) Rau, H.; Rabenau., A. J. Cryst. Growth 1968, 3, 417 (b) Rabenau, A. Angew. Chem., Int. Ed. Engl. 1985, 24, 1026. (6) Rabenau, A.; Rau, H. Naturwissenschaften 1968, 55, 336. (b) Pan, P.; Wood, S. A. Geochim. Cosmochim. Acta 1991, 55, 2365. (c) Honma, H.; Shikazono, N.; Nakata, M. Can. Mineral. 1991, 29, 217. (d) Rabenau, A. Angew. Chem., Int. Ed. Engl. 1985, 24, 1026. (7) JCPDS card 4-545. (8) Schlecht, S. Angew. Chem., Int. Ed. 2002, 41, 1178. (9) Graeff, C. F. O.; Stutzmann, M.; Eberhardt, K. Philos. Mag. B 1994, 69, 387. (10) Villars, P.; Calvert, L. D. Pearson’s Handbook of Crystallographic Data for Intermetallic Phases; American Society for Metals: Metals Park, OH, 1985.
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(11) Villars, P.; Prince, A.; Okamoto, H. Handbook of Ternary Alloy Phase Diagrams; ASM International, Materials Park, Ohio, 1995. (12) Schubert, V. K.; Brandauer, G. Z. Metall. 1952, 43, 262. (13) (a) Cu3Ge JCPDS cards 6-693 and 89-1146 (b) CuGe JCPDS card 36-1134. (14) (a) Boyle, T. J.; Alam, T. M.; Rodriguez, M. A.; Zechmann, C. A. Inorg. Chem. 2002, 41, 2574. (b) Abrahams, I.; Grimes, S. M.; Johnston, S. R.; Knowles, J. C. Acta Crystallogr. 1996, C52, 286. (c) Sita, L. R.; Xi, R.; Yap, G. P.; Liable-Sands, L. M.; Rheingold, A. L. J. Am. Chem. Soc. 1997, 119, 756. (15) Evans, O. R.; Lin, W. Acc. Chem. Res. 2002, 35, 511. (16) Zheng, X.; Xie, Y.; Zhu, L.; Jiang, X.; Jia, Y.; Song, W.; Sun, Y. Inorg. Chem. 2002, 41, 455. (17) Arivuoli, D.; Gnanam, F. D.; Ramasamy, P. J. Mater. Sci. Lett. 1987, 6, 249. (18) Arivuoli, D.; Gnanam, F. D.; Ramasamy, P. J. Cryst. Growth 1986, 79, 432. (19) Ma, R.; Bando, Y.; Sato, T.; Tang, C.; Xu, F. J. Am. Chem. Soc. 2002, 124, 10668. (20) Kunjomana, A. G.; Mathai, E. Mater. Res. Bull. 1991, 26, 1347. (21) Vayssieres, L.; Keis, K.; Hagfeldt, A.; Lindquist, S. Chem. Mater. 2001, 13, 4395.
Communications (22) Palmans, R.; MacQueen, D. B.; Pierpont, C. G., Frank, A. J. J. Am. Chem. Soc. 1996, 118, 12647. (23) Wagner, R. S.; Ellis, W. C. Appl. Phys. Lett. 1964, 4, 89. (24) Nanev, C.; Milchev, A. Phys. Status. Solidi 1972, 12, 291. (25) Nanev, C.; Iwanov, D. J. Cryst. Growth 1968, 3, 530. (26) Sharma, S.; Sunkara, M. K. J. Am. Chem. Soc. 2002, 124, 12288.. (27) Crystal Data: 1 Sn0.25Cu0.73Ge0.02, (nominal), FW ) 77.78, hexagonal, space group P63/mmc, a ) b ) 2.7603(1), c ) 4.3275(6) Å, R ) β ) 90.0, and γ )120.0°, with V ) 28.555(4) Å3, Z ) 2, and dcalc ) 9.047 g/cm3, F(000) ) 69, µ ) 38.250 cm-1, T ) 293 K. R1 ) 0.032. 3 Sn6O8GeNH3, (nominal), FW ) 929.76, cubic, space group F23, a ) b ) c ) 10.4959(6) Å, R ) β ) γ ) 90.0°, with V ) 1156.3(1) Å3, Z ) 4, and dcalc ) 5.341 g/cm3, F(000) ) 1624, µ ) 15.33 cm-1 T ) 93 K R1 ) 0.055. 4 Ag0.70Sn0.30, (nominal), FW ) 109.82, hexagonal, space group P63/mmc, a ) b ) 2.9491(6), c ) 4.773 (1) Å, R ) β ) 90.0, and γ ) 120.0°, with V ) 35.955(2) Å3, Z ) 2, and dcalc ) 10.46 gm/cm3, F(000) ) 95, µ ) 41.47 cm-1, T ) 293 K. R1 ) 0.020.
CG025590M
