Quadrupole splitting and Eu partial lattice dynamics in europium orthophosphate EuPO 4

Transcription

1 Hyperfine Interact (2016) 237:31 DOI /s y Quadrupole splitting and Eu partial lattice dynamics in europium orthophosphate EuPO 4 B. Klobes 1 Y. Arinicheva 2 S. Neumeier 2 R. E. Simon 1 A. Jafari 1,3 D. Bosbach 2 R. P. Hermann 1,3,4 Springer International Publishing Switzerland 2016 Abstract Hyperfine interactions in europium orthophosphate EuPO 4 were investigated using 151 Eu Mössbauer spectroscopy from 6 to 300 K. The value of the quadrupole splitting and the asymmetry parameter were refined and further substantiated by nuclear forward scattering data obtained at room temperature. The temperature dependence of the relative absorption was modeled with an Eu specific Debye temperature of 221(1) K. Eu partial lattice dynamics were probed by means of nuclear inelastic scattering and the mean force constant, the Lamb-Mössbauer factor, the internal energy, the vibrational entropy, the average phonon group velocity were calculated using the extracted density of phonon states. In general, Eu specific vibrations are characterized by rather small phonon energies and contribute strongly to the total entropy of the system. Although there is no classical Debye like behavior at low vibrational energies, the average phonon group velocity can be reasonably approximated using a linear fit. This article is part of the Topical Collection on Proceedings of the International Conference on the Applications of the Mössbauer Effect (ICAME 2015), Hamburg, Germany, September 2015 B. Klobes b.klobes@fz-juelich.de 1 Jülich Centre for Neutron Science JCNS and Peter Grünberg Institute PGI, JARA-FIT - Forschungszentrum Jülich GmbH, Jülich, Germany 2 Institute of Energy and Climate Research (IEK-6) Nuclear Waste Management and Reactor Safety, Forschungszentrum Jülich GmbH, Jülich, Germany 3 Faculté des Sciences, Université deliège, 4000 Liège, Belgium 4 Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA

2 31 Page 2 of 7 Hyperfine Interact (2016) 237:31 Keywords Quadrupole splitting Lattice dynamics Nuclear resonance scattering Europium orthophosphate PACS y e 1 Introduction Monazite type rare earth containing orthophosphates REPO 4 (RE = rare earth) have increasingly drawn attention due to low thermal conductivity [1], resistance to radiation damage and aqueous durability [2, 3]. These properties render REPO 4 ceramics potential refractory materials and nuclear waste forms (particularly with RE = La - Gd), which could incorporate trivalent actinides. Usually, Eu is used as proxy for the latter e.g. in La 1 x Eu x PO 4 compounds, which provides the opportunity to utilize 151 Eu Mössbauer spectroscopy for further characterization. However, reports about the hyperfine interactions in EuPO 4 are scarce and partially contradicting [4, 5]. At least to some extent, this may be attributed to the rather small quadrupole splitting as compared to the natural linewidth of the 151 Eu Mössbauer transition, which also impedes the determination of the exact value of the asymmetry term. Moreover, experimental studies of lattice dynamics in these systems have concentrated on Raman spectroscopy [6] or macroscopic thermal conductivity measurements [1] and, thus, information about the total and/or element specific lattice dynamics is missing. Herein, we report on a study of the quadrupole splitting in europium orthophosphate using a combination of conventional energy-domain Mössbauer spectroscopy and nuclear forward scattering. Additionally, Eu specific lattice dynamics were probed by means of nuclear inelastic scattering. 2 Materials and methods Europium-151 Mössbauer spectra were obtained on a constant-acceleration spectrometer using a 25 mci 151m SmF 3 source. The velocity calibration was performed with α-fe at room temperature utilizing a 57 Co/Rh source. All Mössbauer spectra presented herein were measured using powderized samples and a closed-cycle He cryostat. Isomer shifts are reported with reference to EuF 3 at 300 K. Room temperature nuclear resonance scattering (NRS) measurements were carried out in 16-bunch mode at the nuclear resonance beamline ID18 [7] of the European Synchrotron Radiation Facility in Grenoble, France. Nuclear forward scattering (NFS) and nuclear inelastic scattering (NIS) spectra were analyzed using the software CONUSS [8] and a modified version of the software DOS [9]. NRS, NFS, and Mössbauer spectroscopy samples were all obtained from the same sample batch. Europium orthophosphate with monazite structure (space group P 2 1 /n and Eu site symmetry 1) was obtained using a hydrothermal method described in Ref. [10] with subsequent calcination at 873 K for 2 hours and hot pressing (maximal temperature 1673 K, maximal pressing force 3.9 kn, 2 hours). The monazite phase formation was confirmed by X-ray diffraction (not shown here) and no indications for impurity phase formation were found [3, 10].

3 Hyperfine Interact (2016) 237:31 Page 3 of 7 31 Fig. 1 Mössbauer spectra of EuPO 4 obtained at different temperatures 3 Results and discussion 3.1 Hyperfine interactions in EuPO 4 - conventional Mössbauer spectroscopy and nuclear forward scattering Mössbauer spectra of EuPO 4 measured at different temperatures are shown in Fig. 1. Particularly at low temperatures a small asymmetry on the high velocity side of the spectra is visible indicating a negative sign for quadrupole splitting. As there are no impurity phases present, the quadrupole splitting can thus be considered an intrinsic feature of EuPO 4.All spectra were consistently fitted with a linewidth Ɣ = 2.5 mm/s and an asymmetry parameter η = 0.2 although the fit quality assuming 0 η 0.4 was almost constant. Isomer shifts, δ, and quadrupole splittings, E Q,areshowninFig.2. As expected, δ is indicative of Eu 3+ and no other Eu species could be detected as confirmed by a measurement with increased velocity range (not shown here). The absolute value of E Q decreases with increasing temperature, which might be correlated to the crystal field splitting observed in EuPO 4 [11] similar to observations in Ref. [12]. Below 100 K, the quadrupole splitting is in good agreement with Ref. [5], whereas the room temperature value is more than two times lower than reported by Ref. [4]. Moreover, Jin et al. [4] also reported a very high asymmetry parameter η = 0.9, which could not be verified. Nuclear forward scattering (NFS) was used to check the results obtained by conventional Mössbauer spectroscopy, as there is no source contribution to the overall linewidth observed in a NFS spectrum. The room temperature NFS spectrum of EuPO 4 (see Fig. 3) is affected by a significant dynamical contribution to the hybrid beat [13] due to sample thickness, but the best fit was obtained with rather low values for quadrupole splitting and asymmetry parameter consistent with Mössbauer data. The relative absorption area in a Mössbauer spectrum is proportional to the Lamb- Mössbauer factor and the corresponding temperature dependence can be analyzed using a

4 31 Page 4 of 7 Hyperfine Interact (2016) 237:31 Fig. 2 Temperature dependent isomer shift, δ, and quadrupole splitting, E Q,inEuPO η = 0.15(11), ΔE Q = -2.08(3) mm/s 10 3 counts time (ns) Fig. 3 Nuclear forward scattering spectrum of EuPO 4 measured at room temperature and corresponding fit Debye model [14]. In the case of EuPO 4, the relative absorption can be well modeled with a (Eu specific) Debye temperature D = 221(1) K, which is significantly below the Debye temperature of 495 K as derived from macroscopic specific heat measurements [5]. 3.2 Eu partial lattice dynamics - nuclear inelastic scattering Eu partial DPS, g(e), derived from NIS and the corresponding reduced DPS, g(e)/e 2,are depicted in the top and bottom part of Fig. 5, respectively. In general, the Eu partial DPS does not exhibit any striking features and spectral weight is predominantly concentrated around 15 mev. Considering that phonon mode energies from 11 to 136 mev were observed

5 Hyperfine Interact (2016) 237:31 Page 5 of 7 31 Fig. 4 Lamb-Mössbauer factor f LM of EuPO 4 determined using a Debye model for the relative absorption area in conventional Mössbauer spectroscopy. The room temperature f LM derived from the NIS spectrum is also shown using Raman spectroscopy [6] with most energies well above 30 mev, Eu specific vibrations seem to be rather decoupled from those modes involving P and/or O movements. This notion is also supported by the rather low Eu specific Debye temperature as determined from the temperature dependence of the Lamb-Mössbauer factor (see Section 3.1). Based on g(e), thermodynamic, Eu specific quantities such as the mean force constant F, the internal energy E int, the vibrational entropy S vib and the Lamb-Mössbauer factor f LM can be calculated using integral relations [9]. The calculated values specific for Eu in EuPO 4 are summarized in the top part of Fig. 5. f LM is also shown in Fig. 4 and is in good agreement with the results based on conventional Mössbauer spectroscopy. Gavrichev et al. [15] reported the total room temperature entropy in EuPO 4 to be 117 J/mol/K which amounts to 14.1 k B /(formula unit). Thus, the contribution of Eu specific vibrations, S vib = 4.7(3)k B /atom, to the total entropy of the system is significant. Besides, the low energy limit of the reduced DPS, i.e. lim E 0 [g(e)/e2 ], can be used to calculate the average phonon group velocity, vs NIS [16]. However, g(e)/e 2 does not exhibit a constant low energy limit (see bottom part of Fig. 5), which would be expected assuming classical Debye like behavior. Instead, the low energy region of g(e)/e 2 was fitted using a linear function and lim E 0 [g(e)/e2 ] was estimated by the corresponding (g(e)/e 2 )-axis intercept. Moreover, the impact of the fit region was assessed using three different upper levels, which are depicted by arrows in the bottom part of Fig. 5 along with the obtained average phonon group velocities. As the systematic error of this approach exceeds the statistical error of every individual linear fit, the average phonon group velocity is calculated as the arithmetic mean of the values shown in the bottom part of Fig. 5 and the error is calculated accordingly. Thus, vs NIS = 3510(110) m/s. Using ab-initio calculations, the average

6 31 Page 6 of 7 Hyperfine Interact (2016) 237:31 Fig. 5 (top) Density of phonon states, g(e), derived from a room temperature NIS spectrum. (bottom) Reduced density of phonon states. Data below 5 mev was omitted in this representation as it is potentially biased by the analysis procedure. The low energy was estimated by linear fits using different fit regions, which are marked by the arrows. The average phonon group velocities calculated from these Debye levels are also shown phonon group velocity in EuPO 4 was determined to be 3594 m/s [17] and to be 3678 m/s based on thermal conductivity measurements [1]. Both values are in good agreement with, which supports the applied procedure to linearly extrapolate the Debye level. v NIS s 4 Conclusions Hyperfine interactions and Eu partial lattice dynamics in EuPO 4 were investigated using conventional 151 Eu Mössbauer spectroscopy and nuclear resonance scattering. The temperature dependence of the quadrupole splitting might be related to the thermal occupation of different atomic levels due to crystal field splitting. The asymmetry parameter was determined to be close to 0 using a combination of energy-domain Mössbauer spectroscopy and time-domain NFS. The Eu partial DPS obtained by NIS shows that Eu vibrational modes are rather confined to low energies around 15 mev. Using a linear extrapolation of the reduced DPS, the average phonon group velocity could be determined to be 3510(110) m/s.

7 Hyperfine Interact (2016) 237:31 Page 7 of 7 31 Acknowledgments These experiments were performed using beamline ID18 at the European Synchrotron Radiation Facility (ESRF), Grenoble, France. Dr. D. Bessas (ESRF) and Dr. A. I. Chumakov are acknowledged for assistance in using beamline ID18. Partly, this research was supported by the German Research Society (DFG) in the framework of priority program SPP References 1. Du, A., Wan, C., Qu, Z., Pan, W.: J. Am. Ceram. Soc. 92, 2687 (2009) 2. Lee, W.E., Ojovan, M.I., Stennett, M.C., Hyatt, N.C.: Adv. Appl. Ceram. 105, 3 (2006) 3. Brandt, F., Neumeier, S., Schuppik, T., Arinicheva, Y., Bukaemskiy, A., Modolo, G., Bosbach, D.: Progr. Nucl. Energ. 72, 140 (2014) 4. Jin, M.Z., Jia, Y.Q.: Sci. China Ser. A 33, 430 (1990) 5. Golbs, S., Schappacher, F.M., Pöttgen, R., Cardoso-Gil, R., Ormeci, A., Schwarz, U., Schnelle, W., Grin, Y., Schmidt, M., Anorg, Z.: Allg. Chemie 639, 2139 (2013) 6. Begun, G.M., Beall, G.W., Boatner, L.A., Gregor, W.J.: J. Raman Spectr. 11, 273 (1981) 7. Rüffer, R., Chumakov, A.: Hyper. Interact , 589 (1996) 8. Sturhahn, W.: Hyper. Interact. 125, 149 (2000) 9. Chumakov, A.I., Sturhahn, W.: Hyper. Interact , 781 (1999) 10. Arinicheva, Y., Bukaemskiy, A., Neumeier, S., Modolo, G., Bosbach, D.: Progr. Nucl. Energ. 72, 144 (2014) 11. Thiriet, C., Konings, R., Javorsky, P., Magnani, N., Wastin, F.: J. Chem. Thermodyn. 37, 131 (2005) 12. Bauminger, E., Diamant, A., Felner, I., Nowik, I., Ofer, S.: Phys. Lett. 50A, 321 (1974) 13. Shvyd ko, Yu.V., van Bürck, U., Potzel, W., Schindelmann, P., Gerdau, E., Leupold, O., Metge, J., Rüter, H.D., Smirnov, G.V.: Phys. Rev. B 57, 3552 (1998) 14. Gütlich, P., Bill, E., Trautwein, A.X.: Mössbauer Spectroscopy and Transition Metal Chemistry. Springer-Verlag, Berlin (2011) 15. Gavrichev, K., Ryumin, M., Tyurin, A., Gurevich, V., Komissarova, L.: Russ. J. Phys. Chem. A 83, 901 (2009) 16. Hu, M.Y., Sturhahn, W., Toellner, T.S., Mannheim, P.D., Brown, D.E., Zhao, J., Alp, E.E.: Phys. Rev. B 67, (2003) 17. Feng, J., Xiao, B., Zhou, R., Pan, W.: Acta Materialia 61, 7364 (2013)