860
J. Phys. Chem. C 2008, 112, 860-866
Probing the Local Environments of Fluorine in Ce-Based Fluorite-Type Oxyfluorides with 19F MAS NMR Spectroscopy Laetitia Sronek,† Je´ roˆ me Lhoste,† Manuel Gaudon,† Christophe Legein,‡ Jean-Yves Buzare´ ,§ Monique Body,§ Guillaume Crinie` re,| Alain Tressaud,† Stanislav Pechev,† and Alain Demourgues*,† ICMCB, CNRS, UniVersite´ Bordeaux 1, 87 AVenue du Dr. A. Schweitzer, 33608 Pessac Cedex, France, Laboratoire des Oxydes et Fluorures, CNRS UMR 6010, Institut de Recherche en Inge´ nierie Mole´ culaire et Mate´ riaux Fonctionnels, CNRS FR 2575, UniVersite´ du Maine, AVenue OliVier Messiaen, 72085 Le Mans Cedex 9, France, Laboratoire de Physique de l’Etat Condense´ , CNRS UMR 6087, Institut de Recherche en Inge´ nierie Mole´ culaire et Mate´ riaux Fonctionnels, CNRS FR 2575, UniVersite´ du Maine, AVenue OliVier Messiaen, 72085 Le Mans Cedex 9, France, and Rhodia, Centre de Recherche d’AuberVilliers, 52 Rue de la Haie-Coq, 93308 AuberVilliers Cedex, France ReceiVed: July 2, 2007; In Final Form: September 17, 2007
Ce-based oxyfluorides Ce1-xCaxO2-x-y/2Fy, with 0.13 e x e 0.29 and 0.03 e y e 0.24, adopting the fluorite structure, were prepared by coprecipitation in basic fluorinated medium followed by an annealing under air at T ) 600 °C. XRD profile and Rietveld analyses allowed the determination of the crystallite sizes as well as the unit cell parameters. In this series, fluorine atoms are in tetrahedral environments of cations. 19F magic angle spinning (MAS) NMR spectroscopy was used to study the local structure and fluoride ion environments. Four distinct 19F resonances were observed and assigned to four different types of environments, FCa4, FCa3Ce, FCa2Ce2, and FCaCe3, whose proportions vary with the calcium content. In these Ce-Ca oxyfluorides, F- anions have a great affinity for Ca2+ cations leading to an increase of the F amount with the Ca content. The absence of FCe4 environment is explained from a steric standpoint: the F-Ce bond lengths in the network are too short to accept fluorine ions in the vicinity of four Ce4+ cations. Finally, the increase of the average Ce-X (X ) O, F) bond ionicity with the Ca and F contents was correlated to the evolution of the UV shielding properties of these new compounds, which exhibit outstanding UV absorption and scattering (in the visible range) properties.
Introduction Ceria and derived compositions have been widely investigated as oxygen buffers for three way catalysts (TWC),1 as electrolytes or electrodes in solid oxide fuel cells (SOFC’s),2 and as inorganic UV filters.3 The band gap energy around 3.2 eV4 and the broad absorption spectrum in the UV range show that CeO2 exhibits outstanding UV shielding properties. The refractive index of nanocrystalline cerium oxide films is strongly dependent on the film density.5,6 For instance, for the density of bulk CeO2 (7.21 g‚cm-3) the refractive index measured by ellipsometry is around 2.355 in the visible range. However, for a lower calculated density around 6.7 g‚cm-3, due to the polycrystalline character of the films and the occurrence of voids, the refractive index is found at 2.12.6 Electronic density calculation coupled with electron energy loss spectroscopy measurements also show that the refractive index remains rather high, that is, around 2.457 in the visible range, and it is necessary to get highly divided particles or aggregates with sizes around 30 nm in order to limit scattering and reach a high transparency in the visible range.7-9 However, CeO2 appears with a pale-yellow coloration and a slight absorption in the visible range. For various industrial * Corresponding author. Tel: 05-40-00-26-55, 05-40-00-27-61; e-mail:
[email protected]. † ICMCB, CNRS, Universite ´ Bordeaux. ‡ Laboratoire des Oxydes et Fluorures. § Laboratoire de Physique de l’Etat Condense ´. | Rhodia, Centre de Recherche d’Aubervilliers.
applications, it is relevant to keep a white color and avoid any yellowish hue. Then the incorporation of Ca2+ with a larger ionic size, a lower formal charge, and a smaller electronegativity than Ce4+ ions10 as well as F- ions with a higher electronegativity than O2- anions into ceria were attempted because CaF2 and CeO2 compounds adopt the same fluorite network with an almost identical cell parameter equal to 5.46 Å11 and 5.41 Å,12 respectively. Furthermore, the refractive index of CaF2 around 1.5 in the visible range and the contribution of fluorine because of its high electronegativity allow for the reduction of the electronic polarizability and consequently the refractive index of ceria. The occurrence of Ca2+ and F- ions in the vicinity of Ce4+ and O2- should contribute to strongly modify the absorption and scattering properties at the origin of the evolution of the band gap associated with the 2p (O) f 4f (Ce) charge transfer involving O2- and Ce4+ ions. Various Ce1-xCaxO2-x-y/2Fy compositions were prepared by coprecipitation in a basic fluorinated medium and were characterized by several techniques mentioned in a previous paper.13 Magnetic measurements showed the absence of Ce3+ paramagnetic species, and the chemical composition was determined by WDS analysis and F- titration with a specific electrode. These compounds exhibit an affinity for water and carbonates, which increases with the Ca content. The fluorite-type structure of the compounds (SG: Fm3hm) was investigated by X-ray diffraction, allowing us to determine the cell parameters and the crystallite size. A solubility limit corresponding to 30% of
10.1021/jp075130+ CCC: $40.75 © 2008 American Chemical Society Published on Web 12/23/2007
Local Environments of Fluorine the Ca2+ ions was deduced from the appearance of CaF2 observed by XRD analysis and from the occurrence of Vegard’s law. HRTEM coupled with EELS measurements gave evidence for nanoparticles (ø ) 10 nm) with a rather good homogeneity in composition identified on isolated particles. In this paper, a Rietveld analysis of powder XRD data was carried out as well as 19F NMR study in order to identify the local environments of fluorine. The evolution of Ce-X (X ) O, F) chemical bonding in this series was described and discussed considering the variation of UV absorption properties and the energy shift of the optical band gap versus the chemical composition. The oxidation catalytic activity of these Ce-based oxyfluorides has not yet been evaluated.
J. Phys. Chem. C, Vol. 112, No. 3, 2008 861 TABLE 1: Rietveld Data and Experimental Conditions for Data Collection formula symmetry space group radiation peak shape function FWHM function reflections collected 2θ range (deg) parameters used in refinement
Ce1-xCaxO2-x-y/2Fy cubic Fm3hm Cu KR1 (λ ) 1.54059 Å), graphite monochromator pseudo-Voigt function using the Thompson-Cox-Hastings formulas H2G ) (U + DST2) tan2 θ + V tan θ + W + IG/cos2 θ HL ) X tan θ + Y/cos θ 13 15 < 2θ < 120 16
Experimental Methods Preparation of Ce1-xCaxO2-x-y/2Fy and Chemical Composition Analysis. Ce1-xCaxO2-x-y/2Fy solid solutions were prepared as detailed elsewhere13 by coprecipitating cerium and calcium oxyhydroxides in a basic medium at pH > 12 followed by the maturation of the obtained particles during 3 h (samples b-e) or 20 h (sample a), and then fluorinated during 1 h with a basic solution (pH > 12) from HF and NaOH. The precipitate was collected, and crystalline compounds with pale-yellow coloration were obtained after annealing under air at 600 °C during 12 h. The Ce/Ca atomic ratio was determined by wavelength dispersive spectrometry (WDS) using a CAMECA SX 100 microprobe and the fluorine content by F- titration with a specific electrode. X-ray Powder Diffraction. The compounds were characterized by X-ray powder diffraction using a PANalytical X’Pert Pro diffractometer equipped with a X’Celerator detector in a Bragg-Brentano geometry with monochromated Cu KR radiation (KR1 ) 1.54059 Å). The patterns were collected at room temperature over a 2θ range of 15-120° with 0.017° steps. Accurate lattice parameters and crystallite sizes were determined by a Rietveld14 refinement using the Thompson-CoxHastings15 function (function 7 in the Fullprof package)16 for the profile-fitting. 19F Solid-State NMR. The 19F MAS NMR spectra were recorded on an Avance 300 Bruker spectrometer (7 T) with a Larmor frequency of 282.2 MHz for 19F, using a high-speed CP MAS probe with a 2.5 mm rotor. The external reference chosen for the isotropic chemical shift determination was C6F6 (δisoC6F6 vs CFCl3 ) -164.2 ppm).17 The spectra were acquired using a single pulse sequence (1 µs), followed by the free induction decay acquisition. The delay between two acquisitions was 1 s. For such a delay, the quantitativity of the spectra was checked. The discrimination of isotropic peaks from spinning sidebands was achieved by recording spectra at two different spinning frequencies (20 and 25 kHz). The reconstructions of the 19F NMR spectra were performed with the DMFIT software,18 including spinning sidebands, using six adjustable parameters: isotropic chemical shift δiso, chemical shift anisotropy δaniso, chemical shift asymmetry parameter η, line width, relative line intensity, and line shape. δiso, δaniso, η, the relative line intensity, and the line shape were assumed to be independent of the spinning rate. In this study, the δiso values, the relative intensities, and the line widths are the relevant parameters; the other parameters will not be discussed at all in the following. Results and Discussion Structural Analysis of Ce1-xCaxO2-x-y/2Fy Compounds by the Rietveld Refinement. All of the Ce-Ca based oxyfluorides
TABLE 2: Compositions, Ca/Ce Atomic Ratio, Lattice Parameters (Å), (Ce,Ca)-(O,F) Distances (Å), and Particle Sizes τc (nm) of Ce1-xCaxO2-x-y/2Fy Samples samples
formula
a b ca d e
Ce0.71Ca0.29O1.59F0.2400.17 Ce0.75Ca0.25O1.67F0.1700.16 Ce0.80Ca0.20O1.77F0.0700.16 Ce0.87Ca0.13O1.86F0.0300.11 Ce1-xCaxO2-x-y/2Fy0z + CaF2
Ca/Ce atomic ratio 0.410 (4) 0.342 (8) 0.252 (5) 0.156 (6) 0.40 (3)
a (Å) 5.4237(2) 5.4233(2) 5.4162(2) 5.4170(2) 5.4244(1) 5.4636(2)
dM-X (Å)
tc (nm)
2.3485 2.3484 2.3453 2.3456
7 9 9 9 8
a For this composition, the lattice parameter and the particle size was determined from diffractogram recorded on a Philips PW 1050 diffractometer in a Bragg-Brentano geometry with Cu KR radiations (KR1 and KR2).
adopt the cubic fluorite-type structure (SG: Fm3hm), which can be described as a face-centered cubic unit cell in which eightfold coordinated cations are distributed at the corner of a cube and fourfold coordinated anions occupy all of the tetrahedral sites. Cations (Ce/Ca) and anions (O/F) were assumed to be randomly distributed in 4a (0,0,0) and 8c (1/4,1/4,1/4) sites, respectively. The Thompson-Cox-Hastings function was used for the profile fitting because of the broadness of the diffraction peaks, a consequence of the very small crystallite sizes. The experimental conditions for data collection are summarized in Table 1. By comparison with CeO2, the XRD diffractograms of Ce1-xCaxO2-x-y/2Fy show a shift of the diffraction peaks toward lower 2θ in accordance with the expansion of the lattice resulting of the substitution of Ca2+ ions for Ce4+ ions with a larger ionic radius (rCa2+ ) 1.12 Å, rCe4+ ) 0.97 Å).19 The a lattice parameters and the τc crystallite sizes of four compositions determined by the Rietveld refinement are listed in Table 2. The Ce, Ca, and O/F occupancies are fixed to the values found on the basis of chemical analyses. The (Ce,Ca)-(O,F) distances deduced from the refined lattice parameters are also reported. The crystallite sizes of the compounds are around 7-9 nm, and the lattice parameters range from 5.4170(2) Å for Ce0.87Ca0.13O1.86F0.03 to 5.4237(2) Å for Ce0.71Ca0.29O1.59F0.24. One sample, whose elemental analysis has given 29 mol % Ca2+ and 36 mol % F- (sample e), appears to be a mixture of two phases whose lattice parameters are 5.4244(1) Å and 5.4636(2) Å in good agreement with the occurrence of Ce-Ca based oxyfluoride Ce1-xCaxO2-x-y/2Fy and CaF2 compound (a ) 5.46 Å),11 respectively. The refinement of this two-phase mixture is illustrated in Figure 1 and compared with the refinement of pure phases: Ce0.75Ca0.25O1.67F0.17 and Ce0.71Ca0.29O1.59F0.24. Because of the very close lattice parameters of CaF2 and CeO2 compounds (5.46 Å and 5.41 Å,12 respectively), the CaF2 impurity is difficult to identify. Nevertheless, the enlargement of the diffractograms between 43° < 2θ < 63° clearly shows
862 J. Phys. Chem. C, Vol. 112, No. 3, 2008
Sronek et al. TABLE 3: Atomic Positions, Isotropic Thermal Displacements (Å2), Occupations, and Reliability Factors of Ce0.75Ca0.25O1.67F0.17 and Ce0.71Ca0.29O1.59F0.24 Samplesa cRp ) 8.42
cRwp ) 7.88
RBragg ) 1.82
Biso (Å2)
occupancy (%)
0 0 0 0 0 0 0.5 0.5 0.5
0.44(2) 0.44(2) 1.16(8)
0.75 0.25 0.92
cRp ) 6.87
cRwp ) 6.67
RBragg ) 1.64
Biso (Å2)
occupancy (%)
0.61(2) 0.61(2) 1.35(9)
0.71 0.29 0.92
Ce0.75Ca0.25O1.67F0.17 atoms
site
Ce Ca O/F
4a 4a 8c
x
Ce0.71Ca0.29O1.59F0.24 atoms
site
Ce Ca O/F
4a 4a 8c
x
y
y
z
z
0 0 0 0 0 0 0.5 0.5 0.5
a Estimated standard deviations multiplied by Berar’s factor are indicated in parentheses.
TABLE 4: Bond Lengths (Å), Cationic/Anionic Coordination Number, and Brown and Altermatt r0 Constant (Å) Estimation in CaO, CeO2, CeF4, and CaF2 Binary Systems
compound 12
CeO2 CeF422 CaF211 CaO23
Figure 1. X-ray diffraction patterns of Ce0.75Ca0.25O1.67F0.17 (top), Ce0.71Ca0.29O1.59F0.24 (middle), and a mixture of Ce1-xCaxO2-x-y/2Fy + CaF2 (bottom) samples: (a) observed (...), calculated (-) signals, and (b) difference diagram; the tick marks represent the Bragg position of the diffraction lines.
the presence of CaF2 defined by straight diffraction lines on the left of the Ce1-xCaxO2-x-y/2Fy-phase peaks. The presence of CaF2 as an impurity was also evidenced using 19F MAS NMR.13 The Rietveld refinement does not allow quantifying of
chemical bond
bond lengths (Å)
[cation]/[anion] coordination numbers
r0 (Å)
Ce-O Ce-F Ca-F Ca-O
2.343 2.200-2.304 2.365 2.404
8/4 8/2 8/4 6/6
2.0870 1.9866 1.8541 1.9965
the CaF2 content because, as suggested by Brindley,20 the calculated intensity must be corrected by a particle absorption contrast factor. Otherwise as seen in Figure 1 for pure phase samples, a good agreement was obtained between the experimental and calculated diffractograms. The Rietveld data resulting from the refinement of X-ray diffractograms corresponding to Ce0.75Ca0.25O1.67F0.17 and Ce0.71Ca0.29O1.59F0.24 compositions, such as the RBragg and the conventional agreement cRp and cRwp factors are collected in Table 3. The isotropic thermal displacements, Biso, were refined by fixing the cation and anion occupancies from the elemental chemical composition. In this model, Ce4+ and Ca2+ cations are randomly distributed in the fluorite network (4a Wickoff position), and taking into account the presence of oxygen vacancies, cations can be six-, seven-, or eightfold coordinated to oxygen and fluorine at bond distances around 2.35 Å. As the fluorine and calcium contents increase, the isotropic thermal displacements, Biso, of both cations and anions seem to increase slightly. This evolution can be explained by the local deviations of cations from Wickoff positions leading to various (Ce,Ca)-(O,F) chemical bondings with different bond lengths, as developed in the next section, and not by the anionic vacancies whose molar concentration remains stable around 0.15 whatever the composition. Evaluation of Cation-Anion Bond Lengths in FluoriteType Networks Based on the Brown-Altermatt Model. CeF, Ca-F, Ce-O, and Ca-O distances in a fluorite-type structure were estimated using the bond-valence calculation proposed by Brown and Altermatt.21 This model describes the relationship between the bond length (rij) and the bond valence (sij) associated with i and j elements: Vi ) ∑jsij ) ∑j exp((r0 rij)/(B)) where B is an empirical parameter whose value is estimated for an oxide to be 0.37. Vi represents the oxidation state of the i element, and r0 is a constant characteristic of the i-j bonds: here r0 was refined from the reported bond lengths in CeF4,22 CaF2,11 CeO2,12 and CaO23 (see Table 4). Then, the average rij bond length in the fluorite-type structure can be
Local Environments of Fluorine
J. Phys. Chem. C, Vol. 112, No. 3, 2008 863
TABLE 5: Estimated M(Ce/Ca)-X(O/F) Chemical Bond Lengths (Å) in Fluorite Network Based on the Brown-Altermatt Model21
M-X
rcation (Å) (to respect the cation valence)
ranion (Å) (to respect the anion valence)
Ce-O Ca-O Ce-F Ca-F
2.34 2.51 2.24 2.37
2.34 2.25 2.50 2.37
deduced in order to respect either the cation or the anion valence that can lead to a different result, that is, a pair of rij values can be proposed for each bond length (Table 5). Nevertheless, for systems with equilibrated effective charges, that is, for Ce-O and Ca-F distances in CeO2 or CaF2 fluorite-type structures, only one rij value is obtained per compound. Otherwise, in a hypothetical fluorite-type structure with nonequilibrated charges as in “CeF2” or “CaO2” systems, Ce4+ cations require Ce-F bond lengths of 2.24 Å, whereas F- anions need higher Ce-F distances (2.50 Å); and in the same way Ca2+ ions in eightfold coordination can be predicted at 2.51 Å from oxygen ions, whereas a Ca-O distance at 2.25 Å is calculated for O2- ions in Ca2+ tetrahedral coordination. Thus, several opposite trends have to be taken into account when CeO2 is partially substituted by CaF2, and it appears to be difficult to predict the average Ce-F and Ca-O bond lengths inside the Ce1-xCaxO2-x-y/2Fy fluorite-type structure. Nevertheless, in all cases, the occurrence in the same compound of Ce-F and Ca-O bonds will induce some imbalance either on the cation side or on the anion side, so these chemical bonds will induce some constrains and relaxations. On the contrary, because CeO2 and CaF2 are both equilibrate charge systems with roughly the same bond lengths (2.343 and 2.365 Å, respectively), one can already predict a segregation of the fluoride ions around the calcium cations; indeed it is the best way to limit bond discrepancies inside Ce1-xCaxO2-x-y/2Fy compounds. Finally, one should notice that the Ce-F average bond length value is about 2.5 Å and in the same way Ca-O bond lengths are about 2.25 Å, that is, the bond lengths predicted from the anionic center. Indeed, because the tetrahedral coordination sphere of the anions is less flexible than the eightfold coordination sphere of the cations, it was considered that the respect of the anions valence governs the bond lengths. Local Environments of the Fluorine Ions Probed by 19F MAS NMR. With the aim of studying the local environments of fluorine ions and more particularly by identifying the distributions of Ca2+ or Ce4+ cations around fluoride ions located in 8c sites, 19F NMR measurements were undertaken on the Ce1-xCaxO2-x-y/2Fy samples. The 19F MAS NMR spectra recorded at 25 kHz are reported in Figure 2. The signal-to-noise ratio is low for all of the samples and decreases from spectrum a to spectrum d, in agreement with the related decreasing low F contents. The reconstructions of the spectra, recorded at 20 and 25 kHz, were achieved with four or three, for the lowest Ca content, broad lines between 60 and 190 ppm. An example is given in Figure 3. The isotropic chemical shift values, relative intensities, and line widths deduced from the reconstruction are gathered in Table 6. The line widths do not depend on the spinning frequency, which indicates that heteronuclear and homonuclear dipolar interactions are fully averaged at 20 kHz. Then, the line widths of the NMR resonances, which increase with the Ca, F, and vacancy contents, mirror the isotropic chemical shift distributions. This may be related to the occupancy of Ce4+ and Ca2+ cations as well as
Figure 2. 19F MAS NMR spectra of (a) Ce0.71Ca0.29O1.59F0.24, (b) Ce0.75Ca0.25O1.67F0.17, (c) Ce0.80Ca0.20O1.77F0.07, and (d) Ce0.87Ca0.13O1.83F0.03 samples at 25 kHz. The number of scans is equal to 2k for samples a and b, 8k for sample c, and 36 k for sample d. The twofold arrow points out the 19F δiso value range in CeF4. The sticks indicate the four isotropic 19F NMR lines. The width of the sticks is proportional to the δiso variation with composition.
Figure 3. Experimental (top) and calculated (bottom) 19F MAS NMR spectrum of Ce0.71Ca0.29O1.59F0.24 at 20 kHz. The unnumbered lines are the spinning side bands.
O2- and F- anions on the same crystallographic sites (4a and 8c, respectively) and the occurrence of anion vacancies that lead to various F-Ce and F-Ca bond lengths. The isotropic chemical shifts as well as the relative intensities of the NMR lines depend on the Ca and F contents as shown in Figures 4 and 5, respectively. In the following discussion, the resonances at 63-65, 113116, 150-160, and 180-190 ppm, corresponding to four different fluorine environments, will be referred as lines 1-4, respectively (Table 6). The corresponding chemical shifts will
864 J. Phys. Chem. C, Vol. 112, No. 3, 2008
Sronek et al.
TABLE 6: Isotropic Chemical Shift Values (ppm), Relative Intensities (%), and Line Widths (ppm) of the 19F NMR Lines, with Assignments, as Deduced from NMR Spectrum Simulations for Ce1-xCaxO2-x-y/2Fy Samples line 1 samples
δiso ((1)
I ((0.5)
a b c d attribution
63.5 64.0 65.0
12.9 13.0 2.8 0.0 FCa4
line 2 width ((0.5) 11.0 9.0 6.0
δiso ((1) 113.5 114.5 116.0 115.5
I ((0.5) 28.3 24.4 24.8 25.8 FCa3Ce
be noted δiso(i) with i ) 1-4. Because the δiso(1) value is close to the 19F δiso value in CaF2 (58 ppm),17 line 1 has been tentatively assigned to fluorine ions surrounded by four Ca2+ cations. The other δiso values appear to be closer to the 19F δiso value in CaF2 than 19F δiso values in CeF4 (between 360 and 399 ppm)24 Moreover, in CeF4, each fluorine ion is twofold coordinated to cerium cations. Then, the attributions of the resonances to different distributions of Ca2+ and Ce4+ cations around fluorine atoms cannot be straightforward. In order to achieve these attributions, we have to take into account the cation-fluorine bond distances using the superposition model, proposed by Bureau et al.,17 which assigns isotropic chemical shifts to fluorine crystallographic sites through the use of phenomenological parameters. In this model, the 19F isotropic chemical shift is considered as a sum of one constant diamagnetic term and several paramagnetic contributions from the l neighboring cations and is calculated according to the main following formula: δiso/C6F6 ) -127.1 - ∑lσl (ppm) and σl ) σl0 exp[-Rl(d - d0)]. d0 is the characteristic F-M distance, which is taken equal to the bond length in the related basic fluoride, and σl0 is the parameter that determines the order of magnitude of the cationic paramagnetic contribution to the shielding and was deduced from measurements in the related basic fluoride where δiso/C6F6 ) -127.1 - nσl0 (n is the coordination number of the fluorine atom). The Rl, d0, and σl0 values for Ca2+ and Ce4+ cations17,24 have been determined previously and are reported in Table 7. This model allows us to explain that the isotropic chemical shift of fluorine ions surrounded by four Ca2+ cations in CeCa-based oxyfluorides is slightly higher than that in CaF2. Indeed, a δiso value equal to 64 ppm, which corresponds to line 1, was obtained using the superposition model with four F-Ca bond lengths equal to 2.355 Å. This is in agreement with the fact that, in the investigated samples, the Ca-F bond lengths are, on average, slightly shorter (Table 2) than those in CaF2 (Table 4). A δiso value equal to 115 ppm, which corresponds to line 2, was obtained using the superposition model with several sets of F-Ca and F-Ce bond lengths, for example, three F-Ca bond lengths equal to 2.355 Å and one F-Ce bond length equal to 2.54 Å or three F-Ca bond lengths equal to 2.39 Å and one F-Ce bond length equal to 2.50 Å. These F-(Ca, Ce) distances are in fine agreement with those estimated with the Brown and Altermatt model (Table 5). Then, line 2 may be assigned to fluorine ions surrounded by three Ca2+ cations and one Ce4+ cation. The same approach allowed us to attribute lines 3 and 4 to fluoride ions surrounded by two Ca2+ and two Ce4+ cations and one Ca2+ and three Ce4+ cations, respectively. Moreover, as shown in Figure 4, δiso(2) - δiso(1) > δiso(3) - δiso(2) > δiso(4) - δiso(3). In other words, successive Ce4+ substitutions for Ca2+ cations in the vicinity of fluoride ions result in decreasing increments of the δiso values. Then, it may be inferred that F-(Ca,Ce) distances increase with the number of Ce4+ ions
line 3 width ((0.5)
δiso ((1)
22.0 21.0 18.0 13.5
150.5 152.5 156.0 160.5
I ((0.5) 47.8 45.2 67.6 71.1 FCa2Ce2
line 4 width ((0.5)
δiso ((1)
24.0 24.5 25.0 22.0
179.0 181.5 185.0 188.0
I ((0.5) 11.0 17.5 4.9 3.1 FCaCe3
width ((0.5) 16.0 18.0 10.0 6.5
surrounding fluoride ions. According to the higher Ce4+ charge, the Ce4+ substitution for the Ca2+ cation in the vicinity of fluoride ions results in a higher repulsion between cations and should explain the evolution of the F-(Ca,Ce) distances. As shown also in Figure 4, for all lines, that is, all fluorine ion environments, the isotropic chemical values decrease when the Ca content increases (Table 6) in agreement with an increase of the cell parameter (Table 2). Nevertheless, δiso(2) and δiso(1) are less altered than δiso(3) and δiso(4) with the Ca/Ce atomic ratio. This can be explained by the higher σl0 value for Ce4+ cations. In other words, paramagnetic contributions of the Ce4+ cations to the 19F isotropic chemical shifts are more sensitive to the F-cation distance than those of the Ca2+ cations. In the Ce1-xCaxO2-x-y/2Fy samples, five environments can be proposed for fourfold coordinated fluoride ions: F- surrounded by four Ce4+, three Ce4+ and one Ca2+, two Ce4+ and two Ca2+, one Ce4+ and three Ca2+ or four Ca2+ cations. The probability of each situation calculated on the basis of a random distribution of anions in the 8c sites and cations in the 4a sites is reported in Table 8 for the studied samples. It appears that whatever the Ca content the more probable environments correspond to fluorine sites surrounded by at least two Ce4+ cations in agreement with the fact that the Ce content is always higher than the Ca one in this series. Moreover, when the Ca/Ce atomic ratio decreases, the number of Ca2+ cations becomes so low that the probability to get F- ions surrounded by three or four Ca2+ cations becomes negligible. Nevertheless, this calculation based on probabilities does not take into account either the formal charges and ionic radii of Ca2+ and Ce4+ cations or the stronger affinity of the Ca2+ cations for F- ions in fourfold coordination. Experimentally, two main contributions corresponding to FCa2Ce2 and FCa3Ce are evidenced for the samples with low F/Ca ratios (Table 6). For the sample with the lowest F content, only three peaks are observed (Figure 2). The disappearance of line 1 can be explained easily assuming that the Ca and F contents are too low to give the FCa4 environment, despite the strong affinity of F- ions for Ca2+ cations. As the F/Ca atomic ratio increases, the proportion of FCa3Ce environments remains nearly constant (around 25% whatever the Ca and F contents) and for samples b-d, the proportion of FCa2Ce2 environments decreases whereas those of FCa4 and FCaCe3 environments increase (Table 6). Consequently, for the high F/Ca ratio compositions, four environments with significant proportions have been identified. The particular behavior of sample a will be discussed further. Then, the proportions of fluoride ion environments determined by the 19F NMR study (Table 6) are clearly different from those predicted by the statistical calculations (Table 8). The proportions of fluorine ion environments with at least two Ca2+ cations are systematically higher than those predicted by the statistic calculation. Alternatively, the proportions of FCaCe3 environments are systematically lower than those predicted by the statistic calculation and FCe4 environments are missing whatever
Local Environments of Fluorine
J. Phys. Chem. C, Vol. 112, No. 3, 2008 865 TABLE 9: Predicted and Experimental (Italic) Contents (%) of Fluoride Ion Environments for the Studied Ce1-xCaxO2-x-y/2Fy Samples (The Total F Content Is Equal to y/2 Considering That There Are Twice as Many Anionic Sites) samples a b c d
Figure 4. 19F isotropic chemical shifts versus Ca/Ce atomic ratio in the Ce1-xCaxO2-x-y/2Fy series.
Figure 5. Content of the different types of fluorine environments as a function of F atomic ratio in the Ce1-xCaxO2-x-y/2Fy samples.
TABLE 7: rl (Å-1), d0 (Å), and σl0 (ppm) Parameters for CaF217 and CeF424 for Application of the 19F δiso Superposition Model compounds
Rl
d0
σ l0
CaF2 CeF4
2.976 3.15
2.366 2.243
-46.5 -253.9
TABLE 8: Calculated Probabilities of Anion Environments (%) for Ce1-xCaxO2-x-y/2Fy Samples samples (O,F)Ca4 (O,F)Ca3Ce (O,F)Ca2Ce2 (O,F)CaCe3 (O,F)Ce4 a b c d
0.71 0.39 0.16 0.03
6.93 4.69 2.56 0.76
25.44 21.09 15.36 7.67
41.52 42.19 40.96 34.24
25.41 31.64 40.96 57.29
the Ca content. Because the F atomic ratio is different from one sample to another, it seems more appropriate to discuss the results versus the contents of the fluoride ion environments. These contents, which are equal to the relative line intensities (Table 6) multiplied by y/2 (y corresponds to the F molar content), are gathered in Table 9 with those obtained by multiplying by y/2 the probabilities of anion environments on the basis of a random distribution of anions in the 8c sites and cations in the 4a sites (Table 8). The comparison of these values shows that the more numerous the Ca2+ cations in the vicinity of F- ions, the higher the discrepancy between experimental and predicted contents. On the basis of the bond lengths determined from the Brown and Altermatt model (Table 5), the
FCa4 0.085 0.033 6 × 10-3 0.00
FCa3Ce 1.55 1.10 0.10 0.00
0.83 0.40 0.090 0.011
3.40 2.07 0.87 0.39
FCa2Ce2 3.05 1.79 0.54 0.12
5.74 3.84 2.37 1.07
FCaCe3 4.98 3.59 1.43 0.51
1.32 1.49 0.17 0.046
FCe4 3.05 2.69 1.43 0.86
0.00 0.00 0.00 0.00
average value of the (Ce,Ca)-(O,F) distances in the FCa4-nCen environments can be calculated with a linear combination of F-Ca and F-Ce bond lengths. These distances increase from 2.37 to 2.50 Å when n increases from 0 to 4. Moreover, this latter distance appears underestimated considering isotropic chemical shift calculations (see above). The more numerous the Ce4+ cations in the vicinity of F- ions, the larger the average F-(Ce,Ca) distances, the stronger the constrains for the network, and the less stable the environments of the fluoride ions. This is confirmed by the absence of FCe4 sites, which would create too many constrains for the network of the studied Ce1-xCaxO2-x-y/2Fy compounds where the average (Ce,Ca)-(O,F) distance is around 2.35 Å. The observed contents of the fluoride ion environments may be understood as the result of a compromise between the strong affinity of fluoride ions in fourfold coordination for Ca2+ cations in the fluorite structure, the F and Ca contents, and the network constrains induced by longer Ce-F bonds. For instance, in samples a and b, the contents of the FCa4 environments are higher (1.55 and 1.10%, respectively, considering that the total F environments distributed into the anionic sites are 12% and 8.5%, respectively, in each sample) (Table 9) than the calculated probability of (O,F)Ca4 environments (0.71 and 0.39%, respectively) (Table 8), clearly showing that cations are not randomly distributed. Moreover, the occurrence of fluorine stabilized into ceria framework should contribute to radically change the distribution of cations compared to the homologous oxides because of the strong preference of fluorine for Ca2+ cations. Then high F and Ca contents lead to the segregation of the F- and Ca2+ ions into the network of the studied Ce-Ca oxyfluorides and finally to the formation of CaF2 as observed in sample e, which has the same Ca/Ce ratio as sample a (Table 2) but has been prepared as samples b-d (see the experimental section). Because the proportions of FCa4 environments are similar for samples a and b, it may be inferred that a longer duration of maturation of the oxyhydroxides (sample a) allows us to synthesize a sample with a more homogeneous distribution of Ca2+ cations corresponding to a larger content of FCa4 (1.55% and 1.10%, respectively, considering that the total F environments distributed into the anionic sites are 12% and 8.5%, respectively, in each sample) thus avoiding the formation of CaF2 during fluorination. The highest F content of sample a can also be explained by the increase of the proportions of the more stable environments for fluorine ions (FCa3Ce and FCa2Ce2) and the decrease of the proportions of the less stable environment for fluorine ions (FCaCe3) compared with sample b. Table 6 shows that for samples a and b corresponding to the highest Ca and F contents, the proportion of FCa4 environments remains stable around 13%. Moreover, the proportions of FCa2Ce2 and FCa3Ce sites tend toward 50% and 25% of the total F sites, respectively. Then, the proportion of FCa4 sites seems to reach a limit rate from the atomic ratio Ca/Ce ) 1/3 corresponding to the Ce0.75Ca0.25O1.67F0.17 composition,13 whereas
866 J. Phys. Chem. C, Vol. 112, No. 3, 2008 the anionic vacancy content reaches a limit equal to 1/12 starting from Ca/Ce atomic ratio around 1/4 with a lower F content (Table 2). Then, the Ca and F contents as well as the proportion of each fluorine environment seem to be independent of the creation of anionic vacancies into the ceria matrix. UV-Shielding Properties. The diffuse reflectance spectra of various Ce1-xCaxO2-x-y/2Fy compositions were presented and discussed in a previous paper13 illustrating the gradual increase of the optical band gap with the Ca and F contents. On the basis of the various refractive indices of CeO2, CaO, and CaF2 equal to 2.45, 1.8, and 1.5, respectively, at λ ) 550 nm and taking into account the Gladstone-Dale equations between refractive index, weight fraction, and density, it is reasonable to consider that the refractive index decreases with increasing Ca and F contents leading to an increase of transparency in the visible range. F- ions being bound to mainly one (FCa3Ce1) and two (FCa2Ce2) Ce4+ cations whatever the chemical composition, a large number of Ce sites are affected by fluorine. The occurrence of F- ions in the vicinity of Ce4+ cations and the increase of the proportion of Ce atoms bound to fluorine show that the Ce(O,F) chemical bonding becomes more and more ionic as Ca2+ and F- ion content raises. In other terms, the global electropositive character of Ce4+ cations increases with Ca and F contents leading to the progressive shift of the optical band gap to higher energy associated with the 2p (O) f 4f (Ce) charge transfer. Moreover, because of the occurrence of Ce-F and Ca-F ionic bonds, the electronic polarizability should be reduced then contributing to the attenuation of scattering in the visible range. Conclusions New oxyfluorides with Ce1-xCaxO2-x-y/2Fy compositions were prepared by coprecipitation in basic fluorinated medium, followed by annealing under air at T ) 600 °C. Nanoparticles were obtained with a crystallite size of around 10 nm determined on the basis of XRD data analysis. Rietveld analysis allowed us to confirm the crystallite size, determine the unit cell parameter, and detect the occurrence of CaF2 in these samples. In this series, F atoms are tetrahedrally coordinated with Ce and Ca atoms located at vertices. On the basis of the Brown-Altermatt model, the F- anions force Ce4+ cations to be bound at 2.50 Å whereas O2- anions constrain Ca2+ cations to be linked at 2.25 Å. Then the average bond length is around 2.35 Å and corresponds to the Ce-O and Ca-F distances in CeO2 and CaF2, respectively. 19F MAS NMR spectroscopy was used to study the local structure and fluoride ion environments. Four distinct 19F resonances have been observed. They were assigned, using the superposition model proposed by Bureau et al.,17 to four different types of environment for the fluoride ion: FCa4, FCa3Ce, FCa2Ce2, and FCaCe3. The contents of these environments are clearly different from those calculated on the basis of a random distribution of anions in the 8c sites and cations in the 4a sites. It shows that, in these Ce-Ca oxyfluorides adopting the fluoritetype structure, F- anions have a great affinity for Ca2+ cations leading to an increase of the F amount with the Ca content. The absence of the FCe4 environment is explained from a steric
Sronek et al. standpoint: the F-Ce bond lengths in the network are too short to accomodate four fluorine ions in the vicinity of Ce4+ cations. A critical composition was identified corresponding to Ca/Ce ) 1/3, a situation where the probability of getting a FCa4 environment becomes high. Considering the various proportions of FCe4-nCan environments and the chemical composition, one can point out that the Ce-(O, F) chemical bonding exhibits a more ionic character as the Ca and F contents increase. Consequently, the absorption threshold shifts to the UV range. The development of ionic bonds into ceria contributes therefore to the reduction of both electronic polarizability and refractive index. Thus, new inorganic UV filters with nanosized particles exhibiting a white color and broad absorption edge at the frontier between the UV and visible range as well as a reduced refractive index in the visible range can thus be designed. Acknowledgment. This work has been supported by the Rhodia Company. References and Notes (1) Trovarelli, A.; de Leitenburg, C.; Boaro, M.; Dolcetti, G. Catal. Today 1999, 50, 353-367. (2) Minh, N. Q.; Takahashi, T. In Science and Technology of Ceramic Fuel Cells; Elsevier: New York, 1995. (3) Li, R.; Yabe, S.; Yamashita, M.; Momose, S.; Yoshida, S.; Yin, S.; Sato, T. Solid State Ionics 2002, 151, 235-241. (4) Skorodumova, N. V.; Ahuja, R.; Simak, S. I.; Abrikosov, I. A.; Johansson, B.; Lundqvist, B. I. Phys. ReV. B 2001, 64, 115108. (5) Nair, P. J.; Wachtel, E.; Lubomirsky, I.; Fleig, J.; Maier, J. AdV. Mater. 2003, 15, 2077-2080. (6) Patsalas, P.; Logothetidis, S.; Sygellou, L.; Kennou, S. Phys. ReV. B 2003, 68, 035104. (7) Tsunekawa, S.; Fukuda, T.; Kasuya, A. J. Appl. Phys. 2000, 87, 1318-1321. (8) Li, L.; Chen, Y. Mater. Sci. Eng. A 2005, 406, 180-185. (9) Masui, T.; Fujiwara, K.; Machida, K.; Adachi, G.; Sakata, T.; Mori, H. Chem. Mater. 1997, 10, 2197-2204. (10) Sato, T.; Katakura, T.; Yin, S.; Fujimoto, T.; Yabe, S. Solid State Ionics 2004, 172, 377-382. (11) Cheetham, A. K.; Fender, B. E. F.; Cooper, M. J. J. Phys. C: Solid State Phys. 1971, 4, 3107-3121. (12) Ku¨mmerle, E. A.; Heger, G. J. Solid State Chem. 1999, 147, 485500. (13) Sronek, L.; Majimel, J.; Kihn, Y.; Montardi, Y.; Tressaud, A.; Feist, M.; Legein, C.; Buzare´, J.-Y.; Body, M.; Demourgues, A. Chem. Mater., in press. (14) Rietveld, H. M. J. Appl. Crystallogr. 1969, 2, 65-71. (15) Thompson, P.; Cox, D. E.; Hastings, J. B. J. Appl. Cryst. 1987, 20, 79-83. (16) Carjaval, J. R. FULLPROF Program, Rietveld Pattern Matching Analysis of Powder Patterns, ILL, Grenoble, 1990. (17) Bureau, B.; Silly, G.; Emery, J.; Buzare´, J.-Y. Chem. Phys. 1999, 249, 89-104. (18) Massiot, D.; Fayon, F.; Capron, M.; King, I.; Le Calve´, S.; Alonso, B.; Durand, J. O.; Bujoli, B.; Gan, Z.; Hoatson, G. Magn. Reson. Chem. 2002, 40, 70-76. (19) Shannon, R. D. Acta Crystallogr., Sect. A 1976, 32, 751-767. (20) Brindley, G. W. Philos. Mag. 1945, 36, 347-369. (21) Brown, I. D.; Altermatt, D. Acta Crystallogr., Sect. B 1985, 41, 244-247. (22) Schmidt, R.; Mu¨ller, B. G. Z. Anorg. Allg. Chem. 1999, 625, 605608. (23) Ganguly, R.; Siruguri, V.; Gopalakrishnan, I. K.; Yakhmi, J. V. J. Phys.: Condens. Matter 2000, 12, 1683-1689. (24) Legein, C.; Fayon, F.; Martineau, C.; Body, M.; Buzare´, J.-Y.; Massiot, D.; Durand, E.; Tressaud, A.; Demourgues, A.; Pe´ron, O.; Boulard, B. Inorg. Chem. 2006, 45, 10636-10641.
