samarskites and gadolinites

Transcription

1 Hyperfine Interact (2010) 195:85 91 DOI /s Fe Mössbauer spectroscopy of radiation damaged samarskites and gadolinites Dariusz Malczewski Agnieszka Grabias Grzegorz Dercz Published online: 26 September 2009 Springer Science + Business Media B.V Abstract We report the results of 57 Fe Mössbauer spectroscopy, gamma-ray spectrometry and X-ray diffraction of two fully metamict samarskites and two partially metamict gadolinites. The absorbed α-dose for these minerals are found to range from α-decay/mg for one of the gadolinite samples to α- decay/mg for one of the samarskite samples. The Mössbauer spectra of samarskites and gadolinites show increasing line widths of the Fe 2+ doublets with absorbed α-dose. We also observe that the increase in average quadrupole splitting of the Fe 2+ components correlates better with absorbed α-dose from 232 Th than with total α-dose. Keywords Samarskite Gadolinite Mössbauer spectroscopy Metamict minerals 1 Introduction Metamict minerals such as samarskites and gadolinites are a class of natural amorphous materials that were initially crystalline but have become amorphous. These minerals contain radioactive elements that cause structural damage mainly by progressive overlapping nuclear recoil nuclei collision cascades from α-decays of D. Malczewski (B) Faculty of Earth Sciences, University of Silesia, Bedzińska 60, Sosnowiec, Poland dariusz.malczewski@us.edu.pl A. Grabias Institute of Electronic Materials Technology, Wolczyńska 133, Warszawa, Poland G. Dercz Institute of Materials Science, University of Silesia, Bankowa 12, Katowice, Poland

2 86 D. Malczewski et al. Fig. 1 XRD patterns of investigated metamict samples 238 U, 232 Th, 235 U and their daughter products. Because of the natural occurrence of these actinide elements, they serve as analogs for radiation effects in high-level nuclear waste [1]. Samarskite is a complex niobium-tantalum-titanium oxide. Samarskite has the general structural formula A 3+ B 5+ O 4 where A = Ca, Ti, Fe 2+,Fe 3+, rare earth elements (REE), U and Th, and B = Nb and Ta [2]. Analysis of average site charges and cation radii indicate that both A and B sites have octahedral coordination [2]. Gadolinite has a formula REE 2 Fe 2+ Be 2 Si 2 O 10, where REE means both rare earth elements and yttrium. Structurally, gadolinites consist of sheets of SiO 4 and BeO 4 tetrahedra interconnected by layers of distorted octahedral Fe and 8-coordinate Y, REE and actinides (U and Th) [3]. The purpose of this work is to show that certain changes in hyperfine parameters are associated with the absorbed α-dose and are common for the investigated complex metamict oxides and silicates. 2 Samples and experimental procedures A large, massive brownish black sample of samarskite, SAM1, was collected in pegmatites in Centennial Cone, Jefferson Co., Colorado. A massive chunk of dark brown samarskite, sample SAM2, was collected from the Ross Mine, Yancy Co., North

3 57 Fe Mössbauer spectroscopy of samarskites and gadolinites 87 Table 1 Ages of the samples, Fe, U and Th concentrations and calculated α-doses Samarskite Samarskite Gadolinite Gadolinite (SAM1) (SAM2) (GSP1) (GSP2) Age (Ma) 1,400 1,700 a 1,000 1,200 b 328(12) c 328(12) c Fe (wt.%) U (wt.%) Th (wt.%) Calculated total dose 7.1(8) (8) (2) (2) (α-decay/mg) d Calculated dose from 232 Th 2.1(2) (3) (4) (6) (α-decay/mg) a [4] b Mesoproterozoic rocks c [5] d ( Dose has been calculated from the equation: D = 8 N 238 e tλ238 1 ) ( + 7 N 235 e tλ235 1 ) + ( 6 N 232 e tλ232 1 ),wheren 238, N 235 and N 232 are the present number of atoms of 238 U, 235 U and 232 Th per milligram, λ 238, λ 235 and λ 232 are the decay constants of 238 U, 235 Uand 232 Th, and t is the geologic age Carolina. Figure 1 shows XRD patterns of each of the samples. The lack of crystalline peaks for SAM1 and SAM2 indicates complete metamictization (amorphization) of the samples. The gadolinite samples (GSP1 and GSP2) are from pegmatites of Szklarska Poręba in southwestern Poland. The sample GSP1 is dark grayish matte while the sample GSP2 is glassy black. Despite being from the same location, the two samples exhibit different degrees of metamictization, as indicated by differing numbers of crystalline peaks as seen in Fig. 1. This discrepancy is due to different U and Th concentrations. Ages, Fe, U and Th concentrations and calculated α-doses of the four samples are given in Table 1. The concentrations of 232 Th, 238 Uand 235 U were calculated for each sample based on the gamma-ray activities of 228 Ac ( 232 Th), 226 Ra, 214 Pb, 214 Bi ( 238 U) and 235 U. Gamma-ray spectra were collected using an HPGe detector (32% efficiency with energy resolution of 0.86 kev at 122 kev) and analyzed using the Genie 2000 v.3 software package. Iron concentrations were obtained using a JEOL JSM Scanning Electron Microscope. The samples were ground into powder and prepared in the shape of a thin disc absorber. The Mössbauer transmission spectra were recorded at room temperature using a constant acceleration spectrometer, a multichannel analyzer with 512 channels and a linear arrangement of a 57 Co/Rh source (=50 mci) absorber and detector. All Mössbauer spectra were numerically analyzed by the fitting software programs Recoil and MEP. X-ray powder diffraction (XRD) patterns were obtained using a PHILIPS X Pert diffractometer in the Θ Θ system and Cu Kα radiation in scan mode with step size Results and discussion The Mössbauer spectra of the samarskite and gadolinite samples with the corresponding quadrupole splitting distributions (QSD) are shown in Fig. 2.The hyperfine parameters derived from the fitting procedure are summarized in Table 2.

4 88 D. Malczewski et al.

5 57 Fe Mössbauer spectroscopy of samarskites and gadolinites 89 Fig. 2 (Lefthand plots) 57 Fe Mössbauer spectra at room temperature of metamict samarskite and gadolinite samples. Solid dots experimental data, thick solid line fitted curve, thin solid line fitted doublets. (Righthand plots) Corresponding QSD Table 2 Parameters for 57 Fe Mössbauer spectra (shown in Fig. 2) for investigated metamict samples Sample Doublet χ 2 δ Δ Ɣ Assignment Intensity no. (mm/s) (mm/s) (mm/s) (CN) a Samarskite (SAM1) (2) 2.43 (2) 0.30 (2) Fe 2+ (6) 0.24 (2) (5) 1.21 (3) 0.22 (2) Fe 3+ (6) 0.35 (3) (3) 0.77 (3) 0.21 (1) Fe 3+ (6) 0.41 (3) Samarskite (SAM2) (4) 2.51 (3) 0.19 (1) Fe 2+ (6) 0.23 (3) (3) 2.08 (2) 0.19 (3) Fe 2+ (6) 0.34 (4) (6) 1.63 (2) 0.22 (1) Fe 2+ (6) 0.28 (3) (2) 0.92 (3) 0.31 (2) Fe 3+ (6) 0.15 (1) Gadolinite (GSP1) (1) (3) (2) Fe 2+ (6) 1.0 Gadolinite (GSP2) (5) 1.66 (2) 0.24 (1) Fe 2+ (6) 0.35 (4) (3) 2.15 (2) 0.29 (1) Fe 2+ (6) 0.65 (4) Isomer shift values (δ) are given relative to the α-fe standard at room temperature a Coordination number The Mössbauer spectrum of the sample SAM1 is fitted to three quadrupole doublets assigned to Fe 2+ (labeled with a number 1 in Fig. 2; details given in Table 2) and Fe 3+ (labeled 2 and 3) in octahedral positions with a total relative contribution of 0.24 Fe 2+. Based on annealing experiments recently performed in our laboratory, it appears that nearly the entire content of Fe 2+ in SAM1 is a result of the metamictization process. The main features seen in the spectrum of SAM2 are two broad, asymmetric peaks from divalent iron components. The corresponding QSD has also two maxima at 0.93 and 2.10 mms 1. Based on this QSD, the Mössbauer spectrum can be fitted to three Fe 2+ doublets (nos. 1, 2 and 3) and one Fe 3+ doublet (no. 4). Unlike in SAM1, the total relative content of Fe 2+ is as high as Most likely, the very high content of Fe 2+ in samarskite from Ross Mine reflects the high initial ratio of Fe 2+ to total Fe at the time of its formation rather than a result of the metamictization process. It is known that Fe 2+ / Fe for samarskites often differ from sample to sample [6]. The Mössbauer spectrum of the gadolinite sample GSP1 is fitted to one doublet (no. 1) assigned to Fe 2+ in a uniform octahedral site. The corresponding QSD is very narrow with maximum at 1.77 mm s 1. The sample represents an intermediate stage between the crystalline and metamict state of gadolinite [7]. Similar to the SAM2 spectrum, the spectrum of gadolinite sample GSP2 is characterized by two broad, asymmetric peaks from solely Fe 2+ components. The corresponding QSD is continuous with central maximum at 1.96 mm s 1. Based on this QSD, the Mössbauer spectrum of GSP2 can be fitted to two Fe 2+ doublets (nos. 1 and 2). The doublet no. 1 represents the Fe 2+ octahedra that have undergone a contraction during

6 90 D. Malczewski et al. Fig. 3 The total line widths of high energy peaks of a Fe 2+ and b Fe 3+ components vs. total absorbed α-dose Fig. 4 Average quadrupole splittings for Fe 2+ doublets a vs. total absorbed α-dose and b vs. absorbed α-dose from the 232 Th series metamictization, while the doublet no. 2 represents the Fe 2+ octahedra that have undergone an expansion during metamictization. The total line widths of the high energy peaks of Fe 2+ and Fe 3+ components vs. total α-dose are shown in Fig. 3a, b, respectively. Figure 3a shows that the line widths of the Fe 2+ peaks increase with α-dose both for gadolinites and samarskites. Figure 3b shows that the Fe 3+ line widths increase with α-dose for samarskites. Figure 4a, b show the average of quadrupole splittings for Fe 2+ doublets vs. total absorbed α-dose and vs. absorbed α-dose from the 232 Th series, respectively. As can be seen in Fig. 4, plotting the quadrupole splitting as a function of the α-dose from 232 Th gives the same increasing trend both for gadolinites and samarskites, unlike plotting as a function of total α-dose. The average energy of α particles from the 232 Th series is 6.14 MeV and the average energy of recoil nuclei is about 105 kev. These values are higher than for the 238 U series, where the average energy of α particles and of recoil nuclei is 5.34 MeV and 89 kev, respectively. The α-decays from the 232 Th series cause larger radiation damage than α-decays from 238 U. From

7 57 Fe Mössbauer spectroscopy of samarskites and gadolinites 91 the 232 Th decay, SAM1 has obtained an α-dose six times higher than that of SAM2, whereas this ratio for gadolinites (GSP2/GSP1) is about 1.4 (Table 1). Thus, for metamict minerals, the increase of average quadrupole splittings of Fe 2+ doublets in the high degree may be controlled by the α-dose from 232 Th. 4 Conclusions The line widths of the Fe 2+ components for gadolinites and samarskites and the widths of the Fe 3+ components for samarskites increase with total absorbed α- dose. The increase in average quadrupole splitting for Fe 2+ doublets correlates with absorbed α-dose from 232 Th for both gadolinites and samarskites. Acknowledgement This work was supported by the State Committee for Scientific Research, Poland, through grant no. 2P04D References 1. Weber, W.J., Ewing, R.C., Catlow, C.R.A., Diaz de la Rubia, T., Hoobs, L.W., Kinoshita, C., Matzke, H., Motta, A.T., Nastas, M., Salje, E.K.H., Vance, E.R., Zinkle, S.J.: Radiation effects in crystalline ceramics for the immobilization of high-level nuclear waste and plutonium. J. Mater. Res. 13, (1998) 2. Warner, J.K., Ewing, R.C.: Crystal chemistry of samarskite. Am. Mineral. 78, (1993) 3. Miyawaki, R., Nakai, I., Nagashima, K.: A refinement of the crystal structure of gadolinite. Am. Mineral. 69, (1984) 4. Bryant, B., McGrew, L.W., Wobus, R.A.: Geologic map of the Denver 1 2 degree quadrangle, north-central Colorado: U.S. Geological Survey Miscellaneous Investigations Map I-1163, scale 1:250,000 (1981) 5. Pin, H., Mierzejewski, M.P., Duthou, J.L.: Age of Karkonosze Mts. Granite dated by isochrome Rb/Sr and its initial 87 Sr/ 86 Sr value. Prz. Geol. 10, (1987) 6. Nakai, I., Akimoto, J., Imafuku, M., Miyawaki, R., Sugitani, Y., Koto, K.: Characterization of the amorphous state in metamict silicates and niobatesby EXAFS and XANES analyses. Phys. Chem. Miner. 15, (1987) 7. Malczewski, D., Janeczek, J.: Activation energy of annealed metamict gadolinite from 57 Fe Mössbauer spectroscopy. Phys. Chem. Miner. 29, (2002)