J . Phys. Chem. 1988, 92, 2939-2944
2939
Structural Characterization of Bismuth Molybdates by X-ray Absorption Spectroscopy and Powder Neutron Diffraction Profile Analysis Mark R. Antonio,* Raymond G. Teller, Donald R. Sandstrom,t Meho Mehicic, and James F. Brazdil BP America Research & Development, 4440 Warrensville Center Road, Cleveland, Ohio 441 28-2837, and Boeing Aerospace Company, Seattle, Washington 981 24 (Received: October 28, 1987)
By use of synchrotron radiation, high-quality X-ray absorption data have been recorded for three bismuth molybdate phases with the general composition Biz03-nMo03,for n = 3 (a-phase), n = 2 (@-phase),and n = 1 (y-phase). The results of these bismuth L3-edgeand molybdenum K-edge X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) studies provide structural insights about the local environments of the Bi3+and Mo6+ ions. Also, the structure of @-Bi2O3.2MoO3 was investigated by the use of time-of-flight powder neutron diffraction. The profile analysis of the neutron data for @-bismuthmolybdate (monoclinic, P2,/n, a = 11.9515 (4) A, b = 10.7993 (4) A, c = 11.8805 (4) %.,@ = 90.142 ( 5 ) O , and Z = 8) revealed that the bismuth and molybdenum cations are present in a fluorite-like sublattice.
Introduction At elevated temperatures, many mixed metal oxides exhibit catalytic activity in converting olefins to unsaturated aldehydes. Yet, only a few binary oxide systems, including several in the Bi203-Mo03 phase diagram, are known to be selective catalysts for the partial oxidation of olefins to commercially significant aldehydes. For a number of bismuth-molybdenum oxides, fundamental aspects of their selectivity as heterogeneous oxidation catalysts have been studied, and the solid-state and surface reaction mechanisms are now well understood.'-3 As recently reviewed by Buttrey, Jefferson, and tho ma^,^ seven different bismuth molybdate phases are known; these include five members of the BiZO3.nMoO3series, with n = 3 (a-phase), n = 2 (@-phase),and n = 1 (T-, 7'-, y"-phases), and 3Bi2O3.2MoO3 (&phase) and 19Bi203-7Mo03(e-phase). In order to understand the performance of such bismuth molybdate selective oxidation catalysts, their molecular structures have been characterized by the use of Xneutron,8-" and electron d i f f r a ~ t i o ntechniques, ~,~ as well as by high-resolution electron m i c r o ~ c o p y and ~ ~ ~vibrational ~~'~ spectr~scopies.'~In fact, complete single-crystal X-ray diffraction structure determinations are available for a-BiZO3-3MoO3(Bi2Mo301z),
[email protected](BizMoZO9),l6and y-BiZO3.MoO3 (Bi2Mo06).'7 The aim of the studies reported here is to provide further chemical and structural insights about this Bi-Mo oxide system. By the use of both X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS),we probed the immediate coordination environments of the bismuth and molybdenum cations in each of the BiZO3.nMoO3phases, for n = 1-3. In addition, we investigated the structure of P-Bi203* 2Mo03 by the use of powder neutron diffraction. Experimental Section Materials and Methods. The preparation of a-Bi203-3Mo03 involved the dissolution of Moo3 (Alfa) in 118 mL of concentrated NH40H in 123 mL of distilled water. The solution was heated to 55 O C until a pH of 6.5 was reached, and then it was cooled to room temperature. In a separate flask, Bi(N03)36H20(Alfa) was dissolved in 200 mL of ca. 10% H N 0 3 . This solution was slowly added to the other one with stirring. The pH of the resulting slurry was adjusted to ca. 3 by the dropwise addition of concentrated N H 4 0 H . This slurry was stirred a t room temperature overnight; following further heating and stirring to remove excess water, the precipitate was dried overnight at 120 "C and then denitrified by heating in air at 290 and 425 OC for 3 h each. Final calcination at 500 OC in air for 3 h produced a microcrystalline powder of a-bismuth molybdate. As described by Batist et a1.,I8 @-Bi2O,.2MoO3was prepared by addition of a solution of Bi(NO,),.SH,O to a solution of reagent
'Boeing Aerospace Co.
grade [NH4]6M07024-4HZ0 (Baker). The pH was maintained between 7 and 9 by the addition of ammonium hydroxide. The precipitate was isolated by evaporation and heat treated at 450 OC for 8 h. y-Bi203-Mo03was prepared by the coprecipitation method of Carson et al.,19 in which stoichiometric amounts of reagent grade [NH,] 6M07024-4H20 and Bi(N03)3.5H20 (dissolved in dilute H N 0 3 ) were simultaneously added to a beaker with rapid stirring. The resulting slurry was heated to boiling until dry. The precipitate was heat treated in air at 290 and 425 "C for 3 h each (to effect denitrification) and then calcined at 500 OC for 16 h. The purities of the compounds were checked by powder X-ray diffraction and Raman spectroscopy: No impurity phases were detected. Our subsequent neutron diffraction analysis of (3Bi203-2Mo03revealed the presence of a small amount of yBi203.Mo03. Since the major X-ray diffraction lines of the p(1) Glaeser, L. C.; Brazdil, J. F.; Hazle, M. A,; Mehicic, M.; Grasselli, R. K. J . Chem. SOC.,Faraday Trans. 1 1985, 81, 2903-2912. (2) Grasselli, R. K.; Burrington, J. D.; Brazdil, J. F. Faraday Discuss. Chem. SOC.1982, 72, 203-223. (3) Brazdil, J. F.; Suresh, D. D.; Grasselli, R. K. J . Catal. 1980, 66, 347-367. (4) Buttrey, D. J.; Jefferson, D. A.; Thomas, J. M. Philos. Mag. A 1986, 53, 897-906. (5) Van den Elzen, A. F.; Rieck, G. D. Mater. Res. Bull. 1975, 10, 1163-1168. (6) Miyazawa, S.; Kawana, A.; Koizumi, H.; Iwasaki, H. Mater. Res. Bull. 1974, 9, 41-52. (7) (a) Buttrey, D. J.; Jefferson, D. A,; Thomas, J. M. Mater. Res. Bull. 1986, 21, 739-744. (b) Wuzong, Z.; Jefferson, D. A.; Alario-Franco, M.; Thomas, J. M. J . Phys. Chem. 1987,91, 512-514. (8) Theobald, F.; Laarif, A.; Hewat, A. W. Mater. Res. Bull. 1985, 20, 653-665. (9) Brazdil, J. F.; Teller, R. G.; Grasselli, R. K.; Kostiner, E. In Solid State Chemistry in Catalysis; Grasselli, R. K., Brazdil, J. F., Eds.;ACS Symposium Series 279; American Chemical Society: Washington, DC, 1985; pp 57-74. (IO) Teller, R. G.; Brazdil, J. F.; Grasselli, R. K.; Jorgensen, J. D. Acta Crystallogr., Sect. C: Cryst. Struct. Commun. 1984, C40, 2001-2005. (1 1) Theobald, F.; Laarif, A.; Hewat, A. W. Ferroelectrics 1984, 56, 219-237. (12) Watanabe, A,; Horiuchi, S.;Kodama, H. J . Solid State Chem. 1987, 67, 333-339. (13) Gai, P. L. J . Solid Stare Chem. 1983, 49, 25-42. (14) (a) Matsuura, I.; Schut, R.; Hirakawa, K. J . Catal. 1980, 63, 152-166. (b) Wachs, I. E.; Hardcastle, F. D.; Buttrey, D.; Jefferson, D. A,; Thomas, J. M. Abstracts of Papers, 194th National Meeting of the American Chemical Society, New Orleans, LA; American Chemical Society: Washington, DC, 1987; INOR 328. (1 5) Van den Elzen, A. F.; Rieck, G. D. Acta Crystallogr., Sect. E Struct. Crystallogr. Crysr. Chem. 1973, B29, 2433-2436. (16) Chen, H. Y . ;Sleight, A. W. J . Solid State Chem. 1986, 63, 70-75. (17) Van den Elzen, A. F.; Rieck, G. D. Acta Crystallogr., Sect. E Struct. Crystallogr. Cryst. Chem. 1973, B29, 2436-2438. (18) Batist, Ph. A,; Bouwens, J. F. H.; Schuit, G. C. A. J . Catal. 1972, 25, 1-11. (19) Carson, D.; Coudurier, G.; Forissier, M.; Vedrine, J. C.; Laarif, A.; Theobald, F. J . Chem. SOC.,Faraday Trans. J 1983, 79, 1921-1929.
0022-365418812092-2939$01.50/0 0 1988 American Chemical Society
2940 The Journal of Physical Chemistry, Vol. 92, No. 10, 1988
and y-phases occur a t nearly the same d-spacings, it is difficult to detect the presence of a small amount of one phase in the other one. The colors of the Bi203.nMo03solids for n = 1, 2, and 3 were yellow, beige, and white, respectively. X-ray Absorption Measurements. These were performed on thin layers of the finely powdered bismuth molybdates. The powders were mixed with Devcon Duco cement, which was thinned with acetone, and the resulting slurries were spread uniformly on pure aluminum foil (0.5 mil) and dried in air. Several of these bismuth molybdate/Al layers were stacked as needed to obtain X-ray absorption edge jumps of ca. 1 across the molybdenum K-edge (19999.5 eV) and the bismuth L,-edge (13418.6 eV) photoabsorption thresholds.20 All X-ray absorption data were recorded a t the Stanford Synchrotron Radiation Laboratory (SSRL) on beam line VII-3, a wiggler side station. The SPEAR electron storage ring was operated at 3 GeV with between 40 and 80 mA of dedicated beam current. Monochromatic X-radiation was supplied by separately mounted Si (220) crystals. In order to reduce the harmonic content of the incident X-ray beam, the monochromator was slightly detuned at the beginning of every scan. The entrance slits were adjusted to obtain an energy resolution of ca. 4 and 8 eV for the Bi and Mo XANES/EXAFS data, respectively. The molybdenum and bismuth X-ray absorption data were collected in the transmission mode at ca. 77 K by using a stainless steel, double-Dewar cryostat obtained from The EXAFS Co., Seattle, WA. Conventional flow-type ion chamber detectors (with dinitrogen and argon gases for recording the incident, Io, and transmitted, Z,beam intensities, respectively) were employed for these measurements. The transmission X-ray absorption data, In (Zo/l)versus photon energy ( E , eV), were recorded with an integration time of 2 s/point. The raw experimental data for the Biz0,.nMo03 phases with n = 1-3 are shown as supplementary material (see paragraph at end of paper regarding supplementary material). The XANES data were collected at uniform energy intervals of 0.5 eV/point. Energy calibration was maintained by the use of molybdenum metal foil and bismuth metal powder as reference standards; the relevant Mo and Bi XANES data for elemental molybdenum and bismuth, respectively, are available as supplementary material. In order to obtain uniform intervals of approximately constant k (the photoelectron wave vector) in A-', the EXAFS data were accumulated in three energy regions with ca. 1.2, 2.3, and 4.3 eV/point. X-ray Absorption Data Analysis. The molybdenum and bismuth XANES data were normalized according to a standard procedure.2' Briefly, the pre-edge X-ray absorption data were modeled with a linear function, which was fitted below the onset of the edge (over 85 eV) and extrapolated above the edge. The post-edge X-ray absorption data were also modeled with a linear function, which was fitted above the edge (over 580 eV) and extrapolated below the edge. The XANES data were obtained by first subtracting the pre-edge absorption approximation from the primary data and then dividing this difference by the post-edge absorption approximation. This normalization procedure facilitates a direct and quantitative comparison of the shapes and intensities of the features found in the XANES data obtained for each of the bismuth molybdates. In addition, it provides normalized EXAFS data with an X-ray absorption edge jump equal to unity. The first differentials of the normalized XANES data were also calculated by using an interpolatory cubic spline derivative evaluator and data smoother;z2these first-derivative XANES data for the Bi203.nMo03 solids ( n = 1-3) are available as supplementary material. The EXAFS data reduction and analysis were accomplished with conventional methods.23 In the conversion to k-space, k = ~~~
~
(20) Bearden, J. A.; Burr, A. F. Rev. Mod. Phys. 1967, 39, 125-142. (21) Antonio, M. R.; Brazdil, J. F.; Glaeser, L. C.; Mehicic, M.; Teller, R. G. J . Phys. Chem. 1988, 92, 2338. (22) Local FORTRAN routine employing algorithms from The IMSL Library for cubic spline interpolation (ICSCCU), evaluation (ICSEVU), differentiation (DCSEVU), and smoothing (ICSSCU). See: IMSL Library Reference Manual; IMSL Library: Houston, TX,1982; Vol 1, 2
Antonio et al. [0.263(E - E,)] V 2 in A-', experimental energy thresholds (Eo) of 13 450 and 20 040 eV for the Bi and Mo EXAFS data, respectively, were employed. To extract the EXAFS, x(k), from the primary experimental data, the post-edge background X-ray absorption was approximated by using cubic B-spline functions; these were fitted to the Bi (six sections of 3.322 A-' each) and Mo (six sections of 3.058 A-' each) X-ray absorption data. The normalized, background-subtracted, and k"-weighted ( n = 1 for Bi; n = 2 for Mo) EXAFS data (kflx(k) versus k, A-') for the a-,@-, and y-bismuth molybdates were Fourier transformed without phase correction to obtain modified radial distribution functions, dn(r? versus r' (A). Powder Neutron Diffraction Measurements. Time-of-flight (TOF) neutron diffraction data for @-Bi2O3-2MoO3 were collected under atmospheric conditions by using the special environment powder diffractometer (GPPD) at the intense pulsed neutron source (IPNS) at Argonne National Laboratory. The diffractometer was placed 20 m from the pulsed neutron source; a detailed description of the instrument and its operation has been published." Data from approximately 8.6 X lo5 pulses were used to record the diffraction pattern for @-bismuthmolybdate. The counts were binned in increments of 5 ps, and only those data from the detector bank in the backscattering mode (20 = 150') were employed in the structure refinement. This is the highest resolution data available with the GPPD. The sample (ca. 10 g) was contained in a seamless vanadium tube (ca. in. in diameter and 4 in. long), which was capped with aluminum plugs. A Rietveld-type profile least-squares programZ5for time-of-flight data was used for the structure analyses. Initially, the scale factor and background parameters were refined for 6 cycles. This was followed by successive refinements wherein other parameters (e.g., lattice, absorption, profile, extinction, and x, y , and z , as well as isotropic thermal parameters) were allowed to vary. Starting values were taken from a single-crystal X-ray investigation.16 The minimum T O F value (10 000 ps; approximately d = 1.O A) was chosen based upon a compromise between peak overlap and maximum data usage. As a check on the results, refinements were performed with two other minimum TOF values (8000 and 12000 ks); there were no significant differences between the results of these refinements. In the ensuing discussions, reference will be made to the TOF data with a minimum value of loo00 js.During the course of the structure refinement (Ryp = 10.2%), it became evident that a small amount of an impurity phase, identified as y-bismuth molybdate, was present. Atomic parameters for yBiZO3.MoO3were included in the refinement to account for the extraneous diffraction peaks. As a good structural model for this phase exists,I08" the atomic parameters were included as a fixed contribution to the diffraction pattern; only the scale factor and a single isotropic thermal parameter were allowed to vary. A series of refinements were performed to determine the atomic occupation parameters for ,8-Bi2O3-2MoO3. In one, the oxygen occupancies were fixed at 1.0 and the metal occupancies were allowed to vary. At the conclusion of the refinement, no metal occupation factor differed significantly from unity. In another, the metal occupancy factors were fixed at 1.0 and the oxygen occupations were individually allowed to vary. In this refinement, the occupation factors of two oxygen atoms, 0 4 and 0 5 , converged to values significantly lower than unity, 0.87 (4) and 0.86 (4), respectively. As we could ascribe no chemical justification for this, and lacking any other corroborating analytical evidence, we believe these results are not significant. In the final cycles of refinement (Rwp= 5.3%, Renpt= 4.6%), all atomic occupation factors were fixed at unity. Final atomic coordinates and listings of metal-oxygen, vacancy-xygen, and metal-metal distances (out (23) Teo, B. K. EXAFS: Basic Principles and Data Analysis; SpringerVerlag: Berlin, 1986. (24) Hauman, J. R.; Daley, R. T.; Worlton, T. G.; Crawford, R. K. IEEE Trans., Nucl. Sci. 1982, NS-29, 62. (25) Von Dreele, R. B.; Jorgenson, J. D.; Windsor, C. G. J . Appl. Crysrallogr. 1982, 15, 581. (b) Jorgenson, J. D.; Faber, J. ICANS-II. Proceedings of the VIth International Collaboration on Advanced Neutron Sources; Report No. ANL-82-80, Argonne National Laboratories: Argonne, IL, 1983.
Structural Characterization of Bismuth Molybdates TABLE I: Neutron Data Collection and Profile Analysis Parameters for B-Bi,0,-2MoOl P21/n, Z = 8 space group
cell const
a
b C
P
V
TOF min TOF max agreement factors 3201 points 99 variables
10000 ps 26 000 ps Rp RWP RClPt
11.9515 (4) A 10.7993 (4) A 11.8805 (4) A 90.142 (5)' 1533.38 (6) A' d = 0.95 A d = 2.41 A 3.7 5% 5.33% 4.63%
13398 13488 13418 13420 13438 13448 13458 13466 13478 13488
Energy i n e U Figure 2. Normalized bismuth L,-edge XANES data for a-Bi203. 3Mo0,, @-Bi203.2Mo03, and y-Bi203.Mo0,. Ordinate scale is 0.15
arbitrary units per division.
Figure 1. Representation of the @-Bi20,.2Mo0, structure projected along
(010). The Mo coordination geometries are indicated by tetrahedra. Bi atomic locations and fluorite sublattice cell vacancies are indicated by open and filled circles, respectively. Approximately eight unit cells are displayed. This view clearly illustrates the relationship between the 8-structure and the sublattice (fluorite) whose unit cell vectors are shown. to 5 A) as well as selected bond angles are available as supplementary material. The raw powder neutron diffraction data along with the calculated and difference profiles are shown as supplementary material. Table I contains the pertinent details of the neutron data collection and Rietveld refinement.
Results and Discussion Powder Neutron Diffraction Data. The structure of @Bi2O3.2MoO3 has been adequately described by Chen and Sleight.I6 There are no major discrepancies between the results of the reported single-crystal X-ray refinement and the results of the present powder neutron profile analysis. The coordination geometries of the four crystallographically independent molybdenum atoms consist of regular tetrahedra of oxygen atoms with an average Mo-0 interatomic distance of 1.79 (1) %, and an average 0-Mo-0 bond angle of 109". The lack of distortion of the Mo-04 tetrahedra can be attributed to their isolation from each other; no oxygen atom is bonded to more than one Mo atom. The coordination geometries of the four crystallographically independent Bi atoms differ from one another. Bi(3) displays a square antiprismatic coordination geometry, whereas the other three Bi atoms are bonded to eight oxygen atoms with irregular geometries typical of cations with stereochemically active lone pairs. A projection of the structure of @-Bi203-Mo03 is illustrated in Figure 1. This figure emphasizes the structural relationship between the @phase and a fluorite sublattice. Interestingly, the cation sublattice was adequately described by van den Elzen and Rieck, on the basis of powder X-ray diffraction data.5 According to Buttrey et al.,4 the cation positions
can be described as fluorite related (fluorite stoichiometry is MOz, four molecules per unit cell), in which one-ninth of the metal positions are vacant. The ordering of these vacancies determines the unit cell (approximately 9 times the fluorite unit cell) and defines the stoichiometry of the defect fluorite structure: ( )4M&2, where ( ) = cation vacancy, and M = Bi, Mo. Severe distortions in the oxygen sublattice account for the four- and eight-coordinate geometries of the Mo and Bi cations, respectively. The vacancy coordination is regular and is that of a square prism. Bismuth X-ray Absorption Data. The bismuth L,-edge XANES data for the three BizO3.nMoO3 phases ( n = 1-3) are shown in Figure 2. The edge peaks for both ar-BizO3.3MoO, and fl-Bi2O3.2MoO3are approximately equivalent in shape and position, ca. 13441 eV. Only the broad, post-edge peaks at ca. 13453 and 13 468 eK serve to distinguish the different Bi3+ ion environments in the a- and @-phases,respectively. By comparison, the Bi XANES data for y-BiZ03-Mo03are remarkably different from those noted above. Here, we see a shoulder on the absorption edge at 13 434 eV before the edge peak at 13 442 eV, which is followed by a weak, broad post-edge peak at 13 461 eV. The bismuth L,-edge EXAFS, k x ( k ) versus k, for the Bi203-nMo03phases ( n = 1-3) are shown in Figure 3. These data extend over a large k range (ca. 21 and are of very high quality in terms of the signal-to-noise characteristics. The corresponding Fourier transforms of the k x ( k ) EXAFS are shown in Figure 4. With reference to the known crystal and molecular structures of cy-, @-, and y-bismuth molybdate, it was possible to assign the peaks in these Fourier transforms. For ar-BiZO3~3MoO3, the Fourier transform (Figure 4a) exhibits an intense peak at ca. 1.5 8, (before phase correction), which is due to oxygen coordination, and three significant others at 3.12, 3.54, and 3.80 8, (before correction), which are due to a combination of distant bismuth and molybdenum backscatterings. This latter triplet of peaks can be directly associated with the highfrequency oscillations above ca. 8 A-1 in the corresponding kx(k) EXAFS data, Figure 3a. Below 8 the low-frequency oscillations of the EXAFS data give rise to the strong Bi-0 peak in the Fourier transform (Figure 4a).
2942 The Journal of Physical Chemistry, Vol. 92, No. 10, 1988 0.075
0.050
0.025
0
-0.025
-0.050
0.045
0.030
0.015
n
A! Y
-x
0
A! -0.015
-0.030
Antonio et al. correction) in the Fourier transform of the k x ( k ) EXAFS for @-Bi,03-2Mo03is only half that for the Bi-0 interaction in aBi203.3Mo03. The principal feature of the Fourier transform data for p-Biz0,.2Mo0, is the peak at 3.46 A (before correction), which is due to Bi-Bi interactions in the Bi3O2 chains; similarly, the peak at 3.17 A (before correction) is due to Bi-Bi backscatterings. Although there are distant Mo atoms around bismuth in @Biz0,.2Mo03, they exhibit a broad distribution of interatomic distances (from ca. 3.8 to 4.1 A; supplementary material), which smears out the EXAFS signal, thereby rendering the Bi-Mo interactions unobservable in the Fourier transforms of both the Bi (Figure 4b) and Mo (vide infra) EXAFS data. The complicated peak structure between 2 and 3 A may be due to, in part, backscattering by oxygen atoms in the irregular coordination sphere about bismuth, with known Bi-0 interatomic distances as long as 2.91 (1) A (supplementary material). As for p-Bi203.2Mo03, the principal feature in the Fourier transform of the kx(k) EXAFS data for y-Bi2O3.MoO3is the peak at 3.46 A (Figure 4c), which is due to Bi-Bi backscatterings. This strong scattering by the distant coordination sphere of bismuth cations dominates the k x ( k ) EXAFS data above 8 A-’; these data (Figure 3c) exhibit rapid oscillations with an apparent single frequency. Unlike either CY- or @-bismuthmolybdate, there are just six nearest oxygen atoms about the Bi3+ions in y-Bi203-Mo03. These oxygen atoms are asymmetrically disposed about the bismuth atoms with Bi-0 interatomic distances that fall into two ranges: the first with distances between 2.18 and 2.40 A and the second with distances ranging from 2.52 to 2.61 In line with this division of distances, the Fourier transform of the Bi EXAFS data for y-Bi20,.Mo03 exhibits peaks at 1.46 and 1.92 A (before correction), which are due to oxygen backscatterings. Molybdenum X-ray Absorption Data. The Mo K-edge XANES data for the a-,0-, and y-Bi203*nMo03phases ( n = 3, 2, and 1, respectively) are presented in Figure 5. The remarkable variations in the overall XANES features clearly indicate that the Mo6+ coordination environments for the three bismuth molybdates are substantially different. Note the slight decrease in the pre-edge peak positions (in parentheses) on going from y (20014.2 eV) to a (20012.8 eV) and then to @-bismuthmolybdate (2001 1.9 eV). In contrast, there are no significant differences between the energies of the inflection points of the pre-edges (20008.3, 20008.2, and 20008.1 eV) and the main edges (20021.4, 20021.3, and 20021.2 eV) for y-. cy-, and @-bismuth molybdate, respectively (see supplementary material). Most important, for this progression (7-, CY-,p-) of Bi-Mo phases, is the increase in the pre-edge peak intensities. That is, the Mo K-edge XANES data for y-Bi203-Mo03exhibit a small and not fully resolved peak on the rising K edge, whereas those data for @Bi203.2Mo03display a large and well-resolved peak. The corresponding feature in the Mo XANES data for cy-Bi203.3M00, is of intermediate magnitude. For K- and L,-edge XANES, this well-known type of pre-edge peak, which is due to a 1s nd electronic transition, is a sensitive indicator of the site symmetry of the absorbing atom. For example, .klosll
0.075
0.050
c h
t
A
0.025
0
-0.025
-0.050
-0.075
,
,
,
I
2
4
6
8
I
I
I
IO
12
14
,
18
I
I
20
18
k in Inverse Angstroms
Figure 3. Bismuth L3-edge EXAFS, k x ( k ) , for (a) a-Bi2O3.3MoO,, (b)
P-Bi2O3.2MoO3,and (c) y-Bi203.Mo03. The Fourier transform of the kx(k) EXAFS for P-Bi2O3-2MoO3 is displayed in Figure 4b. In comparison with those data for a-Bi203-3Mo03(Figure 4a), it is clear that the B i a s coordination sphere for @-Bi2O3-2MoO3 is more disordered. That is, although the Bi3+ ions in both a- and @-bismuthmolybdate are eight-coordinate, the magnitude of the Bi-0 peak at ca. 1.4 A (before 2.4-
2.0
b
Figure
-
n
j/
1
2.8
C 2.4-
1,
11
y-Bi,03.Mo03.
The Journal of Physical Chemistry, Vol. 92, No. 10, 1988 2943
Structural Characterization of Bismuth Molybdates 19970
19995
20020
20045
20676
20095
t
I 19970
I
I 20626
19995
I 20645
I 20670
I 26095
Energy i n e U Figure 5. Normalized molybdenum K-edge XANES data for aBi203.3Mo03,8-Bi2O3.2MoO3,and y-Bi203.Mo03. Ordinate scale is 0.165 arbitrary units per division.
for an X-ray absorbing atom in a centrosymmetric environment (e.g., octahedral coordination), the mixing of orbitals of different parities (e.g., d-p and s-p mixing) is symmetry forbidden. In such a case, pre-edge absorption peaks due to 1s nd quadrupole transitions are weak and seldom observed. For an X-ray absorbing atom in a noncentrosymmetric environment (e.g., tetrahedral coordination), substantial d-p orbital mixing can occur. In this instance, pre-edge 1s nd quadrupole transitions intensify and are commonly observed in high-resolution XANES data. Therefore, the intensity of the 1s nd transition can be used to distinguish between a tetrahedral (with no inversion symmetry) coordination environment, on the one hand, and an octahedral (with inversion symmetry) one on the other. It can also be used to determine the distortion of an octahedral complex from perfect Oh symmetry; distortions facilitate d-p orbital mixing, thereby enhancing the pre-edge quadrupole 1s nd transition. With reference to the known structures of the a-,@-, and y-bismuth molybdates, the points discussed above are well-illustrated. For &Biz03-2Mo03,the Mo6+ions are in nearly perfect Mo-04 tetrahedral environments, vide supra. As predicted, the Mo XANES data for this phase exhibits the strongest 1s 4d pre-edge peak. For y-Bi203.Mo03, the Mo6+ions are in somewhat distorted M d 6 octahedral environments with four short and two long Mo-O bonds with average interatomic distances of 1.8 1 and 2.25 A, respectively.lOvll Again, as predicted, the Mo K-edge XANES for y-bismuth molybdate exhibit the weakest 1s 4d peak. Finally, for a-Bi2O3.3MoO3,the Mo6+ions are surrounded by five oxygen atoms in an asymmetric Mo-05 coordination environment, with an average Mc-0 interatomic distance of 1.91 The XANES data for this phase display a pre-edge peak whose intensity is approximately intermediate of the peak intensities for 8- and y-bismuth molybdate. Beyond the pre-edge peaks, the next features observed in the Mo XANES data (Figure 5 ) are assigned as 1s 5p electronic transitions. For /3-Bi2O3-2MoO3,this transition, which appears as an absorption edge shoulder at 20030.4 eV has the lowest intensity, whereas for the a-phase, the corresponding shoulder at 20 03 1.5 eV is somewhat more intense; for the y-phase, a well-
-
-
-
-
-0.844 0
I
,
2
4
, 6
8
10
12
-
16
18
k in Inverse Angstroms
Figure 6. Molybdenum K-edge EXAFS, k2x(k),for (a) a-Bi2O3.3MoO3, (b) @-Bi203.2Mo03, and (c) y-Bi203.Mo03.
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resolved peak at 20030.6 eV becomes visible. This order of increasing 1s 5p absorption intensities (b-, a-,y-) is exactly 4d peak intensities. the reverse of the order of increasing 1s In the tetrahedral ligand field about Mo in /3-Bi20,.2Mo03, the substantial mixing of molybdenum d orbitals with oxygen p orbitals, which leads to the observation of the aforementioned 1s nd quadrupole transition, is expected to delocalize the Mo6+ 5p transition 5p virtual orbital manifold. Therefore, the 1s for /3-Bi203.2Mo03is observed as a weak shoulder. Conversely, in the nearly octahedral ligand field about Mo in y-Bi2O3-MoO3, the slight d-p orbital mixing does not affect the unoccupied Mo6+ 5p orbital manifold. Hence, the 1s 5p transition for yBi203-Mo03is observed as a fully resolved peak. With moderate d-p orbital mixing, such as for the Mo6+ ions in a-Bi203.3Mo03, the 1s 5p transition is observed as a medium intensity shoulder. The principal edge peaks at 20 044.8,20 043.1,and 20 044.6 eV in the Mo K-edge XANES data for a-,/3-, and y-bismuth molybdate, respectively, are due to the first single-scattering resonances (Le., EXAFS) of the Mo-0 interactions. This interpretation is in accord with the recently proposed free-electron wavelength (A = r ) scattering model for the interpretation of XANES.26
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(26) Lytle, F. W.; Greegor, R. B.; Panson, A. J. Phys. Rev. B Condens. Matter, 1988, 37, 1550-1562.
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The Journal of Physical Chemistry, Vol. 92, No. 10, 1988
Antonio et al.
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Figure 7. Fourier transforms, &(r? without phase correction,of the Mo EXAFS for (a) (u-Bi203.3Mo03, (b) @-Bi2O3-2MoO3, and (c) y-Bi,03.Mo03.
The molybdenum K-edge EXAFS, k 2 x ( k )versus k , and the Fourier transforms of these data for Bi203.nMo03(n = 1-3) are shown in Figures 6 and 7, respectively. The Fourier transform data for a-Bi203.3Mo03exhibit a great deal of structural complexity (see Figure 7a). The irregular oxygen coordination about the Mo6+ ions gives rise to three peaks at 1.26, 1S3, and 1.97 8, (before correction). Also, the distant Bi and Mo cations around molybdenum give rise to three peaks at 2.86, 3.30, and 3.65 A (before correction), which are due to Mo.-Mo, Mo-Bi, and Mo.-Mo/Bi backscatterings, respectively. The k 2 x ( k )EXAFS data and the Fourier transform of these data for @-Bi203-2Mo03 (Figures 6b and 7b, respectively) are much less complex than those data for either a- or y-bismuth molybdate. The EXAFS data for @-Bi203.2M00,are a damped, single-frequency sinusoid; the corresponding Fourier transform data exhibit a single, intense peak at 1.35 A (before correction), which is indicative of strong oxygen coordination about molybdenum. There is little evidence for the presence of distant (ca. 3-5 A) Mo-M ( M = Mo, Bi) interactions, such as those observed for a-Bi203.3Md3(Figure 7a). These EXAFS data indicate that the tetrahedral Mo-O, groups in @-Bi203.2Mo03 are isolated from neighboring ones by a lack of direct oxygen atom connectivities (i.e., M&Mo scattering pathways). Also, as was noted before, the large spread of Mo.-Bi interatomic distances reduces both the coherence of the Moq-Bi backscatterings and, thereby, the intensity of the associated EXAFS signal. The Fourier transform of the Mo EXAFS for y-Bi203-Mo03 shows (Figure 7c) an intense peak at 1.38 8, and a weak one at 1.75 8, (before correction); both are due to oxygen coordination about molybdenum, which is in approximately C, symmetry. The first peak is due to four short Mo-O bonds and the second to two long ones. Finally, the distant, medium intensity peak at 3.46 A (before correction) in Figure 7c is predominately due to M w M o backscatterings. Backscattering by bismuth atoms provides only a minor contribution to the peak intensity.
Summary A descriptive analysis of the bismuth L3-edge and molybdenum K-edge X-ray absorption data for the a-, @-, and y-bismuth molybdate phases was presented. With reference to the crystal structures of the Bi2O3.nMoO3phases (for n = 3, 2, and l ) , the complex, multishell radial structure functions obtained by Fourier
transformation of the EXAFS data were interpreted. The Bi and Mo EXAFS results are summarized as follows: For a-Bi2033MoO3, we observed (i) a strong, single-shell Bi-08 coordination sphere; (ii) a distant, triple-shell environment attributable to Bi-M ( M Bi, Mo) interactions; (iii) an irregular, three-shell oxygen coordination environment about Mo; and (iv) a three-shell arrangement of distant cations about Mo. For @-Bi2O3-2MoO3, there was (i) a weak, single-shell B i a s coordination sphere; (ii) a double shell of distant Bi atoms around Bi; (iii) a single-shell Mo-O4 coordination sphere; and (iv) no evidence for backscattering by metal cations about Mo. For y-Bi2O3.MoO3,we found (i) double-shell coordination environments about both Bi and Mo attributable to six nearest oxygen neighbors and (ii) single-shell coordination spheres of distant Bi and Mo/Bi cations around Bi and Mo, respectively. The Bi and Mo XANES data for the Bi20,.nMo0, phases were shown to be sensitive to and characteristic of the different structural environments about the Bi3+ and Mo6+ ions. Finally, the results of a high-resolution powder neutron diffraction profile analysis of @-Bi203-2Md3 confirmed the previously reported single-crystal X-ray refinement and emphasized the structural relationship between the cation/vacancy positions and a fluorite sublattice.
Acknowledgment. We thank Drs. V. A. Biebesheimer and R. K. Grasselli for assistance and Professor J. M. Thomas (The Royal Institution) for useful discussions. SSRL is supported by the United States Department of Energy, Office of Basic Energy Sciences, and the National Institutes of Health, Biotechnology Resource Program, Division of Research Resources. Registry No. Bi,03-3Mo0,, 13595-85-2; BizO3.2Mo0,,16229-40-6; Bi03.Mo03, 13565-96-3. Supplementary Material Available: Figures I and 11, showing raw Bi L3-edge and Mo K-edge X-ray absorption data for the Bi203-nMo03phases, Figures I11 and IV, showing the first differential of the Bi and Mo XANES data, also for the BiZO3nMoO, phases, Figures V and VI, showing normalized XANES data for elemental molybdenum and bismuth, Figure VII, showing powder neutron diffraction data, calculated profile, and difference profile for /3-Bi2O3.2MoO,, and Tables I-IV, listing fractional atomic coordinates, interatomic distances, and bond angles also for P-Bi,03.2Mo03 (25 pages). Ordering information is given on any current masthead page.
