Stabilization of Face-Centered Cubic High-Pressure Phase of REH3

Apr 5, 2018 - Synopsis. The structure of YH3 GdH3 and DyH3 shows a hexagonal structure under ambient condition while it transforms to a FCC at a GPa ...
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Stabilization of Face-Centered Cubic High-Pressure Phase of REH3 (RE = Y, Gd, Dy) at Ambient Pressure by Alkali or Alkaline-Earth Substitution Riki Kataoka,*,† Toru Kimura,‡ Nobuhiko Takeichi,† and Atsunori Kamegawa*,‡ †

New Energy Carrier Research Group, Research Institute of Electrochemical Energy, National Institute of Advanced Science and Technology, Osaka, Japan ‡ Research Center for Environmentally Friendly Materials Engineering, Muroran Institute of Technology, Muroran, Japan ABSTRACT: Heavy rare-earth trihydrides such as GdH3, DyH3, and yttrium trihydide (YH3) usually show a hexagonal crystal structure under ambient pressure. This structure is known to transform to a face-centered cubic (FCC) one at higher pressures (at the order of GPa), and the FCC one returns to the hexagonal structure when the applied pressure is released. In this study, we investigated the structure of alkaline or alkalineearth (A)-substituted REH3 (RE = Y, Gd, Dy; A = Li, K, or Mg) using X-ray diffraction, and measured the phase transition pressure. We found that this FCC high-pressure phase can be stabilized by 10−33 mol % A substitution for RE in the REH3. The mechanism of phase stabilization is simply explained by the ionic radius ratio between the cation and anion (rcat/rani), as well as the stabilities of other ionic crystals such as perovskite materials. For all considered REH3 samples, the FCC phase becomes stable when rcat/rani > 0.856, such as in the case of 10 mol % Li-substituted YH3.

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INTRODUCTION Rare-earth hydrides have attracted attention due to their unique electronic structures that vary depending on their hydrogen content. As the rare-earth metals are hydrogenated to dihydrides, they retain their metallic feature (optically reflective) and then start to show insulating character, as the chemical formula is close to trihydrides (becoming transparent). In other words, there is a metal-to-insulator transition in the rare-earth species during hydrogen absorption/ desorption. Huiberts et al. utilized this characteristic to demonstrate the potential of rare-earth hydrides as switchable mirrors.1 Since then, optical properties of rare-earth hydrides have been studied.2−5 There are also extensive studies on the structural stabilities of the rare-earth trihydrides under high pressures. Most of the rare-earth trihydrides (REH3) can be divided into two types by the crystal structure, that is, a face-centered cubic (FCC) BiF3type structure (La to Pm except Nd) and a hexagonal HoH3type structure (Y and Sm to Tm),6−11 while NdH3 is reported to adopt both the FCC and the hexagonal structure probably due to its structural metastability. These structural models are shown in Figure 1. The HoH3-type hydrides are known to change into the former type under high pressure. This pressure-induced phase transformation of REH3 was first reported by Palasyuk et al. in 2004. Since then, their structural stability and evolution have been extensively investigated using X-ray diffractometry, Raman, and infrared spectroscopy as well as first-principles calculation.12−19 © XXXX American Chemical Society

According to Palasyuk, a trigger of phase transformation in hexagonal REH3 is the interatomic distance between H− ions. When this distance is less than an empirical value of 0.21 nm, the hexagonal structure changes to the FCC structure.14 These FCC phases of REH3 are confirmed to be stable at high pressures, but they return to the hexagonal ambient structure according to an in situ X-ray diffraction (XRD) study.16 Before these researches, our group had reported that MgY2H8 (or (Mg0.33Y0.67)H3−δ) with FCC structure can be synthesized by high-pressure and high-temperature treatment and remain stable even at ambient pressure.20 In that report, however, the relevance of Mg0.33Y0.67H3−δ to the FCC-YH3 high-pressure phase was not discussed. Another group reported that this kind of material could be prepared by hydrogenating sputtered Mg− Y thin film and studied its structure and optical property.3 According to their report, the Y-10 mol % Mg thin film keeps the FCC structure during hydrogenation from dihydride to trihydride. On the basis of these previous reports, we consider that ∼10−33 mol % Mg substitution for Y may be able to stabilize the FCC YH3 high-pressure phase. Moreover, a similar FCC phase, (Li0.1Y0.9)H3−δ, was also obtained by 10 mol % Li substitution for Y in our previous high-pressure study.21 In this context, the alkaline or alkaline-earth substitution for rare-earth elements is effective for stabilizing the FCC high-pressure phase, but the key factor for the structural stabilization is still Received: February 13, 2018

A

DOI: 10.1021/acs.inorgchem.8b00409 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 1. Structural models of REH3. (a) HoH3-type structure (hexagonal) and (b) BiF3-type structure (FCC). 1173 K under 2−5 GPa for 2 h and quenched to room temperature afterward. After the high-pressure treatments, the samples were picked up from the cell in the Ar-filled glovebox. Characterization. Phase identification and structural analysis of samples were conducted by an XRD with Cu Kα radiation (λ = 0.150 45 nm, Bruker, D8 ADVANCE) and high-resolution synchrotron radiation XRD (SXRD) (BL19B2 beamline, SPring-8, Japan). For the SXRD measurement, powders of samples were filled in glass capillaries with an outer diameter of 0.3 mm. The SXRD data were collected using a Debye−Scherrer camera with an imaging plate. The diffraction angle, 2θ, ranged between 5.00 and 78.00°, and the data collection time was 300 s. The X-ray wavelength was λ = 0.075 nm. The detailed structure was refined by Rietveld refinement of the SXRD data using the RIETAN-FP program.22 Weight loss and thermal stability during heating were simultaneously measured by thermal gravimetry and differential thermal analysis (TG-DTA; RIGAKU ThermoMass Photo). Raman spectra of the samples were collected using a Horiba JobinYvon HR800 system equipped with a He−Ne laser (λ = 632.8 nm) as the excitation source.

unclear. Here, we investigate the following three issues to reveal the underlying mechanism: • Detailed structural analysis of Li-substituted FCC-YH3. • The structural stability of the FCC high-pressure phase after substitution in YH3 by other elements, such as KH and MgH2. • The structural stability of REH3 materials with the hexagonal structure, such as GdH3 and DyH3, after Li substitution. Our purpose in this study is to understand how the FCC high-pressure phase is stabilized even at ambient pressure. It is found that this phenomenon could be reasonably explained from a geometrical point of view.

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EXPERIMENTAL SECTION

Sample Preparation. The starting materials were LiH, KH, MgH2 (95 mass %), and REH3 (RE = Y, Gd, Dy) prepared by heating RE metal (99.9 mass %) at 673 K under 4.0 MPa hydrogen for 3 d in a stainless steel tube. These starting materials were then mixed according to given nominal compositions by hand milling and pressed into pellets with a diameter of 3.8 mm and ∼3 mm in height. A pellet was put into a BN container as the pressure medium and then capsuled by graphite heater. The capsule was inserted into a pyrophyllite cube. A schematic cross-sectional illustration of the sample cell is shown in Figure 2. All the sample preparation steps were conducted in a glovebox filled with Ar gas to prevent the samples from oxidizing. A high pressure was applied to the sample by a cubic anvil-type apparatus, the details of which are described in other reports.20,21 The samples were heated at

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RESULTS AND DISCUSSION Characterization and Structural Analysis of Li-Substituted YH3 with FCC Structure. Figure 3 shows the XRD patterns of YH3-X mol % LiH (X = 0, 5, 10) prepared at 1173 K for 2 h under 5 GPa. Peaks of the FCC phase were indexed as double circles, and their relative intensities increased with increasing LiH content in the starting mixture. When the amount of LiH was 10 mol % (YH3-10 mol % LiH), only the FCC phase was detected, and the lattice parameter (0.525 438(3) nm) is similar to that of the FCC YH3 high-pressure phase (a = 0.528 nm, without

Figure 3. XRD patterns of YH3-X mol % LiH, where X = (a) 0, (b) 5, and (c) 10, prepared at 1173 K for 2 h under 5 GPa.

Figure 2. Cross-sectional image of the test cell for high-pressure treatment. B

DOI: 10.1021/acs.inorgchem.8b00409 Inorg. Chem. XXXX, XXX, XXX−XXX

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YH3 shrinks by ∼11%16 compared with that at ambient pressure. Since FCC-YLi hydride is stable at ambient pressure, its lattice volume is ∼10% larger than that of FCC-YH3 at 25 GPa. The lattice expansion is due to the pressure release and would decrease band overlap, that is, increase the electric polarity. Second, Li substitution for Y would increase the electric polarity, since Li has a stronger ionic character than Y.26 The Raman signal reflects the electric polarity; therefore, gap elongation and Li substitution would increase the Raman activity. The above discussion seems to be able to explain the substitution of Li ion for Y site in the YH3 structure. Figure 5 shows the TG-DTA curves of FCC-YLi hydride and YH3. The weight losses were observed at ∼573 K for both

substitution).15 (Note that the reported value is extrapolated to ambient pressure.) The FCC profile identified as Y2O3 phase observed from that of YH3-0 mol % sample was very similar to that of FCC phase observed from other samples. However, as reported in other reports, the pure FCC-YH3 phase is reported to transform to hexagonal structure during releasing pressure. Therefore, we consider the FCC phase in YH3-0 mol % sample as Y2O3 phase with FCC structure.23 The total hydrogen content of YH3-10 mol % LiH was previously evaluated to be 3.52 mass %, which approximately corresponds to ideal hydrogen content (3.38 mass %) calculated from the nominal chemical formula Y0.9Li0.1H2.8, indicating that the sample was synthesized successfully without oxidation or hydrogen release.21 On the basis of these results, we consider the FCC phase to be a Y−Li hydride phase. Hereafter, we denote this sample as FCC-YLi hydride. The Raman spectra provide information about the hydrogen site in the structure. Figure 4 shows the Raman spectra of the FCC-YLi hydride and related Y and Li compounds.

Figure 5. TG-DTA curves of (a) FCC-YLi hydride and (b) YH3 measured under He flow.

samples accompanying endothermic reaction, which is typical of the hydrogen release reaction of trihydride to form dihydride. On the basis of the TG curves, the total amount of weight loss was calculated to be 0.75 mass % (0.62 mol equiv of H atom) for FCC-YLi hydride and 0.80 mass % (0.73 mol equiv of H atom) for YH3. Generally, REH3 releases 1 mol equiv of hydrogen atoms from the HO site (not the HT site) during the dehydrogenation process. Therefore, the decreased amount of hydrogen release by Li substitution is attributed to some amount of defects introduced to the HO site. This result supports the decrease in hydrogen-to-metal ratio (H/M) by Li substitution. The synchrotron XRD pattern of FCC-YLi hydride was analyzed by Rietveld refinement to reveal the detailed structure. Figure 6 shows the result using the structural model constructed based on the above discussion, and the values are summarized in Table 1. Note that the occupancy of H sites was fixed to 1.0, because it is difficult to detect H element by the X-ray due to a very low atomic scattering factor of H. The reliability factors of the weighted pattern (Rwp), the Bragg (RB), and the structural factors (RF) converged to ca. 10%, 5%, and 5%, respectively, which we consider satisfactory for the refinement of this sample. While a trace amount of metal Y phase (ca. 1 mass %), which probably contained in YH3 precursor, was detected as an impurity, the main pattern was confirmed to be the FCC phase without any superstructures. This implies that the Li ion randomly (i.e., not regularly) substitutes for the Y site. The precise amount of substituted Li ion is difficult to evaluate at the 4a site, since the atomic scattering factor is too small to detect by even synchrotron X-ray radiation. However, the estimated amount of substituted Li was ∼14% at the 4a site,

Figure 4. Raman spectra of YH-10 mol % LiH with FCC structure, prepared by high pressure−temperature treatment, and the YH3 with hexagonal structure.

These spectral patterns are very similar to the reported spectra of LaH3 with BiF3-type FCC structure24 (the spectrum of LaH3 is not shown). Some peaks are confirmed in the wavenumber range of ∼200, 350−800, and 800−1000 cm−1. Referring to the Raman spectra of LaH3−δ, these peaks can be ascribed to vibrations of the Y sublattice and H ions at the O site (HO) and T site (HT), respectively, indicating that the H ion occupies both tetrahedral and octahedral sites in the FCC phase. It is noteworthy that the ideal FCC-REH3 with BiF3 structure has only one Raman active mode, that is, T2g ascribed to the HT vibrational mode, according to a factor group analysis of the vibrational spectrum of the crystal.25 This indicates either the disordering or long-range ordering of the HO site (8c) or the symmetry of the FCC structure changes, as pointed out in the case of LaH3. By the way, according to Ohmura et al.,19 the FCC-YH3 at pressures above 25 GPa shows metallic character due to band closure by lattice compression. As a result, FCC-YH3 has no Raman-active mode, unlike the FCC-YLi hydride that shows Raman activity in this study. The difference between the FCCYH3 phases with and without Li substitution would be due to the following two factors. First, the band closure is confirmed at more than 25 GPa. At this pressure, the lattice volume of FCCC

DOI: 10.1021/acs.inorgchem.8b00409 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 6. Result of Rietveld refinement of YH3-10 mol % LiH prepared at 1173 K for 2 h under 5 GPa.

Figure 7. XRD patterns of REH3-10 mol % LiH prepared at 1173 K for 2 h under 5 GPa. RE = (a) Y, (b) Gd, and (c) Dy.

Table 1. Structural Parametersa of YH3−LiH Derived from the Rietveld Refinement

Table 2. Lattice Constants of FCC-REH3 (a) with or (b) without Li Substitution (RE = Y, Gd, Dy)

Rwp

11.26

YH3−LiH (Fm3̅m, No. 225)

RE

(a) a, nm

(b) a, nm14 a

reliability factors, %

Y Gd Dy

0.525 438(3) 0.5352(2) 0.526 02(2)

0.528 0.536 0.526

Re

3.06

RB

3.82

RF

2.67

lattice parameters, nm a

a

Extrapolated value to ambient pressure.

0.525 438(3)

atom

site

g

x

y

z

B, Å2

Y Li H1 H2

4a 4a 4b 8c

0.86 0.14(4) 1.0* 1.0*

0 0 1/2 1/4

0 0 1/2 1/4

0 0 1/2 1/4

0.680(8) 0.680(8) 1.0* 1.0*

substitution) in the FCC structure, and it also supports our results that Li can substitute RE. Next, the structural stabilities of REH3-10 mol % LiH samples under different pressures were investigated. Figure 8 shows XRD patterns of REH3-10 mol % LiH treated at 1173 K for 2 h under 2 GPa.

a Numbers marked with asterisks (*) are fixed in the refinement. Rwp, Re, RB, RF: Reliability factors of the weighted pattern, expected, Bragg, and the structural factors, respectively. g: Site occupancy. x, y, z: Atomic coordinates. B: Isotropic displacement parameter.

which does not contradict the experimental composition. Note that the FCC-YLi hydride was confirmed to reversibly desorb/ absorb hydrogen maintaining its FCC structure in our previous study.21 This is why we conclude that the FCC-YLi hydride is “stabilized” by Li substitution. Hereafter, we consider other REH3-AH (RE: Y, Gd, Dy; A: Li, K, Mg), which are in the FCC hydride phases based on the laboratory XRD analysis, while their detailed structural analyses using synchrotron XRD and Raman spectroscopy are omitted. Stabilization of FCC-REH3 by Li Substitution. Now, we investigate the effectiveness of Li substitution on the stabilities of FCC high-pressure phases of other REH3 such as GdH3 and DyH3. The amount of Li addition is fixed as 10 mol %, since the FCC-YH3 was already found to be stabilized by 10 mol % Li substitution. Figure 7 shows XRD patterns of GdH3- and DyH3-10 mol % LiH prepared at 1173 K for 2 h under 5 GPa. Each sample seems to adopt an FCC structure, like the case of YH3. Lattice constants of these FCC phases, as summarized in Table 2, are slightly lower than those of FCC high-pressure phases of REH3 without Li addition reported by Palasyuk et al. at GPa pressure.14 (Again, the reference values are extrapolated to ambient pressure.) This is probably because the relatively smaller Li ion (ionic radius rLi+: 0.092 nm) partially substitutes for the larger RE ions (rRE3+ 0.10 nm), thereby reducing the average cation radius (rcat = ((1 − x)/100 × rRE3+ + x/100 × rLi+)/2, x: amount of Li

Figure 8. XRD patterns of REH3-10 mol % LiH prepared at 1173 K for 2 h under 2 GPa. RE = (a) Y, (b) Gd, and (c) Dy.

Even under this reduced pressure, the FCC phase could be identified for all samples, while those with RE = Y and Dy also contain some amount of hexagonal ambient-pressure phase of REH3. This indicates that the phase transition from hexagonal to FCC structures occurs at least 2 GPa for these rare-earth elements. In the unsubstituted REH3, the transition from hexagonal to cubic structures is reported to occur at 7.8, 5.0, and 7.0 GPa for YH3, GdH3, and DyH3, respectively,14 and the values are much higher than those from our result. There are several possible reasons for the lowered transition pressure: change in (1) the average cation size, (2) the average electronegativity of the cation after the Li substitution, and/ or (3) the synthesis temperature compared to the reported D

DOI: 10.1021/acs.inorgchem.8b00409 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry conditions in ref 14. The size effect by Li substitution seems to be a very important factor, but the transition pressure is independent of the ion size, as will be discussed in the following section. Palasyuk et al. investigated almost all REH3 phases with hexagonal structures and claimed that the pressure at the hexagonal-FCC transition of REH3 depends on the electronegativity of the RE element:15 the less electronegative it is, the lower the transition pressure of the REH3. Since the Li ion is less electronegative than any RE elements,26 the transition pressure of REH3 might become lower after partial Li substitution for the RE site, at least for RE = Y, Gd, and Dy. Thermal treatment is also a plausible cause, since H− anions in the REH3 lattice display more significant vibration at higher temperatures, resulting in an increased interaction between nearest H− ions. From these points of view, it is understandable that the FCC high-pressure phase is obtained below the reported transition pressure. However, to confirm whether Li substitution affects the transition pressure of REH3 in situ measurement (e.g., XRD and/or Raman) should be necessary. Effect of Alkaline or Alkaline Earth Substitution on Structural Stability of YH3. In the previous subsection, we reported that REH3 with hexagonal structure transforms to the FCC structure upon Li substitution. Now, we show the effects of substitution by two other elements, that is, potassium (K) and magnesium (Mg), on the structural stability of YH3. The ionic radius of K is larger than that of Li and Y, and it is monovalent like Li. Therefore, on the one hand, its substitution could reveal the effect of ionic size of the substituent. On the other hand, the Mg ion has an ionic radius similar to Li but a valence of 2+ instead of 1+. So the effect of valence number could be found by comparing the cases of Li and Mg. Figure 9 shows XRD patterns of YH3with 10 mol % LiH (FCC-YLi hydride), 10 mol % KH (FCC-YK), and 33 mol % MgH2 (FCC-YMg).

Table 3. Lattice Constants of AH-Stabilized FCC-YH3 (A = Li, K, Mg) A

X, mol %

a, nm

Li K Mg

10 10 33

0.525 438(3) 0.528 53(6) 0.516 57(2)

From the XRD pattern of FCC-YMg, the FCC phase is also stabilized by Mg substitution, but a higher amount of substitution is required (33 mol % of MgH2). For detailed information refer to the previous report.20 This large amount of Mg substitution shrinks the size of the FCC lattice, that is, from 0.526 nm for Li to 0.516 nm for Mg. From these comparisons, one can conclude that the ionic size of the substituting element hardly affects the stability of the FCC phase, but its valence number seems to be important for the stability of the high-pressure phase. When RE in REH3 is substituted by alkaline/alkaline-earth elements, the average valence number of the cation changes, and the valence of the anion is inevitably reduced for charge compensation. In other words, defects are introduced to the anion site, which reduces the average anion size. This implies that change in the anion size should be taken into consideration when discussing the structural stability of rare-earth hydrides. In the following subsection, we will discuss the structural stability from this point of view. Tolerance Factor to Stabilize FCC REH3. On the basis of the results that several FCC REH3 high-pressure phases can be stabilized by substitution with various alkaline or alkaline-earth elements, we suspect the stabilizing mechanism lies in defects of the hydride ion generated by such substitution with elements having valence numbers lower than that of RE in REH3(3+), because only 10 mol % substitution of alkaline hydride (LiH and KH) is sufficient for stabilizing FCC-YH3, while 33 mol % is required in the case of MgH2. The defects introduced to the H− site reduce the average ionic radius (rani) of anions. For the cations, its average ionic size (rcat) may increase or decrease upon substitution with alkaline and alkaline-earth ions depending on their size. As a result, the ionic ratio (rcat/rani) changes depending on the amount and kind of the substituted element. Table 4 summarizes the ionic sizes (assuming 8-coordination), the rRE3+/rH− ratios, and the stable structures of REH3. The values of rRE3+ are referred to Shannon’s report.27 The values of rH− for REH3 with FCC structure are calculated according to the following equation. rH− =

Figure 9. XRD patterns of YH3-X mol % AH prepared at 1173 K for 2 n under 5 GPa. A = (a) Li (X = 10), (b) K (X = 10), and (c) Mg (X = 33).

3 × a /4 − rRE3+

(1)

where a represents the lattice parameter of REH3 with FCC structure. From the rH− of all REH3 with FCC structure, the average rH− value was calculated to be 0.126 nm, and this value was fixed for the rRE3+/rH− calculation. Note that the calculated rH− values of the Li-substituted FCC-REH3 are also in good agreement with this average rH− value (0.1244, 0.126, and 0.123 nm for YH3, GdH3, and DyH3, respectively). From the table, when rH−/rRE3+ is more than ca. 0.856, the structure of REH3 is cubic, and otherwise it has a hexagonal structure. On the basis of these trends, we now discuss the stabilization mechanism for the FCC phase. When A substitutes for RE, the chemical composition of the material changes as follows:

FCC-YK shows a typical FCC pattern, indicating that K substitution is effective for stabilizing the high-pressure FCC phase. The calculated lattice parameter is 0.528 53(6) nm, which is larger than that of FCC-YLi hydride; see Table 3. This enlarged lattice constant would be simply due to the larger ionic size of K than that of Li. This indicates that stabilization of the FCC YH3 phase is independent of the size of the substituting element. E

DOI: 10.1021/acs.inorgchem.8b00409 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 4. Ionic Radii of RE3+, rcat/rani Ratio, and Stable Structure of REH3 RE3+

rRE3+,a nm

rRE3+/rH−b

structure

Y La Ce Pr Nd Pm Sm Gd Tb Dy Ho Er Tm

0.102 0.116 0.114 0.113 0.111 0.109 0.108 0.105 0.104 0.103 0.102 0.100 0.099

0.809 0.921 0.907 0.894 0.880 0.867 0.856 0.836 0.825 0.815 0.806 0.797 0.789

hexc cubd cubd cubd cub or hexc,d cubd hexd hexc hexc hexc hexc hexc hexc

substituted by Li, K, or Mg, rcat/rani is increased toward the region with stable FCC structure. On the one hand, for Li and K, a stable FCC phase at ambient temperature could be achieved when x > 0.1. On the other hand, for Mg this x value is 0.3 (30 mol %). These calculation results are very consistent with the experimental observations in this study. As previously discussed, substitution of monovalent ions for RE3+ introduces more defects of H− than the substitution of divalent ions. Accordingly, the average rH− value decreases more significantly upon alkaline substitution than alkaline-earth substitution, allowing rcat/rani to reach the stability threshold at a relatively lower x value. The same happens for the other two RE elements, Gd and Dy, that their FCC structures are more stable than the hexagonal ones when x = 0.1. We therefore conclude that the structural stability of FCC REH3 can be explained from a geometrical point of view, that is, the structural stability is determined by rcat/rani. This seems reasonable, because the structural stabilization of stabilized zirconia by Ca or Y28,29 and related materials and perovskite materials had been similarly explained in terms of rcat/rani.30 Further, we suspect that the stabilization of the FCC highpressure phase by alkaline and alkaline-earth substitution could be extended to all REH3 compounds with a hexagonal structure at ambient pressures. We are currently conducting additional experiments to investigate this.

a The radius of 8 coordination, referred from ref 27. brH− is fixed to 0.126 nm. cHexagonal. dCubic (FCC).

(1 − x)REH3 + x AH → RE1 − pA xH3 − p

(2)

After substitution by A, the material’s cationic and anionic radii change according to the following equations. rcat = (1 − x)rRE3+ + xr A+

(3)

rani = (3 − p)rH−

(4)

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CONCLUSION Effects of alkaline or alkaline-earth (A) substitution for RE in REH3 (RE = Y, Gd, Dy) on their transition pressures and stabilities of FCC high-pressure phase were investigated. These REH3 adopt a hexagonal structure at ambient pressure, but their structure transforms to FCC under the GPa order of high pressure. The FCC high-pressure phase is known to reverse to hexagonal ambient pressure phase when the pressure is released. In this study, we found that “A” substitution for RE in REH3 stabilizes FCC-REH3 high-pressure phase even at ambient pressure. Change in the structural stability of REH3 can be easily explained by the ionic ratio rcat/rani. When rcat/rani > 0.856, the FCC phase becomes more stable at ambient pressure.

where the coefficient p is 2x (A = alkaline) or x (A = alkalineearth), and x is a real number between 0 and 1.0. On the basis of these equations, the values of rcat/rani for various RE and A elements as a function of x are plotted in Figure 10. The ionic radii of Li, K, and Mg were assumed to be 0.092, 0.151, and 0.89 nm, respectively, which are referred from Shannon’s data.27 A region between the dashed lines in Figure 10 indicates stable REH3 with FCC structure, as shown in Table 4. One can find that starting from x = 0, as Y is gradually

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. (R.K.) *E-mail: [email protected]. Phone: +81 143 465642. Fax: +81 143 465644. (A.K.) ORCID

Riki Kataoka: 0000-0003-0773-3314 Notes

The authors declare no competing financial interest.

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REFERENCES

(1) Huiberts, J. N.; Griessen, R.; Rector, J. H.; Wijngaarden, R. J.; Dekker, J. P.; de Groot, D. G.; Koeman, N. J. Yttrium and lanthanum hydride films with switchable optical properties. Nature 1996, 380, 231−234. (2) Nagengast, D.; van Gogh, A.; Kooij, E.; Dam, B.; Griessen, R. Contrast enhancement of rare-earth switchable mirrors through microscopic shutter effect. Appl. Phys. Lett. 1999, 75, 2050−2052. (3) Molen, S. J. v. d.; Nagengast, D. G.; Gogh, A. T. M. v.; Kalkman, J.; Kooij, E. S.; Rector, J. H.; Griessen, R. Insulating fcc YH3−δ stabilized by MgH2. Phys. Rev. B: Condens. Matter Mater. Phys. 2001, 63, 235116.

Figure 10. Change in rcat/rani as a function of (a) RE and (b) A substitution. F

DOI: 10.1021/acs.inorgchem.8b00409 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

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DOI: 10.1021/acs.inorgchem.8b00409 Inorg. Chem. XXXX, XXX, XXX−XXX