Chem. Mater. 2003, 15, 1099-1104
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Crystal Structure of La4Si2O7N2 Analyzed by the Rietveld Method Using the Time-of-Flight Neutron Powder Diffraction Data Junichi Takahashi,*,† Hisanori Yamane,† Naoto Hirosaki,‡ Yoshinobu Yamamoto,‡ Takayuki Suehiro,‡ Takashi Kamiyama,§ and Masahiko Shimada† Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai 980-8577, Japan, National Institute for Materials Science, Tsukuba 305-0044, Japan, and High Energy Accelerator Research Organizations, Tsukuba 305-0801, Japan Received September 17, 2002. Revised Manuscript Received December 9, 2002
The polycrystalline single-phase sample of La4Si2O7N2 was prepared at 1773 and 1873 K by the gas-pressured sintering method. The structure refinement was carried out by the Rietveld method using time-of-flight neutron powder diffraction data measured at 297 K. La4Si2O7N2 is isostructural with the Y4Al2O9 high-temperature phase and crystallizes in a monoclinic cell, P21/c (No. 14-1), Z ) 4. The lattice parameters refined for the sample prepared at 1773 K were a ) 8.0375(2) Å, b ) 10.9900(2) Å, c ) 11.1115(2) Å, and β ) 110.9214(14)°. Nitrogen atoms statistically occupy the bridging site and the terminal sites of Si2O5N2 ditetrahedra in the La4Si2O7N2 structure. The nonmetal sites surrounded by La atoms only are fully occupied by oxygen atoms.
Introduction Silicon nitride (Si3N4) ceramics are well-known hightemperature structural materials and are used for engine components. Because of the covalent nature and low self-diffusion rate in Si3N4, it is difficult to densify the ceramics by solid-state sintering without sintering additives. Densification is achieved by liquid-phase sintering with rare-earth oxide additives. During the sintering process, rare-earth silicon oxynitrides are formed by the reaction of Si3N4, SiO2, and rare-earth oxide additives at the grain boundary. Since the grain boundary phases affect the high-temperature mechanical and chemical properties of the sintered-Si3N4 ceramics, many ceramists have investigated the chemistry of rare-earth silicon oxynitrides. A series of rare-earth silicon oxynitrides Ln4Si2O7N2 (Ln ) Y and lanthanoid, La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu), which consist of Si2(O,N)7 ditetrahedra and Ln(O,N)x (x ) 6-8) polyhedra, has been considered to have the cuspidine (Ca4Si2O7F2)-type structure with a monoclinic cell.1-5 Recently, Lu4Si2O7N2 was found as a grain boundary phase of Si3N4 ceramics with the high bending strength of 700 MPa and with * Corresponding author. Phone: (+81) 22-217-5161. Fax: (+81) 22217-5160. E-mail:
[email protected]. † Tohoku University. ‡ National Institute for Materials Science. § High Energy Accelerator Research Organizations. (1) Wills, R. R.; Stewart, R. W.; Cunningham J. A.; Wimmer, J. M. J. Mater. Sci. 1976, 11, 749. (2) Wills, R. R.; Holmquist, S.; Wimmer, J. M.; Cunningham, J. A. J. Mater. Sci. 1976, 11, 1305. (3) Marchand, M. M. R.; Jayaweera, A.; Verdier, P.; Lang, J. C. R. Acad. Sci. Paris, Ser. C 1976, 283, 675. (4) Montorsi, M.; Appendino, P. Am. Ceram. Soc. Bull. 1979, 58, 789. (5) Montorsi, M.; Appendino, P. J. Less-Common Met. 1979, 68, 193.
the excellent oxidation resistance at 1773 K.6,7 We prepared a single phase of Lu4Si2O7N2 and refined the structural parameters including oxygen/nitrogen (O/N) distribution in the nonmetal sites by the Rietveld analysis using the time-of-flight (TOF) neutron and the X-ray powder diffraction (XRPD) data.8,9 The results of our study pointed out the errors of lattice parameters and the miss-indexing of the X-ray diffraction peaks reported in the previous study.5 For the other end member of the lanthanoid silicon oxynitrides, La4Si2O7N2, there are some reports of the lattice parameters and the unit-cell volume.1,3,10,11 However, the values are scattered in particular for the unitcell volume. In addition, the atomic positional parameters of La4Si2O7N2 had not been reported. These situations motivated us to investigate the crystallographic data of La4Si2O7N2. In the present study, a single phase of La4Si2O7N2 was prepared, and the crystal structure was determined by the Rietveld method using the TOF neutron powder diffraction data. It was clarified that La4Si2O7N2 is not isostructural with the cuspidine-type structure of Lu4Si2O7N28 and the low-temperature phase of Y4Al2O912 (LT-Y4Al2O9). The crystal structure of La4Si2O7N2 could (6) Guo, S.; Hirosaki, N.; Yamamoto, Y.; Nishimura, T.; Mitomo, M. Scr. Mater. 2001, 45, 867. (7) Guo, S.; Hirosaki, N.; Yamamoto, Y.; Nishimura, T.; Mitomo, M. J. Am. Ceram. Soc. 2002, 85, 1607. (8) Takahashi, J.; Yamane, H.; Shimada, M.; Yamamoto, Y.; Hirosaki, N.; Mitomo, M.; Oikawa, K.; Torii, S.; Kamiyama, T. J. Am. Ceram. Soc. 2002, 85, 2072. (9) Takahashi, J.; Yamane, H.; Yamamoto, Y.; Hirosaki, N.; Mitomo, M.; Oikawa, K.; Torii, S.; Kamiyama, T.; Shimada, M. Key Eng. Mater. 2003, 237-237, 53. (10) Ii, N.; Mitomo, M.; Inoue, Z. J. Mater. Sci. 1980, 15, 1691. (11) Mitomo, M.; Izumi, F.; Horiuchi, S.; Matsui, Y. J. Mater. Sci. 1982, 17, 2359.
10.1021/cm020930c CCC: $25.00 © 2003 American Chemical Society Published on Web 02/11/2003
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Takahashi et al. K h-1 under 10-2 Pa in a vacuum from room temperature to 1073 K. At this temperature, a nitrogen gas (99.999% purity) was introduced in the furnace. Then the sample was reheated at the same heating rate and maintained at 1773 or 1873 K for 8 h under a nitrogen gas pressure of 1 MPa. After heating, the sample was cooled at a constant rate of 600 K h-1 to 1473 K and then cooled slowly without temperature control in the furnace. The phase purity of the products was confirmed by XRPD. Diffraction Measurements. The TOF neutron powder diffraction measurements were performed using VEGA at the KEK neutron scattering facility (KENS) of the High Energy Accelerator Research Organization (KEK). The La4Si2O7N2 powder was placed in a vanadium sample tube of 9.2 mm in diameter and 42 mm in height. The height of the powder was 13 and 8 mm for the sample synthesized at 1773 and 1873 K, respectively. Data were collected at ambient temperature over a d spacing range from 0.5 to 3.1 Å (constant resolution ∆d/d ∼ 3 × 10-3) with a bank of 3He counters situated at a diffraction angle of 2θ ) 145-175° in a backscattering geometry. The acquisition time was on the order of 13.5 and 15.0 h for the sample synthesized at 1773 and 1873 K, respectively. The collected data were normalized for the measured incident spectrum using the scattering from the standard vanadium sample. The Rietveld refinement was performed using the computer program RIETAN-2001T.14
Results and Discussion
Figure 1. Projection of crystal structure on the a-b plane (z ) -0.1 to 0.6). (a) HT-Y4Al2O9 and La4Si2O7N2; (b) LT- Y4Al2O9 and Lu4Si2O7N2.
be analyzed by using the structure of the Y4Al2O9 hightemperature phase (HT-Y4Al2O9)13 as a starting model. Both HT- and LT-Y4Al2O9 structures have the same space group, monoclinic P21/c. As illustrated in Figure 1, the difference between the structures is briefly explained by the shift of a/2 along the a axis direction for all the atoms in the half unit cell along the unique b axis. Experimental Section The starting materials used were R-Si3N4 (SN-E10, UBE Industries, Tokyo, Japan), SiO2 (99.9%, Kojundo Chemical Lab. Co. Ltd., Sakado, Japan), and La2O3 (99.9%, Shin-Etsu Chemical Co. Ltd., Tokyo, Japan). The La2O3 powder was preheated at 1273 K in air for 24 h. The 10 g of powders weighed in a molar ratio of Si3N4:SiO2:La2O3 ) 1:1:4 (0.9898:0.3424:8.6678 g) was mixed in a silicon-nitride ball-mill for 2 h with 100 mL of ethanol, followed by slurry-mixture drying in a rotary evaporator. Two grams of the dried powder mixture was diepressed into a 13-mm φ pellet under 20 MPa and put into a high-purity boron nitride crucible of 20 mm in diameter and 20 mm in height. The synthesis was carried out using a gaspressure sintering furnace (FVPHR-R-10, FRET-40, Fujidempa Kogyo Co. Ltd., Osaka, Japan) with a graphite heater. The sample was heated at a constant heating rate of 600 (12) Christensen, A. N.; Hazell, R. G. Acta Chem. Scand. 1991, 45, 226. (13) Yamane, H.; Shimada, M.; Hunter B. A. J. Solid State Chem. 1998, 141, 466.
The XRPD results of samples synthesized at both 1773 and 1873 K showed that the phase other than the target substance La4Si2O7N2 was not detected. In the structural refinement using the TOF neutron powder diffraction pattern of La4Si2O7N2, the atomic coordinates of LT-Y4Al2O9 (monoclinic, space group P21/c)12 having the cuspidine-type structure was used as a starting model. The refinement with this model was not satisfied well because of relatively high R values, Rwp ) 6.52, S ) Rwp/Re ) 2.8478, RI ) 2.97, and RF ) 2.49 for the sample prepared at 1773 K and Rwp ) 6.82, S ) Rwp/Re ) 2.9539, RI ) 4.14, and RF ) 2.94 for the sample prepared at 1873 K. The Si-(O,N) bond lengths were dispersed (1.39(1)-1.96(2) Å) and two Si(O,N)4 tetrahedra in Si2(O,N)7 ditetrahedra showed large asymmetric distortion. These facts indicated the LT-Y4Al2O9 structure model was inappropriate. When we adopted the HT-Y4Al2O9 structure model (monoclinic, P21/c),13 R-factors decreased significantly as shown in Table 1. Table 1 also lists the refined cell parameters and the unit-cell volume of La4Si2O7N2 at room temperature. The cell parameters and volumes of La4Si2O7N2 prepared at 1773 and 1873 K agreed with each other within 3σ except the b axis length with a slight difference within 4σ, where σ is the standard deviation of the refined values and shown in the parentheses in the table. The observed TOF neutron powder diffraction pattern of La4Si2O7N2 synthesized at 1773 K is shown in Figure 2, together with the calculated profile with the background curve and the difference patterns of the Rietveld analysis. The O/N occupancies of the nonmetal sites, atomic coordinates, and isotropic displacement parameters are listed in Table 2. The O/N occupancies were refined under the following conditions: (1) the initial values of O/N occupancies at each nonmetal site were 7/9 and 2/9, respectively, (2) the sum of the oxygen and (14) Izumi, F. The Rietveld Method; Oxford University Press: Oxford, 1995.
Crystal Structure of La4Si2O7N2
Chem. Mater., Vol. 15, No. 5, 2003 1101
Table 1. Refined Cell Dimensions and R Factors Resulted from the Rietveld Analysis for TOF (Time-of-Flight) Neutron Powder Diffraction Pattern of La4Si2O7N2 formula symmetry space group synthesis, T (K) measurement, T (K) a (Å) b (Å) c (Å) R (deg) β (deg) γ (deg) V (Å3) Z Rwp (%) Rp (%) Re (%) S RI (%) RF (%)
La4Si2O7N2 monoclinic P21/c (No. 14, first choice) 1773 1873 297 297 8.0375(2) 10.9900(2) 11.1115(2) 90.0 110.9214(14) 90.0 916.79(3) 4 3.22 2.61 2.29 1.4059 1.15 1.08
8.0367(2) 10.9925(3) 11.1095(3) 90.0 110.920(2) 90.0 916.74(4) 4 3.52 2.75 2.31 1.5233 1.29 1.14
nitrogen occupancies at a site was constrained to be unity, and (3) total numbers of O and N atoms were constrained to be 7 and 2, respectively. The atomic coordinates for La4Si2O7N2 synthesized at 1773 and at 1873 K were in good agreement each other within σ. The O/N occupancies and isotropic displacement parameters for both samples also agreed well within 3σ. The selected interatomic distances and bond angles calculated from the refined atom positions are shown in Tables 3 and 4, respectively. The interatomic distances of Si-(O,N) resulted in the range of 1.635(7)1.714(7) Å, which were comparable with the Si-(O,N) lengths of other silicon oxynitrides analyzed by the neutron diffraction method: Si2ON2,15,16 1.647-1.758 Å; Y2Si3O3N4,17 1.661(8)-1.714(7) Å; Lu4Si2O7N2,8 1.625(6)1.697(5) Å. The La-(O,N)x interatomic distances in La4Si2O7N2 (2.298(6)-2.903(5) Å) were close to those in Lacontained oxides, oxynitrides, and nitrides: La2O3,18,19 2.372-2.841 Å; La2Si2O7,20-22 2.284-3.053 Å; La4.67(SiO4)3O,23 2.290-2.792 Å; La10(SiO4)6N2,24 2.3162.843 Å; LaSiO2N,25 2.453-2.618 Å; LaN,26,27 2.6512.652 Å. The structure of La4Si2O7N2 prepared at 1773 K is illustrated in Figure 3. In this figure, the O and N occupancies are represented with white and black areas, respectively. It is clearly seen from Figure 3 that the O/N atoms are distributed in the nonmetal sites with different occupancies. The occupancy of oxygen refined (15) Billy, M.; Labbe, J. C.; Selvaraj, A.; Roult, G. Mater. Res. Bull. 1980, 15, 1207. (16) Baraton, M. I.; Billy, M.; Labbe, J. C.; Quintard, P.; Roult, G. Mater. Res. Bull. 1988, 23, 1087. (17) Wang, P.-L.; Werner, P.-E.; Gao, L.; Harris, R. K.; Thompson, D. P. J. Mater. Chem. 1997, 7, 2127. (18) Koehler, W. C.; Wollan, E. O. Acta Crystallogr. 1953, 6, 741. (19) Aldebert, P.; Traverse, J. P. Mater. Res. Bull. 1979, 14, 303. (20) Dago, A. M.; Pushcharovskii, D. YU.; Strelkova, E. E.; Pobedimskaya, E. A.; Belov. N. V. Dokl. Akad. Nauk SSSR 1980, 252, 1117. (21) Greis, O.; Bossemeyer, H. G.; Greil, P.; Breidenstein, B.; Haase, A. Mater. Sci. Forum 1991, 79, 803. (22) Christensen, A. N. Z. Kristallogr. 1994, 209, 7. (23) Kuz’min, E. A.; Belov, A. N. V. Dokl. Akad. Nauk SSSR 1965, 165, 88. (24) Titeux, S.; Gervais, M.; Verdier, P.; Laurent, Y. Mater. Sci. Forum 2000, 325-326, 17. (25) Morgan, P. E. D.; Carroll, P. J. J. Mater. Sci. 1977, 12, 2343. (26) Olcese, G. L. J. Phys. 1979, 9, 569. (27) Ettmayer, P.; Waldhart, J.; Vendl, A. Monatsh. Chem. 1979, 110, 1109.
for the O/N1 site (bridging site), which connects two Si(O,N)4 tetrahedra, was 6-10%. The terminal sites of the Si2(O,N)7 ditetrahedra are grouped into three pairs: O/N2 and O/N5 sites with a nitrogen occupancy of about 10%, O/N3 and O/N6 sites with almost equal occupation (50%) of O and N atoms, and O/N4 and O/N7 sites with full occupation (100%) of oxygen atoms. The pairs of the terminal sites (O/N2-O/N5, O/N3-O/N6, and O/N4O/N7) having the similar occupancies are face to face along the a axis in the Si2(O,N)7 ditetrahedra. The ionic sites of O/N8 and O/N9, which are connected to La atoms only, are fully occupied by oxygen atoms. The O/N distribution among the different coordination environments (the bridging, terminal, and ionic sites) mostly follows the Pauling’s second crystal rule.28 Since the two ionic sites, O/N8 and O/N9, are fully occupied by oxygen atoms, all nitrogen atoms bond to Si atoms in the Si2(O,N)7 ditetrahedra. This leads to a mean composition of Si2(O5N2) for the ditetrahedra in La4Si2O7N2. Magic-angle spinning nuclear magnetic resonance (MAS NMR) measurements did not confirm the existence of Si-O4, Si-ON3, and Si-N4 tetrahedra,29-31 indicating the presence of Si-O3N and Si-O2N2 tetrahedra as local units. From the O/N occupancies shown in Table 2, the mean compositions of the Si-centered tetrahedra can be expressed, not as Si-O3N or Si-O2N2, but as Si-O2.6N1.4 for Si1 and SiO2.5N1.5 for Si2. These facts imply that the composition of the ditetrahedra varies locally while satisfying the mean, Si2(O5N2), in the whole crystal. When the two local units (Si-O3N and Si-O2N2) are applied to the Si environment, the six local structures of ditetrahedra can be drawn as follows: (1) O3≡Si-N-Si≡O3 (Si2O6N), (2) O2N≡Si-N-Si≡O3 (Si2O5N2), (3) O2N≡Si-N-Si≡O2N (Si2O4N3), (4) O2N≡Si-O-Si≡O2N (Si2O5N2), (5) ON2≡Si-O-Si≡O2N (Si2O4N3), and (6) ON2≡Si-OSi≡ON2 (Si2O3N4). There are four compositions for the ditetrahedra: Si2O6N, Si2O5N2, Si2O4N3, and Si2O3N4. The nitrogen occupancy of 90-94% in the bridging site (O/N1) reveals that the local structures of (1), (2), and (3) mainly exist in La4Si2O7N2. If the O and N atoms are ordered in La4Si2O7N2, the local structure of Si2(O5N2) ditetrahedra is restricted to be O2N≡Si-NSi≡O3 or O2N≡Si-O-Si≡O2N. After the discussion of local structures with the NMR data and the Pauling’s electrostatic rule, Harris and Bodart32 described that the true structure of the ditetrahedra was O2N≡Si-NSi≡O3. However, to explain the disordered structure of Si2(O5N2) ditetrahedra with the statistical O/N occupation in the nonmetal sites, a possible assortment of atomic arrangements should be enumerated. Both La4Si2O7N2 (HT-Y4Al2O9) and Lu4Si2O7N2 (LTY4Al2O9) have the same space group (monoclinic, P21/c), and all atoms are situated at the general position (4e) in the structure. The position of the screw axis, however, differs in both structures, as shown in Figure 1. Figure 4 compares atomic configurations around the (28) Morgan, P. E. D. J. Mater. Sci. 1986, 21, 4305. (29) Dupree, R.; Lewis, M. H.; Smith, M. E. J. Am. Chem. Soc. 1989, 111, 5125. (30) Harris, R. K.; Leach, M. J.; Thompson, D. P. Chem. Mater. 1989, 1, 336. (31) Harris, R. K.; Leach, M. J.; Thompson, D. P. Chem. Mater. 1992, 4, 260. (32) Harris, R. K.; Bodart, P. R. Mater. Sci. Forum 2000, 325-326, 305.
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Figure 2. TOF neutron powder diffraction pattern and Rietveld refinement profile of La4Si2O7N2. Observed (+), calculated (solid line), background (broken line), peak positions (La4Si2O7N2 and vanadium holder), and differences. Table 2. Refined Coordinates and Isotropic Displacement Parameters from TOF Neutron Powder Diffraction Data of La4Si2O7N2 (Upper, Synthesized at 1773 K; Lower, Synthesized at 1873 K) site
occupancy
x
y
z
Biso (Å2)
Lal
4e
La2
4e
La3
4e
La4
4e
Sil
4e
Si2
4e
O/N1
4e
O/N2
4e
O/N3
4e
O/N4
4e
O/N5
4e
O/N6
4e
O/N7
4e
O/N8
4e
O/N9
4e
1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 0.06(1)O/0.94(1)N 0.11(1)O/0.89(1)N 0.95(1)O/0.05(1)N 0.94(2)O/0.06(2)N 0.64(2)O/0.36(2)N 0.64(2)O/0.36(2)N 0.98(2)O/0.02(2)N 0.96(2)O/0.04(2)N 0.88(2)O/0.12(2)N 0.86(2)O/0.14(2)N 0.54(2)O/0.46(2)N 0.51(2)O/0.49(2)N 0.97(2)O/0.03(2)N 0.99(2)O/0.01(2)N 0.98(2)O/0.02(2)N 1.02(2)O/-0.02(2)N 1.00(1)O/0.00(1)N 0.97(2)O/0.03(2)N
0.5887(5) 0.5894(6) 0.0842(5) 0.0831(6) 0.2780(5) 0.2786(6) 0.7811(5) 0.7816(6) 0.9808(8) 0.9799(10) 0.4019(8) 0.4015(9) 0.1887(6) 0.1888(7) 0.9731(6) 0.9737(7) 0.9445(6) 0.9456(7) 0.8432(6) 0.8427(7) 0.5218(7) 0.5226(8) 0.4731(6) 0.4732(7) 0.3939(6) 0.3930(7) 0.3199(8) 0.3190(10) 0.8245(8) 0.8250(10)
0.1228(4) 0.1226(4) 0.1281(4) 0.1285(4) 0.0851(4) 0.0849(3) 0.0692(4) 0.0694(3) 0.1957(7) 0.1974(8) 0.1959(7) 0.1962(8) 0.2167(2) 0.2165(3) 0.2122(5) 0.2123(6) 0.0508(4) 0.0507(4) 0.2389(5) 0.2384(5) 0.2101(6) 0.2102(7) 0.0564(4) 0.0561(4) 0.2372(5) 0.2377(5) 0.0050(4) 0.0050(5) 0.0052(4) 0.0062(5)
0.4203(4) 0.4198(4) 0.4263(4) 0.4264(4) 0.8048(4) 0.8057(4) 0.8214(4) 0.8214(3) 0.1288(6) 0.1284(7) 0.1293(6) 0.1290(7) 0.1263(4) 0.1264(3) 0.7481(4) 0.7478(4) 0.1589(4) 0.1593(4) 0.9863(4) 0.9863(5) 0.7443(4) 0.7442(5) 0.1719(4) 0.1721(4) 0.9860(4) 0.9856(5) 0.3916(5) 0.3919(6) 0.3948(5) 0.3940(6)
0.11(6) 0.17(7) 0.36(6) 0.35(7) 0.66(7) 0.53(8) 0.33(6) 0.50(8) 0.18(11) 0.52(15) 0.20(12) 0.13(13) 1.36(8) 1.42(9) 0.35(11) 0.52(13) 0.57(9) 0.52(11) 0.61(13) 0.71(16) 1.25(13) 1.61(16) 0.47(9) 0.61(11) 0.63(13) 0.61(15) 0.54(11) 0.43(13) 0.63(12) 0.85(15)
atom
Si2(O5N2) ditetrahedra in La4Si2O7N2 and Lu4Si2O7N2. The structure of Lu4Si2O7N was drawn with the crystallographic data given in ref 8. It can be seen from Figure 4 that La4Si2O7N2 and Lu4Si2O7N2 have similar atomic configurations. The lattice parameters, cell volume, and average La-(O,N)x distances of La4Si2O7N2 are respectively larger than those of Lu4Si2O7N2 (a ) 7.4243(1) Å, b ) 10.2728(1) Å, c ) 10.6628(2) Å, β ) 109.773(1)°, V ) 765.28(2) Å3; the average Lu-(O,N)x distances: 2.323 Å (Lu1), 2.463 Å (Lu2), 2.352 Å (Lu3), and 2.269 Å (Lu4)).8 This is consistent with the fact that the ionic radius of La3+ (crystal radius with 8-fold coordination, CR ) 1.300 Å) is bigger than that of Lu3+ (CR ) 1.117 Å).33 The Si1-O/N1 and Si2-O/N1 distances in La4Si2O7N2 are respectively 0.025 and 0.020 Å longer than those in Lu4Si2O7N2. The elongation of Si-O/N1 (33) Shannon, R. D. Acta Crystallogr. 1976, A32, 751.
lengths in La4Si2O7N2 may be due to the volume effect of the larger La3+ ions. It is to be noted that the La4O/N1 distance (3.188 Å) in La4Si2O7N2 is shorter than the corresponding Lu3-O/N1 distance (3.618 Å) in Lu4Si2O7N2. The O/N1 site in La4Si2O7N2 is surrounded by two Si (Si1 and Si2) and three La (La2, La3, and La4) atoms, leading to a semblance of 5-fold coordination, whereas two Si and two Lu atoms coordinate the O and N atoms in the O/N1 site of Lu4Si2O7N2. The short La4-O/N1 distance causes the larger Si1-O/N1Si2 bond angle in La4Si2O7N2. Conclusions We have shown that a rare-earth silicon oxynitride, La4Si2O7N2, possesses the high-temperature Y4Al2O9 structure. The crystal structure of La4Si2O7N2 is related to the structure of Lu4Si2O7N2 isostructural with cus-
Crystal Structure of La4Si2O7N2
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Table 3. Interatomic Distances (Å) in La4Si2O7N2 (d < 3.0 Å) La1-O/N9 La1-O/N8 La1-O/N8 La1-O/N4 La1-O/N7 La1-O/NS La1-O/N6
2.393(7)a 2.386(8)b 2.405(6) 2.404(7) 2.440(7) 2.451(8) 2.441(6) 2.440(7) 2.483(5) 2.492(6) 2.596(7) 2.594(8) 2.681(5) 2.673(6)
La2-O/N9 La2-O/N9 La2-O/N8 La2-O/N2 La2-O/N1 La2-O/N4 La2-O/N7 La2-O/N3
La3-O/N9 La3-O/N3 La3-O/N6 La3-O/N7 La3-O/N5 La3-O/N2 La3-O/N1 Si1-O/N4 Si1-O/N3 Si1-O/N2 Si1-O/N1 a
2.298(6) 2.302(7) 2.471(6) 2.474(7) 2.473(5) 2.468(6) 2.525(6) 2.520(7) 2.668(7) 2.677(8) 2.693(7) 2.693(8) 2.859(4) 2.868(5)
La4-O/N8
1.646(6) 1.634(7) 1.673(8) 1.691(9) 1.686(7) 1.671(8) 1.697(7) 1.701(9)
Si2-O/N7
La4-O/N6 La4-O/N5 La4-O/N3 La4-O/N2 La4-O/N4
Si2-O/N6 Si2-O/N5 Si2-O/N1
2.366(6) 2.380(7) 2.404(7) 2.389(8) 2.468(7) 2.471(9) 2.556(6) 2.556(7) 2.688(5) 2.685(5) 2.693(6) 2.690(7) 2.765(6) 2.762(7) 2.903(5) 2.902(6) 2.357(6) 2.360(7) 2.489(6) 2.492(7) 2.490(7) 2.488(8) 2.511(6) 2.502(7) 2.533(6) 2.533(7) 2.536(6) 2.531(7)
Figure 3. Perspective view of the crystal structure of La4Si2O7N2 along the b axis (y ) -0.06 to 0.35). Areas drawn with white and black colors in the nonmetal sites refer to the occupancies of oxygen and nitrogen atoms, respectively.
1.635(7) 1.635(8) 1.646(8) 1.654(9) 1.659(8) 1.659(9) 1.717(7) 1.714(8)
Prepared at 1773 K. b Prepared at 1873 K. Table 4. Selected Bond Angles (deg) in La4Si2O7N2
O/N1-Si1-O/N2 O/N1-Si1-O/N3 O/N1-Si1-O/N4 O/N2-Si1-O/N3 O/N2-Si1-O/N4 O/N3-Si1-O/N4 Si1-O/N1-Si2 a
104.1(4)a 104.3(5)b 112.0(4) 110.7(5) 105.8(4) 106.4(4) 110.9(4) 110.7(5) 112.4(5) 113.7(6) 111.2(4) 110.9(5)
O/N1-Si2-O/N5 O/N1-Si2-O/N6 O/N1-Si2-O/N7 O/N5-Si2-O/N6 O/N5-Si2-O/N7 O/N6-Si2-O/N7
103.4(4) 103.8(5) 111.8(5) 111.6(5) 104.6(4) 104.6(4) 108.2(4) 107.8(5) 112.7(5) 112.8(6) 115.3(5) 115.5(5)
164.4(3) 165.3(4)
Prepared at 1773 K. b Prepared at 1873 K.
pidine (Ca4Si2O7F2) and low-temperature Y4Al2O9, but the stacking of Si2(O,N)7 ditetrahedra along the b axis is different. The Rietveld analysis using TOF neutron powder diffraction data of La4Si2O7N2 has clarified the oxygen/nitrogen distribution in nonmetal sites. All of the N atoms connect to Si atoms. There are three pairs of terminal O/N sites along the a axis in the Si2(O,N)7 ditetrahedra. The occupancies of O/N sites are almost the same in each pair but different from the occupancies of the other pairs. The O/N atom in the bridging site of Si2(O,N)7 ditetrahedra is surrounded by two Si and three La atoms within the interatomic distances of 3.19 Å. The mean composition of the ditetrahedra is expressed with Si2(O5N2). In the presence of Si-O3N and/
Figure 4. Local structure around Si2O5N2 ditetrahedra in (a) La4Si2O7N2 and (b) Lu4Si2O7N2.
or Si-O2N2 tetrahedra, six local structures with four different compositions are deduced for the Si2(O,N)7
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ditetrahedra. The bridging site of Si2(O,N)7 ditetrahedra is mostly occupied by a nitrogen atom. This means that three local structures, O3≡Si-N-Si≡O3, O2N≡Si-NSi≡O3, and O2N≡Si-N-Si≡O2N, are dominant in the crystal. Since the difference of crystal structure is clarified between the end-member compounds,
Takahashi et al.
La4-Si2O7N2 and Lu4Si2O7N2, in a series of rare-earth silicon oxynitrides, it is interesting to investigate the crystal structures of Ln4Si2O7N2 compounds with other rare earths. CM020930C
