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Crystal growth and structure refinement of a new higher boride NaAlB14
Journal of Alloys and Compounds 395 (2005) 231–235
Crystal growth and structure refinement of a new higher boride NaAlB14 Shigeru Okadaa,∗ , Takaho Tanakab , Akira Satoc , Toetsu Shishidod , Kunio Kudoue , Kazuo Nakajimad , Torsten Lundstr¨omf b
a Faculty of Engineering, Kokushikan University, 4-28-1 Setagaya, Setagaya-ku, Tokyo 154-8515, Japan Advanced Materials Laboratory, National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan c Technical Support Section, National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan d Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba, Sendai 980-0812, Japan e Faculty of Engineering, Kanagawa University, 3-27-1 Rokkakubashi, Kanagawa, Yokohama 221-8686, Japan f The Angstr¨ ˚ om Laboratory, Inorganic Chemistry, Uppsala University, Box 538, SE-75121 Uppsala, Sweden
Received 13 October 2004; accepted 25 October 2004 Available online 7 January 2005
Abstract Single crystal of a new ternary boride NaAlB14 was grown from a high temperature Al solution using Na2 B4 O7 and crystalline boron powders as raw materials under an Ar atmosphere. The crystals obtained showed a plate-like or a trapezoidal shape with well-developed {0 1 0} faces, and were reddish black with a metallic luster. The largest crystal prepared in the present work attained maximum dimensions of approximately 7.3 mm × 3.2 mm × 2.8 mm. The crystals were examined by powder X-ray diffraction and chemical analyses, and the crystal structure of NaAlB14 was refined using data of single crystal X-ray diffractometry. The structure refinement converged at an R1 (F2 ) value of 0.025 with 29 parameters for 381 independent reflections [F0 > 4σ(F0 )] and 0.027 for all 426 independent reflections. The crystal structure is orthorhombic with the space group Imma (number 74), and lattice constants are a = 1.0465(1), b = 0.5844(1), and c = 0.8231(1) nm; V = 0.5034(1) nm3 ; Z = 4. © 2004 Elsevier B.V. All rights reserved. Keywords: NaAlB14 ; Single crystal; Al solution; Structure; Chemical analysis
1. Introduction Boron-rich compounds containing B12 icosahedral structural units are of great interest because of their remarkable physical and chemical properties, which in many cases are of potential applications to thermoelectric materials and photodetectors [1]. MgAlB14 -type structure is rather simple and a typical icosahedral boride structure; thus, after discovery of MgAlB14 [2], many investigations have been carried out and NaBB14 and Mg2 B14 [3], and LnAlB14 (Ln = Y, Tb, Dy, Ho, Er, Yb, Lu) [4–7] were reported. However, ternary AM–Al–B (AM = Li, Na, K) system compounds have not been understood well up to now except for Higashi’s works [8,9] of LiAlB14 crystals grown from a high temperature Al ∗
Corresponding author. Tel.: +81 3 5481 3292; fax: +81 3 5481 3292. E-mail address: [email protected] (S. Okada).
0925-8388/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2004.10.057
solution using a Li metal. Recently, we successfully prepared single crystals of LiAlB14 from the high temperature Al solution [10] using Li2 B4 O7 powder, instead of Li metal, and amorphous boron. Until now there is no report of NaAlB14 , for both crystal growth and structure refinement. In this paper, we report the experimental conditions for growing crystals of a new boride NaAlB14 from the high temperature Al solution using an anhydrous sodium tetra borate Na2 B4 O7 and crystalline boron powders as the raw materials. Na2 B4 O7 is more suitable as a source of sodium element than Na metal that has high vapor pressure [11], because of a high melting point (1016 K) and a high boiling point (1848 K), relatively high chemical stability in air, low reactivity for an alumina crucible at high temperature, and good solubility in the Al solution at high temperature.
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S. Okada et al. / Journal of Alloys and Compounds 395 (2005) 231–235
We describe also the results of the structure refinement of NaAlB14 and discuss the crystal structure comparing with other MgAlB14 -type structure compounds.
2. Experimental details 2.1. Syntheses of NaAlB14 crystals The reagents used to prepare the compounds were anhydrous sodium tetra borate Na2 B4 O7 (purity 99%) powder or sodium metal chips (purity 99%), crystalline boron (purity 99%) powder and aluminum metal sticks (purity 99.99%). Na2 B4 O7 or Na and B were weighed at nominal composition of atomic ratios n = B/Na = 1.0–8.0 (Table 1) by the reaction shown below: Na2 B4 O7 + (2n + 3)B → 2NaBn + 7BO.
(1)
Na + nB → NaBn .
(2)
Al metal was added to each mixture at a mass ratio of 1:15. The amount of Na2 B4 O7 or Na in the starting materials was fixed at 1.5 g throughout all the experiments. The mixture was placed in a dense alumina crucible (37 mm diameter and 45 mm length) together with an Al2 O3 cover and heated in flowing an Ar gas. The mixture was heated at a rate of 300 K/h and kept at 1573 K for 1 h. The solution was cooled to 1073 K at a rate of 50 K/h and then the furnace was switched off. Dissolving the solidified mixture in a mixed solution of hydrochloric acid and ethanol separated the grown crystals. NaAlB14 crystals were selected under a stereomicroscope for the measurements of chemical analyses and X-ray diffraction. Experimental conditions for the single crystal growths are shown in Table 1. However, the crystal of Na–Al–B system compound was not obtained under the conditions of run numbers 1*–3*. 2.2. X-ray diffraction and chemical analyses The chemical composition of NaAlB14 crystals grown in the Al-self flux was determined by the electron probe miTable 1 Typical growth conditions of NaAlB14 Run number
1* 2* 3* 1 2 3 4 5
Composition of the starting material (atomic ratio) Na
B
1 1 1 1 1 1 1 1
2.0 4.0 6.0 1.0 2.0 4.0 6.0 8.0
Table 2 Crystallographic and data collection parameters of NaAlB14 Crystal system Space group a (nm) b (nm) c (nm) Volume (nm3 ) Z Formula weight Dx (g/cm3 ) Data corrections Applied radiation, λ (nm) Linear absorption coefficient (nm−1 ) Crystal dimension (mm3 ) Absorption correction Reflections measured
2θ max (degree) Unique reflections Structure refinement program Number of variables R1 [F0 > 4σ(F0 )] (for 381 F0 ) R1 [all F0 ] (for 426 F0 ) WR2 (F2 )
Orthorhombic Imma (numbers 74) 1.0465(1) 0.5844(1) 0.8231(1) 0.5034(1) 4 201.31 2.656 CCD area detector diffractometer (Bruker SMART APEX) Monochrom Mo K␣, 0.071073 0.34 0.20 × 0.14 × 0.14 Multi-scan; SADABS, Bruker [16] −14 ≤ h ≤ 12 −7 ≤ k ≤ 8 −11 ≤ l ≤ 11 61.23 426 SHELXL-97 (based on F2 ) 29 0.025 0.027 0.080
croanalysis (EPMA)(JXA-8600MX, JEOL Co., Japan) [12] using standards of a single-crystal NaAlSi3 O8 for Na and Al, and a single-crystal B4.5 C for B. Standard deviations of the EPMA measurement were ±1% for each element. The morphological properties and impurities of the crystals were investigated by a scanning electron microscope (SEM) (JEOL, T-20) and an energy dispersive X-ray detector (EDX) (Horiba, EMAX-2770) [13]. Phase identification and determination of unit-cell parameters were carried out using a standard powder X-ray diffractometer (XRD) (Rigaku, RU-200, Japan) with monochromatic Cu K␣ radiation. Single crystal X-ray diffraction data were corrected using a CCD area detector diffractometer (Bruker SMART APEX, Germany) with graphite monochromated Mo K␣ radiation. Crystallographic and data collection parameters are summarized in Table 2. Without using the previous MgAlB14 structure model, an initial structure solution was obtained by SIR92 [14] and the program SHELXL-97 [15] was used for refinement.
Phases identified
3. Results and discussion – – – NaAlB14 NaAlB14 NaAlB14 NaAlB14 , AlB2 NaAlB14 , ␣-AlB12 , AlB2
Run number 1*–3*: Na metal; run number 1–5: Na2 B4 O7 .
3.1. Preparation of NaAlB14 crystal The experimental conditions for crystal growth of NaAlB14 are shown in Table 1. As seen from Table 1, the crystal of Na–Al–B system compound could not be produced under the conditions of run numbers 1*–3* where Na metal was used as the Na source. NaAlB14 crystals could be grown under the conditions of run numbers 1–5 where Na2 B4 O7
S. Okada et al. / Journal of Alloys and Compounds 395 (2005) 231–235
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Chemical analysis for the crystals showed that the composition corresponds to an atomic ratio Na:Al:B = 1:1:14 (Table 3). The impurity content of the NaAlB14 crystals was not analyzed chemically. No evidence was obtained for the presence of an oxygen-containing phase in the crystals, as concluded from EDX and EPMA analyses of as-grown crystals. 3.2. Structure analysis and description of the structure
Fig. 1. Powder XRD pattern of NaAlB14 crystal. : NaAlB14 , : ␣Al2 O3 .
Fig. 2. SEM micrograph of a NaAlB14 crystal (run number 3). Table 3 Chemical analysis data of NaAlB14 Element
Chemical analysis (mass %)
Al Na B In total Chemical composition
13.19 11.90 78.09 103.18 Al0.95 Na1.00 B14
was used as the Na source being adjusted the atomic ratios n = B/Na = 1.0–8.0 (run numbers 1–5) in the starting mixture. Fig. 1 shows the powder XRD pattern of the NaAlB14 crystals obtained. It’s likely that the majority of ␣-Al2 O3 came from minute fragments of the alumina crucible sticking to the crystals and from the Al2 O3 mortar used for pulverizing the crystals. The largest NaAlB14 crystals prepared in the present work attained maximum dimensions of approximately 7.3 mm × 3.2 mm × 2.8 mm. The crystals obtained showed a plate-like or a trapezoidal shape with well-developed {0 1 0} faces (Fig. 2), and were reddish black with a metallic luster.
The powder XRD indexing result indicated that the grown crystals have an orthorhombic crystal structure with lattice constants of a = 1.0465(1), b = 0.5844(1), and c = 0.8231(1) nm. The lattice constant values and the EPMA results suggested that the ternary Na–Al–B compound crystal is isomorphous with MgAlB14 [2,17]. However, we once experienced that YB50 [18] was expected to be isomorphous to ␥-AlB12 because of similarity of crystal system and lattice constants between two compounds. Single crystal structure analysis for YB41 Si1.2 [19] that is isostructural to YB50 revealed that their crystal structure is completely new. Thus, we started our structure analysis without using the MgAlB14 structure model. An initial solution obtained according to the space group Imma gave seven atomic positions in which five B sites, one Na site and one Al site were assigned. The refinement using the assignment gave R1 value of 2.5% with 29 parameters for 381 independent reflections [F0 > 4σ(F0 )] and 2.7% for all 426 independent reflections. Final atomic coordinates, occupancy factors and temperature factors of metal atoms and boron atoms in NaAlB14 are summarized in Tables 4 and 5, respectively. All B sites are fully occupied. On the other hand, partial occupancies of 95 and 94% were observed at the Na and Al site, respectively. Thus chemical composition of the crystal obtained from the structure analysis is Na0.95 Al0.94 B14 , which shows a good agreement with that given by the EPMA measurement. Table 5 Final atomic coordinates and isotropic temperature factors of boron atoms in NaAlB14 Atoma
Site
x
y
z
Ueq (×10−5 nm2 )
B1 B2 B3 B4 B5
8i 8i 16j 16j 8i
0.1645(2) 0.1028(2) 0.1706(1) 0.3105(1) 0.0839(2)
0.2500 0.2500 -0.0005(2) 0.0894(2) 0.2500
0.4178(2) 0.6247(2) 0.2942(1) 0.4130(1) 0.2176(2)
7.1(3) 6.8(3) 7.0(3) 7.1(3) 7.0(4)
a No deviation from full occupancy. U is one third of the trace of the eq orthogonalized Uij tensor.
Table 4 Final atomic coordinates, occupancy factors and anisotropic temperature factors of metal atoms in NaAlB14 Atom
Site
x
y
z
occ
U11
U22
U33
U23
U13
U12
Na Al
4e 4b
0.5 0.0
0.25 0.0
0.5982(1) 0.5
0.95 0.94
9.1(5) 6.3(4)
15.0(5) 8.3(4)
7.3(5) 15.7(4)
0 −6.1(2)
0 0
0 0
Uij (×10−5 nm2 ).
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S. Okada et al. / Journal of Alloys and Compounds 395 (2005) 231–235
Visualization of the crystal structure using the graphic program CrystalMaker [20] immediately showed that the crystal structure is isomorphous to the MgAlB14 structure. Our axis choice (the used space group: Imma) and boron site assignment are different from those (the used space group: Imam) of the previous papers [2–9], however, we will continue our discussion without transforming the axes and adjusting the site assignment. The three-dimensional boron framework of NaAlB14 structure composes of one structurally independent icosahedron formed by B1, B3, B4 and B5, and one bridge site boron B2 as described in detail by Naslain et al. [3]. Na and Al reside in larger and smaller voids of the boron framework, respectively. Fig. 3 shows a perspective view of the NaAlB14 structure where the boron icosahedron unit is depicted as a cluster. The largest and the second largest atoms correspond to the Na and Al atoms, respectively. The crystal structure was described by Naslain et al. in detail [3], here only a comparison with the equivalents in LiAlB14 and MgAlB14 will be made after Higashi’s comparison [9]. Recently Albert et al. [22] reported that NaB15 crystal structure is slightly distorted from orthorhombic symmetry to a monoclinic crystal structure with space group I1m1 (a = 585.92(3), b = 1039.92(6), and c = 833.17(5) nm; β = 90.373(5)◦ from powder data) and should be written as NaB14.5 or Na2 B29 instead of NaB15 .
Fig. 3. Crystal structure of NaAlB14 : three-dimensional perspective view.
Table 6 Comparison of lattice constants and interatomic distances with the equivalents in LiAlB14 and MgAlB14 ˚ Lattice constants (A)
˚ (M1 = Na; M2 = Al) NaAlB14 (A)
a 10.465(1) b 5.844(1) c 8.231(1) Linkage B–B bond lengths within B12 icosahedron B1–B3 1.784(1) B1–B4 1.794(2) B1–B5 1.851(2) B3–B4 1.796(2) B3–B3 1.814(2) B3–B5 1.814(2) B3–B4 1.837(2) B4–B4 1.878(2) B4–B5 1.805(2) B–B bond lengths for linkages between B12 icosahedron B4–B4 1.773(2) B5–B5 1.756(3)
˚ (M1 = Li; M2 = Al) [9] LiAlB14 (A)
˚ (M1 = Mg; M2 = Al) [21] MgAlB14 (A)
10.3542 5.8469 8.1429
10.313 5.848 8.115
1.788 1.791 1.847 1.784 1.800 1.823 1.848 1.864 1.789
1.798 1.787 1.835 1.791 1.793 1.809 1.837 1.853 1.788
1.751 1.724
1.752 1.711
B–B distances for linkages between bridge site and B12 icosahedron/bridge site B2–B3 1.754(1) 1.746 B2–B1 1.821(2) 1.834 B2–B2 2.150(2) 2.081
1.755 1.787 2.040
Metal–boron distances M1–B2 M1–B4 M1–B5 M1–B4 M1–B3 M2–B2 M2–B1 M2–B3
2.363 2.666 2.811 2.787 2.736 2.061 2.324 2.422
2.522(2) 2.672(1) 2.744(2) 2.806(1) 2.814(1) 2.084(1) 2.357(1) 2.461(1)
In this table our axis choice and boron site assignment are used.
2.422 2.641 2.744 2.785 2.771 2.081 2.339 2.434
S. Okada et al. / Journal of Alloys and Compounds 395 (2005) 231–235
However, site coordinates are not given in their paper, thus, we could not include Na2 B29 in our comparison. Actually the monoclinic distortion is so small that the comparison using Naslain’s results may not cause a miss leading of discussion but we dared not to compare NaB15 . In Table 6, lattice constants and interatomic distances of the present NaAlB14 are compared with others. NaAlB14 has larger a and c values than LiAlB14 and MgAlB14 because of larger ionic radius of Na than Li and Mg, however, b values are almost constant for all three compounds. M1 metal site is surrounded by two icosahedra and two bridge site borons in a–c plane but the void of the boron framework including the M1 metal site forms a tunnel along b-axis. Thus lattice constants of a and c are dependent on the ionic radius of M1 but b is not. The tunnel structure allows the M1 site to split into two positions along b-axis when the ionic radius of the M1 site metal is small, i.e. the Mg site of MgAlB14 [21] and the Ln site of LnAlB14 [4–7] have split positions. The inter-icosahedron bond distance increases more correspondingly to the lattice constant increase than the intra-icosahedron bond distances. The B–B ˚ for linkage between the bridge sites seems distance 2.15 A too large as a B–B bonding. The same distances 2.081 and ˚ for LiAlB14 and MgAlB14 , respectively, are also too 2.040 A large as a B–B bonding. Actually the direct B–B bonding between the bridge site boron should not be expected in these M1AlB14 structure. The dominant role of the bridge site B is to bridge three icosahedra, which form a–b plane icosahedron network. On the other hand, the distances 2.084 and ˚ between the bridge site B and Al and Na, respec2.522 A tively, are noticeably shorter than the other Al–B and Na–B distances. As discussed by Higashi [9] a similar tendency is also seen in the other crystals and both Al and Na seem to bond strongly with the bridge site B in order to mediate the connection between the a–b plane networks.
4. Conclusions The single crystals of a new ternary boride NaAlB14 were grown from a high temperature Al solution using Na2 B4 O7 and crystalline boron powders as raw materials under an Ar atmosphere. NaAlB14 is the first compound obtained in the ternary Na–Al–B system and is the first analogue of the MgAlB14 -type structure family. The crystal structure of NaAlB14 is orthorhombic with the space group Imma (number 74), and lattice constants are a = 1.0465(1), b = 0.5844(1), and c = 0.8231(1) nm; V = 0.5034(1) nm3 ; Z = 4. Crystal growth from high temperature metal solution is a very versatile method allowing variation of numerous experimental parameters and most likely boron-rich compounds
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containing Al and the alkali metals K–Cs could be synthesized in a similar method. These crystal growths are under investigation.
Acknowledgments The authors wish to thank Dr. K. Iizumi and Miss Y. Igumi of Tokyo Polytechnic University for help on the crystal growth experiments and to Mr. K. Kosuda of National Institute for Materials Science for EPMA measurements.
References [1] V.I. Matkovich (Ed.), Boron and Refractory Borides, SpringerVerlag, New York, 1977, p. 439. [2] V.I. Matkovich, J. Economy, Acta Crystallogr. B 26 (1970) 616. [3] R. Naslain, A. Guette, P. Hagenmuller, J. Less-Common Met. 47 (1976) 1. [4] Yu.B. Kuz’ma, V.N. Gurin, M.M. Korsukova, N.F. Chavan, S.I. Chikhrij, Proceedings AS of USSR, Inorg. Mater. 24 (1988) 1705. [5] N.B. Brandt, A.A. Gippius, V.V. Moshchalkov, K.K. Nyan, V.N. Gurin, M.M. Korsukova, Yu.B. Kuz’ma, Sov. Phys. Solid State 30 (1988) 797. [6] M.M. Korsukova, V.N. Gurin, Yu.B. Kuz’ma, N.F. Chaban, S.I. Chikhrij, V.V. Moshchalkov, N.B. Brandt, A.A. Gippius, K.K. Nyan, Phys. Stat. Sol. (a) 114 (1989) 265. [7] M.M. Korsukova, V.N. Gurin, J. Alloys Comp. 187 (1992) 39. [8] I. Higashi, Y. Takahashi, J. Less-Common Met. 81 (1981) 135. [9] I. Higashi, J. Less-Common Met. 82 (1981) 317. [10] K. Kudou, S. Okada, T. Mori, K. Iizumi, T. Shishido, T. Tanaka, I. Higashi, K. Nakajima, P. Rogl, Y.B. Andersson, T. Lundstr¨om, Jpn. J. App. Phys. 41 (2002) L928. [11] D. R. Lide (Ed.), CRC Handbook of Chemistry and Physics, 76th ed., 1995–1996, New York, p. 4. [12] T. Tanaka, A. Sato, J. Solid State Chem. 165 (2002) 148. [13] S. Okada, K. Kudou, T. Tanaka, T. Shishido, V.N. Gurin, T. Lundstr¨om, J. Solid State Chem. 177 (2004) 547. [14] A. Altomare, G. Cascarano, C. Giacovazzo, A. Guagliardi, M.C. Burla, G. Polidori, M. Camalli, J. Appl. Crystallogr. 27 (1994) 435. [15] M. Sheldrick, SHELXL-97: a program for the solution and refinement of crystal structures, Universit¨at G¨ottingen, G¨ottingen, Germany, 1997. [16] Bruker (1999). SMART-NT (Version 5.624), SAINT-Plus-NT (Version 6.02a) and SADABS. Bruker AXS Inc., Madison, Winsconsin, USA. [17] S. Okada, K. Kudou, T. Mori, I. Higashi, N. Kamegashira, K. Nakajima, T. Lundstr¨om, Mater. Sci. Forum 449–452 (2004) 365. [18] T. Tanaka, S. Okada, Y. Ishizawa, J. Alloys Comp. 205 (1994) 281. [19] I. Higashi, T. Tanaka, K. Kobayashi, Y. Ishizawa, M. Takami, J. Solid State Chem. 133 (1997) 31. [20] D. Palmer, CrystalMaker, version 6.3.4, CrystalMaker Software, Bicester, Oxfordshire, OX67BS, UK. [21] I. Higashi, T. Ito, J. Less-Common Met. 92 (1983) 239. [22] B. Albert, K. Hofmann, C. Fild, H. Eckert, M. Schleifer, R. Gruehn, Chem. Eur. J. 6 (2000) 2531.
Crystal growth and structure refinement of a new higher boride NaAlB14 Shigeru Okadaa,∗ , Takaho Tanakab , Akira Satoc , Toetsu Shishidod , Kunio Kudoue , Kazuo Nakajimad , Torsten Lundstr¨omf b
a Faculty of Engineering, Kokushikan University, 4-28-1 Setagaya, Setagaya-ku, Tokyo 154-8515, Japan Advanced Materials Laboratory, National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan c Technical Support Section, National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan d Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba, Sendai 980-0812, Japan e Faculty of Engineering, Kanagawa University, 3-27-1 Rokkakubashi, Kanagawa, Yokohama 221-8686, Japan f The Angstr¨ ˚ om Laboratory, Inorganic Chemistry, Uppsala University, Box 538, SE-75121 Uppsala, Sweden
Received 13 October 2004; accepted 25 October 2004 Available online 7 January 2005
Abstract Single crystal of a new ternary boride NaAlB14 was grown from a high temperature Al solution using Na2 B4 O7 and crystalline boron powders as raw materials under an Ar atmosphere. The crystals obtained showed a plate-like or a trapezoidal shape with well-developed {0 1 0} faces, and were reddish black with a metallic luster. The largest crystal prepared in the present work attained maximum dimensions of approximately 7.3 mm × 3.2 mm × 2.8 mm. The crystals were examined by powder X-ray diffraction and chemical analyses, and the crystal structure of NaAlB14 was refined using data of single crystal X-ray diffractometry. The structure refinement converged at an R1 (F2 ) value of 0.025 with 29 parameters for 381 independent reflections [F0 > 4σ(F0 )] and 0.027 for all 426 independent reflections. The crystal structure is orthorhombic with the space group Imma (number 74), and lattice constants are a = 1.0465(1), b = 0.5844(1), and c = 0.8231(1) nm; V = 0.5034(1) nm3 ; Z = 4. © 2004 Elsevier B.V. All rights reserved. Keywords: NaAlB14 ; Single crystal; Al solution; Structure; Chemical analysis
1. Introduction Boron-rich compounds containing B12 icosahedral structural units are of great interest because of their remarkable physical and chemical properties, which in many cases are of potential applications to thermoelectric materials and photodetectors [1]. MgAlB14 -type structure is rather simple and a typical icosahedral boride structure; thus, after discovery of MgAlB14 [2], many investigations have been carried out and NaBB14 and Mg2 B14 [3], and LnAlB14 (Ln = Y, Tb, Dy, Ho, Er, Yb, Lu) [4–7] were reported. However, ternary AM–Al–B (AM = Li, Na, K) system compounds have not been understood well up to now except for Higashi’s works [8,9] of LiAlB14 crystals grown from a high temperature Al ∗
Corresponding author. Tel.: +81 3 5481 3292; fax: +81 3 5481 3292. E-mail address: [email protected] (S. Okada).
0925-8388/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2004.10.057
solution using a Li metal. Recently, we successfully prepared single crystals of LiAlB14 from the high temperature Al solution [10] using Li2 B4 O7 powder, instead of Li metal, and amorphous boron. Until now there is no report of NaAlB14 , for both crystal growth and structure refinement. In this paper, we report the experimental conditions for growing crystals of a new boride NaAlB14 from the high temperature Al solution using an anhydrous sodium tetra borate Na2 B4 O7 and crystalline boron powders as the raw materials. Na2 B4 O7 is more suitable as a source of sodium element than Na metal that has high vapor pressure [11], because of a high melting point (1016 K) and a high boiling point (1848 K), relatively high chemical stability in air, low reactivity for an alumina crucible at high temperature, and good solubility in the Al solution at high temperature.
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S. Okada et al. / Journal of Alloys and Compounds 395 (2005) 231–235
We describe also the results of the structure refinement of NaAlB14 and discuss the crystal structure comparing with other MgAlB14 -type structure compounds.
2. Experimental details 2.1. Syntheses of NaAlB14 crystals The reagents used to prepare the compounds were anhydrous sodium tetra borate Na2 B4 O7 (purity 99%) powder or sodium metal chips (purity 99%), crystalline boron (purity 99%) powder and aluminum metal sticks (purity 99.99%). Na2 B4 O7 or Na and B were weighed at nominal composition of atomic ratios n = B/Na = 1.0–8.0 (Table 1) by the reaction shown below: Na2 B4 O7 + (2n + 3)B → 2NaBn + 7BO.
(1)
Na + nB → NaBn .
(2)
Al metal was added to each mixture at a mass ratio of 1:15. The amount of Na2 B4 O7 or Na in the starting materials was fixed at 1.5 g throughout all the experiments. The mixture was placed in a dense alumina crucible (37 mm diameter and 45 mm length) together with an Al2 O3 cover and heated in flowing an Ar gas. The mixture was heated at a rate of 300 K/h and kept at 1573 K for 1 h. The solution was cooled to 1073 K at a rate of 50 K/h and then the furnace was switched off. Dissolving the solidified mixture in a mixed solution of hydrochloric acid and ethanol separated the grown crystals. NaAlB14 crystals were selected under a stereomicroscope for the measurements of chemical analyses and X-ray diffraction. Experimental conditions for the single crystal growths are shown in Table 1. However, the crystal of Na–Al–B system compound was not obtained under the conditions of run numbers 1*–3*. 2.2. X-ray diffraction and chemical analyses The chemical composition of NaAlB14 crystals grown in the Al-self flux was determined by the electron probe miTable 1 Typical growth conditions of NaAlB14 Run number
1* 2* 3* 1 2 3 4 5
Composition of the starting material (atomic ratio) Na
B
1 1 1 1 1 1 1 1
2.0 4.0 6.0 1.0 2.0 4.0 6.0 8.0
Table 2 Crystallographic and data collection parameters of NaAlB14 Crystal system Space group a (nm) b (nm) c (nm) Volume (nm3 ) Z Formula weight Dx (g/cm3 ) Data corrections Applied radiation, λ (nm) Linear absorption coefficient (nm−1 ) Crystal dimension (mm3 ) Absorption correction Reflections measured
2θ max (degree) Unique reflections Structure refinement program Number of variables R1 [F0 > 4σ(F0 )] (for 381 F0 ) R1 [all F0 ] (for 426 F0 ) WR2 (F2 )
Orthorhombic Imma (numbers 74) 1.0465(1) 0.5844(1) 0.8231(1) 0.5034(1) 4 201.31 2.656 CCD area detector diffractometer (Bruker SMART APEX) Monochrom Mo K␣, 0.071073 0.34 0.20 × 0.14 × 0.14 Multi-scan; SADABS, Bruker [16] −14 ≤ h ≤ 12 −7 ≤ k ≤ 8 −11 ≤ l ≤ 11 61.23 426 SHELXL-97 (based on F2 ) 29 0.025 0.027 0.080
croanalysis (EPMA)(JXA-8600MX, JEOL Co., Japan) [12] using standards of a single-crystal NaAlSi3 O8 for Na and Al, and a single-crystal B4.5 C for B. Standard deviations of the EPMA measurement were ±1% for each element. The morphological properties and impurities of the crystals were investigated by a scanning electron microscope (SEM) (JEOL, T-20) and an energy dispersive X-ray detector (EDX) (Horiba, EMAX-2770) [13]. Phase identification and determination of unit-cell parameters were carried out using a standard powder X-ray diffractometer (XRD) (Rigaku, RU-200, Japan) with monochromatic Cu K␣ radiation. Single crystal X-ray diffraction data were corrected using a CCD area detector diffractometer (Bruker SMART APEX, Germany) with graphite monochromated Mo K␣ radiation. Crystallographic and data collection parameters are summarized in Table 2. Without using the previous MgAlB14 structure model, an initial structure solution was obtained by SIR92 [14] and the program SHELXL-97 [15] was used for refinement.
Phases identified
3. Results and discussion – – – NaAlB14 NaAlB14 NaAlB14 NaAlB14 , AlB2 NaAlB14 , ␣-AlB12 , AlB2
Run number 1*–3*: Na metal; run number 1–5: Na2 B4 O7 .
3.1. Preparation of NaAlB14 crystal The experimental conditions for crystal growth of NaAlB14 are shown in Table 1. As seen from Table 1, the crystal of Na–Al–B system compound could not be produced under the conditions of run numbers 1*–3* where Na metal was used as the Na source. NaAlB14 crystals could be grown under the conditions of run numbers 1–5 where Na2 B4 O7
S. Okada et al. / Journal of Alloys and Compounds 395 (2005) 231–235
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Chemical analysis for the crystals showed that the composition corresponds to an atomic ratio Na:Al:B = 1:1:14 (Table 3). The impurity content of the NaAlB14 crystals was not analyzed chemically. No evidence was obtained for the presence of an oxygen-containing phase in the crystals, as concluded from EDX and EPMA analyses of as-grown crystals. 3.2. Structure analysis and description of the structure
Fig. 1. Powder XRD pattern of NaAlB14 crystal. : NaAlB14 , : ␣Al2 O3 .
Fig. 2. SEM micrograph of a NaAlB14 crystal (run number 3). Table 3 Chemical analysis data of NaAlB14 Element
Chemical analysis (mass %)
Al Na B In total Chemical composition
13.19 11.90 78.09 103.18 Al0.95 Na1.00 B14
was used as the Na source being adjusted the atomic ratios n = B/Na = 1.0–8.0 (run numbers 1–5) in the starting mixture. Fig. 1 shows the powder XRD pattern of the NaAlB14 crystals obtained. It’s likely that the majority of ␣-Al2 O3 came from minute fragments of the alumina crucible sticking to the crystals and from the Al2 O3 mortar used for pulverizing the crystals. The largest NaAlB14 crystals prepared in the present work attained maximum dimensions of approximately 7.3 mm × 3.2 mm × 2.8 mm. The crystals obtained showed a plate-like or a trapezoidal shape with well-developed {0 1 0} faces (Fig. 2), and were reddish black with a metallic luster.
The powder XRD indexing result indicated that the grown crystals have an orthorhombic crystal structure with lattice constants of a = 1.0465(1), b = 0.5844(1), and c = 0.8231(1) nm. The lattice constant values and the EPMA results suggested that the ternary Na–Al–B compound crystal is isomorphous with MgAlB14 [2,17]. However, we once experienced that YB50 [18] was expected to be isomorphous to ␥-AlB12 because of similarity of crystal system and lattice constants between two compounds. Single crystal structure analysis for YB41 Si1.2 [19] that is isostructural to YB50 revealed that their crystal structure is completely new. Thus, we started our structure analysis without using the MgAlB14 structure model. An initial solution obtained according to the space group Imma gave seven atomic positions in which five B sites, one Na site and one Al site were assigned. The refinement using the assignment gave R1 value of 2.5% with 29 parameters for 381 independent reflections [F0 > 4σ(F0 )] and 2.7% for all 426 independent reflections. Final atomic coordinates, occupancy factors and temperature factors of metal atoms and boron atoms in NaAlB14 are summarized in Tables 4 and 5, respectively. All B sites are fully occupied. On the other hand, partial occupancies of 95 and 94% were observed at the Na and Al site, respectively. Thus chemical composition of the crystal obtained from the structure analysis is Na0.95 Al0.94 B14 , which shows a good agreement with that given by the EPMA measurement. Table 5 Final atomic coordinates and isotropic temperature factors of boron atoms in NaAlB14 Atoma
Site
x
y
z
Ueq (×10−5 nm2 )
B1 B2 B3 B4 B5
8i 8i 16j 16j 8i
0.1645(2) 0.1028(2) 0.1706(1) 0.3105(1) 0.0839(2)
0.2500 0.2500 -0.0005(2) 0.0894(2) 0.2500
0.4178(2) 0.6247(2) 0.2942(1) 0.4130(1) 0.2176(2)
7.1(3) 6.8(3) 7.0(3) 7.1(3) 7.0(4)
a No deviation from full occupancy. U is one third of the trace of the eq orthogonalized Uij tensor.
Table 4 Final atomic coordinates, occupancy factors and anisotropic temperature factors of metal atoms in NaAlB14 Atom
Site
x
y
z
occ
U11
U22
U33
U23
U13
U12
Na Al
4e 4b
0.5 0.0
0.25 0.0
0.5982(1) 0.5
0.95 0.94
9.1(5) 6.3(4)
15.0(5) 8.3(4)
7.3(5) 15.7(4)
0 −6.1(2)
0 0
0 0
Uij (×10−5 nm2 ).
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S. Okada et al. / Journal of Alloys and Compounds 395 (2005) 231–235
Visualization of the crystal structure using the graphic program CrystalMaker [20] immediately showed that the crystal structure is isomorphous to the MgAlB14 structure. Our axis choice (the used space group: Imma) and boron site assignment are different from those (the used space group: Imam) of the previous papers [2–9], however, we will continue our discussion without transforming the axes and adjusting the site assignment. The three-dimensional boron framework of NaAlB14 structure composes of one structurally independent icosahedron formed by B1, B3, B4 and B5, and one bridge site boron B2 as described in detail by Naslain et al. [3]. Na and Al reside in larger and smaller voids of the boron framework, respectively. Fig. 3 shows a perspective view of the NaAlB14 structure where the boron icosahedron unit is depicted as a cluster. The largest and the second largest atoms correspond to the Na and Al atoms, respectively. The crystal structure was described by Naslain et al. in detail [3], here only a comparison with the equivalents in LiAlB14 and MgAlB14 will be made after Higashi’s comparison [9]. Recently Albert et al. [22] reported that NaB15 crystal structure is slightly distorted from orthorhombic symmetry to a monoclinic crystal structure with space group I1m1 (a = 585.92(3), b = 1039.92(6), and c = 833.17(5) nm; β = 90.373(5)◦ from powder data) and should be written as NaB14.5 or Na2 B29 instead of NaB15 .
Fig. 3. Crystal structure of NaAlB14 : three-dimensional perspective view.
Table 6 Comparison of lattice constants and interatomic distances with the equivalents in LiAlB14 and MgAlB14 ˚ Lattice constants (A)
˚ (M1 = Na; M2 = Al) NaAlB14 (A)
a 10.465(1) b 5.844(1) c 8.231(1) Linkage B–B bond lengths within B12 icosahedron B1–B3 1.784(1) B1–B4 1.794(2) B1–B5 1.851(2) B3–B4 1.796(2) B3–B3 1.814(2) B3–B5 1.814(2) B3–B4 1.837(2) B4–B4 1.878(2) B4–B5 1.805(2) B–B bond lengths for linkages between B12 icosahedron B4–B4 1.773(2) B5–B5 1.756(3)
˚ (M1 = Li; M2 = Al) [9] LiAlB14 (A)
˚ (M1 = Mg; M2 = Al) [21] MgAlB14 (A)
10.3542 5.8469 8.1429
10.313 5.848 8.115
1.788 1.791 1.847 1.784 1.800 1.823 1.848 1.864 1.789
1.798 1.787 1.835 1.791 1.793 1.809 1.837 1.853 1.788
1.751 1.724
1.752 1.711
B–B distances for linkages between bridge site and B12 icosahedron/bridge site B2–B3 1.754(1) 1.746 B2–B1 1.821(2) 1.834 B2–B2 2.150(2) 2.081
1.755 1.787 2.040
Metal–boron distances M1–B2 M1–B4 M1–B5 M1–B4 M1–B3 M2–B2 M2–B1 M2–B3
2.363 2.666 2.811 2.787 2.736 2.061 2.324 2.422
2.522(2) 2.672(1) 2.744(2) 2.806(1) 2.814(1) 2.084(1) 2.357(1) 2.461(1)
In this table our axis choice and boron site assignment are used.
2.422 2.641 2.744 2.785 2.771 2.081 2.339 2.434
S. Okada et al. / Journal of Alloys and Compounds 395 (2005) 231–235
However, site coordinates are not given in their paper, thus, we could not include Na2 B29 in our comparison. Actually the monoclinic distortion is so small that the comparison using Naslain’s results may not cause a miss leading of discussion but we dared not to compare NaB15 . In Table 6, lattice constants and interatomic distances of the present NaAlB14 are compared with others. NaAlB14 has larger a and c values than LiAlB14 and MgAlB14 because of larger ionic radius of Na than Li and Mg, however, b values are almost constant for all three compounds. M1 metal site is surrounded by two icosahedra and two bridge site borons in a–c plane but the void of the boron framework including the M1 metal site forms a tunnel along b-axis. Thus lattice constants of a and c are dependent on the ionic radius of M1 but b is not. The tunnel structure allows the M1 site to split into two positions along b-axis when the ionic radius of the M1 site metal is small, i.e. the Mg site of MgAlB14 [21] and the Ln site of LnAlB14 [4–7] have split positions. The inter-icosahedron bond distance increases more correspondingly to the lattice constant increase than the intra-icosahedron bond distances. The B–B ˚ for linkage between the bridge sites seems distance 2.15 A too large as a B–B bonding. The same distances 2.081 and ˚ for LiAlB14 and MgAlB14 , respectively, are also too 2.040 A large as a B–B bonding. Actually the direct B–B bonding between the bridge site boron should not be expected in these M1AlB14 structure. The dominant role of the bridge site B is to bridge three icosahedra, which form a–b plane icosahedron network. On the other hand, the distances 2.084 and ˚ between the bridge site B and Al and Na, respec2.522 A tively, are noticeably shorter than the other Al–B and Na–B distances. As discussed by Higashi [9] a similar tendency is also seen in the other crystals and both Al and Na seem to bond strongly with the bridge site B in order to mediate the connection between the a–b plane networks.
4. Conclusions The single crystals of a new ternary boride NaAlB14 were grown from a high temperature Al solution using Na2 B4 O7 and crystalline boron powders as raw materials under an Ar atmosphere. NaAlB14 is the first compound obtained in the ternary Na–Al–B system and is the first analogue of the MgAlB14 -type structure family. The crystal structure of NaAlB14 is orthorhombic with the space group Imma (number 74), and lattice constants are a = 1.0465(1), b = 0.5844(1), and c = 0.8231(1) nm; V = 0.5034(1) nm3 ; Z = 4. Crystal growth from high temperature metal solution is a very versatile method allowing variation of numerous experimental parameters and most likely boron-rich compounds
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containing Al and the alkali metals K–Cs could be synthesized in a similar method. These crystal growths are under investigation.
Acknowledgments The authors wish to thank Dr. K. Iizumi and Miss Y. Igumi of Tokyo Polytechnic University for help on the crystal growth experiments and to Mr. K. Kosuda of National Institute for Materials Science for EPMA measurements.
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