J. Phys. Chem. 1995, 99, 9611-9616
9611
Coordination and Valence of Europium in the Heteropolyanion [EuPsW30011&~L. Soderholm," G. K. Liu,* J. Muntean, J. Malinsky, and Mark R. Antonio* Chemistry Division, Argonne National Laboratory, Argonne, Illinois 60439 Received: December 22, 1994; In Final Form: March 28, 1995@
The synthesis and characterization of the Eu-encrypted heteropolyanion [ E U P ~ W ~ O 1 0O 1 ' ~I - are presented. X-ray absorption near-edge structure experiments were used to determine that Eu is trivalent in the anion. Optical spectra confirm this finding. The unusual cyclic voltammogram obtained for the Eu heteropolyanion is discussed in terms of the stability of Eu" under reducing conditions. Time delayed optical fluorescence data are only consistent with two structurally inequivalent Eu ions, both inside the heteropoly cavity. Lifetime measurements of the 5Do state indicate that the two structurally different Eu sites are not occupied in the same anion. In addition, from complementary measurements on samples made in D20, it is determined that there are three waters coordinated to Eu in one site and only two waters coordinated to Eu in the second site. 31PNMR and cyclic voltammetry data, obtained from samples with either one or two sites occupied, are indistinguishable. Possible models to explain the presence of two sites are discussed.
Introduction
Experimental Section
The Preyssler heteropolyanion [ N ~ P ~ W ~ O O I consists I O ] ~ ~ -of a cyclic arrangement of five -PW6022- units assembled with D5h symmetry. A structural study has revealed that the central cavity formed by these units is occupied by a Na ion.' Although the encapsulated Na ion was initially reported to be nonlabile, further studies showed that under relatively rigorous conditions it can be exchanged for a rare earth, to form the heteropolyoxotungstate anions [ R P ~ W ~ O O I (for R = Ce, Nd-Lu, U; n = 11 or 12).2 Both the parent Preyssler anion and the rare earth exchanged ions are electrochemically active. As shown by cyclic voltammetry, the [ R P ~ W ~ OI#-O I anions exhibit five reversible reduction steps involving two electrons each over the range 0 to -0.6 V versus a AgIAgC1 reference electrode.2 The electrochemistry of the Eu derivative is unique among the lanthanide-encrypted derivatives, exhibiting four reversible reduction steps involving 2 , 2 , 4 , and 2 electrons over the same potential range. This unique reduction behavior of the Eu analogue, together with the unexpectedly large lanthanideinduced shift (LIS) determined from NMR data and the known stability of divalent E u , ~led us to wonder whether Eu was divalent in the [EuPgW300,10]~-anion. In order to provide electronic, as well as structural, information about the Eu in [ E u P ~ W ~ O O I we have obtained X-ray absorption near edge structure (XANES) data, high-resolution excitation and emission spectra in the solid state at liquid helium temperature, and 31P NMR spectra. XANES is an excellent technique for determining the oxidation state of an ion in a solid, or in solution, as demonstrated by its previous use in clarifying the redox behavior of the related anion [ C ~ P ~ W ~IO]'*-.^ O O I Optical spectroscopy was chosen because the spectral response is dependent on both electronic and structural factors. Static structural information can be discerned from transition line widths and crystal field splittings of the electronic ground multiplet, whereas dynamic information is available through excited-state lifetimes. During the course of this investigation, we noticed differences in the optical spectra for different samples that are dependent on details of the synthetic conditions used for the Eu-exchange reaction. We present the results of our findings below.
The parent compound K I ~ . ~ N ~ I . ~ [ N ~1ol.15HzO P~W~O was OI prepared as previously d e ~ c r i b e d . ~ After . ~ two recrystallizations, a yield of 6.4 g (24% based upon [W04l2-) was obtained for the white, crystalline Preyssler salt. The Eu3+-exchange reactions were based upon
@
Abstract published in Advance ACS Absrracts, May 1, 1995.
+
[NaP5W300110]14- nEu3+
-
+
[EUP~W,,,O,~~]'~-(n - l)Eu3+
+ Na'
For 1 equiv of Eu (i.e., n = l), a colorless warm (60 "C) aqueous solution of EuCl3 was added dropwise to a warm aqueous solution of the Preyssler salt. The resulting clear, colorless solution was sealed in a Parr 4746 Teflon-lined digestion vessel and heated at 165 "C for 24-48 h. Upon cooling, an excess of KCl was added to precipitate a fine white powder, which was filtered, washed in ice-cold H20, and dried in air. A similar procedure was followed with the n = 2 preparations. Exchange reactions were also run in D20, starting with a Preyssler ion prepared in D20 from Na~W04.2D20and phosphoric-d3 acid (85 wt % solution in D20, Aldrich). Chemical analyses on the samples prepared with either 1 or 2 equiv of Eu gave similar results. Assuming a chemical formula of K I Z [ E U P ~ W lol.54H20, ~OO~ the analysis results in weight percent are (expected) K 5.56 (5.20), Eu 1.77 (1.67), P 1.69 (1.72), and W 66 (61) with an estimated accuracy of f 3 % . The cyclic voltammograms of [ N ~ P ~ W ~1 O 0 1O ' ~ -Iand [ E u P ~ W ~ O 01101'~- are the same as those described elsewhere.'.* X-ray diffraction experiments performed using a Scintag 6-6 diffractometer operating with a copper tube showed all samples to be very poorly crystallized. Eu Ls-edge XANES data were collected at ambient temperature on beamline 4- 1, equipped with Si< 111> monochromating crystals, at the Stanford Synchrotron Radiation Laboratory. The feedback system was adjusted to provide about 50% of the maximum incident X-ray intensity in order to suppress harmonic contamination. The monochromator vertical slit width was 1 mm. The fluorescence signal was monitored without a filter, except for the EuC136H20 data, which were obtained in electron yield mode. The data reduction was carried out according to conventional methods5
0022-3654/95/2099-9611$09.00/0 0 1995 American Chemical Society
9612 J. Phys. Chem., Vol. 99, No. 23, 1995
Soderholm et al. T
6960
7000 Energy (eV)
7040
Figure 1. Similarity of the Eu XANES L3-edge energies obtained from the Eu-exchanged Preyssler anion and a trivalent Eu standard is used 10]12-. to conclude that Eu is trivalent in [ E u P ~ W ~ O O ~ Divalent Eu has a peak that is shifted approximately 7-10 eV to lower energy. The upper spectrum was measured in the fluorescent mode, whereas the lower spectrum was detected using electron yield.
All cyclic voltammetry data were obtained using a BAS 1OOBN electrochemical analyzer and BAS electrodes: glassy carbon (3 mm diameter) working electrode (MF-2012), platinum wire auxiliary electrode (MW-1032), and Ag/AgCl reference electrode with vycor tip (MF-2063). A 1 M H2S04 solution (99.9999%, Alfa 11000) prepared with 18 MBvm water was used as the electrolyte. CV scans (-0.65 to +0.4 vs Ag/AgCl) of the neat electrolyte, which was sparged and blanketed with nitrogen, revealed no electroactive impurities. The concentration of the heteropolyanions was approximately 1 mM. In optical experiments, a tunable dye laser pumped by a Q-switched Nd:YAG laser was used as the excitation source. The laser line width was 0.3 cm-'. The sample temperature was varied using a continuous flow, heat-exchange gas cryostat. Fluorescence from the sample was selected using long pass filters and a 1 m monochromator (SPEX 1704) and detected by using a cooled photomultiplier (RCA 3 1034). Fluorescence emission was time resolved with two gated boxcars. The optogalvanic effect of a uranium-neon hollow cathode lamp was used to calibrate excitation spectra. The emission spectrum of a neon lamp was recorded to calibrate the Eu"' fluorescence spectra. Fluorescence dynamics were measured using a transient recorder (LeCroy TR8818) connected to a minicomputer for signal averaging. Phosphorus-31 NMR spectra were recorded at 121.65 MHz on a 300 MHz G. E. Omega spectrometer (7.05 T). [EuP5W3001101'~(5 mM) samples were freshly prepared in 1 M HCl solutions made with D20. The following parameters were used to acquire the spectra: a spectral width of 10 kHz, acquisition time of 3.2768 s, 11.7 ms pulse width (70"), and a repetition rate of 15 s. Data block sizes of 32K were zero filled to 64K data points to give 6.4 points/Hz. Chemical shifts were determined by using both external and internal capillaries containing 85% H3P04. Samples were run in either standard NMR tubes or special spherical tubes designed to minimize anisotropic paramagnetic effects6 There was no discernible difference in the chemical shifts obtained from the two different experimental setups. Results
XANES. The XANES of Eu in [EuPsW30011ol"-is compared with a standard Eu"' spectrum obtained from EuCl36H20 in Figure 1. Spectra obtained on a variety of different
l -0.8
:
:
i
-0.4
i 0
i
l 0.4
E (volts)
Figure 2. CVs obtained from acidic solutions of (a) [ N ~ P ~ W ~10]l4-, OOI (b) [YPsW300110]'~-.and (c) [ E U P ~ W ~ O O I I OThe ] ' ~plotted -. potentials are referenced vs Ag/AgCI. The arrow indicates the direction of cathodic current. The R = Y CV (b) is indistinguishable from those obtained from other R-substituted Preyssler ions, including tetravalent uranium.
[ E u P ~ W ~ O O I samples are all indistinguishable, including those prepared with 2 equiv of Eu and demonstrating two optically different Eu sites, as discussed below. In particular, the magnitudes of the edge jumps, which provide an indication of the Eu concentration, are the same for all samples studied. These L3 spectra provide insights about the valence of Eu. The similarity of the edge energies and resonance line shapes obtained from Eu in the heteropolyanion and the standard Euttl spectrum provides clear evidence that Eu is trivalent in [EuPgW300110]'~-. The presence of divalent or intermediatevalent Eu would manifest itself through the appearance of an additional peak shifted approximately 7 eV to lower energy.7 There is no evidence in our spectra for any such peak, ruling out the presence of Eu" or intermediate-valent Eu in this material. Cyclic Voltammetry. The cyclic voltammogram (CV) obtained for [ E U P ~ W ~1O 0 1O ' ~ -~is compared with those obtained for [NaPsW300110]'~-and [YP~W300llo]'~in Figure 2. As previously noted,' the parent NaC ion has different reduction behavior than does the Yrrrion. All of the other trivalent rareearths encrypted in the heteropolyanion, including Cel*I or the tetravalent ion, U1v,s produce CVs similar to that of [YPsW3001IO]'*-. In contrast, the Eu CV is different from that found for the other rare earths. There is a four-electron step that occurs at -0.33 V that is not present in the other CVs. The CVs obtained for all preparations of [EUP~W~OO, 1 0 1 ' ~ - are indistinguishable, including those prepared with 2 equiv of Eu"'. The standard reduction potential tabulated for Eu"' in acidic solution is -0.57 V (vs A ~ j A g c l ) .The ~ other trivalent rare earths all have reduction potentials that are beyond the hydrogen evolution potential, and therefore, they are out of the range of aqueous cyclic voltammetry. Optical Spectroscopy. The optical spectra obtained here are consistent with the conclusion that the Eu is trivalent in the Preyssler ion. There is no evidence for Eu" in any of the
J. Phys. Chem., Vol. 99, No. 23, 1995 9613
Coordination and Valence of Eu in [EuPsW~OOIIO]'*-
16000
16500
17000
Wave Number (cm-1) 16000
Figure 3. Time resolved fluorescence spectra of a solid sample of state was excited with a trivalent Eu (site A) taken at 4.3 K. The 5DI laser energy of 19 030 cm-l. Emission from both the 5D1and 5Dostates was observed. The time delay of the boxcar gate was (a) 250 p s to select the emission from the 5Do only and (b) 15 p s to record the emission from the 5DI emitting state as well. The letters associated with each peak are used to assign the transitions in the accompanying energy level diagram.
samples that were measured. This result, together with the XANES data, led us to conclude that Eu is trivalent in I 01O . I 2I [EuP~W~O The time-resolved fluorescence spectra shown in Figure 3 have been obtained from a sample prepared as described previously.2 The data were obtained at 4.3 K on solid samples. The laser excitation energy of 19 030 cm-' was selected to directly excite the Eu"' (4f6 configuration) 5D1 multiplet. There are two available fluorescence pathways for deexcitation from this multiplet. The first pathway is the direct fluorescent emission primarily to the 7F3multiplet, with a weaker emission also observed to the 7F4 multiplet. The second deexcitation pathway involves a nonradiative decay to the 5Dostate, which then also fluoresces to the 7 Fmultiplets. ~ These two different deexcitation pathways have different decay constants, and therefore they can be separated experimentally by selecting the time delay between the initial laser excitation pulse and the gate of the fluorescence detection. The two different spectra shown in Figure 3 were both obtained using a laser excitation energy of 19 030 cm-I, but observed after different time delays, and correspond to transitions from primarily the sDo multiplet (Figure 3a) and transitions from both the 5 D and ~ the s D ~ multiplets (Figure 3b). In a different set of experiments, the incident laser energy was systematically varied, and fluorescence spectra were obtained at different fixed incident energies. The fluorescence spectrum obtained with a excitation energy of 19030 cm-I discussed above is compared with the spectrum obtained when the excitation energy is reduced by 65 cm-I to 18 965 cm-'. Whereas all samples produced the fluorescence spectrum shown in Figures 3 and 4a when the laser excitation energy was fixed at 19 030 cm-I, samples synthesized with more than 1 equiv of Eu and for longer soak times produced the spectrum shown in Figure 4b, in addition to the one shown in Figure 4a. The two different spectra shown in Figure 4 correspond to two distinctly different absorption energies for the 7F0 sD1 transition. Data taken at 77 K and at room temperature on the same samples also exhibited the two different spectra, but the lines were significantly broadened. This difference in the energy of the 7Fo 5D1 transition can only occur if there are two structurally-inequivalent Eu ions in our sample. In the experiments discussed above, the excitation energy is tuned to the 7Fo - 5 D transition ~ energy while the fluorescent
-
-
16500
17000
Energy (cm-')
Figure 4. Site-selective fluorescence spectra obtained from a solid sample of trivalent ELIions in [ E U P ~ W ~ ~ O at I4.3 ~ K. O ]Spectrum ~~- a is assigned to site A and was obtained with a laser excitation energy of 19 030 cm-' whereas spectrum b is assigned to Eu in site B, and was obtained with a laser excitation energy of 18 965 cm-I. The 5Do 7Foline is plotted at a larger scale in both spectra.
-
18920
18960
19000
19040
19080
Wave Number (cm-1) Figure 5. Site-selective excitation spectra of trivalent Eu ions in a two-site, solid sample at 4.3 K. The transitions are from the ground The top spectrum state 'Fo to three crystal-field levels of the excited 5D~. is obtained from Eu on site A recorded with emission from 5Do 7F0 at 17 278 cm-I. The bottom spectrum is obtained from Eu on site B with emission at 17 213 cm-I.
-
energies are recorded. The indication that all Eu ions are not equivalent in selected samples prompted us to undertake a different optical experiment. In this experiment, the incident excitation energy is varied while the polychromatic fluorescence emitted from the sample is monitored. The spectra obtained in this manner are shown in Figure 5. These experiments confirm that there is a second, structurally-inequivalent,Eu ion site from which different absorption and emission energies are observed. The two Eu sites observed are labeled site A and site B. Site A is observable in all the measured samples and has a 7F0 sDo transition energy of 17 278 cm-I. Site B is only observed when more than 1 equiv of Eu is used in the sample preparation, and the 7F0 5Domultiplet splitting at site B is 17 213 cm-I. It is the energy differences between the 7F0 and SDo states in these two sites that permits us to use site-selective excitations to obtain independent information on the two different Eu sites. The data shown in Figure 5 are the 7Fo sD1 excitation spectra
-
-
-
9614 J. Phys. Chem., Vol. 99, No. 23, 1995
Soderholm et al.
TABLE 1: Observed Russell-Saunders Energy Multiplets and Their Crystal Field Splittings
5D~ QO 7F2
site A
site B
19050.4 19039.4 19031.5 17278.1 1078.7 1062.7 1040.8 996.9
18983.6 18977.4 18966.4 17213.1 1193.3 1059.6 1037.6 985.8 927.9 406.9 379.0 327.1 0
412.1 364.2 326.3 0
-
obtained by monitoring the 5Do 7F0fluorescence emission at energies of 17 278 cm-' and 17 213 cm-I. Assuming that the optical transition probabilities are similar for the two different Eu sites, we estimate the ratio of Eu in the two sites to be approximately 0.6 for site A to 0.4 for site B. The 'FJ multiplets observed are well resolved for J = 0, 1, 2 , and 3. The energies of the multiplets are similar to those observed for Eu in other ionic environments. The energies of the multiplets can be well represented with the free-ion parameters previously obtained for Eu:LaF3,l0 and the spinorbit parameter fitted to the centroid of the multiplets is the same, to within experimental error, as that published for Eu: LaF3'O or E u : L ~ C ~ ~ . "All . ' ~the observed Russell-Saunders multiplets are further split into states by the crystal-field potential. This splitting is observable as the fine structure exhibited within each multiplet. The energies and assignments for all the observed states in the two sites are listed in Table 1. Whereas there is not enough information available to fit the splitting to any crystal-field model, it can be stated that the Eu is in a low-symmetry environment in both sites. Having determined that there are two structurally inequivalent Eu sites in selected samples, we decided to look at their dynamics, initially to confirm the relative ratio of Eu in the two sites. The 5Do decay fluorescence from sites A and B is shown in Figure 6. The fluorescence decays are single exponentials for both sites. The measured lifetimes are 0.29 ms for site A and 0.4 ms for site B. Each of the curves has an initial rise because of the increasing population of the 5Do resulting from the relaxation of the initially populated 5Dl state. From the rising segment of the curve, lifetimes of the 5DI states are estimated to be 12 ps for site A and 8 ps for site B. The lifetime of the Eu"' ion 5Dostate in inorganic crystals is typically 3-5 ms. The lifetimes measured here are surprisingly short and suggest that the encrypted Eu may be in contact with waters of hydration. EulI1 has a radiationless energy-transfer pathway from the 5Dostate of Eu to the IJ = 3 vibrational mode of water.I3 In order to probe further this possibility, we also measured a deuterated sample with only site A occupied and another sample with both site A and site B occupied. The lifetimes of the 5Dostates in these samples prepared from D20 are found to be about 8 times longer than in those prepared from H20. This result unequivocally demonstrates that Eu is coordinated to water in our heteropolyanion samples. The difference in lifetimes found for site A and site B can be attributed to a different number of waters coordinating to Eu. A method for estimating the number of coordinating waters has been previously outlined by Horrocks and Sudnick.I3 The experimentally determined decay constant, or lifetime, of a
0
0.5
1.o
1.5
l i m e ( ms )
Figure 6. Fluorescence decay curves of the 5Do state of Eu in [ E U P S W ~ ~ O I at 4.3 K. Initial excitation was from the ground state 7Foto the excited state 5 D ~Spectrum . a is obtained from site A at the emission energy of 17 278 cm-I. The lifetime measured from these data is 0.29 ms. Spectrum b is obtained from site B at the emission energy of 17 213 cm-'. The lifetime measured from these data is 0.4 ms. The initial rising in each curve results from increasing population with decay from the 5 D state, ~ as shown in Figure 3. The measured lifetime for the sD1 state is 12 ps for site A and 8 ps for site B.
fluorescing state is composed of several components
t-l(obs) = z-l(nat)
+ t-l(nonrad) + z-'(OH)
where z-'(nat) is the natural rate constant for photon emission, t-'(nonrad) represents the rate constants for all nonradiative deexcitation pathways that do not involve OH, and t-'(OH) is the rate constant for energy transfer to OH vibrations. t-'(nat) t-'(nonrad) can be determined by replacing OH by OD. This is because there is a large isotope effect as the OH vibronic deexcitation pathway becomes inefficient. z-'(OD) is very small, therefore z-'(obsoD) FZ z-'(nat) f z-'(nonrad). The measured lifetimes have been shown to be linear with the number of coordinated waters in a wide variety of samples. Therefore, by measuring the lifetimes of Eu in the presence of H20 and of D20, the number of waters in the first coordination sphere of Eu can be estimated from
+
q(H,O) = 1.05[z-'(H20) - z-'(D20)] where 1.05 ms is the experimentally determined linear coefficient for E u . ' ~Using this equation, and the lifetimes measured in H2O and DzO from site A and site B, we determine that there are three waters coordinated to Eu located in site A and two waters coordinated to Eu in site B. Removing the OH decay pathway, the fluorescence intensity decays measured from the D20 samples are a function of the channels available for energy transfer,
where k~is the inverse lifetime in the absence of energy transfer and (..& is the ensemble average over the entire lattice of the energy transfer probability, Wv. If site A and site B were simultaneously occupied in the same sample, the energy transfer between the two sites would influence the fluorescence decay
Coordination and Valence of Eu in [ E u P ~ W ~ O O I I O ] ' ~ via Forster-Dexter energy transfer, which can be estimated from
W, = P/(R$ In the case under study here, Ru represents the distance between two Eu ions. We can use this theory to address the question of whether site A and site B are simultaneously occupied within one cluster, to form [ E U ~ P ~ W ~ O O Ior I Owhether ] ~ - , they are occupied in different clusters. From the fluorescence line widths we assume that both the Eu are encrypted within the heteropoly cluster, replacing Na+,' and therefore if they are both in the same cluster, the Eu-Eu separation cannot exceed about 5 A. The lifetimes of about 3 ms reported here for the deuterated samples are inconsistent with any phonon-assisted energy transfer occurring in our ~amp1e.I~From these results we conclude that the two sites observed are not occupied in the same cluster but are occupied in different clusters. This conclusion is supported by the finding that the site A lifetime in deuterated samples is independent of whether or not site B is occupied. 31PNMR. The 31PNMR spectra observed from an acidic solution of [ E U P ~ W ~ IO]'*O O I is a single line resonance, with a line width of 8.2 Hz and a chemical shift of -9.1 ppm. The spectra from the one-site and two-site samples are indistinguishable. We see no extra lines or splittings in the NMR spectra from samples with two different Eu sites. The phosphorus ions in the heteropolyanion form a planar five-membered ring perpendicular to the cylindrical cavity in which the Eu is postulated to be encrypted. They are all magnetically equivalent. The observation of a single NMR line in the two-site samples indicates that site A and site B are both on the 5-fold rotation axis of the P-0 ring. This chemical shift of -9.1 ppm is very different from the f0.7 ppm reported previously for Eu in the heteropolyanion.2 The shift that we observe is similar to that found for the diamagnetic Y or Lu. Eu"' has a 7F0ground state and therefore has no paramagnetic moment at low temperatures. However, the lowest state within the first-excited multiplet, 'FI, is at 328 cm-' above the ground state, as shown in Table 1, and it is magnetic, even in low s y ~ n m e t r y . ' ~ ~The ' ~ ~occurrence '~ of a paramagnetic moment on the Eu is expected to result in a lanthanide-inducedshift (LIS). The LIS for the Eu ion measured at 300 K can be readily calculated using Boltzmann statistics and the theory outlined by Bleaney,I7 which treats the unsplit multiplet contribution to the pseudocontact term while ignoring the contribution from the 31Pcontact term. This model works well for the other R in this structure.*-* At 300 K the ground state is about 64% occupied, and the 7F1 first excited multiplet is about 35% occupied. Correcting the expected Eu LIS for the contribution from the magnetic 7F1 multiplet results in a calculated LIS of - 1.3 ppm for [ E u P ~ W ~ O1 0O1 'I~ - relative to the Y analog. This value compares well with our observed LIS for the Eu analog of f 0 . 9 8 ppm. The excited state contributions to the LIS are small and negative and in no way could account for the LIS of f10.8 ppm previously reported for [EuP5W300110]'*-.~The LIS for Eu" is also expected to be zero because the reduced matrix element (JI IaJ/.I contributing ) to the LIS is zero for the 'S7/2 ground multiplet of the 4f7 configuration. Therefore, the large LIS previously reported cannot be accounted for by the presence of divalent Eu, whereas the 31PLIS of -1.3 ppm reported here agrees well with the simple calculations outlined here.
Discussion The XANES and the optical spectroscopy together demonstrate that Eu is trivalent in [EuP~W~OOIIO]'~-. The fact that
J. Phys. Chem., Vol. 99, No. 23, 1995 9615 the CVs obtained from the Eu Preyssler ion in solution are unusual with respect to the CVs obtained from the other rare earths is not the result of Ed1 in the sample at rest potential. However, the standard reduction potential of trivalent Eu in acidic solution is -0.57 V vs Ag/AgCl, which is not far removed from the unusual four-electron step measured at -0.33 V. It is possible that the difference in the Eu heteropoly CV may be the result of the reduction of Eu"'. This possibility is currently under investigation.8 The presence of a second inequivalent Eu in some samples appears to be crucially dependent on the synthetic conditions employed. It is possible to obtain samples with only site A present if the initial Eu concentrations are kept relatively low, Le., do not exceed about a 1:l ratio with the Preyssler ion to be exchanged. In addition, shorter soaking times (Le., less than about 24 h) favor the production of a one-site sample. We found that after 48 h soaking time for samples with excess Eu there is a 60% to 40% ratio of Eu occupation of the two sites, assuming their transition probabilities are similar. Chemical analyses of samples found to have either 1 or 2 distinct Eu sites do not show any difference in the Eu concentrations relative to those of P or W. In other words, the samples that show one or two inequivalent sites both have chemical analyses that are consistent with essentially one Eu per heteropoly anion cluster. This result is supported by XANES measurements that indicate no difference in the step height of the L3 edge for one- or twosite samples and optical measurements that show similar 5D fluorescent lifetimes from site A Eu, regardless of the site B occupancy. Whereas the measured CVs and 31PNMR appear insensitive to the Eu site occupancy, the correlation between the observation of a second Eu site and specific preparative conditions provides strong evidence that there is a chemical distinction between the two types of samples and that the observation of inequivalent Eu is not simply the result of laserinduced defects or other experimental artifacts. The widths of lanthanide optical transitions in solids can be used to comment on structural disorder. When studying well crystallized samples, inhomogeneous line broadening may be caused by crystal strain or point defects, and the line width of optical transitions are typically between 0.1 and 10 cm-' depending on the quality of the crystal. In contrast, amorphous samples, including glasses, have inhomogeneous line widths that can exceed 100 cm-' for 4f states of rare earth ions, because of structure disordering, each individual ion or molecule has a slightly different environment. The optical transitions observed from Eu in either site of the Preyssler anion have line widths between 3.5 and 6 cm-'. These line widths are well within the range expected for well-crystallized samples. However, the samples studied here were very poorly crystallized as defined by the very broad X-ray diffraction lines. The disorder that broadens the diffraction lines involves the packing of the [ E U P ~ W ~ O Oclusters. ~ ~ O ] ~These ~ - clusters do not pack in a wellordered fashion, and therefore there is poor long-range ordering resulting in poor quality X-ray diffraction lines. The presence of well defined narrow optical transitions in an apparently poorly crystalline environment is consistent with the Eu"' ions replacing the Na+ encrypted inside the Preyssler ion.',2 The Eu environment over several neighbors is then well defined, and the optical transitions between crystal-field states are sharp. A possible explanation for the occurrence of the two different Eu sites involves the observation that there are three water molecules coordinated to Eu in site A, whereas there are only two waters coordinated at site B. The difference in water coordination would itself result in a different electrostatic environment for the Eu that would manifest itself optically.
9616 J. Phys. Chem., Vol. 99, No. 23, 1995 There would be no change in charge state of Eu or the cluster. The cylindrical cavity in the heteropolyanion into which the Eu is exchanged has a diameter of about 4 AI. This diameter is too small to fit a Eu with waters coordinated in the plane perpendicular to the cylinder axis. We postulate that site A is very near to the end of the cylinder, so that some of the water is not directly encrypted. The loss of the third water permits the Eu to move further down into the cylinder along the 5-fold rotation axis. The remaining two waters could then be accommodated if the HzO-Eu-OHz bond angle was about 180". This argument would explain why the two site samples require a longer soaking time if there is assumed to be an energetic barrier to the loss of the third H20. It would also explain why we were not able to make a sample with only site B occupied. However, we do not understand why we have not been able to prepare a two-site sample when starting from low Eu concentrations in solution. The requirement that there be excess Eu in solution is not understood from our explanation of the origin of the two Eu sites. Clearly more, work is necessary to understand fully this material. Recent interest in the Preyssler ion and its derivatives as catalyst^'^^^^ will encourage further studies to sort out the two-site issue raised here.
Acknowledgment. The authors acknowledge financial support from the DOE-Basic Energy Sciences, Chemical Sciences, under Contract W-31-109-ENG-38. We also thank Ruoxin Cao and Farrel Lytle for technical assistance. Part of the work presented here was done at SSRL, which is operated by the DOE, Office of BES. References and Notes (1) Alizadeh, M. H.; Harmalker, S. P.; Jeannin, Y.; Martin-Frere, J.; Pope, M. T. J. Am. Chem. SOC.1985, 107, 2662.
Soderholm et al. (2) Creaser, I.; Heckel, M. C.; Neitz, R. J.; Pope, M. T. Inorg. Chem. 1993, 32, 1573. (3) Cotton, F. A.; Wilkinson, G. Advanced Inorganic Chemistry Interscience Publishers: New York, 1980, p 1396. (4) Antonio, M. R.; Soderholm, L. Inorg. Chem. 1994, 33, 5988. (5) Teo, B. K. EXAFS: Basic Principles and Data Analysis: Springer-Verlag: Berlin, 1986; p 349. (6) Micskei, K.; Helm, L.; Brucher, E.; Merbach, A. E. Inorg. Chem. 1993, 32, 3844. (7) Rohler, J. In Handbook on the Physics and Chemistry of Rare Earrhs; Gschneidner Jr., K. A., Eyring, L., Hufner, S., Eds.; NorthHolland: Amsterdam, 1987; Vol. 10, p 453. (8) Antonio, M. R.; Williams, C. W.; Soderholm, L. Unpublished results. (9) Bard, A. J.; Parsons, R.; Jordan, J. Standard Potentials in Aqueous Solution; Marcel Dekker: New York, 1985; p 834. (IO) Camall, W. T.; Goodman, G. L.; Rajnak, K.; Rana, R. S. J. Chem. Phys. 1989, 90, 3443. (11) DeShazer, L. G.; Dieke, G. H. J. Chem. Phys. 1963, 38, 2190. (12) Morrison, C. A.; Leavitt, R. P. In Handbook on rhe Physics and Chemistry of Rare Earths; Gschneidner, Jr., K. A,, Eyring, L., Eds.; North-Holland: Amsterdam, 1982; Vol. 5, pp 461-692. (13) W. Dew. Horrocks, J.; Sudnick, D. R. Ace. Chem. Res. 1981, 14, 384. (14) Yen, W. M. In Spectroscopy of Solids; Kaplyanskii, A. A,. Macfarlane, R. M., Eds.; Elsevier Science Publishers: 1987; Vol. 4, pp 189 -191. (15) Staub, U.; Soderholm, L.; Osbom, R.; Guillaume. M.; Furrer, A,; Trounov, V. J. Alloys Compd., in press. (16) Soderholm, L.; Kwok, W. K.; Goodman, G. L.; Loong, C.-K. Eur. J. Solid State Inorg. Chem. 1991, 28, 615. (17) Bleaney, J. J. Magn. Reson. 1972, 8, 91. (18) Harmp, M. K.; Hill, C. L. Inorg. Chem. 1994, 33, 5448. (19) Antonio, M. R.; Malinsky, J.; Soderholm, L. In Marerials Research Society Symposium Proceedings; Boston, MA, in press. JP943388R
