Circumstances of La, Eu, Dy, and Yb Cations Intercalated via Ion

Oct 11, 2018 - Takahiro Takei†* , Kazuki Aoyama† , Sayaka Yanagida† , Nobuhiro Kumada† , and Yasushi Nakajima‡. † Center for Crystal Scien...
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Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Circumstances of La, Eu, Dy, and Yb Cations Intercalated via Ion Exchange in γ‑Zirconium Phosphate Takahiro Takei,†,* Kazuki Aoyama,† Sayaka Yanagida,† Nobuhiro Kumada,† and Yasushi Nakajima‡ †

Center for Crystal Science and Technology, University of Yamanashi, 7-32, Miyamae, Kofu, Yamanashi 400-8511, Japan Daiichi Kigenso Kagaku Kogyo Co., Ltd., 6-38 Hirabayashi Minami 1-Chome, Suminoe, Osaka 559-0025, Japan

‡

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S Supporting Information *

ABSTRACT: Both α- and γ-zirconium phosphate were examined for use as ion exchangers for recovery of rare earth elements. Trivalent rare earth elements can be partially substituted for protons in the interlayer space, and γzirconium phosphate shows a much better ion exchange competency than αzirconium phosphate. The exchanged cation of the rare earth elements might be related to different amounts of oxygen from P−OH and H2O, and these rare earth elements were thus positioned at a different separations from the zirconium phosphate layer. The radial structure function (RSF) curve from extended X-ray absorption fine structure data implied that the calibrated M−O distance and coordination number changed with the ionic radius. The calibrated M−O distances from RSF were 2.52, 2.42, 2.38, and 2.28 for La, Eu, Dy, and Yb, respectively. The coordination numbers of oxygen for Yb were approximately 7 and greater than 10 for La and Eu, respectively. These smaller coordination numbers for smaller cations may result from the strong interaction between the cations and the zirconium phosphate layer. The Debye−Waller factor also increased with an increase in the ionic radius. These factors show a strong relation to the coordination state of rare earth elements in the unit cell of the γ-zirconium phosphate and to the interaction strength.

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INTRODUCTION Most lanthanide elements have electrons localized in f orbitals, which tends to provide them with unique magnetic and luminous properties. Additionally, lanthanides can be used for electronic materials, abrasive compounds, dyes, laser materials, and so on, which are very useful in improving human life.1,2 In the earth’s crust, amounts of the rare earth elements are approximately several ppm, similar to those of Sn or Pb. Particularly, the distribution of heavy rare earth elements is regionally localized in the world. Therefore, the rare earth elements are sometimes expensive given economic rules. Recently, the costs have gradually decreased though they remain basically valuable compared to the ubiquitous elements. Japan is a scientific and technological nation, and there are widespread urban mines that include abundant rare earth elements. Therefore, optimal processes will be able to recycle these rare earth elements to compensate for their rarity and to become a nation that can provide rare earth elements to other countries. Given these considerations, the processes for extraction of rare earth elements from aqueous solutions in which rare earth elements are dissolved become very important for their reuse and recycling.3−5 Generally, rare earth elements can be dissolved in strong acid to form cationic states. Therefore, materials that can adsorb these cations in strong acid are necessary. An inorganic cation exchanger with high durability in acidic solution would be a strong candidate for the recycling of rare earth elements. © XXXX American Chemical Society

Some layered inorganic materials with cation-exchangeability were examined for adsorption of rare earth elements. Xiao et al. investigated the adsorption thermodynamics of La, Nd, and Y on kaolin and the changes in free energy were negative.6 Moldoveanu et al. reported the extraction of rare earth elements from clay minerals such as kaolinite, muscovite, smectite, and so on.7,8 Takei et al. reported that montmorillonite showed a smaller competency for rare earth element adsorption than that of hydroxyapatite (HAp) and γ-zirconium phosphate (γ-ZrP). However, HAp can be readily dissolved in an acidic aqueous solution.9 Therefore, when HAp is used as the adsorbent of rare earth elements, some mechanism that prevents HAp dissolution is necessary. Actually, HApmesoporous silica hybrids have been reported to increase the durability of HAp in acidic solution.10 However, HAp will consequently dissolve in acidic condition. Zhang et al. reported that Sc3+ adsorption in amorphous, α- and γ-titanium phosphates. They concluded that the amorphous titanium phosphate showed the largest amount of recovery.11 However, the reason for this and the coordination circumstance of Sc3+ were not clear. In the previous study, rare earth cations within the interlayer spaces were examined using synchrotron X-ray diffraction (SXRD).9 These patterns imply the formation of four Received: April 12, 2018

A

DOI: 10.1021/acs.inorgchem.8b01003 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry superlattices via the existence of rare earth cations. The superlattices were categorized by ionic radii. In the interlayer space, the site at which the rare earth elements occur depends on the ionic radius. Such results may indicate the possibility of the formation of materials that have a competency for selective adsorption of a particular cation. However, the site positions in the unit cell estimated from SXRD were obtained only as scientific statistics. In such cases, the environment of the rare earth cations, such as coordination number and bond distance, cannot be determined because there are too many sites for adsorption of the rare earth elements derived from the superlattice and each site will not be fully occupied. In particular, lanthanoids sometime show a tetrad effect derived from a nephelauxetic effect. The superlattice previously mentioned might result from the tetrad effect. In this study, the environments of the rare earth elements, particularly La, Eu, Dy, and Yb, were examined via X-ray absorption fine structure (XAFS) experiments because these elements are typical of each tetrad region and are important for optical materials. La can be used for optical lenses and Eu and Dy for phosphorescent materials, and Yb attracts attention as a sensitizer and laser material.

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EXPERIMENTAL SECTION

For consideration of simultaneous ion exchange in a coexistent rare earth aqueous solution, α-zirconium phosphate (α-ZrP) and γ-ZrP were examined for concerted adsorption by 12 rare earth cations provided from their nitrate salts. The concentration of each of these nitrate salts was 1000 mg/L (12 g/L total). Next, the samples were placed into an aqueous solution at a concentration of 200 g/L, and the solution was stirred for ion exchange. A solution of 0.5 mL was selected from the solution at an arbitrary period during stirring. The selected solution was used for measurement of the exchanged amount of the rare earth element. For estimation of the coordination environment of a typical lanthanoid cation, the La, Eu, Dy, and Yb cations were solely exchanged. La, Eu, Dy, and Yb nitrates were used as rare earth cation sources. The nitrate aqueous solutions were prepared with a constant ratio of rare earth element per Zr of 4. Then, samples were added into the solutions and stirred to adsorb rare earth cations. After the adsorption treatments, the samples were examined for their coordination states with oxygen using XAFS measurements in BL14B2 (SPring-8) and for their crystal structures using SXRD measurements in BL02B2 (SPring-8). In the XAFS measurements, ion-exchanged samples were pelletized via uniaxial pressing at 10 MPa at an optimal thickness which was calculated so as to make Δμt = 1. Then, the pelletized sample was arranged on the axis of an X-ray monochromated by Si(311). For both EXAFS and X-ray absorption near edge structure (XANES) measurements, K absorption edges were used because the energies between the LII and LIII edges are relatively close and are difficult to use to calculate the RSF from the EXAFS curves. For XANES, both K and LIII edges are measured at RT and 20 K, respectively. For these measurements, transmission mode was measured using a gas flow ion chamber in which various optimal mixed gases were automatically supplied with He, N2, and Ar. The measured data were analyzed by Athena and Artemis.12 For the synchrotron XRD (SXRD) measurements, the samples were set in the center of the Debye-Sherrer camera with a camera length of 286.48 mm. X-rays of 0.496071 Å in wavelength were irradiated to measure the SXRD patterns at room temperature. The obtained patterns were analyzed by RIETAN-FP.13 The chemical composition of the ionexchanged samples was measured using SEM-EDX (JSM-6500F, JEOL) and ICP (SPS3520UV-DD, Hitachi high-tech Science Corp.).

Figure 1. Concentration changes of La, Eu, Dy, and Yb in reaction time via ion exchange of α-ZrP and γ-ZrP in a 13 lanthanoids coexisting aqueous solution.

time via ionic exchange of α-ZrP and γ-ZrP for a 12 lanthanoids coexisting solution. The aqueous solution shows a slightly acidic property with a pH of 5.2. These plots show only La, Eu, Dy, and Yb concentrations as typical examples. In a comparison of α- and γ-ZrP, the adsorption amounts in γZrP are apparently larger than those in α-ZrP. For adsorption in γ-ZrP, the amount of La was lower than that of the others. This result implies that La may easily separate from some lanthanoid cation coexisting solution. The pH value of the solution after adsorption treatment indicates around 1.0. Such decrease of pH implies that proton in the γ-ZrP was exchanged by rare earth cation to release into the solution. In the sample of the ion-exchanged γ-ZrP, many species of lanthanoid were adsorbed, and it is difficult to analyze the environment of each adsorbed lanthanoid. To clarify the reason for the difference in

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RESULTS AND DISCUSSION Structure of the Ion-Exchanged γ-ZrP with Rare Earth Cations. Figure 1 shows the concentration change in reaction B

DOI: 10.1021/acs.inorgchem.8b01003 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

The crystal structure of γ-ZrP has been refined by Salvado et al. using neutron powder diffraction. The space group is P21 (No. 4), and there are two types of phosphate, PO4 and H2PO4, with two types of proton existing in the interlayer space.17 From our previous study, the interlayer space increased without collapse of the inorganic layer and without displacement parallel to the layered structure by the intercalation of rare earth cations. In addition, a smaller cation tends to adsorb nearby the inorganic layer. However, the detailed structure was not reported. In this paper, the positions of rare earth elements will be considered as follows. Figure S1 shows synchrotron XRD patterns of typical rare earth elements of La-, Eu-, Dy-, and Yb-adsorbed γ-ZrP. From these patterns, there are no impurities and no rare earth hydroxide phases. These patterns confirm that there are some peaks from the existence of diffraction superlattice in the range of 2.5−4.9 in two theta. Figure 3 shows the relative position on

the adsorption amounts in γ-ZrP, ion exchange was completed solely by the existing lanthanoid solution. Figure 2 shows the relationship between atomic number and the adsorbed amount of rare earth cations into the γ-ZrP

Figure 2. Relationship between the atomic number and the adsorbed amount of rare earth cations into the γ-ZrP interlayer space. The empty plots and the error bars are experimental data and the solid line indicates predicted trace line by tetrad effects.

interlayer space. In these plots, the right axis indicates the distribution coefficient. From these plots, there are four indentations in the whole lanthanoid region. The solid line indicates predicted trace line with the four indentations. Such indentations seem to be derived from tetrad effects. Ohta et al. reported that tetrad effects show the ionicity of the rare earth species existing in the sample via refined spin-pairing energy theory (RSPET) as follows. Kawabe reported that the enthalpy difference between the rare earth state before and after adsorption of the rare earth elements can be fitted using the improved RSPET formula14,15 as follows: 9 ΔHr = A + (a + bq)qZ* + n(S)C1Z* + m(L)C3Z* 13 (1)

Figure 3. Relative positions of a, b, and c axes in the γ-ZrP unit cell. Left upper side figures are projection view of the crystal structure of γZrP onto ab, bc, and ca plane corresponding to each plot of x, y, and z coordinates.

where q and Z* are the number of 4f electrons and effective nuclear charge that can be calculated by Z* = q + 25 for 4f electrons, respectively. The constant coefficients, n(S) and m(L), are provided by the total spin angular momentum quantum number (S) and the total orbital quantum number (L) reported in the literature.15 These n(S) and m(L) parameters are negative. C1 and C3 are constants that are a proportional value of the difference between the Racah parameters of the product and reactant for adsorption. The Racah parameter is larger for ionic and smaller for covalent bonds. Therefore, C1 and C3 should be positive as a result of an increase in iconicity and be negative as a result of an increase in covalency. From eq 1, a positive C1 and C3 will result in a downward convex distribution coefficient and vice versa.16 From this theory, the C1 and C3 values must be positive because four indentations exist in the relationship between the ionic radius of the rare earth element and the distribution coefficients in Figure 2. Such positive C1 and C3 parameters may result from an increase in iconicity of the rare earth elements. That is, abundant water molecules exist and rare earth elements may be in an ionic state within the interlayer space of the γ-ZrP.

the a, b, and c axes in the γ-ZrP unit cell. In these figures, their locations and directions are corresponding to the top left projected figures of the crystal structure. Generally, the space group of γ-ZrP is P21 as mentioned above, which has a 21 screw axis along the b axis. In this space group, there is no center of symmetry, and the general equivalent position can be expressed as (−x, y+1/2, −z) meaning that there are two equivalent positions for rare earth adsorption in a unit cell. Therefore, these equivalent positions should be integrated by averaging the absolute values of the x and z coordinates as follows. When the origin is set at the center of the unit cell, negative coordinates of the existing positions of rare earth elements can be integrated into positive coordinates along the a and c axes via sign inversion. However, the equivalent positions can be expressed as y and y + 1/2 for the y coordinate. These cannot be integrated along the b axis because of another symmetry. Therefore, the mean coordinates were used for the y coordinate. That is, the fractional C

DOI: 10.1021/acs.inorgchem.8b01003 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry integrated coordinates of x and z show the expansion of the position of rare earth elements from the center of the unit cell, and the mean fractional coordinate of y indicates the shift of the position. From these reasons, one-half of the unit cell is shown for the a and c axes, and a full unit cell is shown for the b axis in Figure 3. From these plots, the integrated coordinates of z basically changed because of the ionic radius from 0.16 to 0.09. These values are nearly constant during each stage which has other superlattices as shown in the previous literature. In the case of the x coordinate, the value of yttrium is quite small. However, the values of the other elements (lanthanide elements) are 0.2 or larger. Such large values for the rare earth elements remained in the empty space in which initially zeolitic H2O existed. For the y coordinates, the mean relative coordinates are approximately zero. This means that rare earth elements distribute homogeneously toward the b axis except for yttrium. XAFS and EXAFS Calculations. Figure 4 shows K-edge XANES curves of the rare earth elements adsorbed γ-ZrP,

Figure 5. kn-weighted EXAFS oscillations at the La, Eu, Dy, and Yb K edge of rare earth element adsorbed γ-ZrP.

ZrP and γ-ZrP. Therefore, a larger limit of the range k of 12 was used for calculation of the radial structural function of these samples. For metal oxides, there are a small amount of noises and a larger limit of approximately 15 was used. From a comparison of these curves, an absolute intensity in the large wavenumber range is apparently small for those in α-ZrP and γ-ZrP compared to those in metal oxides. Such a difference may result from a somewhat large Debye−Waller factor and/or a small interference of rare earth elements. Figure 6 shows the

Figure 4. K-edge XANES curves of the rare earth element adsorbed α-ZrP, adsorbed γ-ZrP, rare earth chloride, and oxide.

Figure 6. RSF curves of the rare earth elements in the α-ZrP, γ-ZrP, rare earth chlorides, and rare earth oxides.

adsorbed α-ZrP, rare earth chloride, and oxide. For each element, all XANES show similar curves in spite of different crystals. Such a tendency implies that rare earth elements in the γ-ZrP may be similar to the trivalent state. In addition, the higher atomic number shows the broader white lines of the XANES because of an increase in energy during the absorption edge. Figure S2 shows LIII-edge XANES of the rare earth elements adsorbed γ-ZrP, rare earth chloride, and oxide. Only for La-LIII, XANES curve of La metal referred from the literature18 is contained. From LIII XANES, adsorbed rare earth cation, metal chloride, and oxide show similar curves. However, La metal seems to be broader and slightly smaller energy of rising edge. These XANES curves also confirm the rare earth elements adsorbed in γ-ZrP are the trivalent state. Figure 5 shows kn χ(k) of the lanthanoid cation in γ-ZrP with n = 2. Other kn χ(k) curves of metal oxide, chloride, and metal cations in α-ZrP and γ-ZrP are shown in Figure S3. From these curves, the intensity in the high wavenumber region shows a low signal-to-noise (SN) ratio for those in α-

RSF curves of the rare earth elements in the α-ZrP, γ-ZrP, rare earth chloride, and rare earth oxides. Form these functions, all rare earth metal elements show peaks at approximately or less than 2 Å which are derived from coordinated oxygen atoms as nearest neighbors. These values are apparently smaller than the actual values because of the phase shift. In the case of the rare earth chlorides, the first peak is of a slightly longer length than that of the others. In these cases, the M-Cl distance should be somewhat longer than the M−O distance because the chlorine anion is monovalent. The peak from first neighbors in α-ZrP arises on the longer side than in γ-ZrP. Such shorter distances may indicate stronger interactions between rare earth elements and oxygen, which results in the larger amount of adsorption. For rare earth oxides, there are two large peaks that show the nearest neighbor of M−O and a second neighbor M−M. However, other compounds did not tend to provide second peaks composed of an M−M distance. Such a lack of peak was D

DOI: 10.1021/acs.inorgchem.8b01003 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry a result of the previously mentioned reason. In other words, crystal structure, chemical state, and concentration of rare earth cations might have a large effect as follows. For the crystal structure of a rare earth oxide, the coordination numbers (CNs) of the rare earth elements with oxygen are approximately 6−7. A larger CN can be seen for smaller atomic numbers in lanthanoid because of its ionic size. For the ion exchanged sample, the coordination number of rare earth cations with oxygen may be similar because of the similar tendency of the peak area of the nearest neighbor in the RSF. Thus, a short-range order will be formed in the ion exchanged samples. However, a secondarily closed cation from an M−M distance might be a somewhat looser structure than that of the rare earth metal oxide. For the concentration of rare earth elements, the concentration is quite smaller in the ionexchanged sample than that in the rare earth metal oxide as shown in Figure 2. A small amount of rare earth metal cations will provide data with a poor S/N ratio. In addition, another reason for the lack of atomic weight is the second neighbor atom. Lanthanoid has a relatively large atomic weight of more than 138.9. In the case of zirconium phosphate, the oxygen coordinated by the rare earth elements may bond with phosphorus. Phosphorus has smaller atomic weight than that of a rare earth metal. Such a difference must result in a small secondary peak. These curves are very similar at least in the nearest neighbor oxygen region. Such a similar tendency shows that the coordination circumstance with the nearest neighbor ions is very similar in these samples such as metal oxides and ion exchanged samples. Particularly for La and Eu, the peak in coordination with the nearest neighbors seems to be composed of two peaks in the metal oxides and ion exchanged samples. Such bimodal peaks may be a result of the similar structure of the metal oxides. Table S1 shows the bond distances for each rare earth oxide La, Eu, Dy, and Yb. From these distances, larger cations show a larger coordination number and larger amounts of variation in the M−O distance. Thus, the large amount of the variations results in bimodal peaks of M−O. Estimation of Coordination Number and Bond Separation. The rare earth elements in the γ-ZrP are very complex because there are many sites in the superlattice as previously mentioned. Such a complex structure tends to be difficult for estimating structural analysis in the crystal unit cell. Therefore, the quick first shell (QFS) theory was used, in which only the nearest neighbor shell was used to fit the RSF. Figure 7 shows the fitting curves of the RSF by QFS theory. Usually, absorption can be expressed by the following formula:19

χ (k ) = S0 2 ∑

Nj Fj(k) exp( −2k 2σj 2 + 2rj/λ)

j

(rj − Δj ) + ϕj(k)}

krj 2

sin{2k (2)

where S02, Nj, σj, and rj are the amplification factor, coordination number, Debye−Waller factor, and coordination distance, respectively, and which factor can be estimated by fitting. Fj(k) and ϕj(k) are the backscattering factor and phase factor, which are calculated theoretically. In this formula, the amplitude of the EXAFS vibration was determined by both S02 and Σ. Generally, the amplification factor and coordination number cannot be simultaneously estimated. In this study, the amplification factor was fixed at 1.0, and the other factors were estimated using the QFS theory. Table 1 shows the estimated factors via QFS theory. From the fitted curves, most of the peaks seem to be successfully fitted. However, for La and Eu, there are shoulders on the left-hand side, which are difficult to fit accurately using the QFS theory. The R factors are approximately several percent. The factor of Dy is slightly larger than the others because the fitted degree at the region of approximately 3 Å is somewhat worse. From the fitted results, the Debye−Waller factor decreased with a decrease in ionic radius. The smaller Debye−Waller factor means a more stable state within the interlayer space. Previously, the tetrad effect was shown for adsorption amounts in Figure 2. From the figure, because the rare earth metal should have a cationic state within the interlayer space, the Debye−Waller factor may show the stability of rare earth cations. Because smaller cations can usually generate a strong electric field, the electrostatic attractive force will be larger than that of the larger cations. Thus, a smaller cation adsorbed near an inorganic layer tends to have a small Debye−Waller factor. The coordination number also is estimated by these fittings. The number was maximally approximately 15 for La, which seems to be somewhat large.20,21 These coordination numbers were affected by the adsorption position within the interlayer space. Thus, the cation with a larger ionic radius occurs near the center of the interlayer space, as shown in Figure 3. From these results, it can be seen that the cation at the center position might tend to be hydrated with intercalated H2O. For clarification of such hydration, the contents of water were measured via TG. Figure S4 shows the TG curves of the ion exchanged samples. From these curves, a mass loss can be observed as an apparent three steps at lower than 100 and approximately 350 and 800 °C for pure γ-ZrP. These steps are attributed to the H2O vaporization and the first and second stages of decomposition for H2PO4 to form H2P2O7 and PO3, respectively. However, rare earth cations included sample mass loss was completed at approximately 500 °C and became broader. For these samples, such an unclear mass loss must include both water vaporization and decomposition of H2PO4. That is, the general formula of the sample including the rare earth cations can be expressed as Zr(PO4)Lnx(H2−3xPO4)· nH2O. For these samples, the chemical formula should change from Zr(PO4)Lnx(H2−3xPO4) to Zr(PO4)Lnx(PO3+3x/2) in a range from after the first step and at the highest temperature. From these changes, the amounts of H2O, n, can be calculated as 2.2, 2.4, 3.0, and 1.9 for La-, Eu-, Dy-, and Yb-included γZrP, respectively. The x values expressed in Figure 2 are 0.083, 0.069, 0.098, and 0.100 for each sample, respectively. From each coordination number calculated using QFS theory, there

Figure 7. Fitted curves of the RSF curves of the La, Eu, Dy, and Yb in the γ-ZrP by QFS theory. E

DOI: 10.1021/acs.inorgchem.8b01003 Inorg. Chem. XXXX, XXX, XXX−XXX

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Table 1. Factors Fitted Using the QFS Theory of the Intercalated Cations, La, Eu, Dy, or Yb, in the γ-ZrP Interlayer Space element ionic radius (pm) coordination number Debye−Waller factor M−O distance (Å) R factor (%)

La 117 15.1 2.19 × 10−2 2.52 2.1

Eu 109 12.3 1.87 × 10−2 2.42 2.0

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Dy 105 9.0 1.37 × 10−2 2.38 5.0

Yb 101 7.1 0.89 × 10−2 2.28 2.9

CONCLUSION The layered metal phosphates, α-and γ-zirconium phosphate, were examined for their rare earth cation exchangeability of La, Eu, Dy, and Yb using an aqueous solution including coexisting rare earth elements or one type of rare earth element. In the case of a solution with coexisting rare earth elements, γ-ZrP apparently shows better adsorption competency than that of the other materials, from the adsorption amounts of rare earth elements for γ-ZrP. The rare earth elements occur within an interlayer space and the position seems to be dependent on the ionic radius of the rare earth elements, based on our previous results. To analyze the circumstances for the exchanged rare earth elements surrounded by oxygen in P−OH and water, they were examined using XAFS in this study. From the XANES results, the rare earth elements have a trivalent ionic state. In the case of small cations, the ions tend to displace to the inorganic layer because of their strong electric field. That is, a smaller cation can provide a stronger electric field which might provide a greater binding energy to the nanosheet of the zirconium phosphate. Such ion exchange behavior in γ-ZrP, in which smaller rare earth cations tend to exist at a nearer position to the zirconium phosphate layer and vice versa, was confirmed via RSF from EXAFS. Particularly, the Debye−Waller factor for the large lanthanoid, e.g., La, was larger than that of the others. The coordination number of the oxygen was also apparently larger. These phenomena show larger rare earth cations, such as La, have many water subordinates within the interlayer space. Such a difference in the position within the interlayer space may be expected to provide selective ion exchangeability, a formation of new structures with plural cations, and so on, in the future. For a zeolitic structure, a larger cation might result in many water molecules in coordination and positioned at a greater separation from the zirconium octahedral to form a zeolitic framework. For the new structure, plural cations will adsorb at an optimal position within the interlayer space. The structure, including the plural cations with independent occupancy, is very interesting in regards to new properties of multioccurring cations.

are no inconsistencies if most of the H2O is attached to rare earth cations. Actually, the ratio of n and x are approximately 20−35. Thus, approximately one-half of the H2O included in the interlayer space may be coordinated with rare earth cations from these coordination numbers. Given these considerations, rare earth metals within the interlayer space have an ionic state and subordinate a great amount of H2O particularly for rare earth cations with a larger ionic radius. Considering the position of rare earth elements in the unit cell again as shown in Figure 3, the position of the rare earth elements can be estimated by steric hindrance from PO43− tetrahedra. Figure 8 shows a schematic illustration of

Figure 8. Schematic illustrations of the model for mean presumed positions of a lanthanoid cation and its coordination circumstances in the interlayer gallery of γ-ZrP.

the model for the mean presumed position of the lanthanoid cation and its coordination circumstances in the interlayer gallery of γ-ZrP. The left side shows the intercalated γ-ZrP with small rare earth cation, especially as Yb, Er, and so on. The right side shows the intercalated γ-ZrP with other larger cations. A larger cation tends to displace toward the large space and to subordinate many H2O molecules as a ligand. Such large amounts of H2O coordination require to compare the tetrad effect in Figure 2. Habenschuss22 reported that mean coordination numbers for lanthanoid cations with H2O in aqueous solutions are approximately 8 or 9, which are somewhat larger for Yb and apparently smaller for La, Eu, and Dy than those in γ-ZrP estimated from the EXAFS analysis in this paper. Thus, such large coordination numbers from XAFS measurements will have a strong relation to the large ionicity of the rare earth cations within the interlayer space of γ-ZrP estimated by the tetrad effect.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b01003.

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Synchrotron XRD patterns, LIII-edge XANES spectra, XAFS knχ(k) curves, TG curves (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. F

DOI: 10.1021/acs.inorgchem.8b01003 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry ORCID

(17) Salvado, M. A.; Pertierra, P.; Garcia Granda, S.; Barcina, L. M.; Llavona, R.; Rodriguez, J. Hydrogen bond network of the layered phosphates γ-Zr(H2PO4)(PO4)·2H2O and γ-Hf(H2PO4)(PO4)·2H2O determined by neutron powder diffraction. Z. Kristallogr. - Cryst. Mater. 2001, 216, 326−330. (18) Gong, Y.; Wu, J.; Kitano, M.; Wang, J.; Ye, T.-N.; Li, J.; Kobayashi, Y.; Kishida, K.; Abe, H.; Niwa, Y.; Yang, H.; Tada, T.; Hosono, H. Ternary intermetallic LaCoSi as a catalyst for N2 activation. Nat. Catal. 2018, 1, 178−185. (19) Crozier, E. D. A review of the current status of XAFS spectroscopy. Nucl. Instrum. Methods Phys. Res., Sect. B 1997, 133, 134−144. (20) Giester, G.; Ž ák, Z.; Unfried, P. Syntheses and crystal structures of rare earth basic nitrates hydrates: Part III. [Ln6(μ6−O)(μ3− OH)8(H2O)12(η2−NO3)6](NO3)2·xH2O, Ln = Y, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu; x = 3, 4, 5, 6. J. Alloys Compd. 2009, 481, 116−128. (21) Luo, Q.-H.; Howell, R. C.; Dankova, M.; Bartis, J.; Williams, C. W.; Horrocks, W. D., Jr.; Young, V. G., Jr.; Rheingold, A. L.; Francesconi, L. C.; Antonio, M. R. Coordination of rare-earth elements in complexes with monovacant Wells−Dawson polyoxoanions. Inorg. Chem. 2001, 40, 1894−1901. (22) Habenschuss, A.; Spedding, F. H. The coordination (hydration) of rare earth ions in aqueous chloride solutions from xray diffraction. III. SmCl3, EuCl3, and series behavior. J. Chem. Phys. 1980, 73, 442−450.

Takahiro Takei: 0000-0002-5624-2899 Sayaka Yanagida: 0000-0002-4719-5023 Nobuhiro Kumada: 0000-0002-0402-5809 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The experiments at SPring-8 were performed with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (Proposals 2013A1299, 2017A1772, and 2017B1898). Some of the chemicals were provided by Daiichi Kigenso Kagaku Kogyo Co., Ltd., and Toagosei Co., Ltd.

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REFERENCES

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DOI: 10.1021/acs.inorgchem.8b01003 Inorg. Chem. XXXX, XXX, XXX−XXX