Article pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Evidence of Intermediate Hydrogen States in the Formation of a Complex Hydride Toyoto Sato,*,† Anibal J. Ramirez-Cuesta,‡ Luke L. Daemen,‡ Yongqiang Cheng,‡ and Shin-ichi Orimo†,§ †
Institute for Materials Research, Tohoku University, Aoba, Sendai 980-8577, Japan Neutron Scattering Division, Spallation Neutron Source, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States § WPI-Advanced Institute for Materials Research, Tohoku University, Aoba, Sendai 980-8577, Japan ‡
S Supporting Information *
ABSTRACT: A complex hydride (LaMg2NiH7) composed of La3+, two Mg2+, [NiH4]4− with a covalently bonded hydrogen, and three H− was formed from an intermetallic LaMg2Ni via an intermediate phase (LaMg2NiH4.6) composed of La, Mg, NiH2, NiH3 units, and H atoms at tetrahedral sites. The NiH2 and NiH3 units in LaMg2NiH4.6 were reported as precursors for [NiH4]4− in LaMg2NiH7 [Miwa et al. J. Phys. Chem. C 2016, 120, 5926− 5931]. To further understand the hydrogen states in the precursors (the NiH2 and NiH3 units) and H atoms at the tetrahedral sites in the intermediate phase, LaMg2NiH4.6, we observed the hydrogen vibrations in LaMg2NiH4.6 and LaMg2NiH7 by using inelastic neutron scattering. A comparison of the hydrogen vibrations of the NiH2 and NiH3 units with that of [NiH4]4− shows that the librational modes of the NiH2 and NiH3 units were nonexistent; librational modes are characteristic modes for complex anions, such as [NiH4]4−. Furthermore, the hydrogen vibrations for the H atoms in the tetrahedral sites showed a narrower wavenumber range than that for H− and a wider range than that for typical interstitial hydrogen. The results indicated the presence of intermediate hydrogen states before the formation of [NiH4]4− and H−.
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INTRODUCTION Hydrogen is a unique element because it exists in various states in compounds (hydrides), including as interstitial hydrogen, which we refer to as H0; protons (H+); hydride ions (H−); and covalently bound hydrogen, which we refer to as Hcov. Such various hydrogen states in hydrides lead to attractive chemical, physical, and material properties.1−3 In particular, complex hydrides, which are composed of metal cations, such as Li+ and Mg2+, and complex anions, such as [BH4]− and [NiH4]4− with Hcov, have attracted significant attention because of their potential use as hydrogen-storage and ionic-conduction materials as well as other useful energy-storage applications.1−9 Although the formation processes of hydrogen states in complex hydrides are of both fundamental and technological interest, many complex hydrides induce large rearrangements of metal atomic frameworks; thus, the elucidation of the formation process becomes difficult. By contrast, the complex hydride LaMg2NiH7, which is composed of La3+, two Mg2+, [NiH4]4−, and three H−, is formed from an intermetallic LaMg2Ni and maintains the original supporting metal frameworks10−13 as many interstitial hydrides with H0. By focusing on the formation process of LaMg2NiH7, we have discovered an intermediate phase, LaMg2NiH4.6, which forms before the completion of LaMg2NiH7.12,13 The crystal © XXXX American Chemical Society
structures of LaMg2NiH7 and LaMg2NiH4.6 are illustrated in Figure 1. LaMg2NiH4.6 has NiH1.9 and NiH3.3 units with similar atomic arrangements to the complex anion [NiH4]4− in LaMg2NiH7. Though it was difficult to experimentally verify the electricconductivity properties on LaMg2NiH4.6, theoretical calculations confirmed that LaMg2NiH4.6 was a metallic hydride and that the H atoms in the NiH1.9 and NiH3.3 units were covalently bound to Ni as [NiH4]4− in LaMg2NiH7.12 Given that the NiH1.9 and NiH3.3 units in LaMg2NiH4.6 were reminiscent of [NiH4]4− in LaMg2NiH7, they were suggested to be precursors for [NiH4]4−.12 In LaMg2NiH4.6, the hydrogen atoms in the tetrahedral sites, which we refer to as Htet, were also located at similar positions to H− in LaMg2NiH7. Similarly, Htet in LaMg2NiH4.6 was reminiscent of H− in LaMg2NiH7. Although the elucidation of hydrogen states in LaMg2NiH4.6 would bridge the gap of knowledge about the hydrogen states among Hcov, H−, and H0 (including their formation processes), the details are still not properly understood. Received: November 6, 2017
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DOI: 10.1021/acs.inorgchem.7b02834 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry
LaMg2NiH7 and LaMg2NiH4.6 are shown in Figures S1 and S2 in the Supporting Information, respectively. The samples (approximately 1.0 g) for INS were sealed in an airtight, stainless-steel container with an outside diameter of 20 mm. The INS spectra of all the samples were obtained at 5 K for 1 h. The airtight, stainless-steel container showed peaks in the observed INS spectra. Therefore, contributions from the airtight, stainless-steel container were removed from the observed INS spectra by the subtraction of the empty stainless-steel sample container observed at 5 K. The original sample for LaMg2NiH4.6 was heated up to 500 K and maintained for 3 h to decompose LaMg2NiH4.6. Thereafter, the sample was cooled to 5 K, and the INS spectrum was observed again for the subtraction of LaMg2Ni, LaMg2NiH7, and LaH2−3 from the initial INS spectrum. The INS spectra were obtained using an inverted-geometry INS spectrometer, VISION (BL-16B), which has high resolution (1.5% ΔE/E) and a broad frequency range (−15 to 8000 cm−1),22 at the Spallation Neutron Source (SNS), Oak Ridge National Laboratory, Oak Ridge, TN. Density-functional-theory (DFT) calculations were performed using CASTEP.23,24 The generalized gradient approximation (GGA), as implemented by Perdew−Burke−Ernzerhof (PBE), was used to describe the exchange-correlation interactions. Ultrasoft pseudopotentials were employed to account for the effects of the core electrons, with an energy cutoff of 380 eV for the plane waves. In the calculation for LaMg2NiH4.6, the hydrogen atoms in NiH1.9 were fully occupied. Less-occupied hydrogen atoms in NiH3.3 were removed from those hydrogen-atomic sites. Here, we refer to the NiH1.9 and NiH3.3 units as NiH2 (bent) and NiH3 (trigonal-planar), respectively. The details of the starting unit-cell configurations were obtained from ref 12. In the calculations, the atomic coordinates were relaxed under the constraints of the symmetry. The energy tolerance for the electronic-structure calculations was 5 × 10−10 eV, and the energy tolerance for the ionic relaxation was 5 × 10−9 eV. The tolerance for the interatomic forces was 0.001 eV/Å. After convergence was reached, the force constants and dynamical matrix were obtained using the finite displacement method on a supercell (1 × 1 × 2 for LaMg2NiH4.6 and 1 × 3 × 1 for LaMg2NiH7), from which the phonon frequencies and vibrational modes were calculated. The electronicstructure calculations were performed on a Monkhorst−Pack mesh (unit cell: 2 × 2 × 3 for LaMg2NiH4.6 and 1 × 3 × 1 for LaMg2NiH7, supercell: 2 × 2 × 2 for LaMg2NiH4.6 and 1 × 1 × 1 for LaMg2NiH7), and the phonons were sampled on a gamma-centered grid in the Brillouin zone (5 × 5 × 7 for LaMg2NiH4.6 and 3 × 7 × 3 for LaMg2NiH7) for the densities of the states and INS calculations. O’CLIMAX software25 was used to convert the DFT-calculated phonon results to the simulated INS spectra. O’CLIMAX software25 was also used to resolve the calculated total hydrogen vibrations in LaMg2NiH4.6 and LaMg2NiH7 into each of their individually contributing hydrogen vibrations. Phonon frequencies calculated from DFT with a harmonic approximation were slightly scaled by up to 6% in the O’CLIMAX software to compare to the experimental INS spectra observed at 5 K and align and assign the peaks. This scaling procedure did not change the order of the frequencies.
Figure 1. Crystal structures of (a) LaMg2NiH7 and (b) LaMg2NiH4.6. Gray, orange, green, blue, and cyan circles and cyan tetrahedra indicate La, Mg, Ni, H (Hcov), and H− (Htet) atoms and the tetrahedral sites of H− (Htet), respectively. The local atomic structures around Ni ([NiH4]4−, NiH1.9, and NiH3.3) and H (H− and Htet) are illustrated on the right.
To further understand the hydrogen states during the hydrogen-absorption reaction from LaMg2Ni to LaMg2NiH7 (i.e., evidence of intermediate hydrogen states during the formation reaction), we report insights into the hydrogen states in LaMg2NiH4.6 compared with those in LaMg2NiH7. The hydrogen states in LaMg2NiH7 and LaMg2NiH4.6 were investigated by inelastic neutron scattering (INS), which provides access to all hydrogen-vibrational modes in materials. In particular, the hydrogen contributions to hydrogen vibrations, which are related to the hydrogen-state environments in compounds, can be clearly observed by INS14−21 because hydrogen is approximately 10 times larger than other elements in a coherent neutron-scattering cross section (80.26 barns, 1 barn = 1 × 10−24 cm2). Furthermore, INS can assess hydrogen-vibrational dynamics in metallic hydrides, such as LaMg2NiH4.6, whereas it is difficult to observe vibrations in metallic compounds by the conventional Raman and infrared spectroscopies, so that vibrational dynamics in these metallic systems cannot be properly studied by methods other than INS. Therefore, INS can be one of the most suitable experimental methods to obtain additional information on hydrogen vibrations related to the hydrogen states in LaMg2NiH7 and LaMg2NiH4.6.
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METHODS
RESULTS AND DISCUSSION
Figure 2 shows the INS spectra of LaMg2NiH7 and LaMg2NiH4.6 observed at 5 K. The contaminants (LaMg2Ni, LaMg2NiH7, and La hydrides) in the sample of LaMg2NiH4.6 were removed from the INS spectrum at 5 K by heat treatment at 500 K in vacuum because LaMg2NiH4.6 decomposes into LaMg2Ni at 500 K (Figure S2 in the Supporting Information). After the heat treatment, the INS spectrum was again observed at 5 K to subtract the contaminants from the initial INS spectrum at 5 K. The experimentally observed INS spectra were compared with the phonon densities of the states in LaMg2NiH7 and LaMg2NiH4.6, which were calculated using the CASTEP23,24 program, and O’CLIMAX25 was used to assign the vibrational modes of LaMg2NiH7 and LaMg2NiH4.6 and
LaMg2Ni was prepared by the induction melting of a mixture of La ingots (Sigma-Aldrich, 99.9%) and Mg2Ni powder (Kojundo Chemical Laboratory, 99.9%) in a molar ratio of 1:1. The details are described in ref 12. Before the syntheses of LaMg2NiH4.6 and LaMg2NiH7, LaMg2Ni was initially heated above 673 K for 5 h under vacuum. LaMg2NiH4.6 was synthesized from the heat-treated LaMg2Ni in a mixture of hydrogen (0.03 MPa) and argon (0.27 MPa) at room temperature for 10 h. LaMg2NiH7 was synthesized from the heat-treated LaMg2Ni in a hydrogen-gas pressure of 1 MPa at 573 K for 12 h. LaMg2NiH4.6 and LaMg2NiH7 were investigated using a powder X-ray diffractometer (PANalytical X’PERT with Cu Kα radiation (wavelength λ = 1.5406 Å for Kα1 and 1.5444 Å for Kα2)). The samples were covered by Kapton tape to prevent (hydro)oxidation during the measurement. The X-ray-diffraction patterns for B
DOI: 10.1021/acs.inorgchem.7b02834 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry
there is a similar trend in the other complex hydrides,19,21,28 the higher wavenumbers of the Ni−H-stretching modes than of the Pd−H-stretching modes is unusual because the wavenumbers of the stretching modes of central atoms and hydrogen in complex anions with transition metals increase as the metals descend in the group.29,30 Barsan et al. have reported a correlation between M−H-stretching mode frequencies in complex hydrides and ionization energies (the second ionization energy of the cation minus the first ionization energy of the M).18,31 In the case of LaMg2NiH7 and LaMg2PdH7, the differences could originate from Ni and Pd because they have the same cation metals (La3+ and Mg2+). In general, 3d transition metals have larger first ionization energies than 4d transition metals.32 By contrast, the trend between Ni (7.64 eV) and Pd (8.34 eV) is the opposite. Therefore, we speculate that the reason for the higher frequencies of the Ni−H-stretching modes than those of the Pd−H-stretching modes, which is inconsistent with the relationship between M−H-stretching frequencies and placement within a group, could possibly be related to the larger first ionization energy of Pd than Ni. Furthermore, by focusing on the hydrogen states of [NiH4]4− and H− in LaMg2NiH7, the observed INS spectrum of LaMg2NiH7 was addressed by a combination of the INS spectra of Mg2NiH4, which is composed of Mg2+ and [NiH4]4−,33 and MgH2, composed of Mg2+ and H− (see Figure S3 in the Supporting Information).34 These facts suggest that LaMg2NiH7 was surely composed of Hcov and H−. We next discuss the hydrogen vibrations and states in LaMg2NiH4.6. As mentioned above, similar hydrogen states in hydrides show similar INS spectra. The experimentally observed INS spectrum of LaMg2NiH4.6 resembled that of LaMg2NiH7 above 600 cm−1 (Figure 2). This finding suggested that the hydrogen states in LaMg2NiH4.6 and LaMg2NiH7 were similar. To assign each hydrogen vibration in LaMg 2 NiH 4.6 , the theoretical-calculation results of LaMg2NiH4.6 were compared with the experimental results. Hydrogen vibrations in NiH2, NiH3, and Htet were obtained from O’CLIMAX. The theoretical calculations show four vibrational modes: (I) translational modes at
