High-Pressure Phase Transition of Coffinite, USiO ... - ACS Publications

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High-Pressure Phase Transition of Coffinite, USiO4 J. D. Bauer,*,† S. Labs,‡ S. Weiss,§ L. Bayarjargal,† W. Morgenroth,† V. Milman,∥ A. Perlov,∥ H. Curtius,‡ D. Bosbach,‡ H. Zan̈ ker,§ and B. Winkler† †

Goethe-Universität Frankfurt am Main, Institut für Geowissenschaften, Abt. Kristallographie, Altenhöferallee 1, 60438 Frankfurt am Main, Germany ‡ Forschungszentrum Jülich, Institute of Energy and Climate Research (IEK-6) Nuclear Waste Management, 52425 Jülich, Germany § Helmholtz-Zentrum Dresden-Rossendorf, Institute of Resource Ecology, Bautzner Landstrasse 400, 01438 Dresden, Germany ∥ Dassault Systèmes BIOVIA, 334 Science Park, Cambridge, CB4 0WN, U.K. S Supporting Information *

ABSTRACT: Synchrotron powder diffraction patterns and Raman spectra of synthetic coffinite, USiO4, were collected for pressures up to 35 GPa and are complemented with DFT+U-based calculations. USiO4 undergoes a first-order phase transition from a zircontype (space group I41/amd) to a scheelite-type structure (space group I41/a) at ≈15 GPa and ambient temperature. Contrary to earlier reports, the data indicate that this transition is completely reversible upon pressure release. Bulk moduli were obtained from the p−V data for the zircon-type and scheelite-type USiO4 phases. For zircon-type USiO4, the value for B = 186(5) GPa, whereas, for the scheelite-type phase, B = 204(9) GPa, where the latter is significantly lower than a value proposed earlier (B = 274(16) GPa, [Zhang, F. X.; et al. Am. Mineral. 2009, 94, 916]). Lattice dynamical calculations point toward a Γ-point soft mode triggering the pressure-induced phase transition.

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INTRODUCTION A pressure-induced phase transition from zircon-type structures to scheelite-type structures is known for many ABO4 (A = Sc, Y, REE; B = V, As) compounds. It has also been observed in orthosilicates that crystallize in the zircon structure type at ambient conditions, such as zircon, ZrSiO4,2 and hafnon, HfSiO4.3 The phase transition of ZrSiO4 has been extensively studied under dynamic compression and hydrostatic pressures at ambient and at high temperatures.2,4−7 At ambient temperature, the phase transition in ZrSiO4 occurs in the pressure range between 20 and 30 GPa. This transition pressure is strongly influenced by impurities and radiation damage.8−11 HfSiO4 undergoes this transition at ≈19.6 GPa.3 More recently, this pressure-induced phase transition has also been reported for coffinite, USiO4.1 On the basis of force-field calculations, Bose et al.12 suggested that thorite, ThSiO4, will also transform to the scheelite-type at high pressure (p > 3 GPa). The zircon structure has space group symmetry I41/amd, and the high-pressure scheelite structure has space group symmetry I41/a, which is a maximal subgroup of I41/amd. In the case of ZrSiO4, the scheelite-type structure is ≈10% more dense than the zircon-type structure at ambient pressure.6 The structural relationship of the crystal structures may be described by a shift and rotation of the SiO4− 4 tetrahedra (Figure 1). However, during the phase transition, the cell volume abruptly decreases, the length of the c-axis nearly doubles, while the a-axis is shortened by a third, while the number of formula units per unit cell remains constant. This, as well as the fact that the high© 2014 American Chemical Society

Figure 1. Schematic diagram of the structural relationship between the zircon-type structure and the high-pressure scheelite-type structure. For reasons of clarity, only the SiO4− 4 tetrahedra are depicted.

pressure phases are quenchable, implies that the phase transition is of first order. Attempts to reconcile group− subgroup relationships with the observation of first-order phase transitions are widely discussed. For the zircon-scheelite phase transition, this has been done by Smirnov et al.,6 who proposed a route via a cubic intermediate phase. Received: June 26, 2014 Revised: September 22, 2014 Published: September 23, 2014 25141

dx.doi.org/10.1021/jp506368q | J. Phys. Chem. C 2014, 118, 25141−25149

The Journal of Physical Chemistry C

Article

Table 1. Crystal Structure Parameters of USiO4, Coffinite (Space Group I41/amd with Origin Choice 1: U at Wyckoff Position 4a (0,0,0), Si at 4b (0,0,0.5), and O at 16h (0,y,z)), and Lattice Parameters, Fractional Coordinates y and z of the Oxygen Atom, and Bond Lengths from Powder X-ray Diffraction Experiments, DFT+U Calculations, and Earlier Studies lattice parameters/Å

bond lengths/Å

reference

a=b

c

y(O)

z(O)

d(Si−O)

d1(U−O)

d2(U−O)

this study (exp., 85 K) this study (exp., ambient T) this study (DFT+U) Labs et al.15 Labs et al.15 (EXAFS) Reynolds19 Pointeau et al.18 Bose et al.12 (force-field) Mulak20 Fuchs and Gebert17

6.9792(3) 6.9862(2) 6.9941 6.9842(2)

6.2576(4) 6.2610(4) 6.2601 6.2606(2)

0.1787(15)

0.3313(16)

1.634(9)

2.30(1)

2.42(1)

0.1778 0.1809

0.3312 0.3337

1.632 1.64(1)

2.310 2.298(1) 2.272(4)

2.418 2.439(1) 2.395(6)

6.9980(2) 7.0135(4) 6.76 6.986(2) 6.995(4)

6.2720(2) 6.2669(6) 6.21 6.268(2) 6.263(5)

0.174

0.337

1.55

2.27

2.40

0.18(1)

0.347(10)

1.58(9)

2.32(8)

2.52(9)

Zhang et al.1 measured pressure-dependent X-ray diffraction and infrared spectra of USiO4 up to pressures of 40 and 23 GPa, respectively. The pressure for the phase transition of coffinite to the scheelite-type high-pressure phase was found to occur in the pressure range of 14−17 GPa. In that diamond anvil cell-based study, an alcohol mixture was used as a pressure-transmitting medium. It is known that alcohol mixtures do not provide a hydrostatic environment at pressures above ≈10 GPa.13 Nonhydrostatic pressure may shift the critical pressure of pressure-induced phase transitions14 and also may lead to amorphization. Here, we report the first measurement of Raman spectra of USiO4 at high pressure and present structural data from X-ray diffraction experiments obtained with neon as a pressure medium. Neon behaves hydrostatic up to a pressure of 15 GPa. Although neon is not ideally hydrostatic at higher pressures, it shows much less deviation from hydrostaticity than alcohol mixtures.13 In order to further understand the applicability of DFT+U calculations to systems containing actinides, the experimental investigation is complemented by density functional theory based calculations. The crystal structures and the energies of the vibrational modes of the two polymorphs have been calculated in an attempt to elucidate the origin of the phase transformation.

Almax21 type diamond anvil cells. Tungsten foil of an initial thickness of ≈200 μm was used as gasket material. Gaskets were preindented to a thickness of about 45−55 μm, and holes of about 120−150 μm were laser-drilled into the gaskets to serve as sample chambers. Small chips of ruby (easyLab Technologies Ltd., Reading, U.K.) were loaded into the sample chambers for pressure determination with the ruby fluorescence method.22 Neon was used as a pressure-transmitting medium. The gas was loaded with a custom-built gas loader at 0.18 GPa. Raman Spectroscopy. Confocal micro-Raman measurements at ambient pressure and high pressure were carried out with a Renishaw Raman spectrometer (RM-100) equipped with a Leica DMLM optical microscope with a grating with 1800 grooves per mm and a Peltier-cooled charge-coupled device (CCD). For excitation, the 532 nm line of a Nd:YAG laser with a maximum power of 200 mW was used. A 20× objective lens with a long working distance and a quasi-backscattering geometry was employed. The spectra were collected in the range of 100−1100 cm−1 with exposure times of 150−300 s and a laser power of up to 33% of the maximum power at different pressures up to 23 GPa. The system was calibrated using the band at 519 cm−1 of a Si wafer.23 The collected spectra were corrected by subtracting the background and fitted using Gaussian-type functions. The accuracy of the wavenumbers was ≈1 cm−1, and the spectral resolution was 2 cm−1. Synchrotron Powder X-ray Diffraction at High Pressure. In situ powder diffraction experiments of USiO4 with synchrotron radiation were conducted in diamond anvil cells at pressures up to 35 GPa. The experiments were performed at the PETRA III synchrotron radiation source in Hamburg, Germany (Extreme Conditions Beamline, P02.2).24 Synchrotron radiation with an energy of 41.53 keV (wavelength λ = 0.28955 Å) was employed. The beam size was around 2 × 2 μm2 fwhm. Diffraction images were collected with an area detector (PerkinElmer XRD 1621) and a counting time of 30 s. The diffraction data were processed with the software FIT2D.25 A CeO2 standard was used for calibration of detector parameters. The integrated diffraction patterns were background subtracted with the program Fityk.26 Le Bail fits27 were performed with the program GSAS.16 The lattice parameters given by Fuchs and Gebert17 were used as a starting point for the fits. All spectra were fitted with the weighted Le Bail27 method to determine the lattice parameters of the zircon-type and scheelite-type phases. The coffinite phase was included during the whole fitting process; above 9.5 GPa, Ne solidifies and was included as well. We did not attempt to perform

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EXPERIMENTAL METHODS Sample Preparation. Coffinite was synthesized via a hydrothermal method from UCl4 solution. Details of the synthesis are given elsewhere.15 The lattice parameters of the product phase were determined by X-ray powder diffraction (Bruker D8, Cu−Kα1,2) at ambient pressure. Le Bail refinement with the software GSAS16 using the tetragonal unit cell parameters, space group symmetry, and structure published by Fuchs and Gebert17 gave lattice parameters of a = b = 6.9862(2) Å and c = 6.2610(4) Å. These values are in good agreement with the data published earlier.17,18 Additional data obtained on another sample at the PETRA III synchrotron radiation source in Hamburg, Germany (High Resolution Powder Diffraction Beamline, P02.1), at 85 K were analyzed by Rietveld refinement. Further information on this measurement is given in the Supporting Information. The structural parameters are given in Table 1, where they are compared to earlier experimental data and to values obtained from DFTbased calculations. High-Pressure Experiments. For high-pressure experiments, compacted powder samples were loaded in Boehler25142



Article

Table 2. Crystal Structure Parameters of Scheelite-type USiO4 (Space Group I41/a) at Ambient Pressure, and Lattice Parameters and Bond Lengths from DFT+U Calculations and Earlier Studies lattice parameters

bond lengths/Å

reference

a = b/Å

c/Å

V/Å3

d(Si−O)

d1(U−O)

d2(U−O)

this study, DFT+U Bose et al.12 (force-field) Zhang et al.1

4.9502 4.88

11.0750 11.35

271.39 270.29 273.0

1.644 1.57

2.3895 2.328

2.4304 2.474

and Gebert17 is in agreement with the values obtained in the present study; the longer bond length d2(U−O) = 2.52(9) Å is only in agreement within the large margin of error given by Fuchs and Gebert.17 In the case of the uranium oxygen bonds, there is agreement with the values from the force-field calculations given by Bose et al.12 (d1(U−O) = 2.27 Å, d2(U−O) = 2.40 Å). In summary, by a combination of the different experimental techniques and theory, the U−O bond distances are now well constrained to be 2.295(16) and 2.418(18) Å. In the present study, we were not able to experimentally determine crystal structural parameters of the scheelite-type phase of USiO4 at ambient conditions. Some of the structural parameters of the high-pressure polymorph at ambient conditions are given in Table 2. The lattice parameters from Bose et al.12 are not in good agreement with the lattice parameters from the present DFT+U study. On the other hand, the unit cell volumes are in fair agreement, also with the experimental value given by Zhang et al.1 Again, the silicon oxygen bond length from Bose et al.12 is unrealistically short (1.57 Å). Powder X-ray Diffraction at High Pressures and Equation of State. Figure 2 shows a series of background subtracted X-ray powder diffraction patterns. The esd’s of the pressure are estimated to be 2% of the pressure, but at least 0.2 GPa. At low pressures, only coffinite is present in the sample (black curves). During pressure increase, reflections shift to higher 2θ values corresponding to a decrease of the cell volume. At a pressure of ≈18 GPa, reflections belonging to the highpressure phase appear (red curves). When the pressure of the sample is decreased from 23 GPa (blue curves), the zircon-type phase is completely recovered. The phase transformation between zircon- and scheelite-type shows a hysteresis. The scheelite phase is present in the diffraction patterns until 9.4 GPa during decompression. In a second series of high-pressure experiments, the sample was compressed to a pressure of 35 GPa. In the diffraction patterns measured at pressures above 23 GPa, the zircon-type pattern can no longer be identified. At this pressure, the completion of the phase transition was also reported by Zhang et.1 With a further increase of pressure, the reflections broaden significantly and cannot be resolved at pressures above approximately 30 GPa; i.e., there is a pressure-induced irreversible amorphization even if neon is employed as a pressure-transmitting medium. Figure 3 shows the pressure dependence of the unit cell volumes of the different phases of USiO4 from experiment and theory and the fits of second-order Birch−Murnaghan equations of state31 to the data sets. The fits were performed with the EosFit software.32 The program offers the possibility to use the uncertainties of the data points to derive a weighting scheme for the fit. The errors associated with the experimental data points were thus considered for the fit. The values for the bulk moduli and ambient-pressure unit cell volumes for the two

Rietveld refinements. The localization of a relatively light atom that scatters X-rays weakly (oxygen) next to a very strongly scattering heavy atom (uranium) is difficult. In addition, the very small beam diameter and thus the very small scattering sample volume in the high-pressure experiments results in data sets with low counting statistics that are not suitable for a Rietveld refinement. Computational Details. All calculations in the present work were performed using the CASTEP package.28 The PBEsol generalized gradient approximation29 and the on the fly pseudopotentials from the CASTEP database were employed throughout. The kinetic cutoff energy was 610 eV. Distances between k-points for Brillouin zone sampling were 850 cm−1) are ≈50−100 cm−1 lower in the scheelite-type compared to the coffinite-type polymorph at the same pressure. This is explained by the comparison of the SiO4 polyhedra in the two polymorphs. In coffinite, the Si−O bond length is 1.606 Å and the ratio of the two O−Si−O bond angles is 1.145. In the high-pressure form, the SiO4 polyhedron is less strained, the ratio of the two O−Si−O bond angles is 1.097, and the bond length is elongated to 1.623 Å. The calculated Si−O bond population is the same (0.62) in both polymorphs. This relaxation of the SiO4 polyhedron leads to the decrease in the stretching frequencies.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (J.D.B.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the Federal Ministry of Education and Research (BMBF) under grants 02NUK019C, 02NUK019E, 02NUK021F, 05KS7RF1, and 05K10FRA, Deutsche Forschungsgemeinschaft (German Research Foundation, DFG) under grants WI1232 and DESY, Germany, is gratefully acknowledged. Portions of this research were carried out at the light source PETRA III at DESY, a member of the Helmholtz Association (HGF). We thank Hanns-Peter Liermann and his team at the Extreme Conditions Beamline at PETRA III for help during the experiments. The authors thank F. Krauss of Technical University Munich, Chemistry Department (Fluorine Chemistry), for providing the uranium metal turnings.

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CONCLUSIONS In the present work, we present the first high-pressure investigation of synthetic USiO4 without impurities of UO2. The phase transformation of coffinite, USiO4, to the highpressure form was studied by synchrotron X-ray powder diffraction and Raman spectroscopy in diamond anvil cells up to pressures of 35 GPa and about 20 GPa, respectively. Neon was used as a quasi-hydrostatic pressure-transmitting medium. The phase transition from zircon-type to scheelite-type occurs at pressures above 15 GPa. The back transformation from scheelite- to zircon-type was observed in the synchrotron diffraction patterns and Raman spectra in samples compressed to about 23 GPa. No signs of decomposition to UO2 were found in the samples, although the amorphization reported before can be observed at higher pressures. Raman spectra of USiO4 at high pressures were measured for the first time. The pressure shifts are consistent with those measured for zircon, ZrSiO4, and hafnon, HfSiO4. In addition, the crystal structure and the Raman spectra have been modeled by DFT+U

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