Article pubs.acs.org/IC
In Situ XRD Study of La2Ni7Hx During Hydrogen Absorption− Desorption Kenji Iwase,*,† Kouji Sakaki,‡ Yumiko Nakamura,‡ and Etsuo Akiba‡ †
Department of Materials Science and Engineering, Ibaraki University, 4-12-1, Nakanarusawa, Hitachi 316-8511, Japan National Institute of Advanced Industrial Science and Technology (AIST), AIST Central-5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan
‡
ABSTRACT: Structural changes of La2Ni7Hx during the first and second absorption− desorption processes along the P−C isotherm were investigated by in situ X-ray diffraction (XRD). Orthorhombic (Pbcn) and monoclinic (C2/c) hydrides coexisted in the first absorption plateau, but only a monoclinic (C2/c) hydride was observed in the first desorption plateau. Phase transformation of La2Ni7Hx was irreversible between the first as well as the second absorption−desorption process. The lattice parameters and expansion of the La2Ni4 and LaNi5 cells during the absorption−desorption process were refined using the Rietveld method. The lattice parameters a and b of the orthorhombic hydride (Pbcn) decreased, while the lattice parameter c increased with increasing hydrogen content in the first absorption. During the first absorption, the volume of the orthorhombic La2Ni4 cell expanded by more than 50%, while the expansion of the LaNi5 cell was below 10%. The monoclinic La2Ni4 cell expanded to approximately four times the size of the LaNi5 cell in the first absorption. The lattice parameters a, b, and c of the monoclinic hydride (C2/c) decreased with decreasing hydrogen content in the first desorption. These La2Ni4 and LaNi5 cells contracted isotropically in the first desorption.
1. INTRODUCTION Metal hydrides based on La−Ni intermetallic compounds have been extensively studied.1−5 The La2Ni7 alloy has a hexagonal Ce2Ni7-type structure below 1268 K and a rhombohedral Gd2Co7-type structure between 1268 and 1287 K.6 The unit cell of the Ce2Ni7-type structure contains two subunits, each of which consists of a MgZn2-type cell and two CaCu5-type cells stacking along the c-axis. The unit cell of the Gd2Co7-type structure is formed by stacking the three subunits. La2Ni7 and the related R2Ni7 hydride (R = rare earth) with a superlattice structure have been noticed as promising materials to serve as the negative electrode for nickel−metal hydride (Ni-MH) batteries.7 The crystal structure and hydrogen absorption properties of the Ce2Ni7 hydride have been reported by Denys et al.8 The pressure−composition (P−C) isotherm of the hydride showed a flat plateau, in which the maximum hydrogen capacity reached 0.52 H/M at 293 K. The crystal structure of Ce2Ni7D4.7 has been studied by in situ neutron diffraction, and it was found that Ce2Ni7D4.7 belongs to the space group Pmcn. The unit cell expands along the c-axis, and the volume expansion of the Ce2Ni4 cell can be as high as 62−63%, while the volume of the CeNi5 cell remains unchanged. All deuterium atoms are located in the Ce2Ni4 cell and on the boundary between the Ce2Ni4 and CeNi5 cells. Filinchuk et al. investigated the structural changes of Ce2Ni7 during hydrogenation by using synchrotron X-ray and neutron powder diffraction.9 Ex situ diffraction data were used for Rietveld refinement. Using synchrotron XRD data, the space © 2013 American Chemical Society
group of the metal sublattice of Ce2Ni7Hx was determined to be Pmcn. The crystal structure of Ce2Ni7D∼4 was refined using neutron powder diffraction data with the Pmcn and Pmnm models. The final refinement was obtained using Pmcn only. Deuterium atoms were located in the Ce2Ni4 cell or on the boundary between the Ce2Ni4 and CeNi5 cells. The expansion of the Ce2Ni4 cell along the c-axis was approximately 60%, but the CeNi5 cell was nearly unchanged. The present authors previously reported the crystal structure of the metal sublattice of La2Ni7Hx (x = 7.1, 10.8) determined by in situ XRD.10 Two hydride phases were synthesized from a La2Ni7 alloy with a Ce2Ni7-type structure along the P−C isotherm. La2Ni7H7.1 was orthorhombic (space group Pbcn), and the La2Ni4 and LaNi5 cells expanded by approximately 50% and 5% from the alloy, respectively. La2Ni7H10.8 was monoclinic (space group C2/c). The La2Ni4 and LaNi5 cells expanded by 66% and 14%, respectively, from the alloy. The results indicate that a large expansion of the A2B4 cell along the c-axis is commonly observed in both Ce2Ni7 and La2Ni7, but the formation of a hydride phase with a higher hydrogen content, such as La2Ni7H10.8, and expansion in AB5 cells are observed only in La2Ni7. Yartys et al. investigated the crystal structure of La2Ni7D6.5 by ex situ neutron diffraction.11 They reported that La2Ni7D6.5 had the same hexagonal symmetry (P63/mmc) as the original alloy. The La2Ni4 cell expanded along the c-axis by 58.7%, and the Received: June 4, 2013 Published: August 20, 2013 10105
dx.doi.org/10.1021/ic401419n | Inorg. Chem. 2013, 52, 10105−10111
Inorganic Chemistry
Article
LaNi5 cell slightly shrank by 0.5%. Deuterium atoms were located in the La2Ni4 cells and the boundary between the La2Ni4 and LaNi5 cells, while the LaNi5 cells were almost empty. This Article presents recent results of in situ XRD measurements of La2Ni7Hx during hydrogen absorption− desorption. The P−C isotherm of La2Ni7 was completed at 0.7 H/M, and the hydrogen atoms remained in the sample after the first desorption. To clarify the irreversible hydrogenation property between the first absorption and the first desorption, the lattice parameters and expansion of the La2Ni4 and LaNi5 cells with increasing and decreasing hydrogen content were refined by the Rietveld method. The hydrogen occupation is related to the phase transformation during the hydrogen absorption−desorption process. The variation of the unit cell, the La2Ni4 and LaNi5 cell volume, corresponds to the amount of hydrogen inserted into each of the cells. It is necessary to refine the structural parameters during hydrogen absorption− desorption process. This study presents the phase transformation and change of the structural parameters in the hydrogen absorption−desorption process.
2. EXPERIMENTAL SECTION La2Ni7 alloy was prepared by arc-melting La and Ni metals (99.9%) in an Ar atmosphere. The annealing treatment was conducted at 1153 K for 120 h and quenched in ice water to obtain a single-phase alloy with a Ce2Ni7-type structure. The sample for the P−C isotherm measurement was sealed in a stainless steel container, heated in a vacuum at 373 K for 1 h, and then kept at 273 K for 1 h. The P−C isotherm was measured using the Sieverts’ method without pretreatment for activation. Before the second cycle measurement, the sample was evacuated at 273 K for 3 h. The powder sample for XRD measurement was sieved to a particle size of
