Hyperfine Interactions 78 (1993)535-539

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TDPAC and emission M6ssbauer studies of Fe304(99Ru) Y. Ohkubo, Y. Kobayashi, K. Asai a, T. Okada and F. Ambe The instituteof Physicaland ChemicalResearch ( RIKEN) , Wako-shi, Saitama 351-01, Japan aDepartment of AppliedPhysics and Chemistry, The Universityof Electro-Communications, Chofu-shi, Tokyo 182, Japan

Hyperfine interactions of 99Ru( 4-- 99Rh) nuclei in Fe304 were studied by means of TDPAC and emission M6ssbauer spectroscopy. The isomer shift obtained from an emission M6ssbauer spectrum and the temperature dependenceof the hyperfinemagnetic field obtained from TDPAC measurements indicate that 99Ru ions arising from 99Rh nuclei dilutely doped in Fe304 exist as a mixed state of Ru2+ and Ru3+, which is not common in ruthenium oxides. Fe304 (99Ru)is the first examplewhere a Ru ion in a low valencestate exhibitsits own magnetization in oxides. In oxides, ions of 4d transition metals show interesting properties different from those of the corresponding 3d transition metals [1]. The T D P A C nuclide 99Ru( -<---99Rh) [2] serves as a probe of the 4d transition metal, ruthenium. In the present work, we doped radioactive 99Rh ions into magnetite, Fe304, and studied the hyperfine interactions of 99Ru by T D P A C and emission M6ssbauer spectroscopy. This oxide is an inverse spinel, (Fe3+)[Fe2+Fe3+]O4, and is a ferrim a g n e t with Tc = 858 K. A fast electron transfer occurs between the Fe 2+ and Fe 3+ ions in the octahedral (B) site above the Verway temperature, which is between 110 and 120 K for ordinary Fe304 samples. A b o u t 97% enriched 99Ru was irradiated with 12 MeV protons available at the R I K E N 160 cm cyclotron to produce 99Rh. A carrier-free aqueous solution of 99Rh was obtained from the irradiated ruthenium target by radiochemical separation [3]. A solution of Fe 3+ ions was added to the solution and 99Rh was coprecipitated with ferric hydroxide by adding NHaOH. The ferric hydroxide containing 99Rh 3+ was dried and heated in air at 1023 or 1173 K for 2 h. The obtained ~-Fe203 (99Rh) was heated under a reduced pressure at 1423 K for 2 h, and thereby Fe3On(99Rh) was obtained, the 57Fe Mfssbauer spectrum of the sample at 78 K was typical of Fe304 above the Verway temperature. Namely, the Verway temperature of the sample was lower than 78 K. The 99Ru emission M6ssbauer spectrum was obtained at 5 K and analyzed with a reasonable X2-value, assuming a single site for 99Ru ions. Velocity calibration was performed by measuring the M6ssbauer absorption lines of 57Fe in an iron foil © J.C. Baltzer AG, SciencePublishers

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Y. Ohkubo et al. / TD PA C and emission M~issbauer studies o f Fe304(99Ru)

and in sodium nitroprusside against a 57Co/Rh source. The value of the hyperfine magnetic field (Hhf) and the isomer shift are 145 + 5 kOe and -0.76 + 0.01 mm/s, respectively. The isomer shift of 99Ru correlates with the valence state of ruthenium in oxides [4]. The valence state of 99Ru in Fe304 is assignable to 2+. However, the lower limit of the range of the isomer shift for Ru 3+ oxides is not clear, because only two data on the compounds, LaRuO3 and Co2RuO4, for Ru 3+ and none for Ru 2+ are reported. We suppose that dilutely doped Ru ions tend to take the same valence state as iron ions in the matrix do, as long as oxygen ions are closely packed and the coordination number of Ru ions is the same as that for Fe ions. As is discussed below, 99Ru in Fe304 is considered to be located at the B site. In known compounds, Ru 2+'3+ ions are octahedrally coordinated [5] as Fe 2 and Fe 3+ in the B site of Fe304. We thus conclude that 99Ru in Fe304 exists as Ru 2+, Ru 3+, or a mixed state of them. All the Ru 2+ and Ru 3+ compounds so far reported are of the low-spin type [5]. When Ru is in the divalent low-spin state, it is diamagnetic. When Ru is in the trivalent low-spin state, it has one unpaired electron. The magnetic moment of this unpaired d-electron produces a magnetic field at the nucleus, antiparallel to the magnetic moment. When the magnetic moment of this unpaired electron is aligned, the hyperfine magnetic field due to this moment should be observed. The time spectra N(O, t) of the 353-90 keV 3'-3' cascade emitted from the excited states of 99Ru were taken with a conventional fast-slow setup. Three or four BaF2 scintillation detectors were used, depending on which of two directional anisotropies, R1 (t) andA22G22(t), was to be obtained. Here, R1 (t) = [N(-3~/4, t) - N( + 3n/4, t)]/[N(-3x/4, t) + N( + 3zc/4, t)],

A22G22(t) ----2IN(n, t ) - N(n/2, t)]/[N(n,t) + 2N(x/2, t)]. In the R1 (t) measurement, an external magnetic field of 5 kOe was applied perpendicular to the detector plane in order to align the magnetization of the sample during the measurement. This measurement was performed at room temperature. The A22G22(t) measurements were carried out at various temperatures from 10 to 885K. The sign ofwL was determined to be negative from the positive sign of the Fourier components of R1 (t)/A22, where A22 is negative for the 99Ru 3/2+ state. Here, ~L is the Larmor frequency of the 99Ru (90keV, 3/2+) magnetic dipole moment against the Hhf. By considering the negative sign of the nuclear gyromagnetic ratio of the 99Ru (90keV, 3/2+) state, the direction of the magnetic field at the nucleus was found to be antiparallel to that of the externally applied magnetic field or the net magnetization. We can consider two sources for the antiparallel hyperfine magnetic field at 99Ru in Fe304: (1) antiparallel magnetic moments of Fe 3+ at the A sites, moments which produce an antiparallel supertransferred hyperfine magnetic field (HSTHF) at the nucleus: (2) the parallel magnetic moment of an unpaired electron of the 99Ru ion itself, a moment which produces an antiparallel hyperfine

Y. Ohkubo et al. / TDPA C and emission M6ssbauer studies of FeaO~(99Ru)

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magnetic field. Irrespective of the two possible different mechanisms to produce the antiparallel hyperfine magnetic field, we can conclude that the 99Ru ion arising from the 99Rh3+ ion occupies the B site. The A22G22(t)measured at 10,740, and 885 K (> Tc) are shown in fig. la. The frequency spectra of these anisotropies, derived by a method similar to that developed for analyzing Mfssbauer spectra [6], are shown in fig. lb. Except for the data taken at 885 K, we eliminated the w = 0 component. Each frequency spectra calculated from A22G22(t) has two peaks except at 885 K. The first peak is assigned to the frequency corresponding to a,~, and the second to the one corresponding to 2WL, both with possible broadening. At 885 K ( > Tc), there is only one peak at w = 0 with a width corresponding to the quadrupole frequency, 6WQfor the 90 keV level. In order to evaluate the value of ~ more precisely, we first determined the value of WQby a least-squares fitting of the A22G22(t) data measured at 885 K with the expression: A22G22(t)=A22(l+4cos6wQt)/5. Then, we analyzed the other A22G22(t) data, assuming that WQis independent of temperature and WLhas a Gaussian distribution around WLo.Since WQitself is much smaller than aa_~,its small possible temperature dependence does not affect the determination ofwzo. Fig. 2 shows the temperature dependence of the Hhf, calculated from the fitted parameter, WL0, and the known magnetic moment [7] of99Ru(3/2+).

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Fig. 1. (a) TDPAC spectra, A22G22(t), of 99Ru(+--99Rh) in Fe304 measured at 10, 740, and 885 K, and (b) their frequencydistributions.The solid curves in (a) are the one reproduced with the frequency spectra in (b).

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Y. Ohkubo et at. / TD PA C and emission M6ssbauer studies of Fe304(99 Ru )

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T (K) Fig. 2. Temperature dependence of H~f of 99Ru(*-99Rh) in Fe304. The solid squares are the observed values of Hhf. The solid curve represents the scaled A-site magnetization (ref. [8]), and the dot-dashed one the differencebetween the observed Hhf and the sealed A-site magnetization. As is described above, 99Ru is located at the B site and thus is affected by the Hsa'nF due to the magnetic m o m e n t s o f Fe 3+ at the A sites. The solid curve in fig. 2 shows the temperature dependence o f the A-site magnetization [8] scaled to the m e a s u r e d Hhf at 740 K. We assume that the temperature dependence of the HSTnF due to the A - O - B supertransfer mechanism is the same as that o f this A-site magnetization. Then, there is a clear difference between the measured Hhf and the scaled A-site magnetization. This difference (dot-dashed line in fig. 2) m a y be ascribed to the magnetic m o m e n t o f the unpaired 4d electron o f a R u ion itself. I f this speculation is correct, 99Ru ions in Fe304 exist not as R u 2+ but as a mixed state o f R u E+ and R u 3+, and Fe304(99Ru) is the first example where a R u ion in such a low valence state exhibits its own magnetization in oxides. Details o f experimental results and discussion will be described elsewhere.

References

[1] S. Misawa and K. Kanematsu, in: Crystal and Solid State Physics, Vol. 19a, New Series III of Landolt B6rnstein Zahlenwerte und Funktionen aus Naturwissenshaften mad Teehnik, ed. H.P.J. Wijn (Springer, Berlin, 1986)p. 491. [2] H. Haas and D.A. Shirley, J. Chem. Phys. 58 (1973) 3339. [3] D.F.C. Morris and M.A. Khan, Radioehim. Acta 6 (1966) 110. [4] F.M. D a Costa, R. Greatrex and N.N. Greenwood, J. Solid State Chem. 20 (1977) 381; F.E. Wagner and U. Wagner, in: M6ssbauer Isomer Shifts, eds. G.K. Shenoy and F.E. Wagner (North-Holland, Amsterdam, 1978); T.C. Gibb, R. Greatrex andN.N. Greenwood, J. Solid State Chem. 31 (1980) 153.

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[5] F.A. Cotton and G. Wilkinson, in: Advanced Inorganic Chemistry, 3rd Ed. (Wiley, New York, 1972). [6] J. Hesse and A. Riibartsch, J. Phys. E 7 (1974) 526. [7] E. Browne, J.M. Dairiki, R.E. Doebler, A.A. Shihab-Eldin, L.J. Jardine, J.K. Tuli and A.B. Bruyn, in: Table of Isotopes, 7th Ed., eds. C.M. Lederer and V.S. Shirley (Wiley, New York, 1978). [8] T. Riste and L. Tenzer, J. Phys. Chem. Solids 19 (1961) 117; T. Mizoguchi and M. Inoue, J. Phys. Soc. Japan 21 (1966) 1310.