Structural and valence transitions of EuH x exposed to high pressure H 2 conditions Materials Science : Structure 54 Rare-earth metals R, including yttrium (Y) and scandium (Sc), can absorb large amounts of hydrogen (e.g., 300 mol% in YH 3 , at ambient pressure). This has led to extensive studies of their physical and chemical properties for industrial applications and academic interest. This study [1] is conducted to establish a clear connection between the structural phases of EuH x and the other RH x , and to contribute to a full understanding of the interaction between hydrogen and rare-earth metals. In particular, we are interested in phase transformations and valence states of EuH x under H 2 pressures that exceed 1 GPa to identify the known phases of other ‘‘regular’’ RH x . Systematic studies of RH x have revealed common features in the crystal structure. RH x crystallizes into essentially three structural phases, α , β and γ phases, depending on the hydrogen composition of x = H/R. The α phase is a solid solution where H atoms are distributed statistically at the tetrahedral (T) interstitial sites of the metal lattice as impurities. The β phase has an fcc structure. The β phase has been observed for dihydrides and several trihydrides RH 3 (for R = La, Ce, or Pr) [2]. In dihydrides, H atoms occupy T sites, forming an fcc fluorite structure. The γ phase possesses an hcp structure with the ideal composition RH 3 (R = Y, Nd, Sm, Gd, Tb, Dy, Ho, Er, or Lu) [2]. It is known to transform to an fcc structure at high pressures [3]. Europium hydride EuH x has been an irregular member among the RH x family because of its unique structural properties. Reflecting its divalent ground state 4 f n+1 (5 d 6 s ) 2 , the dihydride EuH 2 crystallizes into the orthorhombic Pnma (PbCl 2 -type, Z(Eu) = 2) structure. EuH x is the only rare-earth metal hydride for which other structural phases have not been clearly observed. When EuH 2 is exposed to high- pressure H 2 , one can expect an increase in hydrogen composition and valence changes that lead to structural phase transitions. The chemical potential of hydrogen on hydrogen solubility is significantly enhanced by high pressures exceeding 1 GPa [4]. The valence state of Eu can be changed between 4 f n+1 (5 d 6 s ) 2 and 4 f n (5 d 6 s ) 3 by high pressure or by chemical manipulation. Thus, at a sufficiently high pressure, the β or γ phase may result from additional hydrogen uptake and a valence transition. We have studied the crystal structures of europium hydride compressed in H 2 and He environments by X-ray diffraction (XRD) in BL10XU and DFT calculation. These systems are denoted as EuH x /H 2 and EuH 2 /He, respectively. To investigate the valence state of EuH x , synchrotron Mössbauer spectroscopy measurements of EuH x /H 2 were carried out in BL09XU . Figure 1 shows the typical integrated X-ray diffraction diagram of EuH x /H 2 with the crystal structures of the three new phases denoted as EuH x -II, III and IV. At 7.2 GPa, EuH x -I, which has Pnma structure, transformed to the EuH x -II, which crystalizes in P 6 3 / mmc (Ni 2 In-type) structure. The EuH x -II transforms to EuH x -III with a tetragonal I 4/ m structure at 8.7 GPa and subsequently to EuH x -IV EuH x / H 2 Intensity (arb. units) 2 θ θ (degrees) 6 g g 8 10 12 14 16 6 8 10 12 14 16 18 IV IV IV IV IV II III III III III III III II I I I I I I I I I I I I II II II II II II II II EuH x –III 4.3 GPa EuH x –IV 9.7 GPa EuH x –I 2.7 GPa EuH x –II and EuH x –III 8.7 GPa EuH x –I and EuH x –II 7.2 GPa EuH x – I ( Pnma , 1atm) a = 5.624 Å b = 5.380 Å c = 5.720 Å Z(Eu) = 4 EuH x – II ( P 6 3 / mmc , 7.2 GPa) a = 3.927 Å c = 5.243 Å V = 70.00 Å 3 Z(Eu) = 2 EuH x – III ( I 4/ m , 4.3 GPa) a = 8.351 Å c = 5.352 Å V = 373.19 Å 3 Z(Eu) = 10 EuH x – IV( I 4/ mmm , 9.7 GPa) a = 3.654 Å c = 5.211 Å V = 69.59 Å 3 Z(Eu) = 2 Eu H Eu H (pressure unload) Fig. 1. Integrated XRD profiles of the EuH x / H 2 system with crystal structures of EuH x -I, II, III, and IV phases. The inset graph indicates the XRD profile of EuH x -III at 4.3 GPa when pressure is unloaded. The downward arrows in the inset graph show the satellite peaks of the EuH x -III phase. The “g” labels show the diffraction peaks of the Re-metal gasket. 55 with the I 4/ mmm structure. In the EuH 2 /He system, the EuH x -I to EuH x -II transition was observed at 7.2 GPa where the same transition was observed in the EuH/H 2 system. However, the EuH x -II phase was found to be stable up to 28 GPa. Here, it is clear that the transition I → II is a thermodynamic effect by external pressure and that II → III → IV transformations are induced by the reaction between the sample and surrounding high-pressure H 2 . To investigate the hydrogen composition x in EuH x -III and EuH x -IV, we compared the atomic volume per Eu, i.e., V atom,Eu , values at the transition pressures. V atom,Eu is calculated from the refined unit cell parameters and plotted as a function of pressure in Fig. 2. We observed a volume expansion of 1.1 Å 3 in EuH x /H 2 at 8.7 GPa, where EuH x -II and EuH x -III coexist. We make a rough estimate for it on the basis of the empirical observation that the absorption of a H atom into a rare-earth metal lattice induces 4± 0.5 Å 3 atomic volume expansion Δ V in the host metal lattice at ambient pressure. At high pressures, the anticipated volume expansion should be less than that at ambient pressure. Assuming that the volume expansion results only from the absorption of hydrogen atoms, the increase Δ x in hydrogen composition in the II → III transition can be estimated to be at least 0.2. We investigated the valence states of EuH x /H 2 by synchrotron Mössbauer spectroscopy measurements. Figure 3 shows the typical Mössbauer spectra of EuH x at 2.7 and 14.3 GPa. The velocity scale was calibrated relative to the single line of EuF 3 at ambient pressure. The isomer shift at 2.7 GPa, where the sample is in the EuH x -I phase, was –10.50 mm/s relative to Eu 3+ :EuF 3 , indicating the divalent state. At 14.3 GPa, the isomer shift changed to 0.71 mm/s, showing that EuH x -IV was in the trivalent state. This is clear evidence for the hydrogen-induced valence transition of EuH x . We compare the EuH x -IV phase with other rare-earth metal hydrides. The fcc structure is a bct structure whose c/a ratio is √ _ 2. EuH x -IV with c/a = 1.425 is a slight (0.8%) distortion of fcc. Because of its trivalent character and the small distortion from the fcc structure, EuH x -IV corresponds to the β phase observed commonly for other RH x . This is the first observation of the β phase and the trivalent state for EuH x . Henceforth, EuH x is no longer an irregular member of the rare-earth metal hydride family. EuH x /H 2 EuH x –I EuH x –II EuH x –I EuH x –II EuH x –III EuH x –III P -unload EuH x –IV EuH 2 /He Eu + bcc-Eu Δ Δ V = 1.1 Å 3 P (GPa) V atom, Eu (A 3 /atom) 50 45 40 35 30 25 0 10 20 30 40 50 Intensity (arb. units) Velocity (mm/s) –12 –8 –4 0 4 8 12 Eu 2+ Eu 3+ EuH x -I 2.3 GPa –10.51 mm/sec EuH x -IV 14.3 GPa 0.72 mm/sec Fig. 2. Pressure dependence of V atom,Eu . Solid lines are guides for eyes. The volume data of pure bcc-Eu were taken from Ref. 5. Fig. 3. High-pressure Eu-Mössbauer spectra of EuH x -I at 2.3 GPa and EuH x -IV at 14.3 GPa. Solid lines show the fit of the experimental data. The velocity scale was calibrated relative to the center of a single line of EuF 3 under ambient conditions. [1] Takahiro Matsuoka* and Katsuya Shimizu KYOKUGEN, Center for Quantum Science and Technology, Osaka University *E-mail: matsuoka@cqst.osaka-u.ac.jp References [1] T. Matsuoka, H. Fujihisa, N. Hirao, Y. Ohishi, T. Mitsui, R. Masuda, M. Seto, Y. Yoda, K. Shimizu, A. Machida and K. Aoki: Phys. Rev. Lett. 107 (2011) 025501 . [2] P. Vajda, in Handbook on the Physics and Chemistry of Rare Earths, ed. by K.A. Gschneidner and L. Eyring (Elsevier Science B.V, Amsterdam 20 (1995) 207. [3] M. Tkacz and T. Palasyuk: J. Alloys Compd. 446-447 (2007) 593. [4] H. Sugimoto and Y. Fukai: Acta Metall. 40 (1992) 2327. [5] K. Takemura and K. Syassen: J. Phys. F Met. Phys. 15 (1985) 543.
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