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Isolation of Intermediate-Valent Ce(III)/Ce(IV) Hydrolysis Products in the Preparation of Cerium Iodates: Electronic and Structural Aspects of Ce2(IO3)6(OHx) (x ≈ 0 and 0.44) Richard E. Sykora,† Laura Deakin,‡ Arthur Mar,‡ S. Skanthakumar,§ L. Soderholm,§ and Thomas E. Albrecht-Schmitt*,† Department of Chemistry, Auburn University, Auburn, Alabama 36849, Department of Chemistry, University of Alberta, Edmonton, Alberta, Canada T6G 2G2, and Chemistry Division, Argonne National Laboratory, Argonne, Illinois 60439 Received November 10, 2003. Revised Manuscript Received January 28, 2004
The layered cerium iodates Ce2(IO3)6(OHx) [x ≈ 0 (1) and 0.44 (2)] have been prepared from the reaction of (NH4)2Ce(NO3)6 with I2O5 at 180 °C in aqueous media. The structure of these compounds consists of square antiprismatic Ce centers that are bound by bridging iodate anions and one bridging oxo anion. The structure of 2 differs from that of 1 in that the bridging oxo atom is partially protonated, resulting in significant lengthening of the Ce-O bond from 2.050(1) to 2.212(2) Å. This protonation also results in partial reduction of 1 from a formally all Ce4+ compound to an intermediate-valent ion with both Ce3+ and Ce4+ that is visually observed with a color change in the crystals from bright yellow to dark brown for 1 and 2, respectively. Magnetic susceptibility measurements on 1 show there to be a small paramagnetic impurity phase that is consistent with about 1% Ce3+ in the sample. Similar measurements on 2 show that it contains at least 22% Ce3+, determined from a measured effective moment of 1.19(5) µB/mol Ce. This result corresponds to a formulation of 2 as Ce2(IO3)6(OH0.44). Analysis of Ce L3-edge XANES spectra supports this formulation. UV-vis diffuse reflectance spectra indicate charge-transfer or delocalization of the 4f1 electrons in 2. These results are discussed in terms of an intermediate-valent state in 2 that is composed of f states on Ce and p states on the bridging O. Crystallographic data (193 K): 1, orthorhombic, space group Pnma, a ) 6.8297(4) Å, b ) 16.6725(9) Å, c ) 14.2487(8) Å, Z ) 4; 2, orthorhombic, space group Pnma, a ) 6.8534(4) Å, b ) 16.989(1) Å, c ) 14.2404(8), Z ) 4.
Introduction The assignment of oxidation states for Ce has long been fraught with difficulties owing to the strong oxidizing ability of Ce(IV) (E° ) 1.70 V in 1 M HClO4) that gives rise to both fully reduced Ce(III) products as well as compounds with intermediate- or mixed-valent Ce(III)/Ce(IV) states.1 This phenomenon is particularly pronounced in aqueous media, owing to the kinetically controlled oxidation of water by Ce(IV).2 Reduction of Ce(IV) to Ce(III) has not always been recognized, and incorrect assignment of oxidation states is common, particularly when structural data are of poor quality.3 †
Auburn University. University of Alberta. Argonne National Laboratory. (1) (a) Chan, G. Y. S.; Drew, M. G. B.; Hudson, M. J.; Isaacs, N. S.; Byers, P.; Madic, C. Polyhedron 1996, 15, 3385. (b) Dvorkin, A. A.; Krasnova, N. F.; Simonov, Yu. A.; Abashkin, V. M.; Yakshin, V. V.; Malinovskii, T. I. Kristallografiya 1984, 29, 471. (c) Gun’ko. Yu. K.; Elliot, S. D.; Hitchcock, P. B.; Lappert, M. F. J. Chem. Soc., Dalton Trans. 2002, 8, 1852. (d) Cassani, M. C.; Gun′ko, Y. K.; Hitchcock, P. B.; Hulkes, A. G.; Khvostov, A. V.; Lappert, M. F.; Protchenko, A. V. J. Organomet. Chem. 2002, 647, 71. (2) Cotton, F. A.; Wilkinson, G. Advanced Inorganic Chemistry, 5th ed.; Wiley: New York, 1988; p 976. (3) Matkovic, B.; Grdenic, A. Acta Crystallogr. 1963, 16, 456. ‡ §
Bond-valence sum calculations4,5 offer a reliable and easy method for calculating the oxidation state(s) of Ce in a variety of bonding arrangements, and the bond valence parameter for the Ce-O bond has been well developed.6 These calculations are especially useful when the ancillary ligands are noninnocent.6,7 The terms mixed-valent and intermediate-valent are often used interchangeably and have a variety of operational definitions that involve nonintegral and/or variable valence states exhibited by rare-earth and actinide systems. They are grouped with a number of anomalous electronic behaviors that include the simple Kondo and lattice Kondo effects, screened and unscreened heavy-fermion behavior, nonfermi liquids, and spin glasses.8-12 Intermediate valence is defined herein as a nonintegral formal valence for Ce ions that sit on (4) Brown, I. D.; Altermatt, D. Acta Crystallogr. 1985, B41, 244. (5) Brese, N. E.; O′Keeffe, M. Acta Crystallogr. 1991, B47, 192. (6) Roulhac, P. L.; Palenik, G. J. Inorg. Chem. 2003, 42, 118. (7) Sofen, S. R.; Cooper, S. R.; Raymond, K. N. Inorg. Chem. 1979, 18, 1611. (8) Deakin, L.; Ferguson, M. J.; Sprague M. J.; Mar A.; Sharma, R. D.; Jones, C. H. W. J. Solid State Chem. 2002, 164, 292. (9) (a) Varma, M. C. Rev. Mod. Phys. 1976, 48, 219. (b) Bornick, R. M.; Stacy A. M. Chem. Mater. 1994, 6, 333.
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a single crystallographic site. The complexity of intermediate-valency is aptly illustrated by the cluster compound Ce4O(OiPr)13(iPrOH),13 which contains a Ce4(µ4-O) butterfly core. There are two possibilities for assigning the oxidation states for the cerium atoms in this cluster. If a localized view of the Ce4+15 core is taken, then it implies one Ce(III) and three Ce(IV) atoms. However, it is also possible that a single electron is delocalized over two or more of the Ce sites. Analysis of the single crystal structure of this compound reveals that there are only two crystallographically unique Ce sites, and therefore, complete localization is not possible. However, the coordination numbers and bond valence sums5-7 for the two different Ce2 units within this core are distinct and suggest one Ce28+ unit and one Ce27+ unit, with the electron delocalized over two crystallographically related Ce sites. Cerocene, Ce(η8-C8H8)2, also presents itself as an interesting example of potential ambiguity in oxidation state assignment for a Ce compound. Here the complex was originally formulated as Ce4+ with two (η8-C8H8)2anions. However, subsequent calculations suggested the compound was better formulated as [Ce3+{(C8H8)2}3-].14 Later ab initio calculations supported this assignment with a ground state consisting of 83% 4f1e2uπe2u3 (i.e. Ce3+) and 17% 4f0πe2u4 (i.e. Ce4+),15 making cerocene an intermediate-valent compound. Subsequent X-ray absorption near-edge structure (XANES) measurements also concluded that the ground state of cerocene is primarily Ce3+.16 Herein we disclose the preparation and characterization of a new cerium iodate system that obtains intermediate-valence. Iodate has in fact played an important early role in cerium chemistry and has been used to separate Ce(IV) from Ln3+ (Ln ) lanthanide) cations in aqueous media.17 The compositions and structures of both Ce(III) and Ce(IV) iodates, Ce(IO3)3 and Ce(IO3)4, are both well-established.18,19 However, given the strong acidity of hydrated Ce(IV), hydrolysis often occurs, resulting in oligomer and polymer formation. One such hydrolysis product is Ce2(IO3)6(O) (1), which can be partially protonated to yield Ce2(IO3)6(OH0.44) (2), an intermediate-valent Ce(III)/Ce(IV) compound. We have characterized these new cerium iodates through a combination of single-crystal X-ray diffraction, UV-vis diffuse reflectance spectroscopy, magnetic susceptibility, and Ce L3-edge XANES measurements. (10) (a) Bud′ko, S. L.; Canfield, P. C.; Mielke, C. H.; Lacerda, A. H. Phys. Rev. B 1998, 57, 13624. (b) Kaczorowski, D.; Kruk, R.; Sanchez, J. P.; Malaman, B.; Wastin, F. Phys. Rev. B 1998, 58, 9227. (c) Karla, I.; Pierre, J.; Skolozdra, R. V. J. Alloys Compd. 1998, 265, 42. (11) Coqblin, B. NATO Sci. Ser., II: Math., Physics Chem. 2003, 110 (Concepts in Electron Correlation), 277. (12) Stewart, J. R. Rev. Modern Phys. 2001, 73, 797. (13) Yunlu, K.; Gradeff, P. S.; Edelstein, N.; Kot, W.; Shalimoff, G.; Strieb, W. E.; Vaartstra, B. A.; Caulton, K. G. Inorg. Chem. 1991, 30, 2317. (14) Neumann, C. S.; Fulde, P. Z. Phys. 1989, B74, 277. (15) (a) Dolg, M.; Fulde, P.; Ku¨chle, W.; Neumann, C. S.; Stoll, H. J. Chem. Phys. 1991, 94, 3011. (b) Dolg, M.; Fulde, P.; Stoll, H.; Preuss, H.; Chang, A.; Pitzer, R. M. Chem. Phys. 1995, 195, 71. (16) Edelstein, N. M.; Allen, P. G.; Bucher, J. J.; Shuh, D. K.; Sofield, C. D. J. Am. Chem. Soc. 1996, 118, 13115. (17) (a) Brinton, P. H. M. P.; James, C. J. Am. Chem. Soc. 1919, 41, 1080. (b) Willard, H. H.; Yu, S. T. Anal. Chem. 1953, 25, 1754. (18) Abrahams, S. C.; Bernstein, J. L.; Nassau, K. J. Solid State Chem. 1976, 16, 173. (19) Ibers, J. A. Acta Crystallogr. 1956, 9, 225.
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Experimental Section Syntheses. (NH4)2Ce(NO3)6 (99.0%, Baker), Ni(NO3)2‚6H2O (99.9%, Baker), and I2O5 (98%, Alfa Aesar) were used as received. Distilled and Millipore filtered water with a resistance of 18.2 MΩ was used in all reactions. SEM/EDX analyses were performed using a JEOL 840/Link Isis instrument. Ce and I percentages were calibrated against standards. Typical results are within 5% of the ratios determined from singlecrystal X-ray diffraction experiments. IR spectra were collected on a Nicolet 5PC FT-IR spectrometer from KBr pellets. Ce2(IO3)6(O) (1). (NH4)2Ce(NO3)6 (584 mg, 1.066 mmol), Ni(NO3)2‚6H2O (310 mg, 1.066 mmol), and I2O5 (356 mg, 1.066 mmol) were loaded in a 23 mL PTFE-lined autoclave. Water (2 mL) was then added to the solids. The autoclave was sealed and placed in a box furnace that had been preheated to 180 °C. After 72 h the furnace was cooled at 9 °C/h to 23 °C. The product consisted of a green solution over yellow prismatic crystals of 1. The mother liquor was decanted from the crystals, which were then washed with methanol and allowed to dry. Yield: 325 mg (68% based on I). EDX analysis for Ce2(IO3)6(O) provided a Ce:I ratio of 1:3. IR (KBr, cm-1, Ce-O, I-O): 851 (m), 816 (m), 801 (m), 761 (m), 750 (m), 720 (s, br), 685 (m, br), 527 (s, br). Ce2(IO3)6(OH0.44) (2). (NH4)2Ce(NO3)6 (451 mg, 0.823 mmol) and I2O5 (549 mg, 1.645 mmol) were loaded in a 23 mL PTFElined autoclave. Water (1 mL) was then added to the solids. The autoclave was sealed and placed in a box furnace that had been preheated to 180 °C. After 72 h the furnace was cooled at 9 °C/h to 23 °C. The product mixture consisted of a yellow solution over a small number of large dark brown prisms of 2 and a large number of bright yellow crystals with an approximately octagonal bipyramidal habit. The mother liquor was decanted from the crystals, which were then washed with methanol and allowed to dry. The yellow crystals were found to be Ce(IO3)4 from single-crystal X-ray diffraction measurements.19 Yield of 2: 74 mg (13% based on Ce). EDX analysis for Ce2(IO3)6(OH0.44) provided a Ce:I ratio of 1:3. IR (KBr, cm-1, Ce-O, I-O): 851 (m), 829 (m), 808 (m), 796 (s), 724 (m), 709 (m), 678 (m), 528 (s, br). Crystallographic Studies. Single crystals of Ce2(IO3)6(O) (1) and Ce2(IO3)6(OH0.44) (2) with the dimensions 0.230 mm × 0.110 mm × 0.020 mm and 0.250 mm × 0.182 mm × 0.152 mm, respectively, were mounted on glass fibers, cooled to -80 °C with an Oxford Cryostat, and optically aligned on a Bruker SMART APEX CCD X-ray diffractometer using a digital camera. A rotation photo was taken for each crystal and a preliminary unit cell was determined from three sets of 30 frames with 10 s exposure times using SMART. All intensity measurements were performed using graphite-monochromated Mo KR radiation from a sealed tube with a monocapillary collimator. For both compounds, the intensities of reflections of a sphere were collected by a combination of three sets of exposures. Each set had a different φ angle for the crystal and each exposure covered a range of 0.3° in ω. A total of 1800 frames were collected with an exposure time per frame of 30 s for 1 and 2. For both compounds, determination of integrated intensities and global cell refinement were performed with the Bruker SAINT (v 6.02) software package using a narrow-frame integration algorithm. A face-indexed analytical absorption correction was initially applied using XPREP.20 Individual shells of unmerged data were corrected analytically and exported in the same format. These files were subsequently treated with a semiempirical absorption correction by SADABS.21,22 The program suite SHELXTL (v 6.12) was used for (20) Sheldrick, G. M. SHELXTL PC, Version 6.12, An Integrated System for Solving, Refining, and Displaying Crystal Structures from Diffraction Data; Siemens Analytical X-ray Instruments, Inc.: Madison, WI, 2001. (21) SADABS. Program for absorption correction using SMART CCD based on the method of Blessing: Blessing, R. H. Acta Crystallogr. 1995, A51, 33. (22) Huang, F. Q.; Ibers, J. A. Inorg. Chem. 2001, 40, 2602.
Aspects of Ce2(IO3)6(OHx) (x ≈ 0 and 0.44) Table 1. Crystallographic Data for Ce2(IO3)6(O) (1) and Ce2(IO3)6(OH0.44) (2) formula formula mass (amu) color and habit crystal system space group a (Å) b (Å) c (Å) V (Å3) Z T (°C) λ (Å) Fcalcd (g cm-3) µ(Mo KR) (cm-1) R(F) for Fo2 > 2σ(Fo2)a Rw(Fo2)b
Ce2(IO3)6(O) 1345.66 yellow, prism orthorhombic Pnma (No. 62) 6.8297(4) 16.6725(9) 14.2487(8) 1622.5(2) 4 -80 0.71073 5.509 170.60 0.0160 0.0380
Ce2(IO3)6(OH0.44) 1346.16 brown, prism orthorhombic Pnma (No. 62) 6.8534(4) 16.989(1) 14.2404(8) 1658.0(2) 4 -80 0.71073 5.391 166.94 0.0302 0.0646
a R(F) ) ∑||F | - |F ||/∑|F |. b R (F 2) ) [∑[w(F 2 - F 2)2]/ o c o w o o c ∑wFo4]1/2.
space group determination (XPREP), direct methods structure solution (XS), and least-squares refinement (XL).20 The final refinements included anisotropic displacement parameters for all non-hydrogen atoms and a secondary extinction parameter. Some crystallographic details are listed in Table 1. Additional details can be found in the Supporting Information. Magnetic Measurements. Magnetic data were measured on powders in gelcap sample holders with a Quantum Design PPMS 9T magnetometer/susceptometer between 2 and 300 K and in applied fields up to 9 T. Alternating current susceptibility measurements were made with a driving amplitude of 1 Oe and frequencies between 10 and 2000 Hz. DC susceptibility measurements were made under zero-field-cooled conditions with an applied field of 2.5 T for Ce2(IO3)6(O) (1) and 1.0 T for Ce2(IO3)6(OH0.44) (2). Susceptibility values were corrected for the sample diamagnetic contribution according to Pascal’s constants23 (Ce4+, -17 × 10-6; IO3-, -50 × 10-6; O2-, -12 × 10-6 emu) as well as for the sample holder diamagnetism. XANES. X-ray absorption near-edge structure (XANES) spectra were obtained from powder samples at room temperature on the BESSRC bending magnet beam line 12-BM-B at the Advanced Photon Source (APS). The beam line is equipped with a Si(111) double-crystal monochromator and a Pt mirror that is required to remove higher order harmonics, which are present because of the high critical energy of the APS ring. Data were collected in fluorescence mode using a Canberra multielement Ge detector. The Cr K edge was used for energy calibration, with the Cr foil first-derivative peak set to 5989 eV. At least two spectra were averaged for each sample. No beam reduction was observed. UV-Vis Diffuse Reflectance Spectra. Diffuse reflectance spectra for 1 and 2 were measured using a Shimadzu UV2501 spectrophotometer equipped with an integrating sphere attachment. The Kubelka-Monk function was used to convert diffuse reflectance data to absorbance spectra.24
Results and Discussion Syntheses. The reaction of (NH4)2Ce(NO3)6 with I2O5 in aqueous media results in the precipitation of Ce(IO3)4 under ambient conditions. When mild hydrothermal conditions (180 °C, autogenously generated pressure) are applied to this same reaction for 3 d, Ce2(IO3)6(OH0.44) (2) and Ce(IO3)4 result. The addition of Ni(NO3)2‚6H2O to this reaction mixture results in the formation of Ce2(IO3)6(O) (1) as the sole solid product. The Ni2+ remains in solution, and its role in this (23) Mulay, L. N.; Boudreaux, E. A. Theory and Applications of Molecular Diamagnetism; Wiley-Interscience: New York, 1976. (24) Wendlandt, W. W.; Hecht, H. G.: Reflectance Spectroscopy. Interscience Publishers: New York, 1966.
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reaction is unclear, but it is essential in the formation of 1. For both 1 and 2, the compounds have been isolated in low to moderate yields as large single crystals. The striking difference between these two compounds is that crystals of 1 are bright yellow and those of 2 are dark brown. Structures: Ce2(IO3)6(O) (1) and Ce2(IO3)6(OH0.44) (2). The structures of 1 and 2 are remarkably complex given their rather simple formulas, which is a reflection of the high coordination number of cerium and the binding flexibility of iodate to f-block elements.13,25,26 The structure of 2 is only a minor variation on that of 1, and they will therefore be discussed together. The layered architectures of 1 and 2 are constructed from square antiprismatic Ce atoms that are bound by iodate anions that bridge between Ce centers, as shown in Figure 1. In addition, there is a bridging oxo atom between the Ce centers in 1 that apparently becomes partially protonated to yield 2 (vide infra). The resulting dimer that is formed by the corner-sharing of two [CeO8] square antiprisms is shown in Figure 2. It is important to note for this and future discussions that there is only one crystallographically unique Ce atom in each compound. The Ce-O (IO3-) distances vary from 2.323(2) to 2.531(2) Å in 1 and from 2.327(6) to 2.557(6) Å in 2. There is an average lengthening of the Ce-O (IO3-) distances by 0.005(6) Å between 1 and 2, which is obviously not a large enough change to be statistically significant. However, there is a significant change in the Ce-O (oxo) distances in the Ce-O-Ce linkage between 1 and 2. In 1, the Ce-O (oxo) distance is 2.050(1) Å, whereas in 2 this bond becomes significantly lengthened to 2.212(2) Å. This also corresponds to an increase in the Ce‚‚‚Ce distances from 3.949(1) Å in 1 to 4.203(2) Å in 2. The expansion of the Ce-O-Ce linkage is also reflected in the b-axis length, which increases from 16.6725(9) Å in 1 to 16.989(1) Å in 2, which corresponds well with the increase observed in the Ce-Oxo distance. The increased Ce-Oxo distance can be ascribed to partial protonation of the oxo atom to yield a bridging hydroxo group. This assertion is supported by physical property measurements discussed later in this paper. Furthermore, by using an average bond-valence parameter of 2.094 Å, which does not assume either a Ce(III) or Ce(IV) oxidation state, we arrive at bond-valence sums for Ce in 1 and 2 of 4.29 and 3.71. If we assume that both compounds contain Ce(IV) (R0 ) 2.068(12) Å), then values of 4.00 and 3.46 for 1 and 2, respectively, are found.6 These values support 2 being an intermediate-valent Ce(III)/Ce(IV) compound. The iodate anions in 1 and 2 are present in two different binding modes. The iodate anions containing I(1), I(3), and I(4) bridge between two Ce centers and have one terminal oxygen atom. The I-O bond distances for these iodate anions range from 1.786(2) to 1.843(2) Å for 1, and from 1.790(6) to 1.844(5) Å for 2. On the (25) (a) Gupta, P. K. S.; Ghose, S.; Schlemper, E. O. Z. Kristallogr. 1987, 181, 167. (b) Gupta, P. K. S.; Ammon, H. L.; Abrahams, S. C. Acta Crystallogr. 1989, C45, 175. (c) Abrahams, S. C.; Bernstein, J. L. J. Chem. Phys. 1978, 69, 2505. (d) Abrahams, S. C.; Bernstein, J. L. Solid State Commun. 1978, 27, 973. (e) Abrahams, S. C.; Bernstein, J. L.; Nassau, K. J. Solid State Chem. 1977, 22, 243. (26) Shehee, T. C.; Sykora, R. E.; Ok, K. M.; Halasyamani, P. S.; Albrecht-Schmitt, T. E. Inorg. Chem. 2003, 42, 457.
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Figure 1. A view of the square antiprismatic Ce atoms in Ce2(IO3)6(O) (1) (a) and Ce2(IO3)6(OH0.44) (2) (b) that are bound by iodate anions. 50% probability ellipsoids are depicted.
Figure 2. A depiction of the [CeO8] square antiprisms in Ce2(IO3)6(O) (1) (a) and Ce2(IO3)6(OH0.44) (2) (b) showing the cornersharing of an oxo/hydroxo group. The Ce-O(11) bond becomes lengthened upon protontation from 2.050(1) Å in 1 to 2.212(2) Å in 2. 50% probability ellipsoids are depicted.
basis of these bond distances, there is no evidence for protonation of the terminal oxo atoms. The iodate anion containing I(2) utilizes all three oxygen atoms to bind three Ce atoms, and its I-O bond distances vary from 1.799(3) to 1.824(2) Å for 1 and from 1.800(5) to 1.820(5) Å for 2. The linking of the [Ce2O] core by the iodate anions gives rise to a complex layered structure, shown in Figure 3. There is an additional long interaction between iodate anions and cerium centers in different layers that is on the order of 2.961(3) Å. If this interaction is included, then the cerium atoms can be treated as capped square antiprisms, and the structure becomes three-dimensional in nature. This interaction is approximately 0.4 Å longer than the other Ce-O bond distances, and while is it potentially important for imparting crystallinity to the structure, its substantially longer distance leads us to treat this structure as consisting of interdigitated layers. Selected bond distances for 1 and 2 are given in Tables 2 and 3, respectively. Magnetic Properties. The magnetic susceptibility of Ce2(IO3)6(O) (1), obtained as a function of temperature under an applied field of 2.5 T, is depicted is Figure 4. The data are fit, over the temperature range from 10 to 300 K, to the modified Curie-Weiss law27,28
C + χ0 χ) T+θ
(1)
where C is the Curie constant
Ng2µB2J(J + 1) C) 3k
(2)
θ is the Weiss constant, χ0 is the temperature-independent susceptibility (TIP), and µB is the Bohr magneton. J is the Russell Saunders quantum number for the ground multiplet. The experimentally determined effective moment, which is often used for comparison with the free-ion value, g[J(J + 1)]1/2, is related to C by
µeff )
( ) 3kC NµB2
1/2
(3)
It is important to note that, in a multicomponent system, it is the susceptibilities and therefore (µeff)2 that are additive, and not the effective moments themselves. The fit yields an effective moment of µeff ) 0.26(1) µB/ mol Ce, a TIP of 0.00067(1) emu/mol Ce, and a θ ) 0.71 K. The figure inset shows the same data plotted as χT vs T, a formalism that assumes Curie behavior with a significant temperature-independent susceptibility (TIP) but no significant Weiss contribution. (27) Smart, J. S. Effective Field Theories of Magnetism; W. B. Saunders: Philadelphia, U. S., 1966; p 188. (28) White, R. M. Quantum Theory of Magnetism; Springer-Verlag: Berlin, Germany, 2nd Edition, 1983; p 282.
Aspects of Ce2(IO3)6(OHx) (x ≈ 0 and 0.44)
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Figure 4. The magnetic susceptibility of Ce2(IO3)6O (1) obtained as a function of temperature under an applied field of 2.5 T. The solid line through the data represents a CurieWeiss law fit. The inset shows the same data, plotted assuming a negligible Weiss constant to show the importance of the TIP contribution.
Figure 3. An illustration of the linking of [Ce2O] cores by iodate anions to give rise to a complex interdigitated layered structure for Ce2(IO3)6(O) (1) and Ce2(IO3)6(OH0.44) (2). The structure of 1 is depicted viewed down the c axis (a) and down the a axis (b). The latter part of the figure shows three interdigitated layers. Table 2. Selected Bond Distances (Å) for Ce2(IO3)6(O) (1) Ce(1)-O(1) Ce(1)-O(3) Ce(1)-O(4) Ce(1)-O(5) I(1)-O(1) I(1)-O(2) I(1)-O(3) I(2)-O(4) I(2)-O(5) I(2)-O(6)
2.498(2) 2.476(3) 2.324(2) 2.416(3) 1.843(2) 1.802(2) 1.786(2) 1.806(2) 1.799(3) 1.824(2)
Ce(1)-O(6) Ce(1)-O(7) Ce(1)-O(9) Ce(1)-O(11) I(3)-O(7) I(3)-O(7) I(3)-O(8) I(4)-O(9) I(4)-O(9) I(4)-O(10)
2.531(2) 2.323(2) 2.331(3) 2.050(1) 1.822(2) 1.822(2) 1.803(4) 1.830(2) 1.830(2) 1.796(4)
Table 3. Selected Bond Distances (Å) for Ce2(IO3)6(OH0.44) (2) Ce(1)-O(1) Ce(1)-O(3) Ce(1)-O(4) Ce(1)-O(5) I(1)-O(1) I(1)-O(2) I(1)-O(3) I(2)-O(4) I(2)-O(5) I(2)-O(6)
2.486(5) 2.486(5) 2.327(6) 2.395(6) 1.844(5) 1.795(5) 1.790(6) 1.807(6) 1.800(5) 1.820(5)
Ce(1)-O(6) Ce(1)-O(7) Ce(1)-O(9) Ce(1)-O(11) I(3)-O(7) I(3)-O(7) I(3)-O(8) I(4)-O(9) I(4)-O(9) I(4)-O(10)
2.557(6) 2.332(5) 2.358(6) 2.212(2) 1.819(5) 1.819(5) 1.793(8) 1.822(6) 1.822(6) 1.781(8)
From charge-balance considerations, Ce in 1 is expected to be tetravalent, with no valence electrons, and therefore should have only a negative, temperatureindependent, diamagnetic susceptibility. The observation of a temperature-dependent moment of 0.26 µB indicates the presence of a paramagnetic ion, which is assumed to be an impurity, because there are formally no unpaired spins in the compound as written. The impurity phase most probably involves Ce3+, the amount
Figure 5. The magnetization at 2 K of Ce2(IO3)6(OH0.44) (2) obtained as a function of temperature under an applied field of 1 T. The solid line through the data represents a CurieWeiss law fit. The inset shows the same data, plotted assuming a negligible Weiss constant to show the importance of the TIP contribution.
of which can be calculated assuming a free-ion Curie contribution of 0.804 and 0.0 emu/mol Ce for Ce3+ and Ce4+, respectively. This approach indicates about 1% Ce3+. Following the same approach, but assuming that the impurity is Ni2+, which was used in the preparation, there would be approximately 2% Ni. In either case, this small quantity of impurity is not expected to influence the structural refinement. The magnetic susceptibility of Ce2(IO3)6(OHx) (2), obtained under an applied field of 1.0 T and as a function of temperature, is depicted is Figure 5. Fitting to χ ) C/(T + θ) + χ0 yields a Curie constant of 0.18(2), a TIP of 0.0017(3) emu/mol Ce, and a θ ) 0.37(1) K. The effective moment obtained from this fit is µeff ) 1.19(5) µB. The small θ is confirmed by a plot of χT vs T shown in the inset. The effective moment determined for 2 is intermediate between that expected for trivalent and tetravalent Ce. There are two scenarios that could result in a significantly lower effective moment than that expected from the free ion: (i) the moment is partially quenched by the crystal field or (ii) there is a nonintegral Ce
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valence. Applying Russell Saunders coupling, appropriate for the Ce3+, 4f1 configuration, and Hund’s rule, the 2F 5/2 multiplet is expected to be the ground term. This multiplet is split by an octahedral crystal-field potential into a low-lying Γ7(1) doublet, which has an effective moment of approximately 1.5-2.5 µB, depending on the symmetry and magnitude of the crystal-field potential.29,30 However, as demonstrated by the linearity of the χT vs T plot, there is no evidence of a splitting of the ground term with an excited-state doublet of less than about 400 K, a realistic value for this iodate system. The more likely scenario is that Ce has a nonintegral, or intermediate, valence in Ce2(IO3)6(OHx). The experimental moment can be used to estimate the ratio of Ce3+/Ce4+ if the susceptibilities are assumed to be additive and if the effective moment of Ce3+ is assumed to be near the free-ion value.27,31 This approach will yield a minimum estimate of the amount of trivalent Ce because the free-ion value is the maximum susceptibility and because the measured moment in intermediate-valent systems has been demonstrated to be less than expected from the Ce3+/Ce4+ ratio.32,33 Analysis of the susceptibility data shows there to be at least 22% trivalent Ce in 2, which corresponds to a stoichiometry of Ce2(IO3)6(OH0.44). The TIP contribution to the susceptibility is somewhat larger than expected for a noninteracting, localized, freeion moment. Contributions to a TIP can have two origins. The first, corresponding to van Vleck paramagnetism, arises from second-order terms connecting crystal field matrix elements from different crystal-field levels.30 The second, known as Pauli paramagnetism, arises from the susceptibility of delocalized electrons. A large Pauli paramagnetism has been previously observed for compounds with intermediate-valent Ce.34 XANES. The source of the reduced effective moment on Ce and the large TIP term determined from the fitting of the susceptibility data are resolved by the XANES (X-ray absorption near edge structure) spectra shown in Figure 6. XANES can be used qualitatively to approximate the valence of an absorbing ion, both in terms of its peak shape, for the case of Ce, and the edge position.35 The fingerprint of a Ce3+ spectrum is a single resonance, or white line, centered at 5727 eV, whereas a Ce4+ spectrum has two lines centered to higher energies, 5731 and 5738 eV, with a separation of about 7 eV. The figure compares the XANES spectra of Ce(IO3)3 and Ce2(IO3)6(O) (1) with the spectrum obtained from 2. The target compound has a spectrum that appears as an admixture of the two standard compounds. Such behavior is inconsistent with that expected from a Ce3+ ion with a magnetic moment that is (29) Lea, K. R.; Leask, M. J. M.; Wolf, W. P. J Phys. Chem. Solids 1962, 23, 1381. (30) Staub, U.; Soderholm, L. Handbook on the Physics and Chemistry of Rare Earths; Gschneidner, K. A., Jr., Eyring, L., Maple, M. B., Eds.; North-Holland: Amsterdam, The Netherlands, 2000; Vol. 30, pp 491-545. (31) See for example, Mitchell, K.; Huang, F. Q.; McFarland, A. D.; Haynes, C. L.; Somers, R. C.; van Duyne, R. P.; Ibers, J. A. Inorg. Chem. 2003, 42, 4109. (32) Maple, M. B.; Wohlleben, D. Phys. Rev. Lett. 1971, 27, 511. (33) Soderholm, L.; Antonio, M. R.; Skanthakumar, S.; Williams, C. W. J. Am. Chem. Soc. 2002, 124, 7290. (34) Mishra, S. N. J. Phys.: Condens. Matter 2003, 15, 5333. (35) Antonio, M. R.; Soderholm, L.; Williams, C. W.; Ullah, N.; Francesconi, L. C. J. Chem. Soc., Dalton Trans. 1999, 21, 3825.
Sykora et al.
Figure 6. The Ce L3-edge XANES data from a trivalent standard (Ce(IO3)3) and tetravalent Ce2(IO3)6O (1) compared with the spectrum from Ce2(IO3)6(OH0.44) (2). The ratio of trivalent to tetravalent Ce in 2 is obtained from a least-squares fit of the relative sums of the data from trivalent and tetravalent Ce compounds. The best fit indicates 25(5)% trivalent Ce in 2.
reduced by crystal-field effects. Instead, the spectrum observed is consistent with intermediate-valent Ce.36 The observed XANES spectrum cannot distinguish between an intermediate-valent Ce and a mixed-valent Ce, defined as two independent, electronically static Ce on crystallographically inequivalent sites, one trivalent and the other tetravalent. However, the structural refinement includes only one crystallographically equivalent Ce, ruling out the possibility of a mixed-valent compound. An estimate of the Ce3+/Ce4+ ratio in 2 can be extracted from the XANES data37 by assuming that the intermediate-valent spectrum is a simple linear combination of representative trivalent and tetravalent standard spectra. Using the XANES data from Ce(IO3)3 and 1 as representative tri- and tetravalent standards respectively, a fit of the spectrum from 2,38 shown in Figure 6, indicates about 25(5)% Ce3+. The large error was included because there are large differences observed in the two peak intensities from a variety of Ce4+ spectra that arise from differences in coordination environments.39 Although efforts were made to ensure similar chemical and coordination environments between the standards used herein and 2, the absence of a full understanding of XANES spectroscopy, particularly as applied to Ce(IV), prohibits confidence in the fitting to greater than the error quoted. On the basis of the combination of the aforementioned magnetic susceptibility measurements and XANES data, we provide a final formulation of 2 as Ce2(IO3)6(OH0.44). Diffuse Reflectance. The UV-vis diffuse reflectance spectra of Ce2(IO3)6(O) (1) and Ce2(IO3)6(OH0.44) (2) are shown in Figure 7. From these spectra it is evident that both compounds have band edges near 480 nm. In addition, 2 has a low-lying, broad band with an absorption maximum near 625 nm that spans most of the remaining visible spectrum. This band accounts for the dark brown coloration of 2 that sharply contrasts with (36) Rohler, J. X-ray Absorption and Emission Spectra; Gschneidner, K. A., Ed.; North-Holland: Amsterdam, The Netherlands, 1987; Vol. 10, pp 453-545. (37) Soderholm, L.; Antonio, M. R.; Williams, C.; Wasserman, S. R. Anal. Chem. 1999, 71, 4622. (38) Ressler, T. J. Synchrotron Radiat. 1998, 5, 118. (39) Antonio, M. R.; Soderholm, L. Inorg. Chem. 1994, 33, 5988.
Aspects of Ce2(IO3)6(OHx) (x ≈ 0 and 0.44)
Figure 7. UV-vis diffuse reflectance spectra of Ce2(IO3)6(O) (1) and Ce2(IO3)6(OH0.44) (2).
the bright yellow color of 1. This absorption feature likely corresponds to delocalization of the 4f1 electrons that result from the partial reduction of Ce(IV) to Ce(III) in 2. Since there is only one crystallographically unique Ce site in these compounds, there is no reason to localize the reduced sites. Conclusions The structural, magnetic, XANES, and diffuse reflectance data taken together indicate that Ce2(IO3)6(OH0.44) is an intermediate-valent compound. The origin of the intermediate-valent behavior is revealed by the structure. There are two Ce4+ ions in 1, with a separation of 3.949(1) Å, that are linked via an oxygen ion, which has a Ce-O distance of 2.050(1) Å. The O becomes protonated in 2, as demonstrated by the lengthening of the Ce-O distances in the linked Ce-O-Ce moiety to 2.212(2) Å and the Ce‚‚‚Ce distance to 4.203(2) Å. It is our hypothesis that the addition of H to 1 to form 2 adds an electron into a molecular orbital formed from states in the isolated Ce-O-Ce linkage. Explicitly, this interaction involves the hybridization of the localized Ce f-states with O p-states of the appropriate symmetry to form an intermediate-valent wave function of the form
ΨIV ) 2Rφf + βφp where R and β are the mixing coefficients. The molecular orbital allows the electron to delocalize over both Ce ions to form an intermediate-valent material.40,41 This ex(40) Fulde, P. Electron Correlations in Molecules and Solids; Springer: Berlin, Germany, 1995; Vol. 100.
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planation accounts for the single crystallographic site for Ce because the electronic character of all Ce ions is equivalent. Intermediate-valency is also consistent with the effective moment observed for 2 together with the high temperature-independent paramagnetism and the presence of both Ce3+ and Ce4+, which is needed to interpret the XANES data. The diffuse reflectance results support the localized nature of the molecular orbitals, which are limited to contributions from two Ce atoms and one O atom. Metallic behavior would be expected if the intermediate-valent state was more extended and the electrons were delocalized over all available Ce. Such behavior is not observed. The absence of fully delocalized states in 2, as seen in most intermediate-valent materials, makes it difficult to further interpret the intermediate-valent state of Ce within the standard models, such as those used in Kondo or heavy-fermion systems, which include delocalized band states explicitly in their theories. In this regard, further work on this rather unique Ce compound would provide a different perspective with which to study delocalized f states. The similarity in the structures of 1 and 2 raises the possibility that Ce2(IO3)6(OHx) is a line compound, in which x can vary over a significant range. If such is the case, the small magnetic moment observed in 1 may arise from a slight degree of protonation, corresponding to a small x value of about 0.02 in that particular sample. The existence of Ce2(IO3)6(OHx) as a line compound would permit the synthesis of several compounds that would differ only by the number of electrons added to the ΨIV states. The ability to tune the intermediate-valent character of the sample would provide a further opportunity to probe the metrics associated with the conductivity and stability range of this unusual electronic state. Acknowledgment. This work was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Heavy Elements Program (Grant DE-FG0201ER15187). An acknowledgment is also made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this research. Work at the APS and at ANL was supported by U.S. DOE, OBES, Chemical Sciences, under Contract no. W-31-109-ENG-38. Supporting Information Available: X-ray crystallographic files for Ce2(IO3)6(O) (1) and Ce2(IO3)6(OH0.44) (2) in CIF format. This material is available free of charge via the Internet at http://pubs.acs.org. CM035147E (41) For example, see the following volume: J. Phys. Condens. Matter 1996, 8.
