Interstitial Oxide Ion Migration Mechanism in Aluminate Melilite La1+

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Interstitial Oxide Ion Migration Mechanism in Aluminate Melilite La1+xCa1-xAl3O7+0.5x Ceramics Synthesized by Glass Crystallization jungu xu, Jiehua Wang, Aydar Rakhmatullin, Sandra Ory, Alberto J. Fernández-Carrión, Huaibo Yi, Xiaojun Kuang, and Mathieu Allix ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.9b00224 • Publication Date (Web): 03 Apr 2019 Downloaded from http://pubs.acs.org on April 4, 2019

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ACS Applied Energy Materials

Interstitial Oxide Ion Migration Mechanism in Aluminate Melilite La1+xCa1-xAl3O7+0.5x Ceramics Synthesized by Glass Crystallization Jungu Xu1, Jiehua Wang1, Aydar Rakhmatullin2, Sandra Ory2, Alberto J. Fernández-Carrión 1, 2, Huaibo Yi1, Xiaojun Kuang1,*, Mathieu Allix2,*

1MOE

Key Laboratory of New Processing Technology for Nonferrous Metals and Materials,

Guangxi Universities Key Laboratory of Non-ferrous Metal Oxide Electronic Functional Materials and Devices, College of Materials Science and Engineering, Guilin University of Technology, Guilin 541004, P. R. China 2

CNRS, CEMHTI UPR3079, Univ. Orléans, F-45071 Orléans, France

ACS Paragon Plus Environment

ACS Applied Energy Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Abstract Gallate melilite materials have attracted considerable interest as new interstitial oxide ion conducting electrolytes for solid oxide fuel cells for more than a decade. However, the preparation of aluminate melilite materials as interstitial oxide ion conductors remains a challenge. Here, we show that interstitial oxide ion conducting aluminate melilite materials La1+xCa1-xAl3O7+0.5x (x = 00.5) can be prepared via a full crystallization from a bulk glass process. Rietveld refinements performed from combined neutron and synchrotron X-ray powder diffraction (NPD and SPD) data reveal multiple interstitial defect positions within the pentagonal ring, demonstrating the diversity of local structures around the oxygen interstitial defects in La1+xCa1-xAl3O7+0.5x. Variable temperature solid-state

27Al

nuclear magnetic resonance (NMR) spectroscopy measurements demonstrate the

existence of 5-coordinated AlO5 polyhedra and dynamic exchange processes between these 5coordinated Al sites, representing the first example of evidence for the migration mechanism of interstitial oxide ions in melilites by NMR’mechanism. This latter involves framework and interstitial oxide ions, and is assisted by rotation and deformation of tetrahedra. These calculations reveal reduced mobility of interstitial oxide ions in aluminate tetrahedral network owing to its rigidity.

Keywords: Aluminate melilite, Interstitial oxide ion conductor, Defect structure, HT

27Al

MAS

NMR, Full crystallization from glass process, Aerodynamic levitation coupled to laser heating synthesis.


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1. Introduction Oxide ion conductors attract considerable interest due to their widespread use in developing technologies including solid oxide fuel cells (SOFCs), oxygen sensors and pumps, oxygen separation membrane, syn-gas production from oxidation of natural gas, and so on.1, 2 The trend of lowering the operating temperature down to ~ 500 C for these devices stimulates the discovery of new oxide ion conductors and investigations regarding the precise conduction mechanism at the atomic level. Mobile oxide species are usually vacancies on the anion sublattice, e.g. in the traditional fluorites and perovskites materials showing high symmetries.3-5 However, systems with excess oxide ions as charge carriers and based on various structural types including layered perovskite,6 -SnWO4,7 apatite,4 scheelite,8 mayenite,9 and melilite10 materials have recently been shown to achieve high oxide ion conductivity properties, competing with vacancy conducting electrolytes. Among these interstitial-oxide-ion conducting materials, melilite materials demonstrate the highest properties among linked-tetrahedral networks. Nevertheless, interstitial oxide ion conducting melilites reported so far are exclusively gallates, which suffer the phase instability at temperatures higher than 800 °C due to gallium volatilization. Whether aluminate melilites can exhibit interstitial oxide mobility remains an open question. Gallate melilites can be described by the general chemical formula A3+B2+Ga3O7, where A and B are usually lanthanide and alkaline earth metal elements, respectively. They adopt a layered (3,4)linked (the numbers three and four denote two different kinds of tetrahedra with three and four bridging oxygen atoms in the structure, respectively) Ga3O7 tetrahedral network with pentagonal tunnels that accommodate the large A/B cations.10 The oxide ionic conductivity in La1+xSr1xGa3O7+0.5x

melilites was first discovered by Rozumek et al.11 while their mobile species were later

proved to be interstitial oxide ions by Kuang et al.10 The interstitial oxide anions enter in the coordination environment of a 3-linked GaO4 tetrahedron among the five tetrahedra defining the 5fold ring and transform this tetrahedron into a bipyramidal GaO5 polyhedron (Figure S1, Supporting



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Information). By rotating, deforming and releasing the excess-bonded oxide ion, this GaO5 polyhedron would transport an interstitial oxide ion from one pentagonal ring to a neighboring ring.12-18 Through static-lattice atomistic simulations based on the interatomic potential method, Tealdi et al. proposed a more energy-favorable off-center site for oxygen interstitials in the pentagonal ring. They further detailed the interstitial oxide ion migration mechanism in nonstoichiometric melilite structures through molecular dynamics (MD) simulations,12 which demonstrated that the concerted knock-on motion between the interstitial and framework oxide ions promotes interstitial oxide ion migration. The intrinsic flexibility of the melilite structure, owing to the existence of terminal oxygens which enable rotation and deformation of tetrahedra and changes in Ga coordination, plays a key role in the interstitial oxide ion conduction mechanism. The accommodation of interstitial oxide ions conducting in gallate melilites is highly dependent on the cationic size in the pentagonal tunnel. As for example, in Ln1+xSr1-xGa3O7+0.5x (Ln = La, Pr, Nd, Sm, Eu, Gd, Dy, Yb, Y) melilites14, it is easier to introduce interstitial oxygen via solid state reaction when Ln = La than in the cases of smaller Ln cations; while in substituted-melilites La1+xM1-xGa3O7+0.5x, small Ca2+ and Sr2+ cations render more favorable the incorporation and migration of interstitial oxygen defects compared with Ba2+.19 In our previous study, we showed that Ln1+xSr1-xGa3O7+0.5x melilite materials could be prepared through a full glass crystallization method using an aerodynamic levitator coupled to a laser heating system.20,

21

This innovative process

enabled to retain glass transparency upon crystallization and the obtained ceramics displayed high bulk oxide ion conductivity competing with La-compositions.21 This crystallization from glass approach can be considered as a “soft chemistry” synthetic route (the crystallization temperature takes place at relatively low temperature, below 800 °C), such offering great opportunities to elaborate new metastable crystalline phases which are hardly accessible via the traditional solid state route.21-23


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The aluminate-melilite structure was reported to be synthesizable for LnMAl3O7 (Ln=La, Pr, Nd, Sm, Eu, Gd, Dy, Ho, Er, Y; M = Ca, Sr, Ba) stoichiometric compositions.24 These materials would offer a cost-effective alternative compared with gallate counterparts, as long as an easier synthesis process, given the absence of volatilization issues. However, non-stoichiometric aluminate melilites could not be accessed via solid state reaction. Here, for the first time we report non-stoichiometric melilite aluminates with high excess of oxide concentration and associated ionic conductivity properties. These materials are elaborated via full glass crystallization of La1+xCa1-xAl3O7+0.5x (0 ≤ x ≤ 0.5) compositions. A multiscale structural characterization of the defect structure was performed using a combination of NPD/SPD powder diffraction for average structural model determination and high temperature (HT) NMR for local structure characterization, while the electrical properties and oxide ion conduction mechanism were studied by alternative current (AC) impedance spectroscopy and molecular dynamic (MD) simulations, respectively. 2. Methods Synthesis. CaCO3 (Alfa Aesar, > 99.8 % purity), La2O3 (Alfa Aesar, > 99.997 % purity, dried at 1100 °C for 6 h before used) and Al2O3 (Alfa Aesar, > 99.997 % purity) starting materials were weighed according to the aimed La1+xCa1-xAl3O7+0.5x (0 ≤ x ≤ 0.6) compositions. They were subsequently mixed thoroughly and pressed into pellets. which were placed in a conical nozzle of an aerodynamic levitator (ADL) system (air atmosphere) equipped with two CO2 lasers for heating.23 About 0.1 g pellets were then heated up to ~ 2200 °C, left to homogenize for a few seconds, and subsequently rapidly quenched by turning off laser heating. The resulting glass beads were then crystallized into polycrystalline ceramics by a single heat treatment at 850 °C for 3 h in a furnace running in air atmosphere. Characterization. X-ray powder diffraction (XRD) analyses were performed using a Bragg Brentano D8 Advance Bruker laboratory diffractometer (Cu Kα radiation) equipped with a lynxEye XE detector. Data were collected from 10 to 60° (2θ) at room temperature with a 0.011° step size



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and an acquisition time of 0.5 s per step. In situ variable temperature X-ray diffraction (VTXRD) data were collected every 25 °C from room temperature up to 1000 °C using an HTK1200N Anton Paar oven chamber. Data collection was performed from 10° to 60° (2θ) with a 0.0161° step size and an acquisition time of 1 s per step. Constant-wavelength (λ = 1.225 Å) neutron powder diffraction (NPD) data were collected on the 3T2 diffractometer (Laboratoire Leon Brillouin, France) from 5° to 120° (2θ) with a step size of 0.05°. Synchrotron X-ray powder diffraction (SPD) data were collected at ambient temperature on the 11BM diffractometer (λ = 0.412 642 Å) at the Advanced Photon Source (APS), Argonne National Laboratory. Rietveld refinements were performed using Topas Academic software.25 Selected area electron diffraction (SAED) experiments were performed on a Philips CM20 transmission electron microscope (TEM) fitted with an Oxford EDS analyzer. The sample was first crushed in ethanol, and a drop of the solution with the small crystallites in suspension was deposited onto a carbon-coated copper grid. Differential Scanning Calorimetry (DSC) was performed on a Setaram MULTI HTC 1600 instrument. The glass transition and crystallization temperatures of the various melilite glass compositions were determined from 150 mg powder samples, using argon as a purging gas and a platinum crucible, with a heating rate of 10 °C•min-1. Alternating current (AC) impedance spectroscopy (IS) measurements were carried out by using a Solartron1260A impedance/gain-phase analyzer over the 10−1−107 Hz range. Prior to the IS measurement, the pellet was coated with gold paste, which was then fired at 500 °C for 30 min to burn out the organic components to form electrodes. Solid-state NMR experiments at room temperature were performed using a Bruker Avance III HD spectrometer operating at a magnetic field strength of 17.6 T, using a 2.5 mm resonance probe at a MAS frequency of 30 kHz. The 27Al chemical shifts are referenced to 1 M Al(NO3)3. The

27Al

1D spectra have been acquired using a one-pulse sequence. The pulse angle was


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sufficiently small (0.3 μs) and the recycling delay (0.5 s) sufficiently long to ensure quantitative interpretation. The Multiple Quantum Magic Angle Spinning (MQMAS) experiments have been recorded using the shifted-echo pulse sequence26 with acquisition and processing of the full echo and synchronized acquisition of the indirect dimension.27 The triple quantum excitation and conversion where achieved under high power irradiation (νrf ~ 250 kHz) and the shifted-echo generation with low power pulse (νrf ~12.5 kHz). In situ HT NMR experiments were carried out on the same spectrometer employing a 7 mm Bruker laser MAS probe. The bottom-less MAS rotor is equipped with an inner container made from boron nitride (BN) which carries the sample. Heating of the sample is achieved using a 200 W DILAS diode laser operating at 980 nm. The laser beam is conducted through an optical fiber into the probe, the fiber ending ca. 1 cm underneath the stator, and then directed to the BN container. In situ MAS experiments were performed at 5000 Hz. The

27Al

magic-angle turning phase-adjusted sideband separation

(QMATPASS28) NMR technique was used. The separation of side bands has been carried out with 9- π QMATPASS pulse sequence. Duration of the π pulse was 5.26 μs. 20-step cogwheel phase cycling29 was used to select the alternating coherence transfer pathway in QMATPASS experiments. For each evolution time t1 increment, 640 scans were recorded. The NMR parameters (chemical shifts and quadrupolar parameters) were fitted using the DMfit program.30 Atomistic simulations. The formation energy and migration of interstitial defects in La1+xCa1xAl3O7+0.5x

aluminate melilites were investigated through static lattice and molecular dynamic

(MD) atomistic simulations based on interatomic potential approach.31 The former was performed using the General Utility Lattice Program (GULP).32,

33

The Buckingham potential

function34 was used to model interactions between ions with the shell model34 to describe the electronic polarizability. The interatomic potential parameters used for the atomistic simulation are listed in Table 1. The parameters for La3+-O2−, and O2−-O2− were obtained from previous MD simulations by Tealdi et al.12 The parameters reported by Lewis et al.35 for Ca2+-O2- and Al3+-O2-



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were used as initial parameters, although they were slightly modified (A and  parameters for Ca2+-O2- and C parameter for Al3+-O2-) through the relax fitting procedure for a better reproduction of the experimental structure of LaCaAl3O7.24 The formation energies of interstitial oxygen defect were calculated based on the appropriate combination of dopant and interstitial defect energies and lattice energies of the binary oxides according to the defect reaction 𝐿𝑎2𝑂3 × +2𝐶𝑎𝐶𝑎 →2𝐿 𝑎𝐶∙ 𝑎 + 𝑂′′𝑖 +2𝐶𝑎𝑂. MD simulations for the interstitial oxygen defect migration in

La1+xCa1-xAl3O7+0.5x aluminate melilites were performed with the DL_POLY code.36 The simulation

box

consisted

of

a

446

supercell

containing

2333

atoms

for

the

La1.302Ca0.698Al3O7.151 composition, 2352 atoms for the La1.5Ca0.5Al3O7.25, or 2304 atoms for the parent LaCaAl3O7 used for comparison. The La, Ca and interstitial oxygen atoms were distributed randomly within the simulation box. The systems were equilibrated first under a constant pressure of 1 atm at specific temperatures between 1200 C and 1400 C for 750000 time steps with a time step of 0.03 fs before carrying out the main MD simulation for 150 ps with 5106 time steps in the NVT ensemble. The Visual Molecular Dynamic (VMD) package37 was used to perform MD data analysis and the mean square displacements (MSDs) were calculated with the nMoldyn code.38 Oxygen diffusion coefficients were calculated from the slope of the MSD plots as a function of simulation time.

Table 1. Buckingham Interatomic Potential and Shell Model Parameters used for La1+xCa1xAl3O7+0.5x

atomistic simulations.

Interaction

A (eV)

ρ (Å)

C (eV Å)

Y (e)

K (eV Å-2 )

La3+-O2-

4579.23

0.30437

0.0

3

99999

Al3+-O2-

676.08661

0.354355

0.0

3

99999


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Ca2+-O2-

1228.9

0.3372

0.105852

1.26

34

O2--O2-

9547.96

0.2191

32

-2.869

42

3. Results and Discussions Diffraction data analyses. La1+xCa1−xAl3O7+0.5x (0 ≤ x ≤ 0.5) compositions form transparent glass beads (Figure S2, Supporting Information) after melting of the precursors mix and further quenching to room temperature using an aerodynamic levitation (ADL) coupled to laser heating device. The amorphousness of the synthesized La1+xCa1−xAl3O7+0.5x glass beads was confirmed by XRD as presented in Figure S3 a-b (Supporting Information) for x = 0.3 and 0.5 compositions. Differential scanning calorimetry (DSC) measurements performed on these glass beads show glass transition temperatures at 849, 819 and 803 ± 1 °C, followed by a strong exothermic peak at 880, 868 and 866 ± 1 °C (onset crystallization temperatures), respectively for LaCaAl3O7, La1.3Ca0.7Al3O7.15 and La1.5Ca0.5Al3O7.25 compositions (Figure S3c, Supporting Information). As illustrated by variable temperature X-ray powder diffraction (VTXRD), the diffraction peaks appearing at ~ 800 C correspond to glass crystallization (Figure S3 a-b, Supporting Information). Based on these DSC and VTXRD results, thermal treatments at 850 C were selected for crystallization of the La1+xCa1−xAl3O7+0.5x glasses. A 3 h heat treatment is sufficient to perform full glass crystallization. This is confirmed by XRD data showing only melilite phase crystallization (Figure 1a) and TEM observations during which no amorphous areas were detected. Single phase La1+xCa1−xAl3O7+0.5x crystalline melilite materials can thus be obtained for 0 ≤ x ≤ 0.5 compositions. However, single melilite phase with the usual tetragonal symmetry P421m was only attained for x ≤ 0.3. As a matter of fact, the XRD pattern of the x = 0.3 ceramic can be indexed with cell parameters: atetra = btetra = 7.8366(4) Å, ctetra = 5.1685(4) Å. For x = 0.4, all (hkl) (h and k  0) reflections are broadened, and split into two distinct peaks for x ≥ 0.5. The x ≥ 0.5



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compositions can be indexed with an orthorhombic cell39 (𝑎ortho = 𝑎tetra+ 𝑏tetra, 𝑏ortho = 𝑎tetra- 𝑏tetra, 𝑐 ortho

= 𝑐tetra, where ortho and tetra denotes tetragonal and orthorhombic symmetries, respectively)

using a Cmm2 space group39 (sub-group of P421m). For instance, the x = 0.5 XRD pattern can be indexed with the following cell: aortho = 11.0138(3) Å, bortho = 11.1782(1) Å, cortho = 5.1681(1) Å. This (pseudo-)orthorhombic polymorph could result from partial ordering of oxygen interstitials, as previously observed in Ga-based melilite materials with high interstitial oxide ions content.39 Nevertheless, detailed inspection of the SPD pattern of x = 0.5 (Figure S4a, Supporting Information), and especially of the (hkl) (h and k  0) overlapping reflections, suggested the presence of residual tetragonal polymorph. In fact, both orthorhombic and tetragonal polymorphs have also been observed by SAED in La1.5Ca0.5Al3O7.25. LeBail refinements performed on SPD data of the x = 0.5 composition, and based on a single orthorhombic cell structural model, show poor intensity match in the mediate position, where the (hkl) (h and k  0) reflections of the tetragonal polymorph are showing up. Considering both orthorhombic and tetragonal polymorphs for this LeBail refinement significantly improved the fit, as illustrated Figure S4b (Supporting Information), thus further confirming the co-existence of tetragonal and orthorhombic polymorphs in the x = 0.5 composition. For the x = 0.6 sample, a minor perovskite LaAlO3-like secondary phase was detected, of approximately 1.4(1) wt%, as estimated from multiple phases Rietveld structural refinement. The refined unit cell volume of the La1+xCa1−xAl3O7+0.5x samples (x ≤ 0.5) display linear increase with x (Figure S5, Supporting Information), which is consistent with La3+ (1.16 Å for 8-coordinated) substitution for smaller Ca3+ (1.12 Å for 8-coordinated). The unit cell parameters remain almost constant for x ≥ 0.5, confirming the solid solution limit close to x = 0.5. Actually, it is hardly possible to obtain pure melilite phase for La1+xCa1−xAl3O7+0.5x materials via solid state reaction, even for the parent LaCaAl3O7 sample in which a minor perovskite LaAlO3-like phase is always present, as shown in Figure S6 (Supporting Information). In fact, the glass crystallization route has been proved to be very competitive for the synthesis of new crystalline


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phases and also pure phase materials. These phases can be more easily accessed, as the homogenization of elements in glass is better than samples resulting from solid-state reaction.23, 40-42 Variable temperature XRD (VTXRD) measurements (Figure 1b) were carried out on the fully crystalline mixed-phase La1.5Ca0.5Al3O7.25 sample which revealed that the phase transition from orthorhombic to tetragonal started at ~ 475 °C and finished at ~ 525 °C. The temperature dependency of cell parameters for the orthorhombic polymorph (Figure S7, Supporting Information) indicates that the orthorhombic polymorph undergoes anisotropic thermal expansion in order to achieve the orthorhombic-to-tetragonal phase transition. Above 450 °C, the a axis starts to contract while the b axis remains expanded but with a steeper slope, which decreases the discrepancy between a and b axes before the transformation to the tetragonal polymorph. As reported in previous works for La1+xSr1−xGa3O7+0.5x and La1+xCa1−xGa3O7+0.5x families, the maximum x value exhibiting the parent tetragonal structure with disordered oxide ion interstitials are x = 0.6 and 0.5, respectively. The phase transitions from orthorhombic to tetragonal symmetry for La1+xSr1−xGa3O7+0.5x and La1+xCa1−xGa3O7+0.5x occur at 565 °C and ~650 °C, respectively.39, 43 Thus, the La1+xCa1−xAl3O7+0.5x materials have smaller compositional range for the single tetragonal polymorph and lower orthorhombic-to-tetragonal phase transition temperature than in La1+xSr1−xGa3O7+0.5x and La1+xCa1−xGa3O7+0.5x.



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Figure 1. (a) Room temperature XRD patterns of La1+xCa1−xAl3O7+0.5x materials synthesized by full crystallization of glass process; (b) VTXRD patterns of the La1.5Ca0.5Al3O7.25 sample. The characters T and O denote tetragonal and orthorhombic polymorphs, respectively. In addition, as can be seen in Figure 1b, the La1.5Ca0.5Al3O7.25 sample decompose into stoichiometric melilite and perovskite phases at temperature higher than 800 °C. In order to evaluate the stability of the La1+xCa1−xAl3O7+0.5x materials when used as electrolyte in SOFCs at a lower temperature, the as-made La1.3Ca0.7Al3O7.15 polycrystalline powder sample which has single tetragonal phase was then calcined at 750 °C under pure oxygen atmosphere and pure hydrogen atmosphere, respectively, for 48 h. The subsequent XRD measurements performed on the resultants did not show any secondary phase or noticeable evolution of the original diffractogram, confirming the good stability of aluminate melilites when applied at low temperatures. Combined Rietveld refinement on both NPD and SPD data was then carried out to characterize the structures of single tetragonal polymorph La1.3Ca0.7Al3O7.15. Before the refinement, SAED patterns (Figure S8, Supporting Information) were recorded on La1.3Ca0.7Al3O7.15, confirming its simple tetragonal melilite cell with space group P421m. No sign of any local ordering such as extra reflections or streaks could be detected. Preliminary Rietveld refinements were performed based on the parent LaCaAl3O7 structural model.44 This model


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contains one La/Ca site (4e), two tetrahedral Al sites (Al1 (2a) and Al2 (4e) sites for 4-linked and 3-linked AlO4 tetrahedra, respectively), and three oxygen sites: O1 (bridging, 2c), O2 (terminal 4e) and O3 (bridging, 8f). No interstitial oxide ions were initially considered in this model. Preliminary Rietveld refinements using this oxygen-stoichiometric model but with a refined La/Ca ratio, close to the nominal composition, led to reliability factors of Rwp  4.75%, RB  3.14% for the NPD data and Rwp  8.48%, RB  2.52% for the SPD data. A difference Fourier map calculation was then performed in order to find out the position of the excess oxide ions. From NPD data, a positive nuclear scattering density was located on a 4e position (~0.35, ~0.15, 0). This site appears close to the center of a pentagonal tunnel (Figure S9, Supporting Information) and is at 2.29 Å -2.31 Å (Figure S10a, Supporting Information) from the 3-linked tetrahedral Al center. This extra oxygen site appears positioned similarly to the one in Ga-based nonstoichiometric melilite materials.10 Consequently, an extra oxygen site O4 was added to this 4e site. Subsequent Rietveld refinements led to a final position at around (~0.345, ~0.155, 0) with an occupancy of 0.068(1), corresponding to an extra oxygen content of 0.136(2) per formula, in good agreement with the nominal composition La1.3Ca0.7Ga3O7.15. The addition of the O4 site notably improved the fit (Rwp ≈ 3.44% on NPD data and 7.74% on SPD data). The final refined structural parameters of La1.3Ca0.7Ga3O7.15 are provided in Table 2. The Rietveld fitting plots are demonstrated in Figure 2.

Table 2. Final refined structural parameters for La1.3Ca0.7Al3O7.15; a = b = 7.7976(1) Å, c = 5.1425(1) Å; Space group: P421m. Atom

site

x

y

z

occupancy

Biso (Å2)

La

4e

0.1615(1)

0.6615(1)

0.5073(2)

0.639(1)

0.808(4)

Ca

4e

0.1615(1)

0.6615(1)

0.5073(2)

0.361(1)

0.808(4)



Page 14 of 37

Al1

2a

0

0

0

1

0.36(2)

Al2

4e

0.3568(2)

0.8568(2)

0.9619(1)

1

0.93(1)

O1

2c

0

0.5

0.1794(1)

1

0.99(2)

O2

4e

0.3596(3)

0.8596(3)

0.2956(1)

1

0.59(1)

O3

8f

0.3372(1)

0.4137(3)

0.1917(2)

1

0.91(1)

O4

4e

0.3457(1)

0.1543(1)

0

0.068(1)

1.2(2)

Figure 2. Rietveld fits on the NPD (a) and SPD (b) data of La1.3Ca0.7Al3O7.15


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In order to analyze the interstitial oxide ion ordering in the orthorhombic La1+xCa1-xAl3O7+0.5x polymorphs, many attempts have been made to obtain a pure orthorhombic polymorph for the La1.5Ca0.5Al3O7.25 composition at different temperatures and dwell times. Unfortunately, all these attempts failed to reduce the content of the tetragonal polymorph, as seen in Figure S11 (Supporting Information). Therefore, two-phase Rietveld refinements based on combined NPD and SPD data were then carried out on the mixed-phase La1.5Ca0.5Al3O7.25 sample. Clearly, the two-phase Rietveld refinement led to lower reliability factors Rwp ~ 3.74 % (for NPD) and Rwp ~ 11.24 % (for SPD), compared with Rwp ~ 4.54 % (for NPD) and Rwp ~ 16.10 % (for SPD) based on considering an orthorhombic polymorph only, again adding further weight to the possible coexistence of orthorhombic and tetragonal polymorphs. The refined cell volume of the tetragonal polymorph (317.43(3) Å3), obtained from high-resolution SPD data, is essentially identical to half of the orthorhombic polymorph (317.26(1) Å3), implying that the tetragonal and orthorhombic polymorphs in the La1.5Ca0.5Al3O7.25 sample have similar chemical compositions, which was confirmed by EDS analysis on the x= 0.5 sample. The refinements of the SPD data led to a ~15.5 wt% content of tetragonal polymorph. The La/Ca ratio and the content of interstitial oxide ions in tetragonal and orthorhombic polymorphs were constrained to be equal. In fact, from the SAED patterns (Figure S12, Supporting Information) recorded on the orthorhombic polymorph, a clear measured difference (~0.1 Å) between a and b axes is obvious, agreeing well with the results derived from Rietveld refinements on the La1.5Ca0.5Al3O7.25 composition using the orthorhombic structure. Lowering the symmetry down to P1, as previously reported for the La1.64Ca0.36Ga3O7.32 case, did not significantly improve the fit. The Cmm2 space group was therefore employed to describe the orthorhombic La1.5Ca0.5Al3O7.25 structure. The final refined structural parameters of La1.5Ca0.5Al3O7.25 at room temperature in tetragonal P421m and orthorhombic Cmm2 are summarized in Table S1 and S2 (Supporting Information). The two-



Page 16 of 37

phase Rietveld fits for NPD and SPD data of La1.5Ca0.5Al3O7.25 are presented Figure S13 (Supporting Information). The refinement of the tetragonal La1.5Ca0.5Al3O7.25 structure reveals interstitial oxygens placed at the center of the five-fold rings, similar to the case of La1.3Ca0.7Al3O7.15. However, for the orthorhombic La1.5Ca0.5Al3O7.25 polymorph, two different positions were obtained for the interstitial oxygen: (i) besides the usual O7 site at the center of the tunnel, and (ii) about 42.6% of the interstitial oxygen locates at a position (O8 site in Figure 3) which is off-centered with respect to the pentagonal ring. Unlike the O7 site close to three 3-linked tetrahedral Al centers, the O8 site is close to two 3-linked and one 4-linked tetrahedral Al centers, among which one of the 3-linked tetrahedral Al centers shows the shortest distance with O8 site (1.95 Å). This site is akin to the calculated position from the atomistic simulation12 and pair-distribution functional analysis of total neutron scattering data45 for the gallate La1.5Sr0.5Ga3O7.25 material. These results confirm the diversity of local environments around the interstitial defect in La1+xCa1-xAl3O7+0.5x, as revealed by the structural analysis of the tetragonal phase La1.3Ca0.7Al3O7.15 above. Further description of the defect structures in the (pseudo)-orthorhombic polymorph of La1.5Ca0.5Al3O7.25 and determination of the true symmetry for the interstitial-ordered polymorph would require a phase-pure sample, which is hardly accessible here owing to the slow and complex kinetics on the interstitial defect ordering.


Page 17 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 3. Projection view of La1.5Ca0.5Al3O7.25 in the Cmm2 model along the [001] direction and showing the location of interstitial oxides O7 and O8 within the rings in the average structure. The blue, green, black, and red balls represent Al1, Al2, Al3 and framework oxygen atoms, respectively. The A-site cations and oxygens overlapping with aluminums were omitted for clarity. Solid-state

27Al

NMR. Compare to powder diffraction, solid-state NMR has specific and

important advantages by studying local structure around a selected nucleus, with no limitations due to disorder in the sample. In addition, NMR is a local probe sensitive to all structures regardless of their crystalline or amorphous state. Therefore, to further investigate the local structure in La1+xCa1−xAl3O7+0.5x aluminate melilites, solid-state

27Al

NMR technique was used.

The literature on the subject is rather scarce. Chiara Ferrara et al. studied the local structure of LaSrAl3-xGaxO7 (x = 0, 1, 1.5, 2, and 3) system, which does not exhibit interstitial oxide ions, by 27Al

and

71Ga

NMR in order to establish a correlation between the structure and NMR

parameters.46 Here,

27Al

NMR measurements were performed on LaCaAl3O7 (prepared by

traditional solid-state reaction) and La1.3Ca0.7Al3O7.15 (prepared by ADL and further crystallization heating step) to address the difference between the parent material, without excess oxide ions, and the La-doped materials with interstitials defects. The aim was especially to shed light on the local structure around the Al atoms under variable temperatures, which is expected to be accompanied with the migration of interstitial oxygen. In the LaCaAl3O7 stoichiometric melilite structure, the interatomic Al−O distances are ~1.83 Å and ~1.75 Å for Al1 and Al2 atoms respectively.24 The tetrahedral angles deviate from the ideal tetrahedral angle, 109.47°, by 1.17° and 9.29° in the Al1O4 and Al2O4 units. Thereby, the Al2O4 polyhedron is more distorted than the Al1O4 tetrahedral.



Page 18 of 37

The relative multiplicities ratio of Al1 and Al2 atoms is 1:2. The 27Al quantitative one pulse MAS NMR spectra recorded at room temperature at 17.6 T for the compositions LaCaAl3O7 and La1.3Ca0.7Al3O7.15 are shown in Figure 4. The spectrum of LaCaAl3O7 contains overlapping peaks. To improve the resolution, two dimensional

27Al

MQ-MAS experiments were performed. As

expected, two signals with relative intensity signals of 1:2 are revealed and match the Al1 and Al2 forth-fold coordinated nuclei. The signal (CQ = 3.9 MHz, ηQ = 0.5) peaking at an isotropic chemical shift δiso ~ 78.5 ppm in the MAS spectrum is assigned to the 4-linked Al1 sites and the broad line caused by the second-order quadrupolar interaction (CQ = 8.0 MHz, ηQ = 0.3, δiso  85 ppm) to the 3-linked Al2 sites. This latest assignment, realized from the symmetry of the polyhedra, is in good agreement with the integral intensities discussed previously. Apart from this signal, 27Al NMR experiments also reveal the presence of a secondary phase with an octahedral coordination showing a weak (~ 1 %) signal with δiso ~ 10-20 ppm. This resonance can be ascribed to the perovskite LaAlO3-like impurity and is consistent with the results from the XRD analysis of the LaCaAl3O7 samples prepared by traditional solid state reaction method (1.4 wt% of LaAlO3 from Rietveld quantification). For the La1.3Ca0.7Al3O7.15 composition, the 27Al MAS and MQ-MAS NMR spectra (Figure 4b) recorded at room temperature contain an additional resonance peak (δiso  50 ppm) compared to the stoichiometric LaCaAl3O7 melilite sample. This new resonance is assigned to five coordinated aluminum sites and is consistent with the combined NPD and SPD Rietveld refinements results showing the presence of extra oxygen in a five-fold coordination. The fit enables an estimation of the relative intensity of this peak of 15 %, meaning that 15 % of the Al atoms are in 5-coordination. This is also consistent with the structural model of La1.3Ca0.7Al3O7.15 obtained from powder diffraction considering the longest Al-O bonds (e.g. ~ 2.3 Å in the average


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structure or < 2.5 Å in the site-split model). In other words, the interstitial defects are stabilized by three Al centers in each ring, but could form a strong bond preferably with one Al atom and two weaker bonds with other Al atoms, as indicated by the site-split model.

Figure 4. Room temperature 27Al MAS and MQ-MAS NMR spectra of (a) LaCaAl3O7 and (b) La1.3Ca0.7Al3O7.15 recorded at 17.6 T.

To investigate the dynamic local structure around Al atoms with variable temperature,

27Al

NMR MAS spectra of LaCaAl3O7 and La1.3Ca0.7Al3O7.15 have been collected at a spinning frequency of 5 kHz from RT to 600 °C (Figure 5). The evolution of the spectra with temperature is completely reversible (Figure S14, Supporting Information). To simplify the spectrum and



Page 20 of 37

discriminate spinning side bands from isotropic contribution, the QMATPASS28 method has been applied. It allows reconstructing an “infinite” spinning rate spectrum whose interpretation is more easily. For LaCaAl3O7, no significant changes were observed within the working temperature range. In the case of La1.3Ca0.7Al3O7.15 and upon heating, peak broadening and intensity decrease can be clearly observed for the five-coordinated aluminum signal. These spectra evolutions can be explained by the exchange process of 5-coordinated Al site arising from the interstitial oxide ion migration. To our best knowledge, these solid state 27Al NMR results represent the first NMR evidence of the existence and migration of interstitial oxide ions in melilites.


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Figure 5. 27Al NMR MAS spectra at a spinning frequency 5 kHz in the temperature range from RT to 600 °C of (a) LaCaAl3O7 and (b) La1.3Ca0.7Al3O7.15. Electrical properties. The electrical properties of La1+xCa1−xAl3O7+0.5x materials were investigated by AC impedance measurements. Figure 6a shows a typical complex impedance plot of a La1.3Ca0.7Al3O7.15 pellet measured at 450 °C. The large semicircular arc at high frequency range can be ascribed to grain response, and the following less-distinguishable small arc overlapping with the bulk response arc is due to the grain boundary response. In addition, an inclined tail at low frequency is ascribed to the electrode response with large capacitance of ∼10−7 F/cm, indicative of oxide ionic conduction.47 Arrhenius plots of the conductivities of La1+xCa1-xAl3O7+0.5x for x ≤ 0.2 (Figure 6b) show that the bulk conductivity strongly increases when the substitution increases. The bulk conductivity of La1.2Ca0.8Al3O7.1 at 800 °C is ~1.1 × 10−3 S/cm, which is more than two orders of magnitude higher than the conductivity of the parent LaCaAl3O7 material (~7.2 × 10−6 S/cm). When x increases to 0.3, only a slight increase of the conductivity can be measured compared to the x = 0.2 material. The conductivity becomes essentially constant when the substitution content further increases to x = 0.4 and 0.5. This behavior may be mainly due to the presence of the orthorhombic polymorph in these two samples.



Page 22 of 37

We stipulate that as the interstitial oxide ions in the orthorhombic polymorph are ordered, then a lower conductivity than for the tetragonal polymorphs with the same or even higher interstitial oxide ions content can be expected, as previously observed in the case of Ga-based melilite materials.43 In addition, the conductivity values for the x = 0.3, 0.4 and 0.5 compositions of La1+xCa1-xAl3O7+0.5x are lower than for the gallate melilite La1.2Sr0.8Ga3O7.1 material which contains a lower level of interstitial oxygen content compared with these three aluminate melilite compositions. This suggests that the Al−tetrahedral network has lower flexibility of deformation and rotation of AlO4 tetrahedra when compared to that of GaO4 tetrahedra owing to rigidity of AlO4 tetrahedra. The Arrhenius plots for the mixed-phase samples x = 0.4 and 0.5 show a decrease of activation energies (Table S3, Supporting Information) from ~ 1.36 eV to ~ 1.11 eV for temperatures above ~ 500 C, which could be associated with the orthorhombic-to-tetragonal phase transition, i.e. the disorder-order transition of oxygen interstitial defects, as indicated by the VTXRD data (Figure 1b). The x = 0.1-0.3 tetragonal-phase pellets also display similar curvature on the Arrhenius conductivity plots although these pellets lack orthorhombic-to-tetragonal phase transition. This behavior may be related to the short-range order-disorder transition of oxygen interstitial defects. A similar phenomenon was previously observed in the La1+xSr1-xGa3O7+0.5x system, for which the pseudo-orthorhombic gallate melilites, such as La1.64Sr0.36Ga3O7.32, showed an evident anomaly around the phase transition temperature on the conductivity Arrhenius plots. Compositions

with

tetragonal

symmetry

containing

interstitial

defects,

such

as

La1.54Sr0.46Ga3O7.27, have also much lower activation energy at elevated temperature than at low temperature.


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Figure 6. (a) Typical complex impedance plot of La1.3Ca0.7Al3O7.15 recorded at 450 °C. Rb and Rgb denote bulk and grain boundary resistivity values, respectively. (b) Arrhenius plots of bulk conductivities for La1+xCa1-xAl3O7+0.5x (0 ≤ x ≤ 0.5) materials in comparison with La1.2Sr0.8Ga3O7.1 from the literature.43

Atomistic simulations. In order to further understand the stabilization and migration of interstitial oxygen atoms in the aluminate melilite structure, static lattice and molecular dynamic simulations based on the interatomic potential method were performed. Such calculations were previously successfully applied in gallate melilites[12] and other structural types based on



Page 24 of 37

tetrahedral units e.g. apatite48, LaBaGaO4-based49 and more recently reported Sr-doped BiVO442 oxide ion conductors. Here, static lattice atomistic simulations were first performed on the parent LaCaAl3O7 composition using the P421m tetragonal cell structural model containing disordered La/Ca over the pentagonal tunnels. This model reproduces well the experimental structure (Table S4, Supporting Information), e.g., the difference between the experimental and calculated cell parameters and calculated bond lengths is less than  0.02 Å and  0.07 Å, respectively. In the tetragonal LaCaAl3O7 melilite structure, the tunnel cations La3+ and Ca2+ share the same crystallographic site in a disordered manner. In order to simplify the static lattice atomistic simulations, at next stage, the P421m structure was transformed into a 1×1×1 cell in P1 and an ordered cationic structural model containing pure La or Ca array in each tunnel was employed for the defect energy calculations. The calculated formation energies of interstitial oxygen defects placed in the La-containing and Ca-containing tunnels are 3.87 eV and 3.57 eV, respectively, which are larger than ~1.5-2.2 eV obtained by atomistic simulations in the Sr/Ca-containing gallate melilites and consistent with the hard accessibility of interstitial-containing aluminate melilites from the traditional solid-state method. The small difference between the formation energies of interstitial defects in the La-containing and Ca-containing tunnels suggests that the cationic ordering could have minor effects on the local distribution of oxygen interstitials. However, the presence of oxygen interstitial defects could slightly favor smaller Ca2+ cations in this aluminate melilite structure. MD simulations were then performed on two interstitial-containing compositions, i.e. La1.302Ca0.698Al3O7.151 (this composition, close to La1.3Ca0.7Al3O7.15, was chosen in order to make the total atom number for each atom type in the simulation box of 446 supercell to be integer) and La1.5Ca0.5Al3O7.25, for probing the migration pathways of interstitial oxygen defects in nonstoichiometric aluminate melilites. As both compositions lead to similar MD simulation results,


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we focus here on the La1.302Ca0.698Al3O7.151 composition. Figure 7 shows the scatter plots of oxide ion coordinates of LaCaAl3O7-based aluminate melilites over the simulated time scale at 1400 C, which enables the migration paths to be visualized. In the parent composition, free of interstitial defects, all atoms only show lattice vibration without long-range migration. Among them, the bridging oxygen atoms clearly display anisotropic vibration along the direction perpendicular to the Ga-Ga lines (Figure 7a). When the excess oxygen atoms are introduced in the pentagonal ring via La3+ substitution for Ca2+, the diffuse distribution and overlapping of different oxygen positions indicate that long-range oxide ion migration takes place (Figure 7b). Moreover, numerous oxide ions are moving between the framework and interstitial sites according to a cooperative “knock-on” mechanism among the interstitial and framework oxygen atoms. More precisely, a migrating interstitial oxygen atom displaces a framework oxygen away directly into a neighboring interstitial position, as shown in path 1 in Figure 7b. Alternatively, the “knocking interaction” between interstitial and framework oxygen atoms can initiate a chain “knocking” among the framework oxygen atoms before displacing one framework oxygen into the neighboring ring, as indicated by path 2 in Figure 7b. Both paths illustrated in Figure 7b involve framework oxygen sites and both the center and off-center sites in the pentagonal ring for the interstitial oxygen were identified from NPD data refinements. MD simulations confirm that the interstitial oxide-ion migration is highly anisotropic and restricted exclusively to the cornersharing AlO4 tetrahedral layer, with no evidence of interlayer ion diffusion (Figure S15, Supporting Information). This is consistent with the presence of the large immobile La/Ca cations bounded by oxygen atoms from the pentagonal rings (Figure S16) and located between tetrahedral layers in the voids. These large immobile La/Ca cations appear to block the interstitial-oxide-ion migration along the c axis.



Page 26 of 37

(b)

(a)

Figure 7. Scatter plots for the oxide anions in single tetrahedral layer of (a) LaCaAl3O7 and (b) La1.302Ca0.698Al3O7.151 viewed along the c-axis from the MD simulation at 1400 C. A and B denote the center (O7) and off-center (O8) sites for the interstitial oxygen atoms identified in La1.5Ca0.5Al3O7.25 from the NPD data refinement, respectively. The dots in cyan, orange, red and pink represents the oxygen atoms initially placed on the crystallographic distinct framework O1, O2, O3 and interstitial O sites respectively; while the blue and green for Al1 and Al2 sites, respectively, and adjacent Al sites are connected by black lines as a guidance for the eyes.

The scatter plots of oxygen atoms together with their MSD values (Figure 8a) confirm that the oxide ion migration process involves interstitial oxide ions and all the framework atoms and is a consequence of the rotational motion of the AlO4 tetrahedral units. Among the three crystallographic distinct framework oxygen sites (in tetragonal polymorph), O3 atoms are the most mobile species (comparable to oxygen atoms Oi initially placed on the interstitial position) as evidenced by the more scattered positions and larger MSD values of O3 ions compared with the other framework oxygen sites. As showed in bottom-right inset of Figure 8a, the simulations were performed only at temperatures of 1200 oC, 1300 oC and 1400 oC, which gave similar results on the oxide ion migration, except for smaller diffusion lengths at lower temperature as


Page 27 of 37

expected. Figure 8b shows the partial radial distribution functions (RDFs) for Al1-O interactions calculated over the simulation time, which provides further insight into ionic (dis)order associated with the long-range oxide ion migration. One can notice the emergence of weak peaks centered at about 1.75 Å of Al1 atoms with O1 and O2 atoms (highlighted in Figure 8b with arrows and identical to the Al1-O3 bond length) which were originally bonded with Al2 atoms. These confirm the occurrence of long-range migration of oxide ions between ions originally on different sites exchanging positions with other sites during the cooperative migration process. (a)

3.5 MSD (Å2)

3.0

2.0

7 6 5 4 3 2 1 0

O

Oi O3

O2

0

20

O1 40 60 80 100 120 Time (ps)

1.5

-5

10

2 -1

D (cm s )

2

MSD (Å )

2.5

1.0

Ea = 1.47(9) eV -6

10

-7

10

0.5 0.0

(b)

0

20

40

60 80 Time (ps)

0.60

0.64 -1 1000/T (K )

100

120

0.68

Ca Al La

140

6

Al1-O1

Al1-O2

g(r)

4

2

0 10

Al1-O3

8

g(r)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Al1-Oi

6 4 2 0

0

2

4

6

8

Interatomic separation (Å)

10 0

2

4

6

8

Interatomic separation (Å)


10


Page 28 of 37

Figure 8. (a) MSD values of La3+, Ca2+ and Al3+ cations and oxide anions. The top-left and bottom-right insets show MSD values of individual oxygen atoms at framework and interstitial oxygen sites and Arrhenius plot of oxygen diffusion coefficient, respectively. (b) RDFs for Al1O interactions versus simulation time in La1.302Ca0.698Al3O7.151. These values were obtained from the MD simulation performed at 1400 C. The cations vibrate around their lattice positions without apparent long-range migration.

The oxygen diffusion coefficient, calculated from the MSD values of oxygen atoms, varies within 510-7–210-5 cm2/s in the 1200-1400 C temperature range, although no experimental values are available for comparison. The activation energy derived from the Arrhenius plot of oxygen diffusion coefficients is ~ 1.47 eV (bottom-right inset of Figure 8a), which is perfectly in agreement with experimental values in the low temperature range from conductivity measurements (Figure 6b). Such a higher energy barrier for oxide ion migration in the aluminate melilite compared to gallate melilites (the calculated activation energy for the interstitial oxide ion migration in La1.5Sr0.5Ga3O7.25 is 0.70 eV)12 could originate from the rigidity of AlO4 tetrahedra that show less flexibility on deformation and rotation limiting the oxide ion transport within the Al3O7 tetrahedral layer. The lower mobility of interstitial defects in aluminate melilite is also evidenced by the much weaker and broader peaks for Al1-O1 and Al1-O2 bonds at 1.75 Å in Figure 8b, when compared with the sharp peaks for the corresponding Ga-O bonds from the MD simulation of La1.5Sr0.5Ga3O7.25. Such a contrast between the oxide ion conduction behavior of gallate and aluminate polyhedral network has also been reported in vacancy-conducting LaGaO3 and LaAlO3-based perovskites.50 The more rigid lattice for LaAlO3-based perovskite were also considered to be a main factor that makes the oxide ion migration to adjacent vacant oxygen sites more difficult in the LaAlO3 host lattice and therefore the activation energy for oxide ion migration higher than that in the LaGaO3-based materials.50 Owing to the larger size


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and electronegativity of Ga than Al, Ga-O bonds exhibit larger length and greater covalency than Al-O bonds.51,

52

This could offer more flexibility to the GaOn polyhedra on the rotation and

deformation within the linked polyhedral network and therefore facilitate the oxide ion migration, as compared with AlOn polyhedra.

4. Conclusion A new family of Ga-free interstitial oxide ion conductors, La1+xCa1-xAl3O7+0.5x aluminate melilites, which is not accessible via classical solid state reaction preparation method, was prepared by full and congruent crystallization from glass using aerodynamic levitation and laser heating method. La1+xCa1-xAl3O7+0.5x displays multiple interstitial defect positions in the melilite pentagonal rings, resulting in a diversity of local structures around the oxygen interstitial defects. Variable high temperature solid-state 27Al NMR measurements have shown the first NMR evidence of the existence and migration of interstitial oxide ions in melilite structure from the presence of an additional 5-coordinated Al signal and dynamic exchange process of 5-coordinated Al sites. MD simulations indicated that interstitial oxide ions migrate within the Al3O7 tetrahedral layers through a cooperatively ‘knocking-on” mechanism among the framework and interstitial oxide ions assisted by rotation and deformation of tetrahedra. Both experimental impedance measurements and molecular dynamic simulations showed that La1+xCa1-xAl3O7+0.5x aluminate melilites display lower interstitial oxide ion mobility than gallate melilites. This may mainly be due to the lower flexibility of rotation and deformation of the AlO4 tetrahedra compared to GaO4 tetrahedra. The present work provides a more comprehensive understanding for the stabilization and migration of interstitial oxide ions in melilite materials, which will be helpful for further discovery of new oxide ion conducting materials with better performance toward the practical application. Supporting Information



Page 30 of 37

Local defect structures in La1+xSr1-xGa3O7+0.5x; glass beads of La1.3Ca0.7Al3O7.15; VTXRD patterns, DSC curves; SPD patterns of La1.3Ca0.7Al3O7.15 and La1.5Ca0.5Al3O7.25 samples, LeBail refinement of the SPD data of La1.5Ca0.5Al3O7.25; cell parameters for the La1+xCa1-xAl3O7+0.5x; XRD patterns of

La1+xCa1-xAl3O7+0.5x (x ≤ 0.2) samples synthesized by traditional solid state reaction method; cell parameters versus temperature for the mixed-phase La1.5Ca0.5Al3O7.25 sample; SAED patterns and calculated difference Fourier map for La1.3Ca0.7Al3O7.15; Local structure around interstitial oxide O4; XRD patterns for the prepared La1.5Ca0.5Al3O7.25 samples followed with annealing at 430 oC with different dwell time; SAED patterns for La1.5Ca0.5Al3O7.25; Refined tetragonal P421m and orthorhombic Cmm2 structural parameters for La1.5Ca0.5Al3O7.25; Rietveld fitting plots for NPD and SPD data of La1.5Ca0.5Al3O7.25; activation energy of La1+xCa1-xAl3O7+0.5x; Reversibility of 27Al

NMR MAS spectra evolution versus temperature of La1.3Ca0.7Al3O7.15; calculated and

experimental structural parameters for LaCaAl3O7; Scatter plots for La1.302Ca0.698Al3O7.151 along the a and c axises. This material is available free of charge via the Internet at http://pubs.acs.org/. Author Information Corresponding Authors *E-mail:

Xiaojun

Kuang

(X.K.)

[email protected];

Mathieu

Allix

(M.A.)

[email protected] Acknowledgments National Natural Science Foundation of China (Nos.21601040, 21622101, 21850410458 and 21511130134),

Guangxi

Natural

Science

Foundation

(Nos.2015GXNSFBA139233,

2014GXNSFGA118004 and 2017GXNSFAA198203), CNRS on the bilateral TransLight PICS07091 project, Research Project (No.213030A) of Chinese Ministry of Education, Guangxi Ministry-Province Jointly-Constructed Cultivation Base for State Key Laboratory of Processing for non-Ferrous Metal and Featured Materials (Nos.13AA-8 and 14KF-9), and Guilin University


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of Technology introduces the talented person scientific research start-up funds subsidization project (No.002401003458) are acknowledged for the financial support. This work was also supported by the EQUIPEX PLANEX ANR-11-EQPX-0036-01 grant. Financial support from the TGIR-RMN-THC Fr3050 CNRS for conducting the research is gratefully acknowledged. The authors thank Dr. Franck Fayon, Dr. Roman Shakhovoy, Dr. Florence Porcher, and Dr. Emmanuel Veron for their help on NMR, NPD and SEM techniques.

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Interstitial Oxide Ion Migration Mechanism in Aluminate Melilite La1+

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