CRYSTAL GROWTH & DESIGN 2002 VOL. 2, NO. 2 83-85
Communications Synthesis and Crystal Growth of Two Novel Layered Structures, NaKLaNbO5 and Na2K2Gd4Nb2O13, in Molten Hydroxide Salts Ju-Hsiou Liao* and Ming-Chuen Tsai Department of Chemistry, National Chung Cheng University, 160 San-Hsing, Min-Hsiung, Chia-Yi 621, Taiwan Received December 4, 2001;
Revised Manuscript Received January 17, 2002
ABSTRACT: NaKLaNbO5 (1) and Na2K2Gd4Nb2O13 (2) were synthesized and crystallized by reacting La2O3 or Gd2O3 with Nb2O5 in molten NaOH/KOH hydroxide salts at 400 and 450 °C, respectively. 1 and 2 adopt related two-dimensional structures, containing anionic metal oxide slabs with alkali metal ions located between the layers. To date, various lamellar perovskite-related structures such as A[M2Nan-3Nb3O3n+1] (A ) K, Rb, Cs; M ) Ca, Pb),1-3 AMNb2O7 (A ) K, Rb, M ) La, Nd, Bi)1c,4-6 and A2Ln2Ti3O10 (A ) Na, K, Rb; Ln ) La, Nd, Sm, Gd, Dy)7,8 have been found to exhibit promising chemical and physical properties such as intercalation,3,5 luminescence,6 and photocatalysis.2,8 The synthesis of these compounds is often achieved by a high-temperature ceramic method, which often leads to the most thermodynamically stable phases. We are interested in exploring new metastable phases that are accessible only in a lower temperature regime. Molten alkali metal hydroxide salts have been described as a Lux acid-base solvent system suitable for the synthesis of metal oxides.9 In the melts, the hydroxide ions dissociate into oxide ions and water: 2OH- h O2- + H2O, where water behaves as Lux acid to dissolve metal oxide reactants: m H2O + MnOm h n M(2m/n)+ + 2m OH-. The reaction temperature of the metal oxides is significantly lowered and a homogeneous reaction media can be obtained. During the reaction, crystallization of the products may be achieved via a dissolution-precipitation process in the hydroxide melts. Herein we report the synthesis and X-ray crystal structure characterization of two novel twodimensional structures, NaKLaNbO5 (1) and Na2K2Gd4Nb2O13 (2). Compound 1 and 2 were crystallized in molten NaOH/ KOH salts at 400 and 450 °C, respectively, in high yields.10 Even though 1 can also be prepared by a conventional ceramic method at 750 °C, contamination by LaNbO4 may occur as a consequence of gradual thermal decomposition. 2 decomposes at ∼500 °C and is a thermodynamically less stable phase, which appears to be accessible only by molten salt synthesis. An isomorphous phase of 1, NaKNdNbO5 (3), and their solid solution, NaKLa1-xNdxNbO5 (x ) 0 ∼ 1), can also be prepared by either a molten salt or ceramic method. However, no suitable crystals are found for singlecrystal X-ray diffraction studies. * To whom correspondence should be addressed. E-mail: chejhl@ ccunix.ccu.edu.tw; Tel: 88652728168; Fax: 88652721040.
Table 1. Atomic Coordinates and Equivalent Isotropic Displacement Parameters for NaKLaNbO5 (1) atom
x
y
z
Ueqa
La Nb Na K O(1) O(2)
0.2500 0.2500 0.2500 0.2500 0.0226(4) 0.2500
0.7500 0.2500 0.2500 0.7500 0.4774(4) 0.2500
0.5000 0.25381(8) 0.7590(4) 0.0000 0.3240(4) 0.0329(7)
0.0053(2) 0.0044(2) 0.0072(9) 0.0218(7) 0.0097(6) 0.0146(13)
a
Ueq ) (1/3)∑i ∑jUijai*aj*aiaj.
Table 2. Atomic Coordinates and Equivalent Isotropic Displacement Parameters for Na2K2Gd4Nb2O13 (2) atom
x
y
z
Ueqa
Gd(1) Gd(2) Nb K Na O(1) O(2) O(3) O(4) O(5) O(6) O(7)
0.43745(3) 0.06232(3) 0.34955(5) 0.25074(14) 0.1591(2) 0.3682(4) 0.3643(4) 0.1299(4) 0.2651(4) 0.0000 0.4004(4) 0.5015(4)
0.04541(11) 0.03625(11) 0.0347(2) 0.0550(5) 0.0393(9) 0.2748(16) 0.2296(15) 0.3117(15) 0.0659(15) 0.248(2) -0.2286(16) 0.2535(15)
0.55081(6) 0.17211(6) 0.21785(10) 0.3666(3) 0.0224(5) 0.3605(8) 0.6229(7) 0.1371(8) 0.1230(8) 0.2500 0.6614(7) 0.4777(7)
0.0058(2) 0.0057(2) 0.0047(3) 0.0171(7) 0.0109(11) 0.0093(18) 0.0072(17) 0.0070(17) 0.0101(18) 0.006(2) 0.0081(18) 0.0057(16)
a
Ueq ) (1/3)∑i∑jUijai*aj*aiaj.
Single-crystal X-ray diffraction analysis11 reveals that 1 and 2 have 2-D frameworks. The atomic coordinates for 1 and 2 are given in Tables 1 and 2, respectively. As shown in Figure 1, 1 is constructed by LaO4 slabs comprising edgesharing cubes with LaIII filling 1/2 body-centered sites, and NbO5 square pyramids reside alternatively at one side of the LaIII cavities. Each NbO5 square pyramid edge-shares its four basal edges with four LaO8 cubes, leaving a shorter Nb-O(2) bond pointing toward the interlayered space. The structural difference of 2 from 1 can be perceived if it is formulated as Na2K2(NbO)2(Gd4O11), Figure 2, in which
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Crystal Growth & Design, Vol. 2, No. 2, 2002
Figure 1. (a) Polyhedron representation of NaKLaNbO5, viewed along the c-axis. (b) View along the a-axis. d(La-O) ) 2.527(2) Å × 8, d(Nb-O) ) 1.958(3) Å × 4, 1.829(6) Å.
double-layered Gd4O11 slabs are sandwiched by NbO5 square pyramids in similar fashion as observed in 1. Each GdIII is seven-coordinated, forming distorted cubes with one corner missing. In both 1 and 2, K+ and Na+ ions are located between the metal-oxide slabs, with the Na+ ions found nearer to the open side of those cavities. The coordination environments of Na+ ions are similar in 1 and 2. Each Na+ ion is surrounded by five oxygen atoms, in which four of them are the corners of the open side of a cavity and the other is the apical oxygen atom of an NbO5 pyramid from the adjacent layer. In 1, each K+ ion is surrounded by 12 oxygen atoms, in which eight of them are the corners of two LaO8 cubes of two adjacent layers, and the other four are apical oxygen atoms of four NbO5 pyramids. In 2, unlike 1, each K+ ion is surrounded by only 10 oxygen atoms because two oxygen atoms at the GdO7 corners are distorted away inward the [Gd3Nb2O13]4layers. In summary, utilizing molten hydroxide salts as reaction media provides a promising lower-temperature synthetic route for novel metastable compounds in crystal form. Doping of other lanthanides can also be achieved in the hydroxide melts. Acknowledgment. We thank the National Science Council of Taiwan for financial support.
Communications
Figure 2. (a) Connectivity of [Gd4Nb2O13]4-. (b) Polyhedron representation of Na2K2Gd4Nb2O13, viewed along the b-axis. d(Gd(1)-O) ) 2.414(8), 2.469(8), 2.399(8), 2.368(6), 2.351(8), 2.354(8) Å; d(Gd(2)-O) ) 2.376(8), 2.554(8), 2.404(8), 2.365(6), 2.409(8), 2.329(8), 2.334(8) Å; d(Nb-O) ) 1.987(8), 1.953(8), 1.944(8), 1.935(8), 1.836(10). Supporting Information Available: The X-ray crystallographic files in CIF format and X-ray powder diffraction files are available free of charge via the Internet at http://pubs.acs.org.
References (1) (a) Dion, M.; Ganne, M.; Tournoux, M. Mater. Res. Bull. 1981, 16, 1429-1435. (b) Jacobson, A. J.; Johnson, J. W.; Lewandowski, J. T. Inorg. Chem. 1985, 24, 3727-3729. (c) Subramanian, M. A.; Gopalakrishnan, J.; Sleight, A. W. Mater. Res. Bull. 1988, 23, 837-842. (d) Treacy, M. M. J.; Rice, S. B.; Jacobson, A. J.; Lewandowski, J. T. Chem. Mater. 1990, 2, 279-286. (2) (a) Ebina, Y.; Tanaka, A.; Kondo, J. N.; Domen, K. Chem. Mater. 1996, 8, 2534-2538. (b) Yoshimura, J.; Ebina, Y.; Kondo, J.; Domen, K. J. Phys. Chem. 1993, 97, 1970-1973. (3) (a) Hardin, S.; Hay, D.; Millikan, M.; Sanders, J. V.; Turney, T. W. Chem. Mater. 1991, 3, 977-983. (b) Ram, R. A. M.; Clearfield, A. J. Solid State Chem. 1994, 112, 288-294. (4) (a) Gopalakrishnan, J.; Bhat, V.; Raveau, B. Mater. Res. Bull. 1987, 22, 413-417. (b) Sato, M.; Abo, J.; Jin, T.; Ohta, M. J. Alloy. Compd. 1993, 192, 81-83. (5) Matsuta, T.; Fujita, T.; Miyamae, N.; Takeuchi, M.; Kunou, I. J. Mater. Chem. 1994, 4, 955-958. (6) Kudo, A. Chem. Mater. 1997, 9, 664-449. (7) Gopalakrishnan, J.; Bhat, V. Inorg. Chem. 1987, 26, 42994301. (8) (a) Ikeda, S.; Hara, M.; Kondo, J. N.; Domen, K.; Takahashi, H.; Okubo, T.; Kakihana, M. Chem. Mater. 1998, 10, 7277. (b) Takata, T.; Furumi, Y.; Shinohara, K.; Tanaka, A.; Hara, M.; Kondo, J. N.; Domen, K. Chem. Mater. 1997, 9, 1063-1064.
Communications (9) (a) Keller, S. W.; Carlson, V. A.; Stanford, D.; Stenzel, F.; Stacy, A. M.; Kwei, G. H.; Alario-Franco, M. J. Am. Chem. Soc. 1994, 116, 8070-8076. (b) Stoll, S. L.; Stacy, A. M.; Torardi, C. C. Inorg. Chem. 1994, 33, 2761-2765. (c) Sandford, D.; Marquez, L. N.; Stacy, A. M. Appl. Phys. Lett. 1995, 67, 422-423. (d) Etheredge, K. M. S.; Gardberg, A. S.; Hwu, S.-J. Inorg. Chem. 1996, 35, 6358-6361. (e) Luce, J. L.; Stacy, A. M.Chem. Mater. 1997, 9, 1508-1515. (f) Reisner, B. A.; Stacy, A. M. J. Am. Chem. Soc. 1998, 120, 9682-9683. (10) 1: A mixture of La2O3 (0.163 g), Nb2O5 (0.133 g), NaOH (6.00 g), and KOH (8.40 g) in a molar ratio 1:1:300:300 was heated (rate ) 90 °C/h) in a platinum crucible at 400 °C for 35 h. Upon cooling at a rate of 60 °C/h, 0.343 g of brownish white powder and rectangular thin plates, in a yield of 91.7% based on La and Nb, were obtained by washing excess flux with water. 1 can also be synthesized quantitatively in powder form by heating stoichiometric amount of La2O3, Nb2O5, Na2CO3, and K2CO3 at 750 °C for 6 h. It should be noted that elongated reaction time or elevated temperatures would result in the decomposition of 1 to LaNbO4. 2: A mixture of Gd2O3 (0.362 g), Nb2O5(0.133 g), NaOH (2.00 g), and KOH (2.80 g) in a molar ratio 2:1:100:100 was heated (rate ) 200 °C/h) in a platinum crucible at 450 °C for 10 h. Upon cooling at a rate of 70 °C/h, 0.573 g of yellowish white powder and rectangular thin plates, in a yield of 95.2%
Crystal Growth & Design, Vol. 2, No. 2, 2002 85 based on La and Gd, were obtained by removing excess flux with water. The identity of bulk products is confirmed by the comparison of experimental XRPD pattern and the calculated pattern based on single-crystal X-ray data. (11) Crystal structure analysis: The data were collected on a Bruker P4 diffractometer with graphite monochromated MoKR radiation (λ ) 0.71073 Å) at 50 kV and 40 mA. Crystals dimensions: 0.05 × 0.15 × 0.15 mm (1) and 0.04 × 0.06 × 0.10 mm (2). Crystal data for 1: space group: tetragonal P4/nmm with a ) b ) 5.8135(4) Å, c ) 8.2804(4) Å, V ) 279.85(3) Å3; Z ) 2; Fcalcd ) 4.437 g cm-3; µ(MoKR) ) 10.333 mm-1. Of the 730 reflections collected (2.46 e θ e 29.96o), 272 reflection are unique and 263 reflections are observed (I > 2σ(I)). No. of variables ) 20; final R1/wR2 (all data) ) 0.0250/0.0685; goodness-of-fit (F2) ) 1.162. Crystal data for 2: space group: monoclinic C2/c with a ) 24.060(3) Å, b ) 5.6464(6) Å, c ) 11.181(1) Å, β ) 116.40(2)o, V ) 1360.6(3) Å3; Z ) 4; Fcalc ) 5.600 g cm-3; λ(MoKR) ) 21.584 mm-1. Of the 1555 reflections collected (1.89 e θ e 24.98°), 1181 reflections are unique and 963 reflections are observed (I > 2σ(I)). No. of variables ) 72; final R1/ wR2 (all data) ) 0.0490/0.0918; goodness-of-fit (F2) ) 1.026. The structures were solved with direct method (SHELXS97), and were refined by a full-matrix least-squares on F2 (SHELXL-97).
CG0155705
