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A high resolution powder neutron diffraction study of the novel layered oxide BiMo2O7OD · 2D2O
J. Phys. Chem. Solids Vol 56. No. IO, pp. 1339-1343. 1995
A HIGH
Elsevier Science Ltd Printed in Great Britain. 0022-3697/95 $9.50 + 0.00
0022-3697(95)00067-4
Pergamon
RESOLUTION POWDER NEUTRON DIFFRACTION OF THE NOVEL LAYERED OXIDE BiMoz070D.2D20 J. A. HRILJAC,?
C. C. TORARDIS
STUDY
and T. VOGTg
tDepartment of Applied Science, Brookhaven National Laboratory, Upton, NY 11973-5000, U.S.A $Central Research and Development, DuPont Company, Wilmington, DE 19880-0356, U.S.A. $Department of Physics, Brookhaven National Laboratory, Upton, NY 11973-5000, U.S.A. (Received 22 November 1994; accepted 22 November 1994)
Abstract-High resolution powder neutron diffraction data have been collected on the deuterated analog of the layered oxide BiMoz070H .2Hz0. Analysis using the Rietveld method has provided the locations of the deuterium atoms and led to an extellent structural refinement in space group P21/m with a = 6.3594( 1) A, b = 11.5976(l) A, c = 5.7962(1)A, and 0 = 113.434(l)“. The model refined to give R,, = 6.69%. RI = 7.30%, and x2 = 2.75 for 55 variables (47 structural) and 475 reflections. The location of the deuterium atoms confirms the prediction made in a previous synchrotron X-ray study but the details are more complex than expected. A full description of the deuterium bonding is provided. Keywords: A. oxides, C. neutron scattering, D. crystal structure.
1. INTRODUCTION Continuing
interest
in the bismuth
molybdates
is
due to their well recognized importance as prototypical examples of catalysts for the selective oxidation and ammoxidation of olefins [l-3]. There are several known binaries in the Bi203.MoOj phase diagram [4-71, and the structures of the catalytically active a-BizMo3012 [8, 91, o-BiZMo209 [lo, 111,and y-Bi2Mo06 [12-141 materials have been reported. More recently, the structure of the high temperature form of the y-phase, y’-BiZMo06, was refined from high resolution neutron diffraction data [15]. Attempts have been made to elucidate the similarities of these phases and correlate the structures to the catalytic activity [2, 7, 151. All of the above noted materials are prepared by high temperature ceramic routes. Efforts to prepare new and potentially active materials both in this laboratory and elsewhere [ 16, 171led to the discovery of a new hydrous phase that was synthesized using very mild hydrothermal conditions. The crystal structure was solved using ab initio techniques from high resolution powder synchrotron X-ray diffraction data [18]. This is now recognized as a viable alternative when sufficiently large single crystals are not available [19, 201. One of the reasons for the increasing number of successful ab initio structure solutions is clearly the availability of dedicated instruments such as beam line X7A at the National Synchrotron Light Source (NSLS) at Brookhaven National Laboratory (BNL) [21-231. The heavy atom part
of this novel bismuth molybdate was successfully refined against the X-ray data using the Rietveld method [24], and the formula was deduced to be BiMo20,0H - 2H20. In order to confirm the stoichiometry and determine the nature and extent of the hydrogen bonding it is necessary to work with neutron rather than X-ray data. In this paper we report the results of such a study, using high resolution powder neutron diffraction data collected at the new dedicated instrument located at beam line HlA at the High Flux Beam Reactor (HFBR) at BNL [25].
2. EXPERIMENTAL
An 11.7 g sample of Na2M004 (dried at 300°C for 2 days and cooled to room temperature in a desiccator over PZO,) was dissolved in a solution consisting of 100 ml D20 (99.9 atom % D, Aldrich) and 4 ml of 65 wt % DN03 in D20 (99.9+ atom % D, Aldrich). This clear molybdate solution was added with stirring to a clear, colorless solution of 12.0 g Bi203, 850ml D20, and 63 g of 65 wt % DN03 in a 2 1 Erlenmeyer flask. The resulting slightly cloudy solution was stoppered and stirred overnight at ambient temperature to give a small amount of a flocculent solid. After stirring for an additional day, a white powder was observed that settled well when stirring was paused. The mixture was stirred for another day with no visible change. The product was collected on a filter under flowing NZ, washed with D20, and rinsed with acetone. Then 15.8 g of product were collected after
1339
J. A. HRILJAC ef al.
1340
drying over PZOs for 3 days. TGA analysis showed an 8.8% weight loss between 100 and 300°C. This compares very well with the calculated value of 8.76% for D20 removal from BiMozO,OD. 2D20. An X-ray powder diffraction pattern was identical to that of the hydrogen form, BiMoz0,0H.2H20 [ 181. A second harmonic generation (SHG) measurement [26] using a pulsed Nd : YAG laser (X = 1.06jlm) gave a very weak signal, ca. 10% of that of quartz, indicating a noncentrosymmetric space group. The high resolution powder neutron diffraction data were obtained on the new diffractometer at beamline HlA at the High Flux Beam Reactor (HFBR) at Brookhaven National Laboratory. Details of the instrument are given in the reference of Buttrey et al. [ 151.The sample consisted of ca. 10 g of powder contained in a vanadium can sealed with indium to prevent exposure to the atmosphere. The data were collected at room temperature with a wavelength of 1.8857A using 5’ primary collimation and 5’ collimation in front of each of the 64 3He detectors. A step size of 0.01 o was used and the data cover the range from 18 to 155” in 20. 3. STRUCTURAL
REFINEMENT
The structural model was refined using the Rietveld method [24] and the GSAS program of Larson and Von Dreele [27]. The background was defined by a series of points connected by linear interpolations and was not refined. The starting model consisted of the heavy atom positions as refined in the earlier synchrotron X-ray work in the centrosymmetric space group P2t/m [18]. This proved to be sufficient to phase difference-Fourier syntheses and provide the location of additional scattering density that was attributed to deuterium atoms. Continued alternation of leastsquares refinements and manual inspection of difference-Fourier maps led to the logical placement of four deuterium atoms. During these early leastsquares cycles very few structural parameters were varied, large damping factors were applied, and the temperature factors were constrained to be equivalent for each atom type. Considerations of the positions of these atoms and the expected chemical composition indicated that three of the sites were only partially occupied. For example, D(1) and D(4) are only ca. 0.7 A apart, as is D(3) and one of its symmetry-related positions. If each of these positions is half-occupied and the last position, D(2), is fully occupied then the stoichiometry is as expected, i.e. BiMozO7OD - 2D20. The site occupancies were fixed and the model further refined, using soft restraints on the O-D distances. There continued to be problems with the temperature factors for the deuterium atoms and the weighted
profile R-factor remained near 10%. A visual inspection of the raw data indicated a higher than expected background. This was attributed to incomplete H/D exchange, and the refinement was continued with an additional variable to correct for this that converged to a value of 0.839(3). The restraints were relaxed and removed when possible. For the final model it was necessary to restrain the 0(1)-D(l) and 0(4)-D(4) distances to 0.98(2)A. After the refinement reached convergence the agreement factors [24] for 13,699 data points and 475 Bragg reflections were: R, = 5.30%, R,, = 6.69%, RE = 4.05%, R, = 7.30% and x2 = 2.75. The SHG measurement indicates the true space group is one of the lower symmetry noncentrosymmetric subgroups, P2, or Pm. A consideration of the problems due to the proximity of the deuterium positions suggested P2, as the logical choice. A model was developed in this space group using the prior locations of the deuterium atoms and generating the appropriate second set of atoms due to the loss of the mirror plane. Least-squares refinements including site occupations led to a sensible model with the correct stoichiometry, and slightly lower agreement factors: R, = 5.08%, R,, = 6.34%, RE = 4.05%, RI = 6.36% and x2 = 2.47. However, several problems were noted. First, the temperature factors showed a much greater variation including a physically unreasonable negative value for one of the oxygens and a very large value (9, ca. 0.08) for one of the deuteriums. Also, the variation in MO-O bond lengths for the now independent molybdenum sites became significantly greater, ranging from 1.656 to 2.54Ob; for one and 1.708 to 2.218 A for the other. Due to these problems and the small decrease in agreement factors we prefer to describe the structure as disordered in space group P2, /m. 4. RESULTS AND DISCUSSION
The final details from the Rietveld analysis of the high resolution powder neutron diffraction data in space group P2t /m are presented in Table 1, the values of selected distances and angles in Table 2, and the observed, calculated and difference profiles are shown in Fig. 1. The model provides an excellent fit to the data as is clearly demonstrated by the small and random nature of the difference profile, the small agreement factors, and the low standard deviations on the atomic coordinates and hence, bond distances and angles. The atomic coordinates for the heavy atom part of the structure compare well with those determined from the X-ray work [18] with the differences varying between one and seven standard deviations.
Neutron diffraction study of BiMo?O,OD
.2&O
1341
Table I. Refined fractional atomic coordinates and temperature factors with estimated standard deviations Atom Bi !;, O(2) O(3) O(4) O(5) O(6)
D/HUH D/H(2)t D/H(3)t D/H(4)t
x
Y
z
Ii IS0
0.2618(3) 0.6568(3) 0.0798(6) 0.5917(6) 0.3682(3) 0.8230(4) 0.5151(4) 0.9392(4) 0.9972( 10) 0.7274(5) -0.0516(11) 0.9128(11)
0.25 0.0951(2) 0.25 0.25 0.0661(2) 0.0749(2) 0.1149(2) 0.1200(2) 0.1812(4) 0.0547(2) 0.011 l(6) 0.1417(4)
0.6567(4) 0.1940(3) 0.8817(6) 0.0259(5) 0.8469(4) 0.9000(4) 0.3908(4) 0.3868(4) 0.8885(11) 0.7275(6) -0.0315(12) 0.8809(13)
0.0103(5) 0.0141(4) 0.0273(8) 0.0153(8) 0.0060(5) 0.0156(6) 0.0187(6) 0.0200(6) 0.0242(14) 0.03 17(8) 0.0265(16) 0.0335(17)
Note. Spacegroup: P2,/m (No. 11). Refined cell constants: a = 6.3594( 1) A, b = 11.5976(1) A, c = 5.7962(l) A,B = 113.434(1)“. Cellvolume: 392.232(8)A3.
tThe final D/H ratio refined to 0.839(3). The occupancies of sites 1, 3, and 4 were fixed at 0.5. in detail [ 181 and will therefore only briefly be reviewed here. The structure is composed of irregular six-coordinate bismuth units and distorted octahedrally coordinated molybdenum atoms, Fig. 2. The molybdenum octahedra share edges defined by the O(3) atoms to form dimers, and these dimers are corner-linked through O(2) atoms to form infinite chains which run along [OlO]. These chains are then cross-linked by the bismuth atoms to form sheets
As discussed in detail in the previous section, the true space group is most likely P21. However, the excellent refinement obtained in the higher symmetry one, including reasonable temperature factors, indicates that the structural distortions are very small. It therefore appears that the dominant cause of the lowered symmetry is the deuterium ordering. This is consistent with the very weak SHG signal. The heavy atom part of the structure has already
been described
1200
1000
600
m
5 8
.,
600
-2001
” 15
“, 35
”
” 55
”
75
”
”
”
”
115
”
”
135
’
“‘I
155
28 (de~)g5 Fig. 1. The powder neutron diffraction data showing the observed (dots), fitted (line) and difference (bottom) spectra from the Rietveld refinement. The tick marks indicate the positions of allowed reflections.
J. A. HRILJAC
1342 Table 2. Selected interatomic Bi-O(1) Bi-O(2) Bi-O(3) BikO(6)
2.059(4) 2.326(4) 2 x 2.372(2) 2 x 2.526(3)
MO-O(~) MO-O(~) MO-O(~)’ MO-O(~) MO-O(~) MO-O(6)
2.006(2) 1.884(3) 2.143(3) 2.347(3) 1.727(3) 1.720(3)
0(1)-D(l)
0.965(3) 1.643(4) 1.677(4) 0.970(3) 1.044(5) 1.675(6) 0.995(3)
0(1)-D(4) 0(4)-D(J) 0(4)-D(2) 0(4)-D(3) 0(4)-D(3)’ 0(4)-D(4)
distances
et al.
(A) and angles (“) with estimated
O(l)-Bi-O(2) O(l)-Bi-O(3) O(l)-Bi-O(6)
O(2)-Bi-O(3)
86.90(14) 2 x 80.44(8) 2 x 82.51(12) 2 x 65.56(6) 75.55(11) 146.81(13) 78.61(12) 98.09(14) 97.23(14) 74.23(10) 76.56(10)
Bi-0(1)-D(4) Bi-0(1)-D(l) MO-O(4)-D( 1) MO-0(4)-D(2) MO-0(4)-D(3) MO-0(4)-D(3)’ MO-0(4)-D(4) D(l)-0(4)-D(2) D( 1)-0(4)-D(3)
which are parallel to (101). No direct bonding is observed between the sheets and these are held together strictly by deuterium bonding. The atoms which project between the sheets are O(l), O(4), and O(5), Fig. 3. 0( 1) is bonded solely to the bismuth atom by the shortest of the Bi-0 bonds, 2.059(4) A. Based on bond valence sums in the earlier X-ray work, this was predicted to be an hydroxide oxygen. The current analysis of the neutron work confirms this, with D(1) being the only other atom found directly bonded to O(1) at a distance of 0.965(3)A. In addition, there is a deuterium bond
121.6(2) 118.6(4) 116.8(2) 119.6(3) 106.9(4) 106.8(3) 116.5(4) 106.8(3) 96.9(5)
standard
deviations
O(2)-Bi-O(6) O(3)-Bi-O(3) O(3)-Bi-O(6) 0(3))Bi-O(6) O(6)-Bi-O(6) O(5)-MO-O(~)’ O(6)-MO-O(~)’ O(3)-MO-O(~) O(3)-MO-O(~) O(3)-MO-O(~) O(4)-MO-O(~) O(4)-MO-O(~) O(5)-MO-O(~) D( l))O( 1)-D(4) O(l)-D(l)-O(4) D(2)-0(4)-D(3) D(2)-0(4)-D(4) D(2)-0(4)-D(3) D(3)‘-0(4)-D(4) O(4)-D(3)-O(4) O(4)-D(4)-O( 1)
2 x 141.56(6) 128.04(12) 2 x 76.26(7) 2 x 146.71(10) 73.32(10) 99.44(12) 156.78(14) 81.32(9) 100.07(12) 105.18(12) 175.30(14) 80.39(11) 103.46(14) 105.7(5) 171.5(7) 107.0(5) 103.1(5) 113.6(3) 94.7(5) 171.3(6) 173.7(7)
found between O(1) and D(4), 1.643(4) A. O(4) is bonded to the molybdenum atom by the longest of the Moo0 bonds, 2.347(3jA, and was predicted to be part of a water molecule. This is confirmed with D(2), D(3) and D(4) all within bonding distances at 0.970(3), 1.044(S) and 0.995(3) A, respectively. Since both D(3) and D(4) are each half occupied it appears that only one of the two is occupied at one time. In addition to these bonds, O(4) is also deuterium bonded to D(l), 1.677(4)A, and D(3), 1.675(6) A. Therefore it appears that each O(4) can be involved in one of two bonding schemes. In the first, direct bonds are made to D(2) and D(3), with a deuterium bond to D( 1). In the second, direct bonds are made to D(2) and D(4), with a deuterium bond to D(3). This leads to spiral chains of water molecules and hydroxide ions which run along the b-axis and hold the layers together. The catalytic oxidation of olefins by bismuth molybdates is bimetallic in nature and involves the abstraction of an olefinic hydrogen at a bismuth
--b Fig. 2. A polyhedral representation of the layers. The bismuth atom coordination is shown as a capped pentagon and the molybdenum as an octahedron. The oxygen atoms are the small gray spheres and the deuterium atoms the white ones.
Fig. 3. An ORTEP [28] diagram showing the atomic labeling scheme and a view of the deuterium bonding. In space group P2r /m there is a mirror plane yhich passes through the Bi, O(l), and O(2) atoms perpendicular to the plane of the paper.
Neutron
diffraction
study of BiMozO,OD .2DzO
site followed by an oxygen insertion at an adjacent molybdenum site [2, 31. The title compound clearly contains the necessary arrangement of nearby bismuth and molybdenum atoms in infinite sheets and is a potential candidate for this reaction. Unfortunately, it is not thermally stable and transforms to an amorphous phase between 150 and 250°C [ 161.After heating the material above 300°C we observed a mixture of cr-BizMojOiz and Moos. This study has detailed the complex and extensive nature of the deuterium bonding in BiMoz070D .2Dz0 which holds the structure together. If this could be stabilized by the replacement of some or all of the water molecules by more tightly bound ligands the thermal stability would increase and the title compound might become an active catalyst. Acknowledgemenrs~The work at Brookhaven National Laboratory was performed under contract No. DE-ACOZ76CH00016, U.S. Department of Energy, Division of Materials Sciences and Division of Chemical Sciences.
REFERENCES Grasselh R. K. and Burrington J. D., Adv. Caral. 30,133 (1981). Grasselh R. K., Burrington J. D. and Brazdil J. F., Faraday Discuss. Chem. Sot. 72,203 (1981). Glaeser L. C., Brazdil J. F., Hazle M. A., Mehicic M. and Grasselli R. K., J. Chem. Sot. Faraday Trans. 181,2903 (1985). Kohlmuller R. and Badaud J. P., Bull. Sot. Chim. Fr. 10, 3434 (1969). Chen T. and Smith G. S., J. Solid Stare Chem. 13, 288 (1975). Egashira M., Matsuo K., Kagawa S. and Seiyama T., J. Cutal. 58,409 (1979). Buttrey D. J., Jefferson D. A. and Thomas J. M., Phil. Msg. A 53,897 (1986).
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8. van den Elzen A. F. and Rieck G. D., Acta Crystallogr. B29,2433 (1973). 9. Theobald F., Laarif A. and Hewat A. W., Maler. Res. Bull. 20,653 (1985). 10. van den Elzen A. F. and Rieck G. D., Muter. Res. Bull. 10, 1163 (1975). I 1. Chen H.-Y. and Sleight A. W., J. Solid State Chem. 63, 70 (1986). 12. van den Elzen A. F. and Rieck G. D., Acia Crystallogr. B29,2436 (1973). 13. Theobald F., Laarif A. and Hewat A. W., Ferroelectrics 56,219 (1984). 14. Teller R. G., Brazdil J. F., Grasselli R. K. and Jorgensen J. D., Acta Crystallogr. C40,2001 (1984). 15. Buttrey D. J., Vogt T., Wildgruber U. and Robinson W. R., J. Solid State Chem. 111, 118 (1994). 16. Murakami Y., Ishizawa N. and Imai H.. Powder Difiacrion 5.227 I1 990). 17. Mmakami Y. andimai H. J., J. Mater. Sci. Left. 10, 107 (1990). 18. Hriljac J. A. and Torardi C. C., Inorg. Chem. 32, 6003 (1993). 19. Cheetham A. K. and Wilkinson A. P., J. Phys. Chem. Solids52, 1199 (1991). 20. Cheetham A. K. and Wilkinson A. P.. Angew. Chem. Inc. Ed. EngI. 31, 1557 (1992). 21. Cox Dy E., Hastings J. B., Cardoso L. P. and Finger L. W.. Mater. Sci. Forum 9. 1 (1986). 22. Cox D. E., Toby B. H. and’Eddy M. M., Aust. J. Phys. 41, 117 (1988). 23. Cox D. E., In Handbook on Synchrotron Radiation (Edited by G. Brown and D. E. Moncton), Vol. 3, p. 155. Elsevier Science, Oxford (1991). 24. Rietveld H. M., J. Appl. Crystallogr. 2, 65 (1969). 25. Vogt T., Passe11 L. and Axe J. D., to be published. 26. Kurtz S. K. and Perry T. T., J. Appl. Phys. 39, 3798 (1968). 27. Larson A. C. and Von Dreele R. B., GSAS: General Structure Analysis System. Report No. LAUR 86-748, Los Alamos National Laboratory, Los Alamos, NM (1994). thermal ellip28. Johnson C. K., ORTEP: A FORTRAN soid plot program for crystal structure illustration. Report No. 5138, Oak Ridge National Laboratory: Oak Ridge, TN (1976).
A HIGH
Elsevier Science Ltd Printed in Great Britain. 0022-3697/95 $9.50 + 0.00
0022-3697(95)00067-4
Pergamon
RESOLUTION POWDER NEUTRON DIFFRACTION OF THE NOVEL LAYERED OXIDE BiMoz070D.2D20 J. A. HRILJAC,?
C. C. TORARDIS
STUDY
and T. VOGTg
tDepartment of Applied Science, Brookhaven National Laboratory, Upton, NY 11973-5000, U.S.A $Central Research and Development, DuPont Company, Wilmington, DE 19880-0356, U.S.A. $Department of Physics, Brookhaven National Laboratory, Upton, NY 11973-5000, U.S.A. (Received 22 November 1994; accepted 22 November 1994)
Abstract-High resolution powder neutron diffraction data have been collected on the deuterated analog of the layered oxide BiMoz070H .2Hz0. Analysis using the Rietveld method has provided the locations of the deuterium atoms and led to an extellent structural refinement in space group P21/m with a = 6.3594( 1) A, b = 11.5976(l) A, c = 5.7962(1)A, and 0 = 113.434(l)“. The model refined to give R,, = 6.69%. RI = 7.30%, and x2 = 2.75 for 55 variables (47 structural) and 475 reflections. The location of the deuterium atoms confirms the prediction made in a previous synchrotron X-ray study but the details are more complex than expected. A full description of the deuterium bonding is provided. Keywords: A. oxides, C. neutron scattering, D. crystal structure.
1. INTRODUCTION Continuing
interest
in the bismuth
molybdates
is
due to their well recognized importance as prototypical examples of catalysts for the selective oxidation and ammoxidation of olefins [l-3]. There are several known binaries in the Bi203.MoOj phase diagram [4-71, and the structures of the catalytically active a-BizMo3012 [8, 91, o-BiZMo209 [lo, 111,and y-Bi2Mo06 [12-141 materials have been reported. More recently, the structure of the high temperature form of the y-phase, y’-BiZMo06, was refined from high resolution neutron diffraction data [15]. Attempts have been made to elucidate the similarities of these phases and correlate the structures to the catalytic activity [2, 7, 151. All of the above noted materials are prepared by high temperature ceramic routes. Efforts to prepare new and potentially active materials both in this laboratory and elsewhere [ 16, 171led to the discovery of a new hydrous phase that was synthesized using very mild hydrothermal conditions. The crystal structure was solved using ab initio techniques from high resolution powder synchrotron X-ray diffraction data [18]. This is now recognized as a viable alternative when sufficiently large single crystals are not available [19, 201. One of the reasons for the increasing number of successful ab initio structure solutions is clearly the availability of dedicated instruments such as beam line X7A at the National Synchrotron Light Source (NSLS) at Brookhaven National Laboratory (BNL) [21-231. The heavy atom part
of this novel bismuth molybdate was successfully refined against the X-ray data using the Rietveld method [24], and the formula was deduced to be BiMo20,0H - 2H20. In order to confirm the stoichiometry and determine the nature and extent of the hydrogen bonding it is necessary to work with neutron rather than X-ray data. In this paper we report the results of such a study, using high resolution powder neutron diffraction data collected at the new dedicated instrument located at beam line HlA at the High Flux Beam Reactor (HFBR) at BNL [25].
2. EXPERIMENTAL
An 11.7 g sample of Na2M004 (dried at 300°C for 2 days and cooled to room temperature in a desiccator over PZO,) was dissolved in a solution consisting of 100 ml D20 (99.9 atom % D, Aldrich) and 4 ml of 65 wt % DN03 in D20 (99.9+ atom % D, Aldrich). This clear molybdate solution was added with stirring to a clear, colorless solution of 12.0 g Bi203, 850ml D20, and 63 g of 65 wt % DN03 in a 2 1 Erlenmeyer flask. The resulting slightly cloudy solution was stoppered and stirred overnight at ambient temperature to give a small amount of a flocculent solid. After stirring for an additional day, a white powder was observed that settled well when stirring was paused. The mixture was stirred for another day with no visible change. The product was collected on a filter under flowing NZ, washed with D20, and rinsed with acetone. Then 15.8 g of product were collected after
1339
J. A. HRILJAC ef al.
1340
drying over PZOs for 3 days. TGA analysis showed an 8.8% weight loss between 100 and 300°C. This compares very well with the calculated value of 8.76% for D20 removal from BiMozO,OD. 2D20. An X-ray powder diffraction pattern was identical to that of the hydrogen form, BiMoz0,0H.2H20 [ 181. A second harmonic generation (SHG) measurement [26] using a pulsed Nd : YAG laser (X = 1.06jlm) gave a very weak signal, ca. 10% of that of quartz, indicating a noncentrosymmetric space group. The high resolution powder neutron diffraction data were obtained on the new diffractometer at beamline HlA at the High Flux Beam Reactor (HFBR) at Brookhaven National Laboratory. Details of the instrument are given in the reference of Buttrey et al. [ 151.The sample consisted of ca. 10 g of powder contained in a vanadium can sealed with indium to prevent exposure to the atmosphere. The data were collected at room temperature with a wavelength of 1.8857A using 5’ primary collimation and 5’ collimation in front of each of the 64 3He detectors. A step size of 0.01 o was used and the data cover the range from 18 to 155” in 20. 3. STRUCTURAL
REFINEMENT
The structural model was refined using the Rietveld method [24] and the GSAS program of Larson and Von Dreele [27]. The background was defined by a series of points connected by linear interpolations and was not refined. The starting model consisted of the heavy atom positions as refined in the earlier synchrotron X-ray work in the centrosymmetric space group P2t/m [18]. This proved to be sufficient to phase difference-Fourier syntheses and provide the location of additional scattering density that was attributed to deuterium atoms. Continued alternation of leastsquares refinements and manual inspection of difference-Fourier maps led to the logical placement of four deuterium atoms. During these early leastsquares cycles very few structural parameters were varied, large damping factors were applied, and the temperature factors were constrained to be equivalent for each atom type. Considerations of the positions of these atoms and the expected chemical composition indicated that three of the sites were only partially occupied. For example, D(1) and D(4) are only ca. 0.7 A apart, as is D(3) and one of its symmetry-related positions. If each of these positions is half-occupied and the last position, D(2), is fully occupied then the stoichiometry is as expected, i.e. BiMozO7OD - 2D20. The site occupancies were fixed and the model further refined, using soft restraints on the O-D distances. There continued to be problems with the temperature factors for the deuterium atoms and the weighted
profile R-factor remained near 10%. A visual inspection of the raw data indicated a higher than expected background. This was attributed to incomplete H/D exchange, and the refinement was continued with an additional variable to correct for this that converged to a value of 0.839(3). The restraints were relaxed and removed when possible. For the final model it was necessary to restrain the 0(1)-D(l) and 0(4)-D(4) distances to 0.98(2)A. After the refinement reached convergence the agreement factors [24] for 13,699 data points and 475 Bragg reflections were: R, = 5.30%, R,, = 6.69%, RE = 4.05%, R, = 7.30% and x2 = 2.75. The SHG measurement indicates the true space group is one of the lower symmetry noncentrosymmetric subgroups, P2, or Pm. A consideration of the problems due to the proximity of the deuterium positions suggested P2, as the logical choice. A model was developed in this space group using the prior locations of the deuterium atoms and generating the appropriate second set of atoms due to the loss of the mirror plane. Least-squares refinements including site occupations led to a sensible model with the correct stoichiometry, and slightly lower agreement factors: R, = 5.08%, R,, = 6.34%, RE = 4.05%, RI = 6.36% and x2 = 2.47. However, several problems were noted. First, the temperature factors showed a much greater variation including a physically unreasonable negative value for one of the oxygens and a very large value (9, ca. 0.08) for one of the deuteriums. Also, the variation in MO-O bond lengths for the now independent molybdenum sites became significantly greater, ranging from 1.656 to 2.54Ob; for one and 1.708 to 2.218 A for the other. Due to these problems and the small decrease in agreement factors we prefer to describe the structure as disordered in space group P2, /m. 4. RESULTS AND DISCUSSION
The final details from the Rietveld analysis of the high resolution powder neutron diffraction data in space group P2t /m are presented in Table 1, the values of selected distances and angles in Table 2, and the observed, calculated and difference profiles are shown in Fig. 1. The model provides an excellent fit to the data as is clearly demonstrated by the small and random nature of the difference profile, the small agreement factors, and the low standard deviations on the atomic coordinates and hence, bond distances and angles. The atomic coordinates for the heavy atom part of the structure compare well with those determined from the X-ray work [18] with the differences varying between one and seven standard deviations.
Neutron diffraction study of BiMo?O,OD
.2&O
1341
Table I. Refined fractional atomic coordinates and temperature factors with estimated standard deviations Atom Bi !;, O(2) O(3) O(4) O(5) O(6)
D/HUH D/H(2)t D/H(3)t D/H(4)t
x
Y
z
Ii IS0
0.2618(3) 0.6568(3) 0.0798(6) 0.5917(6) 0.3682(3) 0.8230(4) 0.5151(4) 0.9392(4) 0.9972( 10) 0.7274(5) -0.0516(11) 0.9128(11)
0.25 0.0951(2) 0.25 0.25 0.0661(2) 0.0749(2) 0.1149(2) 0.1200(2) 0.1812(4) 0.0547(2) 0.011 l(6) 0.1417(4)
0.6567(4) 0.1940(3) 0.8817(6) 0.0259(5) 0.8469(4) 0.9000(4) 0.3908(4) 0.3868(4) 0.8885(11) 0.7275(6) -0.0315(12) 0.8809(13)
0.0103(5) 0.0141(4) 0.0273(8) 0.0153(8) 0.0060(5) 0.0156(6) 0.0187(6) 0.0200(6) 0.0242(14) 0.03 17(8) 0.0265(16) 0.0335(17)
Note. Spacegroup: P2,/m (No. 11). Refined cell constants: a = 6.3594( 1) A, b = 11.5976(1) A, c = 5.7962(l) A,B = 113.434(1)“. Cellvolume: 392.232(8)A3.
tThe final D/H ratio refined to 0.839(3). The occupancies of sites 1, 3, and 4 were fixed at 0.5. in detail [ 181 and will therefore only briefly be reviewed here. The structure is composed of irregular six-coordinate bismuth units and distorted octahedrally coordinated molybdenum atoms, Fig. 2. The molybdenum octahedra share edges defined by the O(3) atoms to form dimers, and these dimers are corner-linked through O(2) atoms to form infinite chains which run along [OlO]. These chains are then cross-linked by the bismuth atoms to form sheets
As discussed in detail in the previous section, the true space group is most likely P21. However, the excellent refinement obtained in the higher symmetry one, including reasonable temperature factors, indicates that the structural distortions are very small. It therefore appears that the dominant cause of the lowered symmetry is the deuterium ordering. This is consistent with the very weak SHG signal. The heavy atom part of the structure has already
been described
1200
1000
600
m
5 8
.,
600
-2001
” 15
“, 35
”
” 55
”
75
”
”
”
”
115
”
”
135
’
“‘I
155
28 (de~)g5 Fig. 1. The powder neutron diffraction data showing the observed (dots), fitted (line) and difference (bottom) spectra from the Rietveld refinement. The tick marks indicate the positions of allowed reflections.
J. A. HRILJAC
1342 Table 2. Selected interatomic Bi-O(1) Bi-O(2) Bi-O(3) BikO(6)
2.059(4) 2.326(4) 2 x 2.372(2) 2 x 2.526(3)
MO-O(~) MO-O(~) MO-O(~)’ MO-O(~) MO-O(~) MO-O(6)
2.006(2) 1.884(3) 2.143(3) 2.347(3) 1.727(3) 1.720(3)
0(1)-D(l)
0.965(3) 1.643(4) 1.677(4) 0.970(3) 1.044(5) 1.675(6) 0.995(3)
0(1)-D(4) 0(4)-D(J) 0(4)-D(2) 0(4)-D(3) 0(4)-D(3)’ 0(4)-D(4)
distances
et al.
(A) and angles (“) with estimated
O(l)-Bi-O(2) O(l)-Bi-O(3) O(l)-Bi-O(6)
O(2)-Bi-O(3)
86.90(14) 2 x 80.44(8) 2 x 82.51(12) 2 x 65.56(6) 75.55(11) 146.81(13) 78.61(12) 98.09(14) 97.23(14) 74.23(10) 76.56(10)
Bi-0(1)-D(4) Bi-0(1)-D(l) MO-O(4)-D( 1) MO-0(4)-D(2) MO-0(4)-D(3) MO-0(4)-D(3)’ MO-0(4)-D(4) D(l)-0(4)-D(2) D( 1)-0(4)-D(3)
which are parallel to (101). No direct bonding is observed between the sheets and these are held together strictly by deuterium bonding. The atoms which project between the sheets are O(l), O(4), and O(5), Fig. 3. 0( 1) is bonded solely to the bismuth atom by the shortest of the Bi-0 bonds, 2.059(4) A. Based on bond valence sums in the earlier X-ray work, this was predicted to be an hydroxide oxygen. The current analysis of the neutron work confirms this, with D(1) being the only other atom found directly bonded to O(1) at a distance of 0.965(3)A. In addition, there is a deuterium bond
121.6(2) 118.6(4) 116.8(2) 119.6(3) 106.9(4) 106.8(3) 116.5(4) 106.8(3) 96.9(5)
standard
deviations
O(2)-Bi-O(6) O(3)-Bi-O(3) O(3)-Bi-O(6) 0(3))Bi-O(6) O(6)-Bi-O(6) O(5)-MO-O(~)’ O(6)-MO-O(~)’ O(3)-MO-O(~) O(3)-MO-O(~) O(3)-MO-O(~) O(4)-MO-O(~) O(4)-MO-O(~) O(5)-MO-O(~) D( l))O( 1)-D(4) O(l)-D(l)-O(4) D(2)-0(4)-D(3) D(2)-0(4)-D(4) D(2)-0(4)-D(3) D(3)‘-0(4)-D(4) O(4)-D(3)-O(4) O(4)-D(4)-O( 1)
2 x 141.56(6) 128.04(12) 2 x 76.26(7) 2 x 146.71(10) 73.32(10) 99.44(12) 156.78(14) 81.32(9) 100.07(12) 105.18(12) 175.30(14) 80.39(11) 103.46(14) 105.7(5) 171.5(7) 107.0(5) 103.1(5) 113.6(3) 94.7(5) 171.3(6) 173.7(7)
found between O(1) and D(4), 1.643(4) A. O(4) is bonded to the molybdenum atom by the longest of the Moo0 bonds, 2.347(3jA, and was predicted to be part of a water molecule. This is confirmed with D(2), D(3) and D(4) all within bonding distances at 0.970(3), 1.044(S) and 0.995(3) A, respectively. Since both D(3) and D(4) are each half occupied it appears that only one of the two is occupied at one time. In addition to these bonds, O(4) is also deuterium bonded to D(l), 1.677(4)A, and D(3), 1.675(6) A. Therefore it appears that each O(4) can be involved in one of two bonding schemes. In the first, direct bonds are made to D(2) and D(3), with a deuterium bond to D( 1). In the second, direct bonds are made to D(2) and D(4), with a deuterium bond to D(3). This leads to spiral chains of water molecules and hydroxide ions which run along the b-axis and hold the layers together. The catalytic oxidation of olefins by bismuth molybdates is bimetallic in nature and involves the abstraction of an olefinic hydrogen at a bismuth
--b Fig. 2. A polyhedral representation of the layers. The bismuth atom coordination is shown as a capped pentagon and the molybdenum as an octahedron. The oxygen atoms are the small gray spheres and the deuterium atoms the white ones.
Fig. 3. An ORTEP [28] diagram showing the atomic labeling scheme and a view of the deuterium bonding. In space group P2r /m there is a mirror plane yhich passes through the Bi, O(l), and O(2) atoms perpendicular to the plane of the paper.
Neutron
diffraction
study of BiMozO,OD .2DzO
site followed by an oxygen insertion at an adjacent molybdenum site [2, 31. The title compound clearly contains the necessary arrangement of nearby bismuth and molybdenum atoms in infinite sheets and is a potential candidate for this reaction. Unfortunately, it is not thermally stable and transforms to an amorphous phase between 150 and 250°C [ 161.After heating the material above 300°C we observed a mixture of cr-BizMojOiz and Moos. This study has detailed the complex and extensive nature of the deuterium bonding in BiMoz070D .2Dz0 which holds the structure together. If this could be stabilized by the replacement of some or all of the water molecules by more tightly bound ligands the thermal stability would increase and the title compound might become an active catalyst. Acknowledgemenrs~The work at Brookhaven National Laboratory was performed under contract No. DE-ACOZ76CH00016, U.S. Department of Energy, Division of Materials Sciences and Division of Chemical Sciences.
REFERENCES Grasselh R. K. and Burrington J. D., Adv. Caral. 30,133 (1981). Grasselh R. K., Burrington J. D. and Brazdil J. F., Faraday Discuss. Chem. Sot. 72,203 (1981). Glaeser L. C., Brazdil J. F., Hazle M. A., Mehicic M. and Grasselli R. K., J. Chem. Sot. Faraday Trans. 181,2903 (1985). Kohlmuller R. and Badaud J. P., Bull. Sot. Chim. Fr. 10, 3434 (1969). Chen T. and Smith G. S., J. Solid Stare Chem. 13, 288 (1975). Egashira M., Matsuo K., Kagawa S. and Seiyama T., J. Cutal. 58,409 (1979). Buttrey D. J., Jefferson D. A. and Thomas J. M., Phil. Msg. A 53,897 (1986).
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8. van den Elzen A. F. and Rieck G. D., Acta Crystallogr. B29,2433 (1973). 9. Theobald F., Laarif A. and Hewat A. W., Maler. Res. Bull. 20,653 (1985). 10. van den Elzen A. F. and Rieck G. D., Muter. Res. Bull. 10, 1163 (1975). I 1. Chen H.-Y. and Sleight A. W., J. Solid State Chem. 63, 70 (1986). 12. van den Elzen A. F. and Rieck G. D., Acia Crystallogr. B29,2436 (1973). 13. Theobald F., Laarif A. and Hewat A. W., Ferroelectrics 56,219 (1984). 14. Teller R. G., Brazdil J. F., Grasselli R. K. and Jorgensen J. D., Acta Crystallogr. C40,2001 (1984). 15. Buttrey D. J., Vogt T., Wildgruber U. and Robinson W. R., J. Solid State Chem. 111, 118 (1994). 16. Murakami Y., Ishizawa N. and Imai H.. Powder Difiacrion 5.227 I1 990). 17. Mmakami Y. andimai H. J., J. Mater. Sci. Left. 10, 107 (1990). 18. Hriljac J. A. and Torardi C. C., Inorg. Chem. 32, 6003 (1993). 19. Cheetham A. K. and Wilkinson A. P., J. Phys. Chem. Solids52, 1199 (1991). 20. Cheetham A. K. and Wilkinson A. P.. Angew. Chem. Inc. Ed. EngI. 31, 1557 (1992). 21. Cox Dy E., Hastings J. B., Cardoso L. P. and Finger L. W.. Mater. Sci. Forum 9. 1 (1986). 22. Cox D. E., Toby B. H. and’Eddy M. M., Aust. J. Phys. 41, 117 (1988). 23. Cox D. E., In Handbook on Synchrotron Radiation (Edited by G. Brown and D. E. Moncton), Vol. 3, p. 155. Elsevier Science, Oxford (1991). 24. Rietveld H. M., J. Appl. Crystallogr. 2, 65 (1969). 25. Vogt T., Passe11 L. and Axe J. D., to be published. 26. Kurtz S. K. and Perry T. T., J. Appl. Phys. 39, 3798 (1968). 27. Larson A. C. and Von Dreele R. B., GSAS: General Structure Analysis System. Report No. LAUR 86-748, Los Alamos National Laboratory, Los Alamos, NM (1994). thermal ellip28. Johnson C. K., ORTEP: A FORTRAN soid plot program for crystal structure illustration. Report No. 5138, Oak Ridge National Laboratory: Oak Ridge, TN (1976).