750
J . Phys. Chem. 1984, 88, 750-754
Photochemistry of the Uranyl Ion in Colloidal Silica Solution J. Wheeler and J. K. Thomas* Department of Chemistry, University of Notre Dame, Notre Dame, Indiana 46556 (Received: April 8, 1983)
-
Uranyl acetate, once adsorbed onto a silica particle, exhibits an extremely long excited-state lifetime with t ! , z 440 p s as compared to 11 ps in water. The fluorescence spectrum is slightly blue shifted, and the transient absorption spectrum is quite different, consisting of only one peak on the silica but two peaks in water. The rate constants for several quenchers are determined and used to probe the nature of the silica surface. The interactions of the uranyl ion with various surfactants are also discussed, which further illustrate the uniqueness of the uranylsilica system. A revised energy diagram is presented to account for the various spectral features encountered.
Introduction While the luminescent properties of the excited uranyl ion have been investigated for several decades,' recent interest has been focused on its utilization in schemes for the photoconversion and chemical storage of light energy.24 The uranyl ion has several attractive features: (1) it has an absorption band in the visible (420 nm) range and absorbs strongly in the near-UV; (2) its excited state has a long lifetime (1 1 p s in water); and (3) it is a strong excited-state electron a ~ c e p t o r . ~ - ~ The hydrolysis of the uranyl ion is quite complex,1° and virtually all studies to date have been carried out in strongly acidic solutions, typically of phosphoric, nitric, sulfuric, or perchloric acid. The recent advances in micellar chemistry suggest that organized molecular assemblies could be satisfactorily used to enhance the photochemistry of excited uranyl complexes. Colloidal silicas have been employed as inorganic analogues to micelles," and the results presented herein revealed particularly favorable interactions with the uranyl ion. Experimental Section Absorption and emission techniques for both steady-state and pulsed-laser studies have been described previously," and only brief details are necessary here. Absorption spectra were recorded on a Perkin-Elmer 552 spectrophotometer and fluorescence spectra were recorded on a Perkin-Elmer MPF-44B spectrofluorimeter. Flash photolysis studies were carried out with a Lambda-Physik EMG-100 laser employing either nitrogen gas (X,= 3371 A) or a xenon/hydrogen chloride mixture (Aex = 3080 A). A low-power (70 pJ/pulse) nitrogen laser from Photochemical Research Associates was used briefly to determine laser intensity effects. The NALCO silica employed was No. 1115, pH 10.4, r = 40 A radius. All solutions of Si no. 1115 were run at 20% dilution in doubly distilled deionized water. The uranyl acetate was reagent grade from J. T. Baker Chemicals and was used as received. The ruthenium tris(bipyridy1) dichloride was obtained from the G. Frederick Smith Co. Dimethylaniline obtained from Matheson Colemand and Bell Inc. was purified by vacuum distillation and (1) Rabinowitz, E.; Belford, R. L. "Spectroscopy and Photochemistry of Uranyl Compounds"; Pergmanon Press: New York, 1964. (2) Rosenfeld-Grunwald, F.; Brandels, M.; Rabini, J. J. Phys. Chem. 1982,
86, 4745. ( 3 ) Reisfeld, R.; Jargensen, C. K. Struct. Bonding (Berlin) 1982, 49, 1. (4) Reisfeld, R. Chem. Phys. Lett. 1983, 95, 95. (5) Balzani, V . ; Carassiti, V. 'Photochemistry of Co-ordination Compounds"; Academic Press: London, 1970. (6) Burrows, H. D.; Kemp, T.J. Chem. SOC.Rev. 1974, 3, 139. (7) Burrows, H. D.; Pedrosa de Jesus, J. D. J . Photochem. 1976, 5 , 265. (8) Jargensen, C. K.; Reisfeld, R. J . Electrochem. SOC.1983, 130, 681. (9) Jargemen, C. K.; Reisfeld, R. Struct. Bonding (Berlin) 1982, 50, 121. (IO) Ahrland, S.; Heitanen, S.; Silltn, L. G . Acta Chem. Scan. 1954, 8, 1907. (11) Wheeler, J.; Thomas, J. K. 'Inorganic Reactions in Organized
Media"; Holt, Smith L., Ed.;American Chemical Society: Washington, DC, 1982; ACS Symp. Ser. No. 177, p 97.
0022-3654/84/2088-0750$01 .50/0
degassed with prepurified nitrogen gas prior to use. The various salts employed were all of reagent grade or higher purity and were used as received.
Results and Discussion Spectral Studies. The steady-state luminescence spectra of the M) in water and in a 20% Si No. 1115 uranyl ion (1 X solution are shown in Figure 1. The silica system affords a luminescent intensity 2.8 times that in water and the fluorescence spectrum presents more structure. The excited-state half-lives were 11 p s in water and 440 p s in the silica. When ruthenium tris(bipyridy1) dichloride, Ru(bpy)?+, is incorporated into a porous silica particle, it too shows a much longer lived excited state and more structure,I2 but with the uranyl system the fluorescence enhancement is much less than the 40-fold increase predicted from a consideration only of the corresponding lifetimes. The 7-nm blue shift of the peak maximum from 514 nm in water to 507 nm in silica is also in correspondence with that observed for the R ~ ( b p y ) , ~system + described elsewhere.12 Though data to be presented on the quenching of uranyl with R ~ ( b p y ) , ~on + silica demonstrate that both are tightly held on the particle, this binding may not necessarily be responsible for the spectral feature observed for the uranyl system. Virtually the same spectrum is obtained if 1 X lo4 M uranyl acetate is dissolved in a 2.61 X lo-, M KH2P0,/9.13 X lo-, M Na2HP04pH 7.45 solution. The excited-state decay is not completely exponential, but it has a lifetime of about 40 p s , 4 times that in water but 10 times less than on the silicas. Phosphates quench the uranyl emission at higher concentration, but apparently the environment of the uranyl ion on a silica or in the phosphate buffer is very different from what it experiences in water. If uranyl acetate is frozen in ethanol at 77 K, the emission spectrum is again the same as on silica at room temperature. It has also been reported', that in phosphate glass the uranyl ion likewise has a longer lifetime. The decay is nonexponential, but the longer lived component of a double exponential fit to the decay has a half-life of -300 ps. The emission spectrum is also the same as that for the uranyl ion in silica solution. The induced absorption spectra obtained by fixing the emission wavelength reveals a similar structural enhancement between A,, = 370 and 450 nm (Figure 2). These details are not noticeable from conventional UV-visible spectrometers due to light scattering from the colloidal silica particles. Concentration Effects. Unlike in highly acidic media,2 the lifetime of the excited uranyl ion in water is highly dependent on concentration. At 1 X lo4 M, the decay is a simple exponential with an 11-ps half-life. If the concentration is increased to 0.001 M, the decay is not exponential and requires a double exponential of the form F = Fo(A exp[-k,t] (1 - A ) exp[-k,t])
+
(12) Wheeler, J.; Thomas, J. K. J . Phys. Chem. 1982, 86, 4540. (13) Lieblich-Sofer, N.; Reisfeld, R.; Jargensen, C. K. Inorg. Chim. Acta 1978, 30, 259.
0 1984 American Chemical Society
The Journal of Physical Chemistry, Vol. 88, No. 4, 1984 751
Uranyl Ion in Colloidal Silica Solution
5
V
10
15
20
25 3 0
~~- -.35 40
45
TI m e,,JJsec
Figure 3. Transient decay profiles of uranyl acetate in water at (a) 0.001 M using an excimer laser, (b) 0.001 M with the excimer laser, and (c) 0 001 M using a low-power Nitromite laser. F is fluorescence intenisty in arbitrary units.
475
520 x/nm
Figure 1. Emission spectra of uranyl acetate at the same concentration (1 X IO4 M) and optical density at the exciting wavelength (337 nm) in (a) 20% Si No. 11 15 and (b) water. F is fluorescence intensity in arbitrary units.
t .. b7 :3
t A ,?
mM uranyl acetate Figure 4. Plot of the kinetic parameters using a double exponential model for the fluorescence decay as a fraction of the uranyl acetate concentration.
crease slightly with increasing concentration. Since the excited uranyl ions are few compared to those in the ground state, the reaction
-
1
(U022+)*+ u022+
I I ( ,
a
i I
I'
N
-
1 /nm 490 Figure 2. Induced absorption spectra obtained by monitoring the emission at 505 nm from 1 X IO4 M uranyl acetate solutions in (a) 20% Si No. 11 15 and (b) water. A is the induced absorbance in arbitrary units. Since emission is much greater on the silica than in water, the instrument gain was adjusted in order to examine other spectral features.
190
Figure 3 has the superimposed transient profiles at 0.001 vs. 0.001 M uranyl acetate excited with an excimer laser (Aex = 3371 A, measured 3.75 mJ/pulse) and a Nitromite laser (A, = 3371 A, 70 pJ/pulse output energy from manufacturer's specifications). The data at 0.001 M involve a difference of a factor of 50 in the laser intensity, so interactions between two excited-state uranyl ions are far less important than excited-state-ground-state ones. The transient decay curves can be fitted to the double exponential so that the experimental and calculated curves exactly coincide. The constants A , k l , and k2 are plotted in Figure 4. As the uranyl ion concentration increases, the percentage of the rapidly decaying component increases as well as its rate. The slower component, k2, remains essentially constant but does in-
2U022+
should be pseudo first order, but it is not. It has already been noted that the laser intensity (hence (UO?+)* concentration) has little effect on the rate. Consequently, the data are indicative of ground-state complexes of the uranyl ions with themselves. Transient Absorption. The transient absorption spectra of 0.0015 M'uranyl acetate in water and in a 20% Si No. 11 15 solution are shown in Figure 5. In water the data are taken at the end of the laser pulse, 3.6 ps later, and 8.5 ps from the end of the pulse. Owing to the long life of uranyl excited states on the silica particles, data were taken at the end of the pulse and 161 and 405 ps from the pulse. Although the emission spectra of uranyl ions have more structure o n silica than in water, the reverse is true of the transient absorption spectra, where water has two well-defined peaks and the silica spectrum is composed of a single band. Spectroscopic Implications. The spectroscopy of the uranyl ion has been the subject of considerable debate. McGlynn and Smith'4~1s assigned it the ground-state molecular orbital configuration a totally symmetric singlet state consisting of one u and two x linkages to the axial oxygen. The x g bond is sufficiently weak to be considered essentially nonbonding. The lowest available (14) McGlynn, S. P.; Smith, J. K.; Neely, W. C. J . Chem. Phys. 1961, 35, 105.
(15) McGlynn, S.P.; Smith, J. K. J . Mol. Specirosc. 1961, 6, 164.
752 The Journal of Physical Chemistry, Vol. 88, No. 4, 1984 0.D.
Wheeler and Thomas
a
_I
,045
23,500 cm-l
t
20,500 cm-l
,0301
i Figure 6. Proposed energy-level diagram for the uranyl ion.
L
350
410
470
530
590
650
\/nm 0.D.
b
TABLE I: Quenching Rate Constants (k,) for the Uranyl Ion k,, Li(mo1 s) quencher
water
20% Si KO. 1 1 15
thiocyanate dimethylaniline ethanol
4.8 x 109 2.8 X lo8 2.2 X lo9 1.2 X lo9 2.8 X lo5 dynamic 5.05 X 10' steady state 3.2 x 109
2.4 X lo6 2.5 x 104 3.1 x 105 4.0 x 107 1.3 X lo3 dynamic 3.5 X l o 3 steady state 7 x 108
1-
B r'
.068 1
T l+
350
530 590 650 /nm Figure 5. Transient absorption spectra of 0.0015 M uranyl acetate in (a) water and (b) 20% Si No. 1115. For water: ( 0 )end of pulse and (A) 3.6 and (+) 58.5 ps following the laser pulse. These symbols represent end of pulse and 161 and 405 fis after the pulse, respectively, for the silica. 410
470
A
excited state is considered to be from a IT, to a 5f atomic orbital of the uranium atom. Bell and Biggers16-18 mathematically resolved the absorption and emission spectra, concluding that the excited-state emission comes from two triplet states, one at 21 270 cm-' and the other at 20 502 cm-'. They further calculated that only 4.66% of the emitted light comes from the higher state. Gorller-Wabrand and Van Q u i c k e n b ~ r n e 'argued ~ * ~ ~ that the excited state can only be a singlet state and not a triplet based on the Russell-Saunders coupling scheme, but J0rgensen2' has argued that distinctions between singlet and triplet states are not significant for the uranyl ion. Hill, Kemp, Allen, and Cox22reported the transient absorption spectra of 0.002 M aqueous uranyl sulfate following a microsecond flash. The spectrum reported in Figure 5, however, shows much greater resolution between the two peaks at 475 and 560 nm for 0.0015 M uranyl acetate. The spectrum for uranyl acetate on 20% Si No. 1115 reveals that the two main absorption bands in water are not due to resolution of the vibrational structure as has earlier been reported,22but must be due to absorption from two distinct processes. We propose the energy diagram in Figure 6 to explain the data. Bell and Biggers had originally assigned the lowest excited-state energies to be 20 502 and 21 270 cm-' with the higher state accounting for only 4.66% of the total emission. The upper level (16) Bell, J. F.; Biggers, R. E. J . Mol. Spectrosc. 1965, 18, 247. (17) Bell, J. F.; Biggers, R. E. J . Mol. Spectrosc. 1967, 22, 262. (18) Bell, J. F.; Biggers, R. E. J . Mol. Spectrosc. 1968, 25, 312. (19) GBrller-Waband, C.;Van Quickenborne, L.G. J . Chem. Phys. 1971, 54, 4178. (20) GBrller-Waband, C.;Van Quickenborne, L.G. J . Chem. Phys. 1972, 57, 1436. (21) Jergensen, C. K.; Reisfeld, R. Chem. Phys. Lett. 1975, 35, 441. (22) Hill, R. J.; Kemp, T. J.; Allen, D. M.; Cox,A.J . Chem. Soc.,Furuduy Trunr. 1 1973, 70, 847.
is now assigned a slightly higher energy, 23 500 cm-', and the level at 42 820 cm-I is lowered to 41 500 cm-I. These changes are very slight considering that Bell and Biggers worked with highly acidic media, while our work was carried out under neutral pH in water and pH 10.3 on the silicas. Belford and Belfordz3had proposed a model in which there was significant K bonding to the axial ligands of the uranyl ion, and Newman'sz4 calculations considering relativistic effects confirmed their importance. Although their portrayal of the uranyl ion may or may not be accurate, it does underline the importance of the uranyl 6d orbitals. If these orbitals are tied up in bonding to the SiOz oxygen atoms, then this may account for the unique spectroscopic features experienced with the silica system. Suppose that the main absorption is to the level at 23 500 cm-'. Transition from this level to the one at 20 500 cm-' should be rapid. Since the energy difference is only 8.6 kcal/mol, the rate of interconversion can be estimatedz5to be 2 X 10l2,and is certainly very fast on a microsecond time scale. The upper level may have a very long lifetime, with a half-life of -400 p , but in water interconversion allows for measurement of only the shorter lived species at 20 500 cm-', whose half-life is only 11 f i s . The silicas supposedly tie up this state via d-p interactions between the uranium 6d and the oxygen 2p orbitals of the silica particles. With the uranyl bound to silica one consequently only observes the decay of the longer lived excited-state species, and the emission is slightly blue shifted as observed. Lacking the lower level, it is immediately obvious why the uranyl ion has only one excited-state absorption band on the silica particles but has two distinct bands in water. If the quantum yield for fluorescence is only 0.35 from the lower state and nearly unity for the upper state, this also explains why the factor of 40 increase in the half-life on the silica particle gives rise to only a 2.85 factor of increase in steady-state luminescence. Quenching Studies. The pseudo-first-order rate constants, k,, for various species with (UOz2+)*are given in Table I. The positively charged uranyl ion is electrostatically attracted and bound to the negatively charged silica particle. The approach of an anionic quencher is then repelled from the colloid, giving rise to a much lower reaction rate as compared to water. Dimethylaniline (DMA) quenches (UOz2+)*30 times slower on the silica system than it does in water. This further supports the notion that the uranyl ion is in a unique environment when it is bound to the particles. DMA quenching of 4 4 1-pyreny1)-
-
R. L.; Belford, G. J . Chem. Phys. 1961, 34, 1330. (24) Newman, J. B. J . Chem. Phys. 1965, 43, 1691. (25) Turro, N. J. ''Modern Molecular Photochemistry"; Benjamin/Cummings Publishing Co.:Menlow Park, CA, 1978. (23) Belford,
Uranyl Ion in Colloidal Silica Solution
The Journal of Physical Chemistry, Vol. 88, No. 4, 1984 753
0 0
:i . 3
111
-+---+
0 0
.z
+
0
-..--+-.+--1 .
.
111
-+--
I-;
1:
mM NaOH Figure 8. Plot of emission Zo/Zvs. [NaOH] for 1 X lo4 M uranyl acetate in water. x
104
+
*
L -
210
+
Figure 7. Ground-state absorption spectrum for 1 X lo4 M uranyl acetate in water at (a) 0, (b) 0.0002, and (c) 0.001 M N a O H showing the subsequent hydrolysis of the uranyl ion.
butyltrimethylammonium bromide (PN') yields rate constants of 4.0 X lo9and 1.0 X lo9 L/(mol s) in water and in silica solution, respectively. These rates are indicative of steric hindrance while the uranyl data are not. The pyrene in PN+ is bound to the particle via a butyl chain; hence, it is more exposed than U02*+,which must bind directly to the particle surface. Ethanol reacts with (U022+)*very slowly and gives evidence that at the higher concentrations required to quench the excited uranyl there is also some static component to the kinetics. Oxygen does not affect (U022+)*. The uranyl ion will also quench Ru(bpy)* in water with a pseudo-first-order rate constant of 3.9 X lo8 L/(mol s) in deaerated solutions. Hydrolysis of the uranyl ion initially complicates the rate vs. \UO?+]kinetic plot, but the plot is linear beyond 0.003 M. No quenching could be observed for the silica system due to the relative immobility of the two doubly charged cations, neither of which can move rapidly enough during the Ru(bpy)* lifetime for a reaction to take place. Marcantonatos and Deschaux have recently investigated the quenching of uranyl fluorescence by T1+ and Ag+ in homogeneous s o l ~ t i o n . ~ ~While , ~ ' the high pH of the medium precludes use of Ag', the binding of the positively charged Tl+ to the silicas provides the fastest reaction rate for the silica system. The fact that this rate is still 4.6 times faster in water reinforces the picture of a tightly bound uranyl ion which can only slowly move along the particle surface. In this case, although the doubly charged uranyl ion is tightly bound, the T1+, being only singly charged, can move quickly enough during the long-lived (UO?+)*lifetime to quench the fluorescence. This point is again illustrated by the fact that heptylviologen quenches Ru(bpy)* at only half the rate on the silicas as it does in water: 3.4 X lo8 vs. 7.4 X lo8 L/(mol s). Ion mobility dominates over the proximity effect which would have predicted a quenching rate enhancement when the two cationic species are bound to the silica particles as opposed to being in free solution. In 0.05 M sodium lauryl sulfate (NaLS) the rate vs. U022+ concentration for ( R ~ ( b p y ) , ~ +is ) *completely linear. The k, = 4.0 X lo8 L/(mol s), the same as that for water. There is no quenching of (Ru(bpy)32+)*with UOZ2+on the silicas at 0.008 ( 2 6 ) Marcantonatos, M.D.; Deschaux, M.Chem. Phys. Left. 1980, 76, 359.
(27) Marcantonatos, M.D.; Deschaux, M.Chem. Phys. Letr. 1981, 80, 327.
+
m+--v-+ I r e
.
.
+--c- ' 4 '6
:
,
7
mM NaOH Figure 9. Emission decay rate vs. [NaOH] for 1 X IO4 M uranyl acetate in water.
M U02*+ (roughly eight uranyl ions per particle). The NaLS data indicate that it is the relative immobility of the ions on the silica particles that inhibits the quenching reaction. Hydrolysis of the Uranyl Ion. If sodium hydroxide is added to raise the pH of the 1 X M uranyl acetate solutions, the subsequent hydrolysis introduces large changes in the excited uranyl's spectral properties. The absorption spectrum (Figure 7) shows a huge increase in the optical density in the region below 400 nm and a refinement of the spectral structure. The enhanced absorption coefficient accounts for the increased fluorescence intensity shown in Figure 8. Initially the uranyl ion hydrolyzes to form species with different absorption characteristics and probably fewer ground-state-excited-state interactions. This accounts for the initial decrease in the decay rate (increase in the half-life) of the excited state as shown in Figure 9. Once the equivalence point (pH 7.00) is reached at approximately 4 X lo4 M NaOH, both steady-state and dynamic methods observe quenching from the OH- ion. At basic pH in water, the lifetime falls off rapidly, but on a silica particle at high pH the lifetime is increased 40-fold. Silica is unique in enhancing the lifetime at high pH. Varying the concentration of sodium borate or sodium metasilicate in water M uranyl acetate presented the same overall bewith 1 X havior as did NaOH. Surfactants. Surfactant molecules react with the excited uranyl ion in a variety of ways. Sodium decanoate presents the same basic behavior as NaOH. Initially the steady-state yield and the excited-state lifetime increase until about 1 X lo-, M decanoate is reached, beyond which there is rapid quenching. Sodium oleate shows rapid quenching beyond 3 X lo4 M oleate, but did present the initial increase in the excited-state lifetime. Sodium lauryl sulfate, on the other hand, showed no change in the excited-state lifetime, but did show static quenching monitored both by steady-state analysis and by studying the decrease in the initial concentration of excited-state species following the laser pulse. Quenching is essentially linear to about
754
J . Phys. Chem. 1984,88, 754-756
0.005 M NaLS with k, = 1.1 X lo7 but is very rapid beyond that. Once micelle formation ensues, the positively charged uranyl ion is drawn to the micelle where it rapidly reacts with the sulfate head groups. Similar behavior is seen if various amounts of KH2P04/ Na2HP04are added to form 1 X lo4 M uranyl solutions in water. The phosphate mixture is in fact a pH 7.4 buffer solution. Initially the steady-state emission intensity rises and the lifetime increased from 11 to 54 p s . Further addition of phosphate leads to static quenching only; the lifetime remains the same. Binding. As noted earlier, above the cmc NaLS is a highly effective quencher for the excited-state uranyl ion. Negatively charged quenchers, on the other hand, are electrostatically repelled from the silica particles and consequently show very slow reaction rates. If uranyl acetate is allowed to equilibrate with NaLS, followed by addition of silica to form 20% Si No. 1 115/ 1 X 1O4 M uranyl acetate/0.05 M NaLS solution, the uranyl ions are easily
seen, by means of steady-state fluorescence, to migrate from the NaLS to the silica particles until essentially all of the uranyl is on the silica. Migration is significant within a few minutes and complete within hours. Conclusion Studies on the uranyl ion indicate that it enjoys significantly different photochemical properties when it is bound to a colloidal silica particle as compared to free aqueous solution. This is reflected in a huge increase in the emission lifetime and other spectral features. These properties could not be obtained in free solution or with the group of surfactants studied. Acknowledgment. We thank the Army Research Office via Grant No. DAAG29-80-K-007 POO1, for support of this research, and NALCO Chemicalf Co. for the generous gift of colloidal silica. Registry No. Uranyl ion, 16637-16-4; silica, 7631-86-9.
Homogeneity and Magnetic Susceptibility in Some Substituted Cadmium Spinels Mark Tellefsen, Louis Carreiro, Robert Kershaw, Kirby Dwight, and Aaron Wold* Department of Chemistry, Brown University, Providence, Rhode Island 0291 2 (Received: May 3, 1983)
The normal spinels CdFe204,CdGal,,Feo,204,and CdRhl,8Feo,204 were prepared, and their magnetic susceptibilities were measured. The low perfvalue of 4.72 ( 5 ) p B found for CdFe204can be attributed to hybridization of the 6Sand 4G wave functions resulting from spin-orbit interaction coupled with strong crystal fields having trigonal components at the spinel B sites. Magnetic susceptibility was sensitive to the homogeneity of samples of CdGal,8Feo,204 and CdRhl,xFeo,204, which was dependent upon the method of preparation. For homogeneous samples, the magetic susceptibility approaches the theoretical value for high-spin Fe3+(3ds). The slight remaining discrepancy from spin-only moment is due to the statistical existence of a few small Fe3+clusters.
Introduction The spin-only moment observed for the Fe3+(3d5) ion in a number of oxide systems (Fe2O3, Fe203/Rh203,ZnFe204) has been reported to be unusually low compared to the theoretical In an earlier study, Selwood' indicated that iron-iron interactions between adjacent cations were probably responsible for the low moment observed in a-Fe203. Similarly, low moments were observed by K r h 2 in a study of the solid solution Fe2O3Rh203. However, this system presents a further problem in that complete homogeneous solid solution is difficult to attain.4 Lotgering3 has also reported on the low moment observed for Fe3+ in the normal spinel ZnFe204,which he attributed to the presence of a small number of Fe3+ ions at tetrahedral sites. However, the reported temperature dependence of the magnetic susceptibility for ZnFe204was linear and hence the lowering of the moment could not be attributed to the properties of A-O-Bclusters. Another possible explanation might be interaction of t,, orbitals on neighboring B sites resulting in a trigonal distortion of the cubic field, permitting admixture of the 6S and 4G states. In order to study the role of such t2*-t2, interactions, the systems CdGa2-,Fe,04 and CdRh2-,Fe,04 were chosen. The magnetic susceptibility of the normal spinel CdFe204has not been reported. However, a lowering of the spin-only moment of Fe3+ would be anticipated. In addition, members of the systems CdGa2-xFe,04 and CdRh2-,Fe,04 are expected to crystallize with the normal (1) P. W. Selwood, L. Lyon, and M. Ellis, J . Am. Chem. SOC.,73,2310 (1951). (2) E. KrCn, P. Szabb, and G.Konczos, Phys. Lett., 19 (2), 103 (1965). (3) F. K. Lotgering, J . Phys. Chem. Solids, 27, 139 (1966). (4) L. Carriero, unpublished research.
0022-3654/84/2088-0754$01.50/0
spinel structure, and it may be possible to minimize the trigonal distortion and hence obtain a measured moment per Fe3+ approaching the theoretical value. Experimental Section Preparation. Polycrystalline samples of the systems CdRh2-xFe,04 and CdGa2-,Fe,04 were prepared by the solid-state reaction of the binary oxides. Samples of CdRh,,8Feo,204 were also prepared by a precursor method starting from ammonium hexachlororhodate(II1) and iron(I1) sulfate h e ~ t a h y d r a t e . ~ Solid-State Reaction of the Oxides. Starting materials consisted of Fe203 (Mapico Red, Columbian Carbon Co.), GazO3 (Gallard and Schlesinger, 99.999%), RhzO3, and CdO. Rhodium(II1) oxide was prepared in the high-temperature, ambient pressure form (space group Pbca) by heating finely divided rhodium metal (Engelhard Inc., 99.99%) under flowing oxygen at 800 O C 5 Cadmium oxide was obtained from the decomposition of C d C 0 3 at 450 O C in air.6 Members of the system CdRh2-,Fe,04 were prepared by thoroughly grinding together stoichiometric quantities of the oxides (with a 5% by weight excess of CdO) and heating in a silica boat open to air at 800 OC. Fast-scan X-ray diffraction analysis indicated completion of the reaction by the absence of Fe203 or Rh203after two 24-h heating intervals with intermittent grkding. After complete reaction, excess CdO was removed by washing the product with 50 mL of hot 1 M (aqueous) NH,Cl, followed (5) H. Leiva, R. Kershaw, K. Dwight, and A. Wold, Mater. Res. Bull., 17, 1539 (1982). ( 6 ) V. S. Nguyen, R. Kershaw, K. Dwight, and A. Wold J . Solid State Chem., 36,241 (1981).
0 1984 American Chemical Societv
