Inorg. Chem. 2005, 44, 4421−4426
Ca1-xNa2xAl2B2O7: A Structure with Tunable Density of Na+ Vacancies Meng He,* Hiroki Okudera, and Arndt Simon Max Planck Institute for Solid State Research, Heisenbergstrasse 1, D-70569 Stuttgart, Germany Received January 19, 2005
A series of samples with the composition Ca1-xNa2xAl2B2O7 (0 < x e 1) was investigated and a hexagonal structure with unusually large range of homogeneity (at least from x ) 0.01 to 0.95) was revealed. The hexagonal phase consists of [Al2B2O7]∞2- lamellae stacked along the c axis, as in CaAl2B2O7 and Na2Al2B2O7. Nevertheless, the configuration and stacking sequence of the [Al2B2O7]∞2- lamellae are different in these three structures. In the hexagonal structure of Ca1-xNa2xAl2B2O7, Ca and half Na cations (Na1) statistically occupy the same crystallographic site which is located between the [Al2B2O7]∞2- lamellae, the other half Na cations (Na2) distribute in the planes bisecting the [Al2B2O7]∞2- lamellae. Depending on the composition, the site occupation factor of Na2 site can vary in the same range as x, leading to a tunable density of Na+ vacancies in the structure. The AlO4 tetrahedra and BO3 triangles in the structure tilt in appropriate ways to improve the bond valence sum of Na2 cations which are not sufficiently bonded to the anions.
Introduction CaAl2B2O7 was first reported by Scha¨fer and Kuzel1 in their study of the ternary system CaO-Al2O3-B2O3. Two modifications were observed in their investigation: a monoclinic phase believed to be stable at high temperature and a hexagonal phase (P6322, a ) 4.81 Å, c ) 15.55 Å) formed upon prolonged heating at 830 to 900 °C. Later, Chang and Keszler2 solved the structure of CaAl2B2O7 in space group R3hc (No. 167) with lattice parameters a ) 4.810(6) Å and c ) 46.633(5) Å, the latter being about 3 times that of the so-called low-temperature phase. Chang and Keszler also found that a very low level of Na+ impurities leads to a completely different powder pattern, which is, interestingly, consistent with the data reported for the low-temperature phase. Na2Al2B2O7 crystallizes in space group P3h1c (No. 163) with lattice constants a ) 4.8010(4) Å and c ) 15.2425(16) Å.3 Both CaAl2B2O7 and Na2Al2B2O7 consist of [Al2B2O7]∞2lamellae stacked along the c axis. However, the configuration and the stacking sequence of [Al2B2O7]∞2- lamellae in these two compounds are different. For details see our recent publication.3 * Author to whom correspondence should be addressed. Fax: +49 711 689 1091. E-mail:
[email protected]. (1) Scha¨fer, V. L.; Kuzel, H. J. Neues Jahrb. Mineral. Monatsh. 1967, 131-136. (2) Chang, K. S.; Keszler, D. A. Mater. Res. Bull. 1998, 33, 299-304. (3) He, M.; Kienle, L.; Simon, A.; Chen X. L.; Duppel, V. J. Solid State Chem. 2004, 177, 3212-3218.
10.1021/ic0500863 CCC: $30.25 Published on Web 05/18/2005
© 2005 American Chemical Society
By citing Chang’s Ph.D. dissertation, Chang and Keszler2 stated that the Na+-containing CaAl2B2O7 crystallized in space group P6322, and the stacking of [Al2B2O7]∞2- lamellae in this phase was ABAB... rather than ABCABC... as in pure CaAl2B2O7. However, details were not given. We reinvestigated the series Ca1-xNa2xAl2B2O7 and present our results here. Experimental Section Sample Preparation. Polycrystalline samples with the composition Ca1-xNa2xAl2B2O7 (x ) 0.01, 0.32, 0.90, 0.95, 0.99, and 1.00) were prepared by solid-state reaction using CaCO3, Na2CO3, Al2O3, and H3BO3 in appropriate ratios as starting materials. The mixtures were first preheated at 500 °C for 20 h, then annealed at 900 °C (for x ) 0.01 and 0.32) or 800 °C (for x ) 0.90, 0.95, 0.99, and 1.00) for 4 days, and finally cooled to room temperature in the furnace with the power turned off. Two batches of single crystals were grown. For the first batch, polycrystalline Ca0.99Na0.02Al2B2O7 and NaBO2 prepared in advance were used as starting materials. The mixture of Ca0.99Na0.02Al2B2O7 and NaBO2 in a molar ratio 3:2 was melted in a Pt crucible and then allowed to cool slowly. The detailed heating scheme was as follows. The temperature was raised to 1050 °C, held for 5 h, then decreased to 950 °C at a rate of 2 °C/h, then to 800 °C in 50 h, and at last to room temperature in the furnace with the power turned off. For the second batch, polycrystalline Ca0.05Na1.90Al2B2O7 prepared in advance was heated to 1020 °C, held at this temperature for 2 h, then cooled to 950 °C in 50 h, and to 850 °C at a rate of 2 °C/h. Then the power was turned off and the sample was cooled to room temperature in the furnace. Transparent, platelike single Inorganic Chemistry, Vol. 44, No. 12, 2005
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He et al. Table 1. Crystallographic Data and Details of Structure Determination for Ca1-xNa2xAl2B2O7 (x ) 0.32 and 0.95) formula Mw (g mol-1) space group a (Å) c (Å) V (Å3) Z Dx (mg m-3) µ (mm-1) diffractometer radiation crystal crystal size (mm3) T (K) scan type absorption correction Tmin, Tmax measured reflns independent reflns reflns with I > 2σ(I) Rint Rσ θmax index range reflns used in refinements refined params R[F2 > 2σ(F2)] wR(F2) S weighting scheme (∆/σ)max ∆Fmax, ∆Fmin (eÅ-3) twin laws fraction of twin component
Ca0.69(1)Na0.64(3)Al2B2O7 (1) 230.02 P63/m (No. 176) 4.8238(5) 15.472(3) 311.79(7) 2 2.450 1.069 Stoe IPDS II Mo KR transparent, platelike 0.120 × 0.080 × 0.024 293(2) w numerical 0.8971, 0.9683 3735 375 347 0.0349 0.0164 31.89 -6 e h e 7, -7 e k e 7, -23 e l e 23 375 29 0.0345 0.0867 1.095 ω ) 1/[σ2(Fo2) + (0.0555P)2 + 0.1724P], where P ) (Fo2 + 2Fc2)/3
