Lithium Diffusion Pathway in Li1.3Al0.3Ti1.7(PO4)3 (LATP) Superionic

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Lithium Diffusion Pathway in Li1.3Al0.3Ti1.7(PO4)3 (LATP) Superionic Conductor Mykhailo Monchak,†,‡ Thomas Hupfer,† Anatoliy Senyshyn,*,‡ Hans Boysen,§ Dmitry Chernyshov,∥ Thomas Hansen,⊥ Karl G. Schell,† Ethel C. Bucharsky,† Michael J. Hoffmann,† and Helmut Ehrenberg† †

Karlsruher Institut für Technologie (KIT), Institut für Angewandte Materialien (IAM), 76131 Karlsruhe, Germany Heinz Maier-Leibnitz Zentrum (MLZ), Technische Universität München, Lichtenbergstrasse 1, D-85748 Garching, Germany § Department für Geo- und Umweltwissenschaften, Sektion Kristallographie, LMU München, Am Coulombwall 6, D-85748 Garching, Germany ∥ Swiss-Norwegian Beamlines, ESRF-The European Synchrotron, 71 Avenue des Martyrs, 38042 Grenoble Cedex 9, France ⊥ Institut Max von Laue-Paul Langevin (ILL), 71 Avenue des Martyrs, 38043 Grenoble Cedex 9, France ‡

S Supporting Information *

ABSTRACT: The Al-substituted LiTi 2 (PO 4 ) 3 powders Li1+xAlxTi2−x(PO4)3 (LATP) were successfully prepared by a water-based sol−gel process with subsequent calcination and sintering. The crystal structure of obtained samples was characterized at different temperatures using high-resolution synchrotron-based X-ray and neutron powder diffraction. Possible lithium diffusion pathways were initially evaluated using the difference bond-valence approach. Experimental 3D lithium diffusion pathway in LATP was extracted from the negative nuclear density maps reconstructed by the maximum entropy method. Evaluation of the energy landscape determining the lithium diffusion process in NASICON-type superionic conductor is shown for the first time.

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to air and moisture (the authors2 reported a chemical instability of LTP- and LATP-based materials toward Li metal, with the reduction of Ti4+ to Ti3+ which, correspondingly, will require a protection layer in all-solid-state battery applications). The presence of 3D-connected cavities in a NASICON type of structure, e.g., LiTi2(PO4)3 (LTP), make it well suited for lithium conduction, which can be considerably enhanced by partial substitution of tetravalent cations (Ti4+) by trivalent ones (Al3+, Fe3+, Y3+, etc.).3−5 Among variously doped LTP, the highest bulk conductivity of 10−3 S/cm at 27 °C6 was achieved in the Al-substituted LTP compound. There maximal conductivity in Li1+xAlxTi2−x(PO4)3 was observed at x = 0.3, known as LATP = Li1.3Al0.3Ti1.7(PO4)3. Depending on the synthesis and sintering conditions, the total ionic conductivity of LATP was reported to vary between 10−8 and 10−3 S/ cm.7−10 The low electronic contribution (about 10−11 S/cm11) to the total conductivity makes LATP a promising choice for use as a solid electrolyte in all-solid-state batteries.12,13 Understanding of processes supplemented the 3D lithium diffusion in the LATP class of materials has an obvious relevance for further materials development, and it was actively explored by theoretical methods: Nuspl et al.14 determined the

INTRODUCTION Rapid growth of the portable electronics market is due to portable energy storage solutions with improved characteristics, e.g., energy and power density, temperature, cycling stability, safety, cost, etc. Among various factors defining the performance of a typical Li-ion battery, the issue of electrolytea lithium salt (typically LiPF6) in organic solvents (ethylene carbonate, dimethyl carbonate, diethyl carbonate, or their mixture)is often considered to be a bottleneck in the stateof-the-art Li-ion batteries. Key properties like battery safety, high-voltage operation, use of new electrode materials with improved gravimetric and volumetric energy densities, temperature range of stable operation, and/or cycling stability are severely hampered by the liquid electrolyte. This inspired an active search for new liquid electrolytes with improved characteristics, which is now a subject of intense research. Alternatively, the concept of an all-solid-state batteryan electrochemical cell based on solid-state components only, including the electrolyteis considered. Despite the obvious advantages offered by an all-solid-state approach to Li-ion technology the use of solid-state batteries is defined by the limited selection of suitable solid lithium-based electrolytes.1 Among various types of solid-state lithium-ion conductors the ceramics with NASICON-type of structure has attracted the interest of researchers due to its chemical stability © XXXX American Chemical Society

Received: December 10, 2015

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DOI: 10.1021/acs.inorgchem.5b02821 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry diffusion pathway of Li+ ions in LTP and LATP frameworks by molecular dynamics simulations.15 Lang et al.15 investigated the migration barriers for an interstitial Li ion and a Li vacancy in the rhombohedral structure of LTP using density functional theory. One has to admit the increased role of theoretical methods for determination and prediction of diffusion pathways in ionic conductors. Recently, Wang et al.16 attempted to develop a fundamental design principle in the form of a new structural descriptor capable of predicting the performance of sulfide-based solid-state Li-ion conductors. The crystal structure is the property defining ionic conductivity, and, therefore, the structural properties of LATP and related materials were actively studied using powder diffraction. In the LTP-type structure the Li+ ions tend to preferentially occupy the M1 site (0, 0, 0) in space group R3̅c.17 The doping of LTP by Al causes Li ions to occupy additional interstitial positions within the framework, and different positions for the lithium atom were proposed in the literature, e.g., 18e (x, 0, 0.25) (M2 site), 36f (0.47, 0.31, 0.25) (M2′ site),8 (0.07, 0.34, 0.07) (M3 site),18 etc. In “completely” Liintercalated Li3Ti2(PO4)3 with only Ti3+ and with the rhombohedral space group R3̅, all Li ions were reported to occupy M3/M3′ sites inside M2 cavities(0.030 0.319 0.045) and (0.055 0.373 0.117), respectively.19 The localization of the lithium interstitial position is highly relevant for understanding lithium diffusion in LATP-based materials. However, significant discrepancies in the literature data seriously bound the understanding of diffusion processes in these materials. The observed inconsistencies can be related to various factors, e.g., data quality, sample preparation, morphology, chemical composition, etc. Therefore, in the current manuscript a systematic study of the crystal structure of LATP-based materials was performed on the number of samples with nominally similar composition. Analysis of structural data aiming to determine nuclear density maps and to define the lithium diffusion pathways is reported.

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thin-wall vanadium containers (0.15 mm wall thickness). Altogether, 12 samples were investigated at ambient temperature, and on the basis of the performed study one sample has been chosen for further structural studies at high temperatures. The high-resolution neutron powder diffraction data were collected at 27, 100, 200, 300, 400, 500, 700, and 800 °C in vacuum. Depending on the temperature, different exposure times were set, i.e., the data sets at RT and 400 °C were collected for 2 h and at 700 and 800 °C for 3 h, while short data sets with 30 min exposure were obtained at 100, 200, 300, and 500 °C. Collected 2D diffraction patterns were then corrected for geometrical aberrations and curvature of the Debye−Scherrer rings.22 One more sample (sintered pellet) has been chosen among residual 11 samples for further low-temperature neutron diffraction measurement at −269 °C (4 K) using a closed-cycle refrigerator (SPODI). A small portion of the powder sample has been taken for structural characterization using powder diffraction and synchrotron radiation. Structural studies were performed at the Swiss−Norwegian beamline BM1A (ESRF, Grenoble, France) using a photon energy of 17.792 keV, λ = 0.69687 Å. The sample was filled into a 0.3 mm quartz capillary and mounted on the capillary spinner. The capillary was heated by a nitrogen hot blower, and data collection was performed upon continuous sample rotation and continuous heating with the 10 °C/min ramp in the temperature range 23−600 °C. The 2D diffraction data from a Pylatus 2 M detector were reduced using the SNBL Toolbox.23

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RESULTS AND DISCUSSION The structure models were refined, based on the observed patterns. The refinement was performed using the FullProf software package.24 At the initial state of the refinement, data treatment was carried out for the basic structural framework consisting of Ti/Al, P, and O atoms. The peak profile shape was described using a pseudo-Voigt function, which is a linear combination of a Gaussian and Lorentzian function. The background of the diffraction patterns was fitted using a linear interpolation between selected data points in nonoverlapping regions. The scale factor, lattice parameters, and fractional coordinates of atoms and their anisotropic displacement parameters, zero angular shift, profile shape parameters, and full width at half-maximum (Caglioti) parameters were allowed to vary. Rietveld refinement of structure models, based on the observed X-ray and neutron powder diffraction patterns, revealed the rhombohedral NaZr2(PO4)3 structure type (R3̅c space group) for LATP at all studied temperatures (see Figure S1). Besides the main phase, different AlPO4 phases are often reported to coexist with LATP-type materials as impurity phases.25−28 AlPO4 (cristobalite) was detected and quantified (0.8−1.7% w/w) based on the neutron diffraction data. LATP is known to possess a highly anisotropic thermal behavior of lattice parameters, whereupon heating the lattice strongly expands in the c direction and remains almost unchanged in the ab plane. Analysis of structural data obtained using neutron powder diffraction and synchrotron-based highresolution powder diffraction confirmed this (Figure S2), i.e., at T < 300 °C the obtained temperature evolution of lattice parameters has been found in good agreement with each other and literature data.8,29 At higher temperatures the thermal behavior of lattice parameter c determined using synchrotron radiation tends to diverge from “neutron” values and thus is systematically higher. The observed discrepancy can be attributed to the different sample atmospheres during data collection (vacuum in the case of neutron scattering and air in the experiment using synchrotron radiation). Optical microscopy revealed an appearance of dark inclusions in samples

EXPERIMENTAL SECTION

For sample preparation the lithium acetate (Li(C2H3O)·2H2O, Alfa Aesar) and aluminum nitrate (Al(NO3)3·9H2O, Merck) were completely dissolved in water. Titanium−isopropoxide (Ti[OCH(CH 3 ) 2 ] 4 Alfa Aesar) was added slowly, resulting in TiO 2 precipitations through contact with water. Ammonium phosphate (NH4H2(PO4)3, Merck) was separately dissolved in water and added as an anion source, resulting in a homogeneous white gel. All precursors were mixed in stoichiometric quantities under continuous stirring. After mixing the gel was left for 48 h at room temperature prior to application of thermal treatment at 400 and 900 °C for 8 h at each step (resulting in a white powder) with subsequent sintering. A more detailed experimental description can be found in ref 20. Eight samples of nominal stoichiometry Li1.3Al0.3Ti1.7(PO4)3 and four of Li1.4Al0.4Ti1.6(PO4)3 were prepared (where 50% of samples was prepared in powder form, while the residual 50% underwent a sintering cycle). The reason for this preparation was to optimize the reagents proportions leading to synthesis of a phase-pure LATP phase. An excess of Li and P precursors was used for a similar purpose and found to be affecting the content of secondary phases (Table S1). Elastic coherent neutron scattering experiments were performed at the high-resolution neutron powder diffractometers D2B and SPODI utilizing a similar experimental setup and located at the Institut Laue Langevin (ILL, Grenoble, France)21 and at the neutron research reactor FRM II (Garching, Germany),22 respectively. Monochromatic neutrons with wavelengths of 1.594 (D2B) and 1.5482 Å (SPODI) were used for data collection. Measurements were performed in Debye−Scherrer geometry. Powder samples were filled into cylindrical B

DOI: 10.1021/acs.inorgchem.5b02821 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 1. (a) Bond-valence mismatch in Li1.3Al0.3Ti1.7(PO4)3 (left) and MEM-reconstructed negative nuclear density maps (right), (b) (0 1 −4) sections of bond-valence mismatch and negative nuclear densities.

c. Analysis of Valence Mismatch (Difference BondValence Method). Alternatively to approaches a and b a relatively simple prediction of lithium positions along with the preferable lithium pathways can be performed by the difference bond-valence method (analysis of valence mismatches). The method is based on the assumption that the diffusion in solids might occur on a pathway where deviations between bondvalence and nominal valence ΔV remain as low as possible. Being computationally simple and based on the tabulated empirical bond-valence parameters the method is prone to work for various cationic or anionic conductive species, where only structural information about a steady/stable framework is required as an input.30 The 3D distribution of bond-valence mismatches in LATP (using experimentally determined lattice parameters and atomic positions for Ti, P, and O) was calculated via the method proposed in ref 31, recently implemented in the Bond_Str program implemented in the FullProf package.24 The obtained isosurface for the difference valence ΔV = 0.2 is plotted in Figure 1 and predicts 3D diffusion pathway in LATP. Analysis of a (0 1 −4) section of the three-dimensional difference bondvalence map reveals a probable lithium diffusion pathway in the form of a zigzag Li1 chain with an intermediate lithium position at the zigzag slope. The obtained lithium site (36f) with coordinates (0.03, 0.32, 0.06) correlates well with those determined above and is in fair agreement with literature data.18

heated in air, which can be associated with sample decomposition. Analysis of difference Fourier maps (Fobs − Fcalcd) obtained by modeling of neutron patterns clearly revealed the M1 = 6b (0, 0, 0) position for lithium, while localization of the second site was not as straightforward. Therefore, three different methods were applied for the initial localization of further lithium on the second lithium site. a. Analysis of Difference Fourier Maps on Various Samples with Nominal Composition. Besides the M1 position careful analysis of Fobs − Fcalcd (separation of real signal from the low-intensity peaks of spurious character due to termination effects) revealed the presence of small negative peaks around (0.11, 0.33, 0.08) in most of the samples. By analysis of the difference Fourier maps for different samples (generated without Li) the position (0.11, 0.33, 0.08) besides the Li1 (0, 0, 0) was found in most of the samples (Figure S3). The refined position ((0.05(2), 0.29(3), 0.07(1)) at RT and the obtained position (0.04, 0.32, 0.07) at 800 °C has been found to be very close to the one reported in ref 18 (M3 site). b. Mean Coordinate between Neighboring M1 Atoms. The location of the Li3 atom can be found in another way: lithium diffusion is supposed to involve Li1 ions. In the first approximation the virtual Li3 site can be defined at the middle (1/6, 1/3, 1/12) between two adjacent Li1 atoms. Further refinement of the virtual Li3 site leads to a convergence with Li3 coordinates at (0.06(3), 0.31(5), 0.08(2)). C

DOI: 10.1021/acs.inorgchem.5b02821 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 2. Two-dimensional (0 1 −4) section cut of the lithium one-particle-potential (OPP) and its 1D section along lines connecting four Li atoms in a chain Li1−Li3−Li3−Li1.

of electron/nuclear density maps from the “limited” powder diffraction data sets by the maximum-entropy method (MEM) is applied as an alternative. The method is based on the estimation of 3D scattering densities from a limited amount of information by maximizing information entropy under restraints, consistent with experimental observations. Termination effects often occur to be less pronounced in MEM evaluation, thus leading to more precise scattering density maps.36 LATP contains lithium and titanium, whose neutron scattering lengths are both negative. The negative nuclear density map for LATP reconstructed from experimental structure factors obtained at 800 °C is plotted in Figure 1. The threshold is set to −0.015 fm/Å3, indicating some small artifacts which are neglected in the further discussion. A fair agreement between theory and experiment can be noticed: similar to the bond-valence mismatch analysis the MEM reconstruction reveals a zigzag shape of the lithium diffusion pathway in LATP, occurring between two adjacent Li1 positions through a M3 (Li3) position. Assuming the lithium motion in the single-atom potential to be independent at high temperatures and nuclear density map to have the feature of a probability field, observed lithium motion can be analyzed in the framework of Boltzmann statistics.37 The oneparticle potential (OPP) was recalculated from negative nuclear densities, and it is shown in Figure 2. The established Li1− Li3−Li3−Li1 zigzag chains which formed the 3D diffusion pathway have obvious energy preferences in the lattice. The cut through the potential energy profile along the Li1−Li3−Li3− Li1 chain (Figure 2) indicates an activation barrier of ca. 0.33 eV. This value is in good agreement with the literature,8,20,25,26,38,39 reporting the bulk activation energy of LATP-based materials varying from 0.15 to 0.30 eV.

Three different approaches lead to the M3 (Li3) site, whose coordinates have been found to be very similar after refinement. Heavy lithium disorder in LATP (Table S2) leads to severe correlations between lithium site occupancies and displacement parameters and resulted in unrealistic lithium content. Therefore, atomic site occupancies were constrained as Li1 + Li3 = 1.0 + Al, where Li1 and Li3 are site occupations at M1 and M3 lithium sites; isotropic displacement parameters for Li1 and Li3 were chosen to be the same. Similar to Li4Ti5O12,32 the population of the interstitial M3 (Li3) site exhibits an increase upon heating. The increase of lithium content on the Li3 site occurs at the cost of Li1 site (Table S2), and the isotropic displacement parameter Biso(Li) rises from 4.9(6) Å2 at ambient temperature to 18.2(9) Å2 at 800 °C, thus indicating pronounced structural disorder. Among the series of Li−, Ti/Al−, and P−O interatomic distances in the first coordination sphere, the largest thermal elongation has been noticed for the Li1−O bond (Table S3, Figure S4); the [Ti/Al]O6 polyhedra become more regular upon heating, i.e., Ti/Al−O1 distance increases while the Ti/ Al−O2 distance is reduced; the P−O bonds tend to decrease systematically. The temperature-driven P−O bond contraction is considered as an artifact due to increased librational motions of PO4 units. The M3 site (Li3) has been found 4-fold coordinated to oxygen, while further considerations (bond length distortion, etc.) are seriously hampered by the localization precision. Analysis of the difference bond-valence distribution revealed the 3D diffusion pathway in LATP involving both Li1 and Li3 sites. However, being restricted to compounds with localized bonds, the difference bond-valence method cannot be used for prediction of activation energies in its standard form. The calculation of the 3D distribution of bond-valence mismatches in four known modifications of NASICON-type materials, e.g., LiZr2(PO4)3 (α (space group R3̅c33), α′ (space group C1̅34), β (space group Pbna), and β′ (space group P21/n35), yield 3D frameworks for Li diffusion in four different structures (Figure S5). The observed 3D frameworks indicate that geometrical prerequisites for lithium diffusion are fulfilled and the diffusion process in these materials is mediated by activation energies.35 The relation of structure and activation energies of the diffusion process is nontrivial especially based on powders. Even direct analysis of the diffusion pathways in disordered systems is in this case often seriously limited by termination effects in Fourier maps, caused by powder averaging and limited data statistics. More and more often the determination

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CONCLUSIONS The crystal structure of Li1.3Al0.3Ti1.7(PO4)3 ionic conductor was systematically studied using a combination of neutron and synchrotron-based high-resolution powder diffraction. The system is characterized by highly anisotropic thermal expansion, whose magnitude seems to be affected by the atmosphere of data collection. The structural evolution of lithium−oxygen coordination polyhedra has been found mostly affected by temperature. While Ti/Al polyhedra become more regular at elevated temperatures, PO4 tetrahedra display a weak tendency to contract upon heating due to increasing libration motions. Significant lithium disorder (both static and dynamic) is noticed for Li1.3Al0.3Ti1.7(PO4)3. Different approaches were D

DOI: 10.1021/acs.inorgchem.5b02821 Inorg. Chem. XXXX, XXX, XXX−XXX

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(12) Birke, P.; Salam, F.; Döring, S.; Weppner, W. Solid State Ionics 1999, 118, 149−157. (13) Nagata, K.; Nanno, T. J. Power Sources 2007, 174, 832−837. (14) Nuspl, G.; Takeuchi, T.; Weiβ, A.; Kageyama, H.; Yoshizawa, K.; Yamabe, T. J. Appl. Phys. 1999, 86 (10), 5484−5491. (15) Lang, B.; Ziebarth, B.; Elsässer, C. Chem. Mater. 2015, 27, 5040−5048. (16) Wang, Y.; Richards, W. D.; Ong, S. P.; Miara, L. J.; Kim, J. C.; Mo, Y.; Ceder, G. Nat. Mater. 2015, 14, 1026−1032. (17) Qui, D. T.; Hamdoune, S.; Soubeyroux, J. L.; Prince, E. J. Solid State Chem. 1988, 72, 309−315. (18) Arbi, K.; Hoelzel, M.; Kuhn, A.; García-Alvarado, F.; Sanz, J. Inorg. Chem. 2013, 52, 9290−9296. (19) Arbi, K.; Hoelzel, M.; Kuhn, A.; García-Alvarado, F.; Sanz, J. Phys. Chem. Chem. Phys. 2014, 16, 18397−18405. (20) Bucharsky, E. C.; Schell, K. G.; Hintennach, A.; Hoffmann, M. J. Solid State Ionics 2015, 274, 77−82. (21) Suard, E.; Hewat, A. Neutron News 2001, 12 (4), 30−33. (22) Hoelzel, M.; Senyshyn, A.; Juenke, N.; Boysen, H.; Schmahl, W.; Fuess, H. Nucl. Instrum. Methods Phys. Res., Sect. A 2012, 667, 32−37. (23) http://www.esrf.eu/home/UsersAndScience/Experiments/ CRG/BM01/bm01-a/image.htm/snbl-tool-box.html. (24) Rodrıguez-Carvajal, J. Commission on powder diffraction (IUCr). Newsletter 2001, 26, 12−19. (25) Huang, L.; Wen, Z.; Wu, M.; Wu, X.; Liu, Y.; Wang, X. J. Power Sources 2011, 196, 6943−6946. (26) Arbi, K.; Mandal, S.; Rojo, J. M.; Sanz, J. Chem. Mater. 2002, 14, 1091−1097. (27) Wong, S.; Newman, P. J.; Best, A. S.; Nairn, K. M.; MacFarlane, D. R.; Forsyth, M. J. Mater. Chem. 1998, 8 (10), 2199−2203. (28) Narváez-Semanate, J. L.; Rodrigues, A. C. M. Solid State Ionics 2010, 181, 1197−1204. (29) Best, A. S.; Forsyth, M.; MacFarlane, D. R. Solid State Ionics 2000, 136−137, 339−344. (30) Dolotko, O.; Senyshyn, A.; Mühlbauer, M. J.; Boysen, H.; Monchak, M.; Ehrenberg, H. Solid State Sci. 2014, 36, 101−106. (31) Adams, S. Acta Crystallogr., Sect. B: Struct. Sci. 2001, 57, 278− 287. (32) Laumann, A.; Boysen, H.; Bremholm, M.; Fehr, K. T.; Hoelzel, M.; Holzapfel, M. Chem. Mater. 2011, 23 (11), 2753−2759. (33) Catti, M.; Comotti, A.; Di Blas, S. Chem. Mater. 2003, 15, 1628−1632. (34) Catti, M.; Stramare, S.; Ibberson, R. Solid State Ionics 1999, 123, 173−180. (35) Catti, M.; Morgante, N.; Ibberson, R. M. J. Solid State Chem. 2000, 152, 340−347. (36) Senyshyn, A.; Boysen, H.; Niewa, R.; Banys, J.; Kinka, M.; Burak, Ya.; Adamiv, V.; Izumi, F.; Chumak, I.; Fuess, H. J. Phys. D: Appl. Phys. 2012, 45, 175305. (37) Boysen, H. Z. Kristallogr. - Cryst. Mater. 2003, 218, 123−131. (38) Adachi, G.-Y.; Imanaka, N.; Aono, H. Adv. Mater. 1996, 8 (2), 127−135. (39) Kosova, N.; Devyatkina, E.; Osintsev, D. J. Mater. Sci. 2004, 39, 5031−5036.

consistent in the localization of an interstitial lithium site, whose occupation has been found to be thermally populated. A comparison of the lithium diffusion pathways obtained by the means of theory (difference bond valence) and negative nuclear density maps after MEM reconstruction revealed a similar character, thus indicating that Li migration in LATP likely occurs between Li1 (6b) and Li3 (36f) sites. The obtained activation energy (determined in the framework of Boltzmann statistics) has been found to be in good agreement with the literature.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.5b02821. Examples of powder diffraction patterns, short characteristics of LATP samples studied at room temperature, difference Fourier maps for Li1.3Al0.3Ti1.7(PO4)3 at room temperature, tables and pictures revealing the structure features (lattice parameters, Biso(Li), Li sites occupancies, interatomic distances, etc.), bond-valence isosurface in four LiZr2(PO4)3 modifications (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Deutsche Forschungsgemeinschaft (Projects EH 183/15-1, SE 2376/1-1, and HO 1165/18-1) and a Hans L. Merkle scholarship. The authors thank Dr. M. J. Mühlbauer, Dr. O. Dolotko, and Dr. V. Dyadkin for their assistance with synchrotron measurements.

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REFERENCES

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DOI: 10.1021/acs.inorgchem.5b02821 Inorg. Chem. XXXX, XXX, XXX−XXX