Mössbauer Spectral Properties of Yttrium Iron Garnet, Y3Fe5O12, and

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Mössbauer Spectral Properties of Yttrium Iron Garnet, Y3Fe5O12, and Its Isovalent and Nonisovalent Yttrium-Substituted Solid Solutions Gary J. Long* and Fernande Grandjean* Department of Chemistry, Missouri University of Science and Technology, University of Missouri, Rolla, Missouri 65409-0010, United States

Xiaofeng Guo and Alexandra Navrotsky Thermochemistry Laboratory, University of California, Davis, California 95616, United States

Ravi K. Kukkadapu Environmental Molecular Sciences Laboratory, Pacific Northwest Laboratory, Richland, Washington 99352, United States S Supporting Information *

ABSTRACT: Several high-resolution Mössbauer spectra of yttrium iron garnet, Y3Fe5O12, have been fit as a function of temperature with a new model based on a detailed analysis of the spectral changes that result from a reduction from the cubic Ia3̅d space group to the trigonal R3̅ space group. These spectral fits indicate that the magnetic sextet arising from the 16a site in cubic symmetry is subdivided into three sextets arising from the 6f, the 3d, 3d, and the 1a, 1b, 2c sites in rhombohedral-axis trigonal symmetry. The 24d site in cubic Ia3̅d symmetry is subdivided into four sextets arising from four different 6f sites in R3̅ rhombohedral-axis trigonal symmetry, sites that differ only by the angles between the principal axis of the electric field gradient tensor and the magnetic hyperfine field assumed to be parallel with the magnetic easy axis. This analysis, when applied to the potential nuclear waste storage compounds Y3−xCa0.5xTh0.5xFe5O12 and Y3−xCa0.5xCe0.5xFe5O12, indicates virtually no perturbation of the structural, electronic, and magnetic properties upon substitution of small amounts of calcium(II) and thorium(IV) or cerium(IV) onto the yttrium(III) 24c site as compared with Y3Fe5O12. The observed broadening of the four different 6f sites derived from the 24d site results from the substitution of yttrium(III) with calcium(II) and thorium(IV) or cerium(IV) cations on the next-nearest neighbor 24c site. In contrast, the same analysis applied to Y2.8Ce0.2Fe5O12 indicates a local perturbation of the magnetic exchange pathways as a result of the presence of cerium(IV) in the 24c next-nearest neighbor site of the iron(III) 24d site.

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INTRODUCTION

proposed that yttrium iron garnet may serve as an excellent host for the long-term storage of radioactive lanthanide and actinide ions.3−5 Although there have been several recent papers that have investigated the various uses of yttrium iron garnet, including its derivatives containing traces of other lanthanides and actinides,3−7 in all these papers that report Mössbauer spectral results, the observed spectra are of marginal use6−10 or the published analysis of the spectra is cursory and sometimes of limited value.3,4,11−14 Thus, because it seems that many authors either do not know about our earlier paper,2 a paper that reported a detailed analysis of the Mössbauer spectra of yttrium iron garnet in

The Mössbauer spectral properties of yttrium iron garnet, Y3Fe5O12, are important for several reasons. First, this compound and several of its substitutional derivatives have been found to have important applications as magnetooptical, thermochromic, microwave, magnetic bubble, and composite ceramic materials. Second, in all of these applications, the iron57 Mössbauer spectra are very useful both in checking for the presence of iron(II) impurities in pure and substituted yttrium iron garnets and in understanding the magnetic properties of the iron(III) ions present. The presence of diamagnetic yttrium(III) on the 24c crystallographic site1 also helps to provide a better understanding of the role that the various trivalent magnetic rare-earth ions in compounds, such as Eu3Fe5O12 and Dy3Fe5O12, play in producing magnetic spin reorientations at low temperatures.2 Finally, it has recently been © XXXX American Chemical Society

Received: December 3, 2015

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DOI: 10.1021/acs.inorgchem.5b02769 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry terms of a space group symmetry reduction15,16 from Ia3̅d to R3̅, or do not accept this analysis, we have revisited the Mö ssbauer spectral results presented previously2−4 and, whenever profitable, fit the new spectral results presented in several recent papers.11−14

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three and four components derived from the cubic 16a and 24d sites. For these sites, the angle, θ, between the principal axis of the electric field gradient and the magnetic hyperfine field takes on three and four different values, respectively. In the cubic structure of Y3Fe5O12 with its magnetization along [111], the θ angles are easily calculated2,19 to be 54.7° for the 24d sites and 70.5° and 0° for the 16a sites now divided into 16a12 and 16a4 sites, respectively. Because the rhombohedral distortion is small, the θ angles for the 6f, 3d, and 3d sites and for the 1a, 1b, and 2c sites have been constrained to those calculated for the 16a12 and 16a4 cubic sites, respectively, and only one quadrupole interaction for the rhombohedral sites derived from the 16a cubic site has been adjusted with these constrained θ angles. Within the rhombohedral structure, with an unknown small distortion from cubic symmetry, the θ angles for the 24d-derived sites are expected to be distributed around 54.7°. Hence, in the fits presented herein, only one quadrupole interaction, ΔEQ, compatible with that measured2 for Y3Fe5O12 above its Curie temperature and four θ angles distributed around 54.7° have been adjusted to reproduce the obvious observed broadening of the 24d site absorption, a broadening that is only consistent with a rhombohedral distortion. The asymmetry parameter, η, of the electric field gradient tensor is assumed to be zero. In view of the very small rhombohedral distortion present in yttrium iron garnet, this assumption is strictly correct for the 6f, 3d, and 3d sites and is reasonable for the 1a, 1b, and 2c sites. In addition, there is one isomer shift, δ, each for the 16a- and 24d-derived sites, one spectral line width, Γ, one total spectral absorption area, one relative area, and one baseline. The fit of each spectrum thus involves the adjustment of 16 variables, including 12 hyperfine parameters; to support the viability of the resulting fits, one must ensure that the parameters exhibit a reasonable dependence as a function of temperature and/or sample composition. No texture was observed in any of the spectra under study herein. The work of Sawatzky et al.20 has shown the essential equality of the f-factors for the different iron sites in yttrium iron garnet between 7.5 and 295 K, and this equality has been assumed in reporting percent areas in the present fits. The code also calculates the correlation coefficient between each pair of variables used in the fit, yielding knowledge of which parameters are highly correlated. Rather surprisingly, with the expected exception of the baseline, total spectral absorption area, and line width and, to a lesser extent, a moderate correlation among the three hyperfine fields for the 6f, the 3d, 3d, and the 1a, 1b, 2c sites derived from the 16a site and the ΔEQ and four θ parameters for the four 6f sites derived from the 24d site, there is very little correlation between the remaining parameter pairs, the correlation of which is typically between zero and approximately ±0.08. In all cases, the final reduced χ2 values of the Mössbauer spectral fits were in the range of approximately 1.1−2.5. For the spectra of the yttrium iron garnets containing substitutional calcium(II), cerium(III), cerium(IV), and thorium(IV), the stoichiometric compositions reported previously3,4 are used herein. However, it should be noted that these stoichiometries have been determined both with the use of a cerium(IV) content obtained from the nearedge X-ray absorption spectra, a content that, as the authors admit, is highly prone to large inaccuracies, and with a hidden assumption that adds excess oxygen dianions to ensure charge balance, yielding a resulting total elemental composition that is larger than 100%. As a consequence, it should be noted that these stoichiometric compositions are subject to rather limited accuracies.

METHODS

The Mössbauer spectra evaluated in this work are either the same spectra as reported previously2−4,11,13,14 or additional spectra obtained more recently from the same samples and absorbers or from the authors of other papers. The new spectral analysis takes into account the reduced R3̅ trigonal symmetry, as described previously;2 the reduction in space group from cubic Ia3d̅ to trigonal R3̅ divides the cubic octahedral 16a and tetrahedral 24d iron(III) sites1 into six and four trigonal iron(III) sites, respectively. Further, this spectral analysis has been conducted with a new fitting code developed in conjunction with our colleagues, a very robust fitting code17,18 that calculates the Mössbauer spectral profile expected for each spectral component in terms of the hyperfine parameters of a given crystallographic and magnetic iron(III) site rather than fitting a sum of Lorentzian lines at the observed positions from which the hyperfine parameters are extracted. This calculation of each spectral component profile involves the evaluation of the iron-57 nuclear ground and excited state Hamiltonians, their eigenvalues and eigenvectors, and, thus, the evaluation of the absorption line positions and their intensities. The combination of high-quality spectra,2−4,11−14 the use of the R3̅ trigonal space group symmetry, and the use of a robust fitting program17,18 provides excellent spectral fits that are in complete agreement with the crystallographic and magnetic structure of yttrium iron garnet. These fits of the yttrium iron garnet Mössbauer spectra (see Table 1) use four independent internal hyperfine fields, Hint, three for the 6f, the 3d, 3d, and the 1a, 1b, 2c crystallographic sites in R3̅ rhombohedral-axis symmetry derived from the cubic 16a site and one field for each of the four 6f equivalent sites derived from the cubic 24d site. There is also one quadrupole interaction, ΔEQ = e2Qq/2, where e is the electron charge, Q is the iron-57 nuclear quadrupole moment, and eq is the principal axis of the electric field gradient, for each of the

Table 1. Mössbauer Spectral Parametersa for Yttrium Iron Garnet, Y3Fe5O12 siteb 16a

24d

parameterc

7.5 K

85 K

295 K

Γ, mm/s area, % δ, mm/sd ΔEQ, mm/s Hint,6f, T Hint,3d,3d, T Hint,1a,1b,2c, T θ6f, deg θ3d,3d, deg θ1a,1b,2c, deg area, % δ, mm/sd ΔEQ, mm/s Hint, T θ6,1, deg θ6,2, deg θ6,3, deg θ6,4, deg

0.290(1) 41.1(2) 0.489(1) −0.404(3) 55.39(1) 54.64(1) 54.51(2) 70.5 70.5 0 58.9(2) 0.262(1) −0.960(1) 47.07(1) 43.3(1) 53.1(1) 56.7(1) 70.0(1)

0.286(4) 42.0(2) 0.487(1) −0.407(5) 54.90(2) 54.37(2) 54.08(3) 70.5 70.5 0 58.0(2) 0.252(1) −0.976(3) 46.59(1) 42.5(1) 52.8(1) 56.7(1) 72.0(1)

0.285(4) 41.4(2) 0.384(1) −0.366(5) 49.51(2) 48.88(2) 48.74(2) 70.5 70.5 0 58.6(2) 0.156(1) −0.961(3) 39.60(1) 38.7(1) 51.5(1) 60.8(1) 76.5(1)

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RESULTS AND DISCUSSION The fits of the Mössbauer spectra2−4 of yttrium iron garnet between 7.5 and 295 K are shown in Figure 1, and the corresponding fit parameters are listed in Table 1; the temperature dependence of the fit parameters is shown in Figure S1 of the Supporting Information. The initial fits used the paramagnetic 550 K values2 of ±0.37 and ±1.00 mm/s for the 16a and 24d sites, respectively, as a starting value for ΔEQ. The subsequent refinement of ΔEQ for the sextets revealed a

Statistical fitting errors are given in parentheses. The actual errors are 2−3 times as large. The parameters with no associated error have been constrained to the value given. bThe Wyckoff site in the cubic Ia3d̅ space group. cThe Wyckoff site in the trigonal R3̅ space group. dThe isomer shifts, δ, are reported relative to 295 K α-iron foil. a

B

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temperature of 545 K and saturation fields equal to the 7.5 K fields have been used in this plot. There is an excellent agreement between the measured reduced hyperfine fields and the Brillouin curve in black for S = 5/2, as expected for iron(III) cations. The 295 K Mössbauer spectra of the potential radioactive waste storage compounds,4 Y3−xCa0.5xTh0.5xFe5O12, have been fit for x = 0.2−1.6; the resulting spectral parameters as a function of x are listed in Table S1. Fits of the spectra for x > 0 with a model identical to that used for Y3Fe5O12 were somewhat poor. Hence, additional variable parameters were used, namely, four different isomer shifts and hyperfine fields for the four 6f sites derived from the 24d sites. These additional parameters are justified by the additional disorder introduced by the calcium(II) and thorium(IV) cation substitutions on the yttrium(III) 24c site, a site that is a next-nearest neighbor site of the 24d site. Further, two different line widths were used for the derived 16a and 24d sites. In total, 22 parameters have been adjusted. The spectra for x = 0.2 and 1.6 are shown in Figure 2,

Figure 1. Mössbauer spectra of yttrium iron garnet, obtained at the indicated temperatures, and fit with the parameters listed in Table 1. The spectral data and the total fit are colored black, and the sextets arising from the 16a and 24d sites are colored green and red, respectively.

small reduction in their magnitude upon cooling and, further, indicated that the ΔEQ for both sites must be negative. The revised fits have a substantially reduced misfit associated with the cubic 16a sextets because it has now proven to be possible to fit this site with three sextets in the R3̅ trigonal space group rather than using only two sextets.2−4,21 These three sextets have hyperfine fields designated as Hint,6f, Hint,3d,3d, and Hint,1a,1b,2c in Table 1 and also have θ values at the known2 fixed angles given in the table. These site assignments have been based on the expected local environment in R3̅ symmetry and are to some extent tentative; however, any attempt to change the relative areas of these sextets from 6:6:4 led to far poorer fits. The subdivision of the cubic 24d sites into four trigonal 6f sites and hence sextets substantially improves the fit and replaces the artificial broadening of the 24d sextet introduced in some of the earlier Mö ssbauer work on yttrium iron garnet.3,4,11,13,14,21 Although limited to four temperatures, the temperature dependence of the isomer shifts, hyperfine fields, and quadrupole splittings (see the top portion of Figure S1) are typical of those found for iron(III). As expected, both the isomer shifts and the hyperfine fields decrease with an increase in temperature and the quadrupole splittings are essentially temperature-independent. A reduced plot of the temperature dependence of the average hyperfine fields for the 16a and 24d sites is shown in the bottom portion of Figure S1. A Curie

Figure 2. 295 K Mössbauer spectra of x = 0.2, Y2.8Ca0.1Th0.1Fe5O12, and x = 1.6, Y1.4Ca0.8Th0.8Fe5O12, showing the fit obtained with the parameters listed in Table S1. The spectral data and the total fit are colored black, and the sextets arising from the 16a and 24d sites are colored green and red, respectively.

a figure that reveals that the major change is an increase in the spectral line width, Γ, from 0.285(4) to 0.305(1) mm/s for the 16a sites and from 0.285(4) to 0.416(2) mm/s for the 24d sites, as x increases from 0.0 to 1.6, respectively. The compositional dependence of the Mössbauer spectral fit parameters from x = 0.0 to 1.6 is shown in Figure S2 and confirms the uniform changes with x found for all the parameters for the spectra of Y3−xCa0.5xTh0.5xFe5O12. Thus, the Mössbauer spectra of Y3−xCa0.5xTh0.5xFe5O12, in which the yttrium(III) cations are replaced with an equal mixture of divalent and tetravalent cations that is isovalent with yttrium(III), reveal little, if any, change in the basic structural properties of Y3Fe5O12 except for a broadening of the line width, a broadening that is more substantial for the 24d sites than for the 16a sites because the 24d sites have six 24c C

DOI: 10.1021/acs.inorgchem.5b02769 Inorg. Chem. XXXX, XXX, XXX−XXX

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7.5 to 550 K for each site is poor. The difference between the ΘM value projected from the Debye approximation in the magnetically ordered phase from 7.5 to 295 K and the observed value in the paramagnetic phase at 550 K (see Figure S1) points toward changes in the crystals at or near the 545 K Curie temperature of Y3Fe5O12. The existence of such a change in Y3Fe5O12 is supported by a change1 in slope in the temperature dependence of the a0 lattice parameter and in its elastic properties28,29 in the vicinity of its Curie temperature. The Mössbauer spectra obtained for Y3−xCa0.5xCe0.5xFe5O12 have been fit for x = 0.2−1.4 (see Table S3), and the resulting spectra and fits are virtually identical to those obtained for Y3−xCa0.5xTh0.5xFe5O12 (see Tables S1 and S3 and Figure S4). Once again, as for Y3−xCa0.5xTh0.5xFe5O12, the Mössbauer spectra reveal little if any change in the basic structure from that of Y3Fe5O12 except for a broadening of the line width of the sextets corresponding to the 24d site. All of the remaining parameters are fully consistent with those observed for Y3Fe5O12. It is apparent that for both Y3−xCa0.5xTh0.5xFe5O12 and Y3−xCa0.5xCe0.5xFe5O12 the insertion of calcium(II) and thorium(IV) or cerium(IV) onto the yttrium(III) site has virtually no effect upon the observed 295 K Mössbauer spectrum. Again, no iron(II) component is detected in the Mössbauer spectra of Y3−xCa0.5xCe0.5xFe5O12, with x = 0.2−1.4. The Mö s sbauer spectra and related properties of Y3−xCexFe5O12, with x = 0.1 and 0.2, have been reported previously.3 The 295 K Mössbauer spectrum of Y2.9Ce0.1Fe5O12 is virtually identical to that of Y3Fe5O12 as might be expected because this compound is reported to contain only substitutional cerium(III) on the yttrium(III) 24c crystallographic site (see the parameters obtained for this fit in Table S4). In contrast, the 295 K spectrum of Y2.8Ce0.2Fe5O12 is rather different from that of Y3Fe5O12 and all of the Mössbauer spectra discussed above because it exhibits an additional sextet, a sextet that is most obvious as the black component easily observed at approximately ±6 mm/s (see Figure S5). The appearance of this added sextet is accompanied by a corresponding decrease in the area of the four sextets that comprise the 24d site in Y3Fe5O12; the temperature dependencies of the Mö s sbauer spectral fit parameters for Y2.8Ce0.2Fe5O12 obtained at 77, 150, and 295 K are included in Figure S6. The additional sextet in black, which is harder to observe but is also present at 77 and 150 K, arises because Y2.8Ce0.2Fe5O12 is the only compound studied herein that is reported3 to have both cerium(III) and cerium(IV) substituted onto the yttrium(III) 24c crystallographic site. Thus, an average nonisovalent substitution on the yttrium(III) 24c crystallographic site leads to observable changes in the Mössbauer spectra. The iron(III) on the 16a site in Y2.8Ce0.2Fe5O12 has two nextnearest neighbor 24c sites, whereas the iron(III) on the 24d site has six next-nearest neighbor 24c sites. As a consequence of this difference, the presence of cerium(IV) on the 24c sites is observed to have its major influence on the sextets associated with the iron(III) 24d sites, an influence that reduces the hyperfine field of the sextet somewhat at 77 and 150 K and more significantly at 295 K (see Figure S6). If we assume that the cerium(III) and cerium(IV) cations are not randomly distributed on the 24c site, an iron(III) 24d cation could have as next-nearest neighbors on the 24c site either 5.6 yttrium(III) and 0.4 cerium(III) with a probability of 0.76 or 5.6 yttrium(III) and 0.4 cerium(IV) with a probability of 0.24. In this nonrandom case, the black sextet is expected to have a

crystallographic sites containing yttrium(III), as well as the calcium(II) and thorium(IV) substitutional ions, as next-nearest neighbors, whereas the 16a sites have only two 24c site nextnearest neighbors. Finally, no iron(II) magnetic sextet component is observed above the detection limit estimated to be ∼1% of the spectral area if the sextet is not fully hidden by the sextets found in the Mö ssbauer spectra of Y3−xCa0.5xTh0.5xFe5O12 or Y3−xCa0.5xCe0.5xFe5O12 with x = 0.2 to 1.6 or 1.4, respectively. The Mössbauer spectra of both x = 0.2, Y2.8Ca0.1Th0.1Fe5O12, and x = 1.6, Y1.4Ca0.8Th0.8Fe5O12, have also been measured at 7.5, 150, and 295 K and fit with the parameters listed in parts A and B of Table S2. Although limited to three temperatures, the temperature dependence of the isomer shifts, hyperfine fields, and quadrupole splittings (see Figure S3) is that expected for iron(III). Both the isomer shifts and hyperfine fields decrease with an increase in temperature, and the quadrupole splittings are essentially temperature-independent. Often a fit of the temperature dependence of the logarithm of the spectral absorption area of iron-57 Mössbauer spectra with the Debye model22 for the second-order Doppler shift will yield information about the lattice properties of a compound. Of course, the description of the lattice vibrations in a compound as complex as yttrium iron garnet with a single Debye temperature, ΘD, is at best a weak approximation. Further, the Mössbauer temperature, ΘM, which is similar to ΘD and obtained from the temperature dependence of the isomer shift, δ, over the limited temperature range of 7.5−295 K, is also at best an approximate description of the lattice dynamics. Thus, the Debye temperatures determined from heat capacity or sound velocity measurements and from the temperature dependence of δ or of the spectral absorption area must be compared with caution. Nevertheless, a fit of the 7.5−295 K temperature dependence of the δ values obtained for Y3Fe5O12, Y2.8Ca0.1Th0.1Fe5O12, and Y1.4Ca0.8Th0.8Fe5O12 with the Debye model22 for the second-order Doppler shift (see Figures S1 and S3), although limited to three or four temperatures, indicates that ΘM decreases, respectively, from 520(15) to 485(35) to 460(5) K for the 16a sites and from 530(34) to 502(3) to 490(4) K for the 24d sites. The ΘM values of 520(15) and 530(34) for Y3Fe5O12 fall within the range of ΘD of 454−599 K determined23−25 from heat capacity and sound velocity measurements. In addition, these values are larger than the ΘD values of 366(15) and 406(15) K for the 16a and 24d sites, respectively, obtained20 from the temperature dependence of the observed spectral absorption area. However, it should be noted that the ΘM values obtained from δ are usually found26,27 to be higher than the ΘD values obtained from the spectral absorption area. This difference occurs because higher-frequency vibrations are probed through the temperature dependence of δ than through the temperature dependence of the absorption area. Hence, although the fits between 7.5 and 295 K of the isomer shift may best be considered as guides to the eye in Figures S1 and S3, the results do seem both to agree reasonably well with previously reported ΘD values for Y3Fe5O12 and to indicate a decrease in ΘM and a corresponding small softening of the lattice upon substitution of calcium and thorium ions onto the yttrium 24c site. The inclusion of the isomer shifts measured2 at 550 K for Y3Fe5O12 in Figure S1 shows that the measured values of 0.096 and −0.077 mm/s for the 16a and 24d sites, respectively, are lower than projected from the Debye approximation with the above ΘM values. Unfortunately, a fit of all four δ values from D

DOI: 10.1021/acs.inorgchem.5b02769 Inorg. Chem. XXXX, XXX, XXX−XXX

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Table S5 may be considered as the most accurate Mössbauer parameters for Y3Fe5O12 at 295 K.

percent area of 14%, a value that is slightly larger than the observed values of 9.8−12.4%. Because the average valence of the cerium cations determined3 from the X-ray near edge spectral study of Y2.8Ce0.2Fe5O12 varies from 3.11 to 3.24 depending on the fitting model used, the agreement between the expected and measured percent area of the black sextet is reasonable and the sextets colored black in Figure S5 may be assigned to the iron(III) 24d site with a maximal number of cerium(IV) cations in their next-nearest neighbor 24c sites. The small observed decrease in the hyperfine field of the iron(III) 24d site with cerium(IV) next-nearest neighbors likely results from local structural distortions around the 24c site because of the presence of the small cerium(IV) cation in the expanded3 lattice of Y2.8Ce0.2Fe5O12. These distortions and the occupation of the 24c site by different cations negatively affect the ferrimagnetic exchange pathways between the 16a and 24d iron(III) cations through the oxygen anions, and a reduced hyperfine field is observed for some of the 24d iron(III) sites, yielding the added sextet colored black in the spectra of Y2.8Ce0.2Fe5O12. Alternatively, the presence of the black sextet with its reduced hyperfine field could result from the presence of a nonstoichiometric oxygen anion content perhaps associated with a charge balance achieved through electron delocalization, the less likely possible presence of trace amounts of iron(II) on the 24c site at levels that are largely below the Mössbauer detection limit, or some combination of all of these possibilities. In all cases, the fits of the Mössbauer spectra of the substituted yttrium iron garnet compounds discussed above are dramatically better than those reported previously.3,4,21 Finally, we have undertaken a reanalysis of six additional recently reported2,3,11,13,14 high-quality Mössbauer spectra of Y3Fe5O12 as prepared and studied in five different laboratories for which the original data were available. Many of these papers use very different and sometimes questionable spectral fitting models, which makes difficult any comparison of the resulting spectral parameters. The results of these additional spectral analyses are shown in part in Figure S7, and the resulting parameters are listed in Table S5. As is indicated in this table, in general there is excellent agreement in the spectral hyperfine parameters for the 295 K spectra reported previously.2,3,11,13 The one exception is the reported14 spectrum that has a line width much larger than those of the other spectra because the sample used has been prepared with the spark plasma sintering technique, a technique that quite naturally leads to the larger line widths. As a consequence of the large line width, the uncertainties in the hyperfine parameters are larger than for the other spectra.2,3,11,13 Because in some cases the exact temperature at which a spectrum has been measured is not reported, there may be small differences between room temperature and 295 K. The hyperfine field is the parameter expected to be the most sensitive to temperature, and this is exactly what is observed in Table S5; all the Hint values in this table increase in the order shown for refs 2, 3, 11, and 13. On the basis of the smaller hyperfine fields reported in refs 3 and 11, one is tempted to conclude that the temperature in these references was somewhat higher than 295 K. Nevertheless, even in the possible presence of slight temperature differences, the spectral parameters reported in Table S5 at 295 K are essentially identical. The averaging in this table of the spectral parameters over the four spectra obtained in different laboratories yields a very good estimate of the experimental accuracies of these parameters; the average values listed in

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CONCLUSIONS Via incorporation of the details of the crystallographic structure of yttrium iron garnet, Y3Fe5O12, in its trigonal R3̅ space group into the analysis of its Mössbauer spectra as a function of temperature, it is possible to obtain essentially perfect spectral fits. This approach makes possible a comparison of the spectra obtained with four separate samples by four different laboratories, a comparison that indicates, as one would hope, that all the spectra are identical within the experimental accuracy. The analysis also indicates the importance of the reduction in symmetry from the cubic Ia3̅d to the trigonal R3̅ space group and permits the identification in Y3Fe5O12 of three magnetically inequivalent sextet components arising from the 6f, the 3d, 3d, and the 1a, 1b, and 2c sites in the rhombohedralaxis trigonal R3̅ space group as well as four sextets arising from the four different, magnetically inequivalent, 6f sites in rhombohedral-axis trigonal R3̅ space group. No evidence is observed in the Mössbauer spectra of these compounds for the presence of any iron cations on the yttrium 24c site or of any secondary iron-containing phases. However, as is apparent to the authors and as noted by a perceptive reviewer, there is a very small misfit in some of the spectral refinements near 1.5−1.7 mm/s, a misfit that is also apparent in the plot of the unpublished residuals and a misfit that was somewhat more apparent in our earlier fits.2 Various attempted fits have shown that it is essentially impossible to associate this misfit with the presence of any sextet that is consistent with the presence of iron(III) on the yttrium 24c site. In contrast, the misfit may be the result of our assumption that the single sextet assigned to either the two 3d sites and/or the three 1a, 1b, and 2c crystallographic sites in yttrium iron garnet is only partially correct and that one or more of these sites has slightly different hyperfine parameters that might yield, at least in part, some absorption at approximately 1.5−1.7 mm/ s. Unfortunately, the addition of one or more magnetic sextets is far from simple and rather fraught with difficulties, and we have elected not to undertake this addition. An alternative explanation may be the reduced internal re-absorption of the γrays in the weaker but broader absorption at 1.5−1.7 mm/s as compared with that at the very sharp absorption at −1.0 mm/s. This possibility is further supported by the observation that the absorption at approximately 1.5−1.7 mm/s is always slightly larger than the fit, a fit that is predominantly determined by the remaining far more intense absorption lines found in the rest of a spectrum. Of course, both possibilities may play a part in the small observed misfit. Further, as reported previously,3 it is possible to incorporate equal amounts of calcium(II) and thorium(IV) or cerium(IV) cations that, in part, replace the yttrium(III) cations on the 24c site of Y3Fe5O12 to form solid solutions that mimic the content of possible radioactive nuclear waste storage materials. The Mössbauer spectra of these materials, with the replacement of up to approximately half of the yttrium(III) cations with a mixture of divalent and tetravalent cations that is isovalent with yttrium(III), are virtually identical to the spectra of pure Y3Fe5O12 and show only a substantial broadening of the four sextets assigned to the four 6f sites, sites that have six nextnearest neighbor 24c sites occupied by a distribution of yttrium(III), calcium(II), and thorium(IV) or cerium(IV) cations. Hence, the structure of Y3Fe5O12 is very stable upon E

DOI: 10.1021/acs.inorgchem.5b02769 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

(9) Ristic, M.; Nowik, I.; Popovic, S.; Felner, I.; Music, S. Mater. Lett. 2003, 57, 2584−2590. (10) Kashif, I.; Rahman, S. A.; Ibrahim, E. M.; Sanad, A. M. Adv. Appl. Phys. 2014, 2, 15−26. (11) Serier-Brault, H.; Thibault, L.; Legrain, M.; Deniard, P.; Rocquefelte, X.; Leone, P.; Perillon, J.-L.; Le Bris, S.; Waku, J.; Jobic, S. Inorg. Chem. 2014, 53, 12378−12383. (12) Fechine, P. B. A.; Silva, E. N.; de Menezes, A. S.; Derov, J.; Stewart, J. W.; Drehman, A. J.; Vasconcelos, I. F.; Ayala, A. P.; Cardoso, L. P.; Sombra, A. S. B. J. Phys. Chem. Solids 2009, 70, 202− 209. (13) Paiva, D. V. M; Silva, M. A. S.; Ribeiro, T. S.; Vasconcelos, I. F.; Sombra, A. S. B.; Góes, J. C.; Fechine, P. B. A. J. Alloys Compd. 2015, 644, 763−769. (14) Gaudisson, T.; Acevedo, U.; Nowak, S.; Yaacoub, N.; Grenèche, J.-M.; Ammar, S.; Valenzuela, R. J. Am. Ceram. Soc. 2013, 96, 3094− 3099. (15) Chenavas, J.; Joubert, J. C.; Marezio, M.; Ferrand, B. J. LessCommon Met. 1978, 62, 373−380. (16) Dong, J.; Lu, K. Phys. Rev. B: Condens. Matter Mater. Phys. 1991, 43, 8808−8821. (17) Dattagupta, S.; Blume, M. Phys. Rev. B 1974, 10, 4540−4550. (18) Herman, R. Ph.D. Thesis, University of Liège, Liège, Belgium, 2004. (19) Alff, C.; Wertheim, G. K. Phys. Rev. 1961, 122, 1414. (20) Sawatzky, G. A.; van der Woude, F.; Morrish, A. H. Phys. Rev. 1969, 183, 383. (21) In refs 3 and 4, the Mössbauer spectrum of yttrium iron garnet was analyzed with two sextets for each of the 16a and 24d sites, for a total of four sextets. The use of these four sextets is not supported by the crystallographic and magnetic structure of yttrium iron garnet and leads to rather poor fits especially at approximately − 4 and 2 mm/s, in spite of the use of an unjustified hyperfine field distribution hidden in the Voigt-based line shape profile. It should be noted that in both Table 3 in ref 3 and Table 1 in ref 4 the unit of the average field, ⟨H⟩, and the width of the field distribution or of the Voigt profile, ⟨|H|⟩ and sd ⟨H⟩, is tesla and not millimeters per second as inadvertently indicated. (22) Shenoy, G. K.; Wagner, F. E.; Kalvius, G. M. Mössbauer Isomer Shifts; Shenoy, G. K., Wagner, F. E., Eds.; North-Holland: Amsterdam, 1978; p 49. (23) Shinozaki, S. Phys. Rev. 1961, 122, 388. (24) Edmonds, D. T.; Petersen, R. G. Phys. Rev. Lett. 1959, 2, 499. (25) Kunzler, J. E.; Walker, L. R.; Galt, J. K. Phys. Rev. 1960, 119, 1609. (26) Owen, T.; Grandjean, F.; Long, G. J.; Domasevitch, K. V.; Gerasimchuk, N. Inorg. Chem. 2008, 47, 8704. (27) Möchel, A.; Sergueev, I.; Nguyen, N.; Long, G. J.; Grandjean, F.; Johnson, D.; Hermann, R. P. Phys. Rev. B: Condens. Matter Mater. Phys. 2011, 84, 064302. (28) Burenkov, Y. A.; Nikanorov, S. P. Phys. Solid State 2002, 44, 318. (29) Novikov, N. N.; Petrichenko, N.; Ya; Khimenko, M. V. Izves. Vysshikh Uchebnykh Zavedenii, Fizika 1971, 14, 117.

substitution of isovalent cations on the yttrium(III) 24c site and may serve as the basis for viable radioactive storage materials. In contrast, the Mössbauer spectra of a nonisovalent substitution of cerium(III) and cerium(IV) on the yttrium(III) 24c site reveal the presence of an additional broad sextet assigned to iron(III) 6f sites, sites that have cerium(IV) cations as nextnearest neighbor 24c sites. Again, no evidence of the presence of any iron(II) cations or any secondary iron-containing phases is observed in the Mössbauer spectra of these compounds.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.5b02769. Plots and tables of the temperature and compositional dependence of both the Mössbauer spectra and the spectral fit parameters (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Author Contributions

G.J.L. and F.G. are responsible for all the new spectral analysis reported and for writing the paper. R.K.K. provided the earlier spectral data and some additional unpublished spectra. X.G. and A.N. provided the samples for the initial studies but were not otherwise involved in the reported work. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank H. Serier-Brault, P. B. A. Fechine, and J.-M. Grenèche for providing the Mössbauer spectral data reported in their earlier papers and R. P. Hermann for assistance with the new fitting code. Part of the work reported herein has been carried out at the Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the U.S. Department of Energy’s Office of Biological and Environmental Research, which is located at the Pacific Northwest National Laboratory in Richland, WA.

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REFERENCES

(1) Geller, S.; Gilleo, M. A. J. Phys. Chem. Solids 1957, 3, 30−36. (2) Vandormael, D.; Grandjean, F.; Hautot, D.; Long, G. J. J. Phys.: Condens. Matter 2001, 13, 1759−1772. (3) Guo, X.; Tavakoli, A. H.; Sutton, S.; Kukkadapu, R. K.; Qi, L.; Lanzirotti, A.; Newville, M.; Asta, M.; Navrotsky, A. Chem. Mater. 2014, 26, 1133−1143. (4) Guo, X.; Kukkadapu, R. K.; Lanzirotti, A.; Newville, M.; Engelhard, M. H.; Sutton, S. R.; Navrotsky, A. Inorg. Chem. 2015, 54, 4156−4166. (5) Guo, X.; Rak, Zs.; Tavakoli, A. H.; Becker, U.; Ewing, R. C.; Navrotsky, A. J. Mater. Chem. A 2014, 2, 16945−16954. (6) Naik, S. R.; Salker, A. V. J. Alloys Compd. 2014, 600, 137−145. Naik, S. R.; Salker, A. V. Phys. Chem. Chem. Phys. 2012, 14, 10032− 10040. (7) Niyaifar, M.; Mohammadpour, H. J. Magn. Magn. Mater. 2015, 396, 65. Niyaifar, M.; Mohammadpour, H.; Aezami, A.; Amighian, J. Phys. Status Solidi B 2016, 253, 554. (8) Lee, Y. B.; Chae, K. P.; Lee, S. H. J. Phys. Chem. Solids 2001, 62, 1335−1340. F

DOI: 10.1021/acs.inorgchem.5b02769 Inorg. Chem. XXXX, XXX, XXX−XXX