Chemistry Publications Chemistry 2003 A study of the Phase Mg2Cu6Ga5, Isotypic with Mg2Zn11. A Route to an Icosahedral Quasicrystal Approximant Qisheng Lin The Ames Laboratory, [email protected] John D. Corbett Iowa State University Follow this and additional works at: http://lib.dr.iastate.edu/chem_pubs Part of the Inorganic Chemistry Commons, and the Materials Chemistry Commons The ompc lete bibliographic information for this item can be found at http://lib.dr.iastate.edu/ chem_pubs/956. For information on how to cite this item, please visit http://lib.dr.iastate.edu/ howtocite.html. This Article is brought to you for free and open access by the Chemistry at Iowa State University Digital Repository. It has been accepted for inclusion in Chemistry Publications by an authorized administrator of Iowa State University Digital Repository. For more information, please contact [email protected]. Inorg. Chem. 2003, 42, 8762−8767 A Study of the Phase Mg2Cu6Ga5, Isotypic with Mg2Zn11. A Route to an Icosahedral Quasicrystal Approximant Qisheng Lin and John D. Corbett* Department of Chemistry, Iowa State UniVersity, Ames, Iowa 50011 Received June 9, 2003 The new title compound was synthesized by high-temperature means and its X-ray structure refined in the cubic space group Pm3h, Z ) 3, a ) 8.278(1) Å. The structure exhibits a 3-D framework made from a Ga14 and Mg network within which large and small cavities are occupied by centered GaCu12 icosahedral and Cu6 octahedral clusters, respectively. The clusters are well bonded within the network. Electronic structure calculations show that a pseudogap exists just above the Fermi energy, and nearly all pairwise covalent interactions remain bonding over a range of energy above that point. Analysis suggests that the compound is hypoelectronic with a four-electron deficiency per unit cell, and such a derivative with Sc substituting for Mg is an appropriate quasicrystal approximant (Im3h). Such characteristics seem to be key factors in the formation of icosahedral quasicrystals. Introduction that have compositions that lie close to those of quasicrystals. 2,3 Quasicrystal phases are a novel class of intermetallic They commonly serve to model quasicrystal structures. compounds that exhibit rotational symmetries in their dif- Quasicrystals are also generally recognized as electron - 10 fraction patterns that are incommensurate with translational phases, as described by Hume Rothery stabilization rules, periodicity.1-3 Recently, intensive studies focusing on their and they thus exhibit restricted ranges of valence electron discovery, structures, and properties have been carried out counts per atoms (e/a) and probable band gaps or pseudogaps 11,12 in a variety of binary, ternary, and quaternary systems.4-9 at or near the Fermi energy. Comparable sizes for However, to date there is still no general set of rules that component atoms also play an important role in forming tight 5,13 can be used to predict which alloys and which compositions 3-D networks that contain all atom types. are susceptible to the formation of quasicrystalline phases. Numerous studies of alkali-metal compounds of the triels Examples that are extensively used to generalize experience Ga, In, and Tl have shown that these intermetallic compounds in the search for new icosahedral quasicrystals encompass provide a rich collection of examples of isolated, centered, the known quasicrystals plus their presumed approximants. interbridged, and fused icosahedra.14-17 These properly The latter are translationally normal crystalline compounds periodic compounds exhibit relatively flexible structures in with large unit cells that contain condensed, high symmetry order that the alkali-metal countercations can be accom- building blocks, such as icosahedra and dodecahedra, and modated in voids. But they also open promising avenues of exploration for new quasicrystals through substitution of * To whom correspondence should be addressed. E-mail: jcorbett@ iastate.edu. better network-forming cations such as Mg, Ca, Zn, Cd, etc., (1) Shechtman, D.; Blech, I.; Gratias, D.; Cahn, J. W. Phys. ReV. Lett. that also lie among the so-called “icosogen”18 elements. 1984, 53, 1951. (2) Goldman, A. I.; Kelton, R. F. ReV. Mod. Phys. 1993, 65, 213. (3) Janot C. Quasicrystals: A Primer, 2nd ed.; Oxford University Press: (10) Hume-Rothery, W. J. Inst. Met. 1926, 35, 295. Oxford, U.K., 1994. (11) Fujiware, T. Phys. ReV.B1989, 40, 942. (4) Guo, J. Q.; Abe, E.; Tsai, A. P. Jpn. J. Appl. Phys. 2000, 39, L770. (12) Takeuchi, T.; Sato, H.; Mizutani, U. J. Alloy Compd. 2002, 342, 355. (5) Guo, J. Q.; Abe, E.; Tsai, A. P. Phys. ReV.B2000, 62, R14605. (13) Guo, J. Q.; Tsai, A. P. Philos. Mag. Lett. 2002, 82, 349. (6) Tsai, A. P.; Guo, J. Q.; Abe, E.; Takakura, H.; Sato, T. J. Nature (14) Corbett, J. D. Angew. Chem., Int. Ed. 2000, 39, 670. 2000, 408, 537. (15) Belin, C.; Tillard-Charbonnel, M. Prog. Solid State Chem. 1993, 22, (7) Kaneko, Y.; Arichika, Y.; Ishimasa, T. Philos. Mag. Lett. 2001, 81, 59. 777. (16) Tillard-Charbonnel, M.; Manteghetti, A.; Belin, C. Inorg. Chem. 2000, (8) Kaneko, Y.; Maezawa, R.; Kaneko, H.; Ishimasa, T. Philos. Mag. Lett. 39, 1684. 2002, 82, 483. (17) Dong, Z.-C.; Corbett, J. D. Angew. Chem., Int. Ed. Engl. 1996, 35, (9) Tsai, A. P. MRS Bull. 1997, 43; Tsai, A. P. In Physical Properties of 1006. Quasicrystals; Stadnik, Z. M., Ed.; Springer: New York, 1999; p 5. (18) King, R. B. Inorg. Chem. 1989, 28, 2796. 8762 Inorganic Chemistry, Vol. 42, No. 26, 2003 10.1021/ic030191h CCC: $25.00 © 2003 American Chemical Society Published on Web 11/21/2003 Mg2Cu6Ga5, Isotypic with Mg2Zn11 One thing that aroused our interest in these systems was Table 1. Lattice Parameters of Mg2Cu6Ga5 the evolution of structure and bonding from Na15K6Tl18M loaded composition 17 19 (M ) Mg, Zn, Cd, Hg) through Na2Au6In5 to Mg2Cu6- a 3 at. % (MgxCu6Gay) a (Å) V (Å ) Al 20 and then to Mg Zn .21 Nothing has been reported on 5 2 11 18/36/46 (3/6/7.7) 8.2751(4) 566.66(5) the last two compounds since the original 1949 reports by 10/40/50 (1.5/6/7.5) 8.2764(9) 566.9(1) Samson. All of these compounds have primitive cubic 16.7/36.7/46.6 (2.7/6/7.6) 8.2761(6) 566.86(7) symmetry and are closely related to the Mg Zn -type 15/40/45 (2.2/6/6.7) 8.2751(8) 566.66(9) 2 11 16/29/55 (3.2/6/11.3) 8.2754(5) 566.72(6) structure, which consists of three different substructures: 12/32/55 (2.3/6/10.2) 8.2754(9) 567.7(1) polyhedra of centered icosahedral Zn13 and octahedral Zn6 a ) 2+ From Guinier film data with Si as internal standard, λ 1.540598 Å; plus a Zn14 network link with nominal Mg cations. 23 °C. Naturally, the nature of the interactions among the linked networks of spacers and polyhedra in the above series Table 2. Some Crystal Data and Structure Refinement for Mg2Cu6Ga5 undergoes dramatic evolution from Na15K6Tl18MtoMg2Zn11 fw 778.46 as the bonding character changes from predominantly ionic cryst syst, space group, Z cubic, Pm3h (No. 200), 3 unit cell dimension, Å 8.278(1) through heteronuclear covalent to mainly homonuclear V,Å3 567.3(1) covalent, a direction that also seems to be clearly associated d (calcd), Mg/m3 6.836 - with the formation of quasicrystalline phases. In this paper, abs coeff (Mo KR), cm 1 340.6 final R indices [I > 2σ(I)]a R1 ) 0.0477, wR2 ) 0.1282 we describe the structure and bonding of Mg2Cu6Ga5, the [all data] R1 ) 0.0480, wR2 ) 0.1288 first gallium compound in the Mg Zn family, and consider 2 11 a R1 ) ∑||F | - |F ||/∑|F |; wR2 ) [∑w(|F |2 - |F |2)2/∑w(F 2)]1/2. how to possibly tune the structure to that of a quasicrystalline o c o o c o phase. This pursuit is, in fact, encouraged by the discovery 22 of the quasicrystalline Mg (Li)-Cu-Al and Mg32(Cu,Zn,- and regular morphology and growth terraces, and the largest was 23 3 Al)49 phases, which are described in terms of Bergman about 0.3 × 0.3 × 0.3 cm . Crushed pieces were ground together 24 clusters and more or less give connection to the Mg2Zn11- with Si (NIST) as internal standard for Guinier powder diffraction type structure.25 examination. Products were found to contain two phases, ∼70% 27 Mg5Cu6Ga4 and ∼30% of the title phase. Further experiments Experiment Section showed that higher yields of the title phase (∼90-95%) were obtained using the same techniques if the atomic proportions of Synthesis. Experience shows that gallium compounds generally Mg/Cu/Ga were kept in the range ∼(10-16)/(29-40)/(50-55). The cannot be obtained directly when a synthesis is carried out with highest yield came from the loaded composition Mg3.3Cu6Ga11.3 the nominal composition sought; rather a shift of stoichiometries (∼Mg16Cu29Ga55), at which point >95% was obtained. Table 1, obtained in the process toward compounds poorer in gallium is which lists the cell parameters of Mg Cu Ga crystals obtained from 15 2 6 5 generally observed. This may have to do with gallium’s particu- different reactions, shows how the cubic cell parameters of the larly low melting point, and the physical segregation that results filtered crystals remain substantially the same as the loaded therefrom. Therefore, we have always used a self-flux method to compositions are changed (always with excess Ga). Even though grow crystals, and an internal sieve to separate crystals from the 26 the atomic radii of Cu and Ga are close (12 bonded radii: Cu, melts, as has been described elsewhere.