Crystal structure and ion conducting properties of La5NbMo2O16

Crystal structure and ion conducting properties of La5NbMo2O16

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Crystal structure and ion conducting properties of La5NbMo2O16 T.D. Vu a,n, F. Krichen a, M. Barre a, R. Busselez a, K. Adil b, A. Jouanneaux a, E. Suard c, F. Goutenoire a a

IMMM (Institute of Materials and Molecules of Mans), UMR-CNRS 6283, University of Maine, 72085 Le Mans Cedex 9, France KAUST (King Abdullah University of Science and Technology), Thuwal, Saudi Arabia c ILL ((Institute Lauë-Langevin), 6 J. Horowitz street, B P 156, 38042 Grenoble Cedex 9, France b

art ic l e i nf o

a b s t r a c t

Article history: Received 5 June 2015 Received in revised form 4 January 2016 Accepted 9 January 2016

The new compound La5NbMo2O16 with high ionic conduction has been discovered during the study of the ternary phase diagram of La2O3–MoO3–Nb2O5. The material crystallizes in the cubic space group Pn 3¯ n (no 222) with the unit cell parameter a ¼11.2250(1) Å. La5NbMo2O16 is a new analogue of the R5Mo3O16 series (R¼ Pr, Nd). The structure was refined from a combined data X-ray and neutron powder diffraction. The ionic conductivity of the compound is then measured on sintered pellets, by means of complex impedance spectroscopy. & 2016 Elsevier Inc. All rights reserved.

Keywords: Oxide Electronic X-ray and neutron diffraction Impedance spectroscopy

1. Introduction La2Mo2O9, the first oxide of a rare-earth metal and W or Mo displaying high ionic conductivity has been discovered by Lacorre et al. [1]. The exceptional properties of this material are due to the disordered oxygen network in its cubic phase at high temperature [2]. In the purpose of discovering new phases with the same property as La2Mo2O9, different chemical systems have been investigated. Indeed, in M2O3–(Mo/W)O3 systems (M¼rare-earth metal), many compounds were evidenced with the same structure as the title phase: Pr5Mo3O16 [3], Nd5Mo3O16 [4], CdTm4Mo3O16 (considered as a substituted phase). All these materials can be described from a supercell of Fluorite cell CaF2. According to the papers, these structures exhibit mixed oxidation state of metals, an important property in catalysis chemistry. In Nd5M3O16 (M ¼Mo, W) [4], the coordination number of rareearth cations and Mo (or W) are 8 and 6, respectively in oxide compounds. Thus, if the oxide of a rare-earth metal and Mo (W) presents a Fluorite structure, the anions have to change their positions to allow the metal coordinations. Three possible anionic arrangements are also noted in the paper. Only one of them (anions moved from the center to the faces of cation tetrahedra) was observed in structure of La5NbMo2O16 compound. This paper addresses the synthesis of the new material La5NbMo2O16, its substitutions, the electron diffraction analysis n

Corresponding author. E-mail address: [email protected] (T.D. Vu).

(TEM), the Rietveld refinement of powder X-ray and neutron diffraction (XRD and ND) data and the results of impedance spectroscopy of the materials.

2. Experimental The compound and its substitutions were prepared in solid state route. The starting oxides are La2O3, Nb2O5 and MoO3 for the title compound and Y2O3, SrCO3, BaCO3 and Ta2O5 for substituted phases. Lanthanum oxide powder was decarbonated at 1000 °C overnight prior to use. The oxides were weighed in stoichiometric proportions and ground together in an agate mortar. The samples were annealed in many steps (Table 1) with the heating and cooling rates of 6 °C/min. The room temperature Powder X-ray diffraction (PXRD) patterns were collected on a Bragg–Brentano diffractometer (Empyrean PANalytical) equipped with a linear detector PIXEL in the angular range from 8 to 155° (2θ), with a step size of 0.013° for 10 h. The neutron diffraction (ND) pattern was obtained at room temperature using the D2B high-resolution/high-flux powder diffractometer at the Institute Lauë–Langevin in Grenoble, France. The data were acquired at 2θ intervals of 0.05° from 5° to 160° in 2 h at λ ¼1.5960 Å. The sample was packed in a vanadium can. The electron diffraction study was performed on a 200 kV side entry JEOL 2100 transmission electron microscope with a doubletilt specimen holder at room temperature. For specimen preparation, a small amount of powder was ground in an agate mortar and pestled under dry methanol to produce a suspension. A drop of the

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Table 1 Temperatures and durations of annealing procedure. 1st time 2nd time 3rd time 4th time 5th time

at at at at at

1200 °C 1300 °C 1300 °C 1300 °C 1300 °C

in in in in in

88 h 30 h 57 h 34 h 58 h

suspension was deposited on a holey carbon film supported by a 1000 mesh copper grid and dried. The simulations results presented in this work are based upon lattice energy minimization procedures as implemented in GULP package [5]. In our studies, the force field developped by Preux et al. [6] with ionic polarization treated as a core–shell model first introduced by Dick and Overhauser [7] has been used. The interatomic interactions consist of Coulombic long-range and Buckingham short-range interactions. The lattice parameters and Gibbs free-energy were obtained using the following method: first the crystallographic structure in Table 7 was used as input parameters, then an energy minimization was performed to refine the atomic positions and lattice parameters, afterward the calculation of cell parameters and Gibbs free energy was conducted using zero static internal stress approximation [8] at 300 K and 1 bar. In the case of substituted compounds, the partial site occupancies as implemented in GULP were used. Finally, the transport property of the compound was studied by impedance spectroscopy using a Schlumberger Solartron SI 1260 frequency response analyzer with 0.1 V amplitude signal over the 32 MHz–0.1 Hz frequency range. Five millimeters diameter pellets with, as electrodes, a thin platinum layer deposited on both faces were used for the measurements.

3. Results and discussion 3.1. Electron diffraction The electron diffraction diagrams of various crystallites showed sharp spots which present a good crystallization. According to these diagrams, first routine information about the cell parameters is shown, a ¼b¼ cE 11.2 Å. The reconstruction of reciprocal lattice was also interpreted on two basal planes of (001)* (on which → → → → contains a⁎ and b⁎) and (0 1¯ 1)* (on which contains a⁎ þ b⁎ direction) (Fig. 1). The analysis of the reciprocal lattice allowed us to deduce simultaneously the space group. Indeed, the existence conditions hk0, h + k = 2n and hhl, l = 2n (or hll, h = 2n because of the cubic cell) were derived from (001)* and (0 1¯ 1)* projection respectively. These conditions yielded to the unique space group of Pn 3¯ n (no 222). 3.2. Crystal symmetry The indexation of the powder X-ray diffraction was carried out using the auto-indexing program Treor [9] implemented in the Fullprof program [10]. A satisfactory solution was obtained : a ¼b¼c ¼11.2180(5) Å, V¼1411.69 Å3 with a figure of merit M20 ¼11 [11]. These cell constants are in a good agreement with the values determined from electron diffraction analysis above. From the first Search-Match procedure using X'Pert Highscore Plus [12], the La3Mo2O10 phase (Fluorite-like structure; pdf 00-0230315 [13]) was found with all main peaks identified. Thus, the cell parameters of La5NbMo2O16 seem to be related to the fluorite-type structure of CaF2 (aF ¼5.5 Å, Fm 3¯ m, Z¼ 4). The comparison of the X-ray diffraction diagrams between CaF2 and La5NbMo2O16 also

Fig. 1. Electron diffraction diagrams of (001)* and (0 1¯ 1)* planes.

provided evidences for this hypothesis. Most of high-intensity peaks on the XRD pattern of La5NbMo2O16 are superposed with main peaks on the pattern of CaF2, both in intensity and 2θ position. Other small peaks are due to the superstructure. Because the cell parameters of the new phase are two times more than that of the fluorite cell, they can be expressed as:

⎛ a ⎞ ⎛ 2 0 0 ⎞ ⎛ aF ⎞ ⎜ ⎟ ⎜ ⎟ ⎜ ⎟ ⎜ b⎟ = ⎜ 0 2 0⎟ × ⎜ bF ⎟ ⎝ c ⎠ ⎝ 0 0 2⎠ ⎝ cF ⎠ AX2 × 8 → A 8 X16 → La5NbMo2 O16 The number of chemical formula units per unit cell (Z¼4, the same as CaF2) was extrapolated using the average volume of oxygen (18–22 Å3) in oxides combined with the nominal

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composition of the compound La5NbMo2O16.

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Table 3 Bond-valences of atoms.

3.3. Crystal structure refinement

La1 La2

Firstly, the atomic positions of resolved structures Pr5Mo3O16 were applied in a combined refinement of the XRD and ND pattern of La5NbMo2O16. The only modification is that the Mo position was replaced by a mixed site of (Nb/Mo). The final combined Rietveld refinement of the appropriate atomic coordinates, occupancies, temperature factors along with the lattice constants, scale factors, zero-points, peak-shape parameters (Thompson–Cox–Hastings [14] Pseudo-Voigt function for XRD pattern and classic Pseudo-Voigt function for ND pattern) and background parameters converged to the R-factor values gathered in Table 2. Structural parameters are reported in Table 7. The observed, calculated and difference profiles are illustrated in Fig. 6. The bond valence was calculated by the BondStr tool implemented in the Fullprof Suite for fully occupied sites and manually for mixed sites. The calculated values show a good agreement (according to Brown and Shannon [15]) with the expected values: þ3 for La, þ 5.67 for (Nb/W) and  2 for O (Table 3). 3.4. Structure analysis The unit cell La5NbMo2O16 is a cubic structure of 8 Fluorite cells which are illustrated in Fig. 7. These 8 cubes are represented in 2 types. The first type is a pure Fluorite: La atoms in regular facecentered cubic and O atoms in tetrahedral vacancies (Fig. 7a). The second type is also a Fluorite cell but the positions of O atoms are very distorted. This second type of cell has 3 forms. One form can rotate 90 around the b or a-axis to create another (Fig. 7b–d). The oxygen arrangements are in a good agreement with that mentioned in the paper of Alekseeva et al. [4]. In type 1, oxygen atoms locate at the center of the tetrahedra composed from only one kind of cation (La). In type 2, some oxygen atoms shift from centers to faces of the tetrahedra which are composed from two kinds of cation (La and Nb/Mo). The distances of O–La and O–(Nb/ Mo) were presented in Table 4. 3.5. Substitutions As described above, the La5NbMo2O16 does not possess any vacancy of oxygen. In the purpose of increasing the number of defaults, many types of cations have been chosen for the substitution of La5NbMo2O16: Ba2 þ , Ta5 þ , Y3 þ and Sr2 þ . Among them, Sr2 þ seems to be the most appropriate because its ionic radii is the closest to that of La3 þ (1.18 Å via 1.032 Å for the coordination number of 6). Thus Sr2 þ was used to synthesize two series of doped compounds: stoichiometric with fixed number of oxygens (16) and non-stoichiometric with varied number of oxygens. Table 5 shows the general formula of substituted compound and their corresponding sample names.

3.22(6) 2.94(5)

(Nb/W)

5.28(4)

O1 O2

1.90(4) 2.17(5)

Table 4 Inter-atomic distances (Å). La1–O1 La1–O2 La2–O1 La2–O2 (Nb/Mo)–O1

2.620(2) 2.424(2) 2.649(2) 2.334(2) 1.805(2)

    

4 4 6 2 4

Minimum oxygen–oxygen distance: O1–O1: 2.859(3).

The evolution of cell parameter (or cell volume) shows whether the doping element can be inserted into the main lattice. Illustrated in Fig. 2, the linear and significant decreasing of the cell parameter from 11.22 Å to 10.81 Å means that Y3 þ have been doped into the structure. The solubility of Y3 þ is limited to 80%, leading to the formula LaY4NbMo2O16. In the highest doping ratio (x ¼5), all La atoms were replaced by Y atoms so that the structure of the new phase was not observed. However, in this case of Y series, the impurity of LaNbO4 cannot be eliminated. The formation of this phase is favored in the synthesis condition (solid state synthesis). Another successful substitution is Sr-16 samples. Indeed, the cell parameter in these samples increases linearly (11.22–11.27 Å) without appearance of impurities. On the contrary, the Ta samples do not show any changes in the cell parameter but they present high quantities of impurities (LaNbO4 and La3NbO7). It means that Ta5 þ cannot go inside the structure, although its radii is the same as that of Nb5 þ (0.64 Å for the coordination number 6). Finally, in the Ba and Sr series, the cell constants of the new compound La5NbMo2O16 do change slightly but the impurity quantities are significant. The impurities (La2MoO6 and BaMoO4 for Ba; LaNbO4 and La3NbO7 for Sr, respectively) grows with the doping ratio and can not be eliminated. Thus, in these cases, the doping ions can not create the solid solution of La5NbMo2O16. As mentioned above about Sr2 þ , the results on two series of Sr and Sr-16 are discussed here. About the Sr-16 set, the substitution was successful. The limit where x ¼1 can be reached, leading to the formula La4SrMo3O16. The Sr set was thought to be more interesting because of creating anionic vacancies in the structure and consequently could improve the oxygen conduction. However, this target has not been reached. Instead of non-stoichiometric phases with oxygen vacancies, we synthesized the stoichiometric (Sr-16) samples and 2 impurities (LaNbO4 and La3NbO7). It can be explained that the Sr-16 substitution is possible, due to the (Nb/Mo) mixed site which can compensate the aliovalent doping in the La site. Expected phase EP: La5 − x Srx NbMo2 O16 − x /2 □ x /2 Obtained phase OP: La5 − x Srx Nb1 − x Mo2 + x O16

Table 2 Combined Rietveld refinement results. Diffractometer

X-ray

Neutron

Radiation 2θ range (°) RBragg (%)

Cu–Kα 8–155 4.71

1.59600(5) 5–160 2.30

Rp (%)

11.5

7.82

Rwp (%)

9.41

9.07

R exp (%)

1.78

1.41

Number of reflections

258

233

Table 5 Sample names, ionic radii of the doping elements and the corresponding formula of substituted compound. Sample name

Ionic radii (Å)

Global formula

Ba Ta Y Sr-16 Sr

1.35 0.64 0.9 1.18

La5 − x Bax Nb1 − x Mo2 + x O16 La5Nb1 − x Tax Mo2 O16 La5 − x Yx NbMo2 O16 La5 − x Srx Nb1 − x Mo2 + x O16 La5 − x Srx NbMo2 O16 − x /2 □ x /2

Total number of refined parameters: 58.

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Fig. 2. Evolution of cell parameters of La5NbMo2O16 in the function of doping ratio.

Fig. 4. Complex impedance diagram in the Nyquist plane for La5NbMo2O16 measured under air at 600 K with the equivalent electrical circuit used for fitting.

Fig. 5. The Arrhenius plot of La5NbMo2O16, in comparison with La2Mo2O9 and YSZ 8%.

(EP - OP) ×

( ) → La 2 2+x

5 − x Srx Nb3 O15 − x

In this formula, if x value varies from 0.1 to 1, the percentage of Nb (3/(8-x)) will vary from 38% to 43%. These values are between 25% (La3NbO7) and 50% (LaNbO4). That explains why we always obtain these impurities. The Srx in the formula shows that Sr has entered into the LaNbO4 lattice because its cell parameters evolute with the doping ratio. Meanwhile, there is no evolution in La3NbO7 cell parameters. The percentages of phases and cell parameters of main compound and impurities are provided in supplementary data.

Fig. 3. Top: Lattice parameter as a function of doping ratio for simulated Sr-16 (circles), simulated Sr (squares) compounds and Sr-16, Sr experimental compounds (rhombuses and triangles, respectively). Bottom: Lattice free energy as a function of doping ratio for Sr-16 and Sr simulated systems.

3.6. Atomistic simulations For different doping ratios (0–1), the cell parameters and the Gibbs free energy of the two series of Sr and Sr-16 at 300 K and 1 bar were simulated and presented in Fig. 3. The experimental

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Table 6 Thermal expansion coefficients α of some materials in K  1. 14.7  10  6 18.1  10  6 14.4  10  6 11.6  10  6 12.2  10  6 11.2  10  6

α-La2Mo2O9 β-La2Mo2O9 La18W10O57 La2WO6 La10W2O21 La7Nb3W4O30

Table 7 Crystallographic positions refined from mixed X-ray and neutron diffraction data of La5NbMo2O16: a ¼ 11.2250(1) Å; Pn 3¯ n (no 222); Z¼ 4.

Fig. 6. Rietveld refinement on (a) XRD and (b) ND data: observed (points), calculated (line), difference profiles and reflection positions (ticks).

Atom

Multiplicity

x

y

z

Biso (Å2)

Occupation

La1 La2 Mo1 Nb1 O1 O2

12e 8c 12d 12d 48i 16f

0.0067(2) 0 0 0 0.5848(2) 0.1201(2)

0.25 0 0.75 0.75 0.3618(1) 0.1201(2)

0.25 0 0.25 0.25 0.8286(1) 0.1201(2)

0.15(2) 0.15(2) 0.49(5) 0.49(5) 2.2(2) 2.2(2)

1 1 0.33 0.67 1 1

cell parameters were also plotted for comparison purpose. In the top section of the figure, except to the experimental Sr samples, all series present a linear expansion of cell parameters. Both simulated and experimental lattice parameters of the initial phase of La5NbMo2O16 (x ¼0) are in good agreement (11.251 Å and 11.221 Å, respectively). Nevertheless, the agreement becomes poorer as the doping ratio increases. At high doping ratio, i.e. x ¼1, the simulated lattice constant reaches 11.32 Å for Sr sample and 11.49 Å in the case of Sr-16. Although the quantitative agreement is not reached, the overall behavior concerning lattice parameters obtained from the simulation is compatible with the experimental results. In addition, we see that the lattice parameter of Sr-16

Fig. 7. Illustration of an unit cell of La5NbMo2O16.

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samples are much more sensitive to the doping ratio than the Sr samples. The Gibbs free energy simulated at 300 K and 1 bar of the two series (the bottom section of Fig. 3) leads to the phase stability. The free energy rises strongly and linearly in the case of Sr samples (square) as the doping ratio increases. To the contrary, the Gibbs energy of Sr-16 samples (circle) decreases in a non-linear fashion with the increase of the Sr2 þ substitution and oxygen vacancies. This phenomena may explain the purity of the Sr-16 samples and the raise of impurity amount in the Sr series (see supplementary information).

non-stoichiometric phase) is thought to be promising but did not been successfully synthesized. The explanation for this is the instability of the substituted compounds and the presence of impurities in our experimental condition. Another route of synthesis should be considered.

Appendix A. Supplementary data Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.jssc.2016.01.005.

3.7. Impedance measurement and thermal expansion The impedance spectroscopy measurements of the compound La5NbMo2O16 were realized in the temperature range of 600– 1000 K with the step of 50°. In the impedance diagram plotted in the Nyquist plane (Fig. 4), two semicircles at high frequency correspond to the ionic motion of the grains and grain boundaries. In particular the line at low frequency represents the electrode polarization and confirms the ionic nature of the conductivity. The characteristic resistances of these two contributions were determined from an equivalent electrical circuit (Fig. 4, inset). The Arrhenius plot (Fig. 5) showed that the total ionic conductivity La5NbMo2O16 is comparable to that of the low temperature form of La2Mo2O9. Thus, this new material seems to be very promising. The thermal expansion coefficient of La5NbMo2O16 was deduced using high-temperature X-ray diffraction in the temperature range of 30–1000 °C:

β=

ΔV Vo × ΔT

α = β/3 = 10.62 × 10−6 (K−1) The value is similar to that of other materials (Table 6).

4. Conclusion During the investigation of the ternary phase diagram of La2O3–MoO3–Nb2O5, only one new material has been discovered. It has a Fluorite-related structure, like other materials previously reported (Pr5Mo3O16, Nd5Mo3O16,…). It does not possess any vacancies but presents a remarkable ionic conductivity in comparison to La2Mo2O9 and YSZ 8%. In order to ameliorate the oxygen conduction of this new phase, many series of substitution have been realized. Among them, the Sr batch (doping ion of Sr2 þ in

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