Synthetic Access to Cubic Rare Earth Molybdenum ... - ACS Publications

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Synthetic Access to Cubic Rare Earth Molybdenum Oxides RE6MoO12−δ (RE = Tm−Lu) Representing a New Class of Ion Conductors Daniel Schildhammer,† Gerda Fuhrmann,† Lucas Petschnig,† Simon Penner,§ Michaela Kogler,§ Thomas Götsch,§ Andreas Schaur,‡ Nikolaus Weinberger,‡ Andreas Saxer,‡ Herwig Schottenberger,† and Hubert Huppertz*,† †

Faculty of Chemistry and Pharmacy, Institute of General, Inorganic and Theoretical Chemistry, Leopold-Franzens-University Innsbruck, Innrain 80-82, 6020 Innsbruck, Austria § Institute for Physical Chemistry, Leopold-Franzens-University Innsbruck, Innrain 80-82, 6020 Innsbruck, Austria ‡ Institute of Material Technology, Leopold-Franzens-University Innsbruck, Technikerstrasse 11-13, A-6020 Innsbruck, Austria S Supporting Information *

ABSTRACT: Materials crystallizing in highly symmetric structures are of particular interest as they display superior physical properties in many relevant technological areas such as solid oxide fuels cells (SOFCs), catalysis, or photoluminescent materials. While the rare earth molybdenum oxides RE6MoO12 with the large rare earth cations RE = La to Dy crystallize in a cubic defect fluorite structure type (Fm3̅m, no. 225), the compounds with the smaller cations RE = Tm− Lu could hitherto only be synthesized in the rhombohedral defect fluorite structure type (R3,̅ no. 148). In the following, new low temperature access to the rare earth molybdenum oxides RE6MoO12−δ (RE = Tm−Lu) crystallizing in the highly symmetric cubic bixbyite structure type (Ia3̅, no. 206) will be discussed. The three-step method comprises preparation of the rhombohedral phases by solution combustion (SC) reactions, their reduction including simultaneous structural transitions from the rhombohedral to the cubic phases, and subsequent reoxidations while preserving their cubic structures. Detailed studies on this process were performed on the compound Yb6MoO12−δ using TG-DTA, XPS, EDX, and X-ray powder diffraction (XRPD) measurements. In contrast to the rhombohedral phase Yb6MoO12, which does not show any ionic conductivity, the cubic bixbyite structured compound can be classified as a promising ionic conductor. Electrochemical impedance spectroscopy (EIS) revealed that bulk and grain boundary activation energy determined to be 144.6 kJ mol−1 and 150.4 kJ mol−1, respectively, range in the same regime as the conventional ionic conductor 8-YSZ. Furthermore, the new cubic phase Yb6MoO12−δ displays improved coloristic properties (UV−Vis spectroscopy) with a yellow hue value (CIE-Lab) being enhanced from b* = 26.0 of the rhombohedral to b* = 46.1 for the cubic phase, which is relevant for the field of inorganic pigments.

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INTRODUCTION

high temperature solid state oxide fuel cells (SOFCs) and membrane applications.5,6 Furthermore, they have been demonstrated to be excellent hosts as well as sensitizers for photoluminescence materials.7−11 Most rare earth molybdenum oxides exhibit an intense yellow color due to the characteristic O2p−Mo4d charge transfer transitions of Mo6+ in a fluorite structure. This makes them suitable for use as nontoxic, environmentally benign pigments.12 In addition, some of the systems, such as Y6−xDxMoO12+δ (D = Nd, Pr, Si) have been found to show a remarkable high reflectance in the NIR region of the solar spectrum.13,14 This, along with the excellent

Rare earth molybdenum and tungsten oxides are of great interest and significant importance in solid state and crystal chemistry as they offer a remarkably wide range of diverse compositions and structure types.1,2 Nowadays, remarkably growing attention is being devoted toward the structure− property relationships of these materials and their potential technological applications.3 The compounds with the general formula RE6MO12 (RE = rare earth, M = W, Mo) are characterized by highly symmetric structures exhibiting excellent chemical and thermal stabilities that are connected with some unique physical properties.4 Due to their relatively high proton and mixed proton-ion conductivity under a hydrogen-containing atmosphere at elevated temperature, they are considered to be good candidates for electrolytes of © 2016 American Chemical Society

Received: August 5, 2016 Revised: September 21, 2016 Published: September 22, 2016 7487

DOI: 10.1021/acs.chemmater.6b03252 Chem. Mater. 2016, 28, 7487−7495

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Chemistry of Materials

SC method offers a simple and rapid application of the advanced chemical solution process that enables the fabrication of complex oxides of homogeneous composition at remarkably low temperatures.22 For the preparation of RE6MoO12 (RE = Tm−Lu), the corresponding soluble nitrates and glycine were dissolved in deionized water. The combustion was carried out at 400 °C followed by a calcination of the precursor at 700 °C. For the purpose of performing a detailed study, we analyzed the phase formation of Yb6MoO12 via a standard solid state reaction as well as using the “Yb6MoO12” precursor obtained from the SC reaction at different calcination temperatures. The XRPD patterns of the SC products obtained after calcining for 20 h at 1000, 1400, and 1550 °C are shown in Figure 1.

thermal and chemical stabilities, renders the compounds promising for application in the energy sector as “cool pigments.” All rare earth molybdenum oxides with the composition RE6MoO12 crystallize in the defect fluorite structure type. In detail, symmetry systematically depends on the ratio of the radii RE/Mo.15 With RE = La to Ho, the rare earth molybdenum oxides form cubic defect fluorite structures (Fm3̅m, no. 225), while the rare earth cations Er to Lu and Y, having smaller ionic radii, crystallize in rhombohedral defect fluorite structures (R3̅, no. 148). In this series, Er and Y mark the line adopting both the cubic and the rhombohedral structure. In the case of the known rare earth tungsten oxides RE6WO12, only Y shows both defect fluorite modifications. The larger cations Nd, Eu, and Gd exhibit the cubic phase, while all compounds with the smaller rare earth cations (Ho to Lu) form the rhombohedral structure. There are several additional studies on novel high- and lowtemperature phases related to those of the rare earth tungsten oxides, whereas much less has been reported on the corresponding molybdenum oxides. In 1998, Yoshimura et al. reported a new method for the synthesis of Y6WO12 in a low temperature cubic defect fluorite structure crystallizing in the space group Fm3̅m (no. 225).16 The described metastable cubic phase could be obtained from a precursor prepared by a modified Pechini method. In the next step, the precursor was calcined at temperatures between 600 and 1000 °C. At temperatures above 1000 °C, the metastable cubic phase transformed into the above-mentioned rhombohedral phase crystallizing in the space group R3.̅ A cubic high-temperature modification for the rare earth tungsten oxides RE6WO12 (RE = Y, La−Yb, except Ce, Pm, Eu, Tb, and Tm) was already described in 1967. These compounds crystallize in the cubic bixbyite structure type in the space group Ia3̅ (no. 206) when applying calcination temperatures above 1960 °C.17 The cubic bixbyite structure, which is closely related to the fluorite structure, is of general interest because it affords a high symmetric atomic ordering and materials with improved properties in various applications, e.g., photoluminescence materials or for solid electrolytes.18,19 However, no investigations regarding the properties of the ternary cubic high-temperature phases RE6WO12 (RE = Y, La−Yb except Ce, Pm, Eu, Tb, Tm) have been carried out, most probably due to the lack of economical viability and the difficulties in preparing the materials in the cubic bixbyite structure.17,20 A similar high-temperature study has been carried out on the molybdenum related compounds RE6MoO12.21 Cubic hightemperature phases have been obtained at 1350 °C with the rare earths having large ionic radii (RE = La−Ho, except Ce, Pm). For the phase formation of the cubic Er and Y compounds, the authors claimed the need for an increased temperature of 1500 °C. With the smaller rare earth cations RE = Tm, Yb, and Lu, the cubic high-temperature structure could not be reached even at temperatures of 1525 °C. To date, no cubic bixbyite-structured phase is known for these compounds.

Figure 1. XPRD patterns of the product obtained after calcination of the “Yb6MoO12 precursor” at different temperatures.

As expected, the rhombohedral defect fluorite-type R3̅ structure of Yb6MoO12 already formed at 1000 °C; this phase can also be observed upon calcination at 1400 °C. In the diffraction patterns of the SC product calcined at 1550 °C for 20 h, a mix of reflections assignable to the rhombohedral phase Yb6MoO12 and cubic Yb2O3 were detected. The Rietveld refinement of the powder pattern revealed a composition of 65% rhombohedral Yb6MoO12 and 35% Yb2O3. This is an indication for the starting decomposition of Yb6MoO12 into Yb2O3 and MoO3 at temperatures above 1500 °C, followed by a sublimation of MoO3. Nearly identical results were obtained for the thermal behavior of the product, which was synthesized via the standard solid state reaction. These results evidently demonstrate the synthetic difficulties in preparing a cubic bixbyite-type structured phase of an ytterbium−molybdenum oxide by a classical high-temperature, solid-state route or via the SC synthesis. Moreover, it clearly indicates the importance of a new synthetic access, which will be presented in the following. Starting from the rhombohedral defect fluorite compounds RE6MoO12 (RE = Tm−Lu; Figure 2, top), obtained in a prior step by calcining the “RE6MoO12” precursors from the SC reaction at 1200 °C for 20 h, we were able to prepare rare earth molybdenum phases (RE = Tm−Lu) crystallizing in the cubic space group Ia3̅ (no. 206) as described by the following route: first, a reduction was executed with forming gas (90% N2/10% H2) at 1000 °C (Figure 2, arrow 1). In each case, the color changed for all three derivatives from pale yellow to a blackcolored material. For a comprehensive elucidation, the reduced ytterbium compound was characterized by electron dispersive X-ray spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS), and X-ray powder diffraction (XRPD).

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RESULTS AND DISCUSSION RE6MoO12 (RE = Tm−Lu) can usually be prepared by classical solid-state reactions from stoichiometric mixtures of the corresponding oxides RE2O3 and MoO3.15 In our studies, we have employed an alternative route and used the solution combustion (SC) methodology.22 To the best of our knowledge, this is the first time that this method has been used for the syntheses of rare earth molybdenum oxides. The 7488


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Figure 2. Schematic process of the new reduction−oxidation method. Red atoms indicate the mixed rare earth molybdenum positions, whereas the green and yellow spheres show the molybdenum and rare earth atoms, respectively. In blue, the coordination spheres of the cations at Wyckoff position 8e (Ia3̅, octahedra) and Wyckoff position 18f (R3̅, 7-fold polyhedra) are depicted.

The EDX analysis evidenced the expected Yb/Mo ratio of 6:1 (details in the Supporting Information). According to the XPS data (Figure 3a), the molybdenum species were predominantly present in form of Mo(VI) (with the 3d5/2 component located at a binding energy of 232.8 eV),23 making up 78% of the total Mo. Additionally, a prominent Mo(IV) peak at 229.1 eV is visible in the spectrum (16%).24 A small amount (6%) of Mo(V) can also be seen between these signals, with the 5/2 peak residing at 231.6 eV.25 All chemical states of molybdenum feature a doublet with a spin−orbit splitting of 3.1 eV, as expected.26 A more complex peak shape is found for the Yb 4d region, which can be seen in Figure 3b. In this case, the LS coupling of the Yb(III) oxidation state gives rise to a pattern of five peaks, as described in the literature.27,28 The main peak (a) of the Yb3+ component is located at 185.0 eV, while peak b is shifted by 3.5 eV relative to peak a, with the other peaks exhibiting binding energy differences of 7.8 eV (c), 14.6 eV (d), and 21.0 eV (e). These data, as well as the relative intensity values, are also listed in Table 1. In addition to Yb(III), a doublet corresponding to Yb(II), which does not exhibit any LS coupling, has been fitted to the data. These reduced species make up about 4% of the total ytterbium. A quantification of the Mo 3d, Yb 4d, and O 1s signals for the reduced sample yields a composition of 25 atom % Yb, 9 atom % Mo, and 66 atom % O, which is different from the expected values for Yb6MoO12 (32 atom % Yb, 5 atom % Mo, and 63 atom % O)the Yb/Mo ratio is only 2.7 instead of 6, indicating a pronounced conglomeration of Mo to the surfacenear region of the specimen. However, the lower oxidation states of the metallic constituents indicate the presence of oxygen vacancies in the lattice, resulting in the sum formula of Yb6MoO12−δ. A calculation of the total charge of the cations, taking into account the respective amount of each oxidation state, can be used to determine the oxygen deficiency δ. In particular, this can be calculated to be 0.32.

Figure 3. High-resolution X-ray photoelectron spectra of the Mo 3d (left column) and Yb 4d regions (right column). The top row shows the spectra acquired for the reduced sample (panels a and b), the mid row after oxidation at 450 °C (panels c and d), and the bottom spectra were recorded after oxidation at 700 °C.

Table 1. Yb(III) Peaks (Caused by LS Coupling) for the Reduced and Re-Oxidized Yb6MoO12 Samples reduction at 1000 °C

reoxidation at 450 °C

reoxidation at 700 °C

Yb(III) peak

Ebind rel. to a/eV

relative. Intensity

Ebind rel. to a/eV

relative intensity

Ebind rel. to a/eV

relative intensity

a b c d e

0 3.5 7.8 14.6 21

1 0.32 0.68 0.73 0.16

0 4.9 7.8 14.4 21.3

1 0.05 0.44 0.62 0.04

0 2.9 7.9 14.7 21.2

1 0.65 0.69 0.87 0.15

Assuming the same behavior for the Tm and Lu derivatives, the three reduced black products could thus be denoted with the formula RE6MoO12−δ (RE = Tm−Lu). Rietveld refinements of the obtained powder patterns revealed a cubic bixbyite structure for all three reduced RE6MoO12−δ (RE = Tm−Lu) compounds crystallizing in the space group Ia3̅ (no. 206; Supporting Information). Naturally, these compounds possess great similarities to the bixbyite-structured sesquioxides RE2O3. The comparison of the lattice parameter of Yb6MoO12−δ with that of Yb2O3 revealed values of a = 1036.09(1) pm and a = 1043.22(5) pm,29 respectively, exhibiting a significantly reduced size of the unit cell of Yb6MoO12−δ with a cell volume of 7489


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Chemistry of Materials 1.115(1) nm3. This shrinkage of the unit cell as compared to Yb2O3 is caused by the sum of various effects. To facilitate the comparison of the fully occupied bixbyite structure of Yb2O330 with the new formed Yb6MoO12−δ, the cation to anion ratio of the sesquioxide was formally extended by a factor of 4 from 2:3 to 8:12. The new Yb6MoO12−δ can be characterized by the cation to anion ratio of (8 − x):(12 − δ). The resulting cation vacancies designated as x arise from the formal substitution of two Yb3+ cations against one Mo6+ cation. The content of anion vacancies δ is induced by the partial reduction of the metal cations, caused by the abovementioned reducing process. The decrease of the cell parameter of Yb6MoO12−δ compared to Yb2O3 can be explained on the basis of two effects: first, cation and anion vacancies that lead to a shrinking crystal structure and, second, the partial substitution of Yb3+ (ionic radius: 100.8 pm with CN = 6) by the smaller Mo6+ (ionic radius: 73 pm with CN = 6). The same considerations also apply to the reduced compounds Tm6MoO12−δ and Lu6MoO12−δ containing Tm3+ (ionic radius: 102.0 pm with CN = 6) and Lu3+ (ionic radius: 100.1 pm with CN = 6).31 The next step of our strategy was to reoxidize the previously introduced, reduced cubic intermediates RE6MoO12−δ (RE = Tm−Lu; Figure 2, arrow 2a). According to this, the reduced materials were calcined in air at 700 °C for 10 h. The resulting intense yellow colored products were analyzed by XRPD and, additionally, the ytterbium derivative by XPS and UV−Vis spectroscopy. In Figure 3e, the XPS data of the Mo 3d region of the reoxidized compound Yb6MoO12−δ are displayed. No remnants of Mo4+ can be observed in this case. However, there is a significant portion of Mo5+ left (25%, with the 3d5/2 peak occurring at 232.6 eV), with the remaining molybdenum ions being present as Mo(VI), the 5/2 component of which is located at a binding energy of 233.8 eV. The Yb 4d spectrum f) now resembles that of the reduced compound b) more than the one annealed at a lower temperature, except for the missing Yb2+. The contributions of peaks b and e for the Yb(III) state are once again more prominent, and b is also shifted closer to peak a (2.9 eV apart from the binding energy value of a, which is 184.9 eV). All other peaks again feature the same shifts relative to a: 7.9 eV for c, 14.7 eV for d, and e is shifted by 21.2 eV. The oxygen deficiency of this sample is δ = 0.13, which is significantly lower than for the previous samples, confirming that oxygen from the air was incorporated into the structure by this oxidizing step. The atomic concentrations in this oxidized sample (Yb, 27 atom %; Mo, 9 atom %; O, 64 atom %) again show an enrichment of the surface-near regions by molybdenum as compared to the bulk levels (EDX measurements), with an Yb/ Mo ratio of 3.1. This ratio, however, is larger than for the other samples, indicating that the oxidation is able to undo the surface segregation of Mo to a certain degree. Consequently, the formula can still be denoted as Yb6MoO12−δ, but now with a smaller portion of oxygen vacancies. Again, the structure refinement of the reoxidized compound Yb6MoO12−δ revealed the structural analogy to Yb2O3. Interestingly, the cell parameter of Yb6MoO12−δ possessing a value of 1045.76(1) pm is slightly increased in comparison with Yb2O3 (a = 1043.60(8) pm). This is due to an increasing oxygen content by the oxidation of the corresponding metal cations leading to a stronger anion−anion repulsion causing an enlargement of the cell parameter from a = 1036.09(1) to a = 1045.76(1) pm.

However, when comparing the reoxidized phase Yb6MoO12−δ with the powder pattern of the isostructural cubic high-temperature phase of Yb6WO12 described by Foex,17 only differences in the intensities of the reflections can be identified. The cell parameter of Yb6MoO12−δ (a = 1045.76(1) pm) is similar to that of Yb6WO12 (a = 1046 pm). The ionic radii of molybdenum(VI) and (V) with values of 73 and 75 pm, respectively, and tungsten(VI) with a radius of 74 pm are nearly identical (at CN = 6). This explains the similarity of the powder patterns and, consequently, the cell parameters and structures. Therefore, the crystal data of Yb6WO12 were used as starting parameters for the Rietveld structure refinement. At the Wyckoff positions 8a and 24d, the Yb/Mo ratio was set to 0.75/0.125 and a content of 12.5% vacancies. At the Wyckoff position 48e, a nearly full oxygen occupation of 98(2)% was found by the Rietveld structure refinement, thus confirming the XPS results of a residual amount of Mo(V). The average Yb/ Mo−O distance of 226.9 pm is comparable to the cation− oxygen distances in Yb6WO12 (225.8 pm; see Supporting Information). In Figure 4, the coordination spheres of the cations at the Wyckoff position 8a (Yb1/Mo1) and 24d (Yb2/Mo2) are

Figure 4. Distorted Yb2/Mo2O6 octahedron (center) coordinated by two corner-shared and two edge-shared Yb1/Mo1O6 octahedra.

presented. The Yb2/Mo2−O1 polyhedron marking the center of Figure 4 is coordinated by two edge-sharing and two cornersharing Yb1/Mo1−O1 octahedra. Whereas the cations at the Wyckoff position 8a have all equal oxygen distances of 212.5(13) pm, the Yb2/Mo2−O1 polyhedron shows three different bond lengths. While the Yb2/Mo2−O1 distance to both corner-sharing Yb1/Mo1−O1 octahedra is 216.4(13) pm, the Yb2/Mo2−O1 bond lengths to the edge-sharing octahedra are 247.5(12) and 231.0(12) pm. An identical situation with respect to the bond lengths was observed for Yb6WO12. In the sesquioxide Yb2O3, the variation of the bond length is significantly smaller. The same structural analysis was performed with the corresponding compounds Tm6MoO12−δ and Lu6MoO12−δ. The obtained data from the Rietveld refinements are listed in detail in the Supporting Information. Due to the lanthanide contraction, the lattice parameters of the three new rare earth molybdenum oxides RE6MoO12−δ (RE = Tm Yb, Lu) with cubic bixbyite structures decreased as expected in the order of the ionic radii of the rare earth cations. Upon heating up the reoxidized rare earth molybdenum oxides RE6MoO12−δ (RE = Tm Yb, Lu) to 1200 °C, a back transformation to the rhombohedral phase occurs (Figure 2, arrow 3). 7490


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Chemistry of Materials To get more detailed insight into the phase evolution during the oxidation process, a TG-DTA study on the reduced compound Yb6MoO12−δ was carried out under an ambient atmosphere (Figure 5 top). The data clearly show an initial

Table 2. Crystallographic Data of the Reduced, Reoxidized (450 °C), and Reoxidized (700 °C) Yb6MoO12−δ Phase from the Rietveld Refinement of the Powder Diffraction Data (See Supporting Information for More Detail) empirical formula crystal system space group a, pm V, nm3

reduced

reox. 450 °C

reox. 700 °C

1036.09(1) 1.1122(1)

Yb6MoO12−δ cubic Ia3̅ (no. 206) 1046.821(4) 1.14714 (2)

1045.758(4) 1.1437(1)

The ytterbium derivative was complementary analyzed by XPS analyses. Upon oxidation of the reduced Yb6MoO12−δ sample at 450 °C in the air, the Mo 3d region separation of the experimental data (displayed by black crosses in Figure 3c) is strongly reduced, which is caused by the presence of a larger amount of Mo(V). The 3d3/2 peak is located right between the Mo(VI) 3d5/2 and 3d3/2 signals; the 3d5/2 component of this state also causes the Mo(IV) state to not be as distinctly separated from the rest of the peaks as for the reduced specimen in a). The Mo species are distributed into 57% Mo(VI), found at a binding energy of 233.1 eV, 22% Mo(V) (231.5 eV), and 21% Mo(IV) (229.2 eV), suggesting that the oxidation at such a low temperature as 450 °C actually causes the Mo to reduce more. What seems to be counterintuitive can be explained by examination of the Yb 4d spectrum in Figure 3d). While all five peaks expected for Yb(III) can be found in the spectrum, albeit with different intensity ratios, no Yb(II) signals are discernible. Before reoxidation, this lower oxidation state made up slightly more than 4% of the total Yb, which, because of the 6:1 Yb/Mo ratio in the crystal structure, is equivalent to 26% relative to Mo. Comparing the spectra of the reoxidized and the reduced compound, the amounts of additional Mo(V) and Mo(IV) species in the oxidized variant are 16% and 5%, respectively. If these “new” states are generated by reduction of Mo(VI), it is obvious that Mo(VI) is converted partially by a one-electron reduction step to Mo(V) and by a further reduction to Mo(IV). Consequently, the molar fraction of the redox partner equals 26% relative to the total Mo species, which corresponds exactly to the amount of Yb(II) that was oxidized upon treatment in the air. This proves that, at low temperatures such as 450 °C, the gaseous environment actually plays no role in the oxidation of Yb(II) but rather is a case of an internal redox process between Mo(VI) and Yb(II), which would be consistent with the respective standard potentials (E0(Mo(VI)/Mo(V)) = −0.5 V; E0(Yb(III)/Yb(II)) = −1.05 V, relative to SHE).32,33 In the Yb 4d spectrum (Figure 3d)), the appearance of the five peaks for the Yb(III) differs drastically from the reduced state spectrum as depicted in b). The single peaks are not as pronounced as for the reduced specimen, with the major contributions now coming from peaks a, c, and d (although the contribution of a is diminished compared to that of b). Peaks b and e are showing a reduced intensity, and b is shifted by 4.9 eV from peak a, which is located at 184.5 eV (binding energy). The other peak shifts remain approximately constant, with c being 7.8 eV away from a, d shifted by 14.4 eV, and e ranging at 21.3 eV to the higher binding energy side of a. The quantification of the three elemental constituents (Yb, 23 atom %; Mo, 10 atom %; O, 67 atom %) confirms the segregation of Mo as well (Yb/ Mo = 2.3). Due to the redox reaction between Yb(II) and Mo(VI), no additional oxygen from the air is incorporated to

Figure 5. Simultaneous thermal analysis (STA) of the cubic reduced compound Yb6MoO12−δ obtained with a heating rate of 10 °C min−1. Isothermic steps of 1 h each at 500 and 1200 °C allows the transformation to the oxidized cubic phase and subsequently the back transformation to the rhombohedral structure. The last hour holding time at 1550 °C shows the beginning of the mass loss. The jumps at 500 °C are assigned to instrumental artifacts.

exothermic reaction with a mass gain at around 400 °C (Figure 5, arrow “a” corresponding with Figure 2, arrow 2b), a second exothermic phase transition with only a slight mass increase at around 650 °C (Figure 5, arrow “b” corresponding with Figure 2, arrow 2c), and a third one (Figure 5, arrow “c”) at around 1200 °C corresponding to arrow 3 in Figure 2. The second exothermic reaction (650 °C) can be assigned to the formation of the reoxidized phase Yb6MoO12−δ described above (Figure 2, arrow 2b); the last transition (1200 °C, Figure 2, arrow 3) was allocated to the phase transformation into the corresponding oxidized rhombohedral starting product Yb6 MoO12, as confirmed by XRPD. On the basis of these findings of an additional unknown phase formation at around 400 °C, we calcined samples of the reduced compounds RE6MoO12−δ (RE = Tm−Lu) in the air at 450 °C for 1 h (Figure 2, arrow 2b). All three formed products were of a similar, green−yellow color. The structural characterization via XRPD indicated nearly the same cell parameter of the products obtained at 700 °C and crystallization in the identical space group Ia3̅ (no. 206; Table 2). 7491


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indicated by an abrupt increase of the reflection starting at 389 nm. The CIE-Lab color coordinates (Table 3) confirmed the

the lattice, resulting in unchanged oxygen deficiency in comparison with the reduced specimen (at 0.32). The compounds RE6MoO12−δ (RE = Tm−Lu) obtained by calcining at 450 °C, however, when subsequently heated to 700 °C (Figure 2, arrow 2c), were further oxidized, resulting in the disappearance of the Mo(IV) species and the formation of the intense yellow products RE6MoO12−δ (RE = Tm−Lu) described above. Upon heating up to 1200 °C, the back transformation to the rhombohedral phase occurs (Figure 2, arrow 3). The evolution of the cell parameters for all three rare earth molybdenum oxides RE6MoO12−δ (RE = Tm−Lu) during the process of the syntheses, which comprises reduction, reoxidation at 450 °C, and reoxidation at 700 °C, is graphically summarized in Figure 6. While the reduced compounds have

Table 3. CIE Lab Values of the Rhombohedral Phase Compared with the Reoxidized (700 °C) New Cubic Phase compound

space group

L

a

b

Yb6MoO12 Yb6MoO12−δ

R3̅ Ia3̅

98.5 95.1

−8.6 −9.5

26.0 46.1

visual observation, namely the pale yellow color of the rhombohedral phase and the intense yellow color of the cubic bixbyite structured material. The yellow hue of the materials is expressed by the b* value, which is significantly increased from 26.0 to 46.1 by the phase transition from the rhombohedral to the cubic structure. Furthermore, both materials exhibit high NIR reflectance, which is significantly increased from 80 to 85% at 1200 nm for the change from the rhombohedral structure to the cubic bixbyite phase, indicating that compositions of these materials could serve as colored “cool pigments”. To gain information on the ionic conductivity of the rare earth molybdenum oxides, frequency dependent EIS measurements (Nyquist plots) were performed for the rhombohedral and the cubic bixbyite structured Yb6MoO12−δ compound. The tests were conducted in the air at temperatures between 873 and 1173 K. The new bixbyite structured Yb6MoO12−δ offers the characteristic semicircles as displayed in Figure 8, whereas

Figure 6. Evolution of the lattice parameters a of RE6MoO12−δ (RE = Tm−Lu) starting from the reduced forms followed by two steps of reoxidation at 450 and 700 °C.

the smallest cell parameters in all three rare earth modifications, the calcined products show an increase of around 8 pm. The difference of the cell parameters for the products reoxidized at 450 and 700 °C is only about 1 pm or lower. In order to characterize the potential abilities of the rare earth molybdenum oxides synthesized here as colored and “cool pigments”, we have studied and compared the optical properties of the rhombohedral compound Yb6MoO12 with the cubic phase Yb6MoO12−δ reoxidized at 700 °C (Figure 7). Both phases show the absorption band of the characteristic O2p− Mo4d charge transfer transition from the UV to around 400 nm,

Figure 8. Nyquist plot (data points) and simulated spectra (continuous lines) using the proposed equivalent circuit fit model for reoxidized Yb6MoO12−δ treated in air at selected temperatures. The lowest frequency of 100 mHz is at the right side and the highest one of 0.1 MHz at the left side of the x-axis. The inset in the upper right corner shows an enlargement of the plots at higher temperatures.

for the rhombohedral Yb6MoO12 no ionic conductivity is measurable. This indicates an enhanced ionic conductivity of the cubic structured material when compared to the rhombohedral one. At all temperatures, two depressed semicircles are apparent: a very large one at high frequencies and a small one at low frequencies with “tailing.” The bulk (b) contribution is usually observed at high frequencies in the

Figure 7. UV−Vis−NIR diffuse reflectance spectra of the rhombohedral Yb6MoO12 (black) and the cubic bixbyite Yb6MoO12−δ phase (red). 7492


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low temperatures, opening up the possibility of accessing and investigating a new family of inorganic functional materials. The new cubic phase Yb6MoO12−δ shows superior coloristic performance, as well as good ionic conductivity, thus holding promise of applications in the field of functional pigments or SOFCs.

Nyquist plot, whereas the grain boundary (gb) and electrode (e) contribution are located at mid and low frequencies, respectively.34,35 The “tailing” of the semicircle at low frequencies is the result of the interaction between the electrode and the sample pellet and is actually the onset of another semicircle, which will appear in the high temperature range. The equivalent circuit model that was used in this study to fit the experimental data is composed of constant phase elements (CPEs) instead of ideal (Debye) capacitors. This is commonly done for polycrystalline samples due to material inhomogeneity, surface defects (e.g., pores), electrode roughness, and ionic transport deviation from Fick’s law. All these parameters usually give rise to a certain degree of frequency dispersion and nonuniformity of the current density. A CPE therefore represents more accurately the capacitive behavior of this kind of cell in the whole studied frequency range, which can also be clearly seen in the depressed semicircles.36−38 With the equivalent circuit model described in refs 39 and 40, it is possible to distinguish the different contributions for the different processes (b, gb, and e). With an increase in temperature, Rb, Rgb, and Re decrease, converging to a final value. The determined b and gb resistances were then converted to electrical conductivity (σ) by considering the thickness and area of the sample (Figure 9). The bulk activation energy for the

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METHODS

Chemicals. As starting materials, stoichiometric amounts of the corresponding nitrates Tm(NO3)3·5H2O, Yb(NO3)3·5H2O, and Lu(NO3)3·6H2O (all from Strem Chemicals 99.9%); ammonium molybdate (NH4)6Mo7O24 ·4H2O (MERCK > 99.0%); and citric acid and glycine (Sigma-Aldrich 99 and 98%, respectively) were used. Syntheses. For all three rare earth molybdenum oxides RE6MoO12−δ (RE = Tm−Lu), the synthetic procedure is identical and is described exemplarily for Yb6MoO12−δ in the following. Ammonium molybdate (30.74 mg) was dissolved in 5 mL of deionized water in a Pyrex beaker (250 mL). Afterward, glycine (200.74 mg) was added as fuel in a molar ratio of metal cations/fuel = 1:2.5. To prevent precipitation, the pH value of the mixture was adjusted to pH = 1 using citric acid (about 5 g). Subsequently, Yb(NO3)3·5H2O (469.26 mg) was added and stirred until a clear solution formed. Afterward, the combustion mixture was placed in a chamber furnace (Nabertherm, HTC 03/15) preheated to 400 °C. After 4 min, the reaction took place. A brownish, highly foamed powder was obtained as the product, which was subsequently transferred in a corundum crucible (5 × 1 cm) and placed again in the chamber furnace. A last oxidation step at 1200 °C for 10 h afforded a pale yellow highly crystalline powder. The subsequent reduction process was carried out by placing the same corundum crucible, charged with the rhombohedral phase Yb6MoO12, into a tube furnace (Carbolite, STF 16/180) operated by purging forming gas (10% H2 and 90% N2, flow rate 90 mL/min). The temperature was raised at 10 °C min−1 to 1000 °C, and after a reaction time of 20 h, the furnace was cooled down by switching off the heat. The resultant reduced product exhibited a grayish to black color showing a good stability in air. In the next step, the reduced cubic Yb6MoO12−δ was put in a corundum crucible and placed into a chamber furnace with a heating ramp of 10 °C min−1 at either 700 (Figure 2a) or 450 °C (Figure 2b) for oxidation of the sample. After 10 h at 700 °C or 1 h at 450 °C, the reaction mixture was quenched to room temperature affording intense yellow or green, respectively, highly crystalline powders. Elementary Analyses (EDX + SEM). Energy dispersive X-ray spectroscopy (EDX) was realized by using a FEI-Quanta 200 3D scanning electron microscope (SEM). An excitation energy of 25 keV was chosen as accelerating voltage. The L-shells of Mo and Yb were used for EDX analysis. EDX measurements, as well as recording of the SEM pictures, was carried out in high vacuum mode (pressure p ≈ 4 × 10−6 mbar). To prevent shifts in the picture caused by charging effects, the sample was coated with a thin gold layer (Agar Sputter Coater) after collecting the EDX spectra. TG-DTA Analysis. Simultaneous thermal analysis was carried out with the instrument STA 449 F5 Jupiter from Netzsch GmbH. Measurements were performed in the air (gas flow 60 mL/min) in the temperature range from RT to 1550 °C with a heating rate of 10 C° min−1. XPS Analysis. X-ray photoelectron spectroscopy (XPS) was carried out using a Thermo Scientific MultiLab 2000 spectrometer with a base pressure in the low 10−10 mbar range. The instrument is equipped with a monochromated Al−Kα X-ray source, an Alpha 110 hemispherical sector analyzer, as well as a flood gun for charge compensation, providing electrons with a kinetic energy of 6 eV. The energy axis shift was calibrated relative to the C−C component of the C 1s peak (set to 284.8 eV). Electrochemical Impedance Spectroscopy (EIS). The operando/in situ impedance cell consisted of an outer quartz tube with two inner quartz tubes to which the sample and the electrodes were attached. Heating was provided by a tubular Linn furnace and

Figure 9. Arrhenius plots with analysis of the bulk and grain boundary contribution of Yb6MoO12−δ treated in air between 773 and 1173 K.

new cubic bixbyite structured Yb6MoO12−δ is 144.6 kJ mol−1, thus only ∼30 kJ mol−1 higher than the activation energy for bulk anion conductivity of the common ionic conductor 8-YSZ (∼110 kJ mol−1).39,40 Similarly, the grain boundary activation energy with a value of 150.4 kJ mol−1 lies in the same range as the values reported in the literature for 8-YSZ (between 110− 163 kJ mol−1, depending on the used gas;35,37,39,41−44 additional details in the Supporting Information)

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CONCLUSION In the present work, we reported the first successful syntheses of three new rare earth molybdenum oxides RE6MoO12−δ (RE = Tm−Lu) crystallizing in the cubic bixbyite structure (Ia3̅, no. 206). In contrast to all other methods, the new route enables the syntheses of the highly symmetric structures at remarkable 7493



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controlled by a thermocouple (K-element) located in the reactor about 5 mm downstream of the sample and a Micromega PID temperature controller. The impedance was measured by an IM6e impedance spectrometer (Zahner Messsysteme), which provided data on the impedance and the phase angle of the current as a function of voltage. The powder samples were pressed into pellets with a pressure of 1.5 torr (5 mm diameter, ∼0.1 mm thick) and placed between two circular Pt electrodes forming a plate capacitor in mechanically enforced contact with the sample pellet. Prior to every EIS experiment, the pellets were sintered in the air at 1273 K for 3 h. A typical Nyquist plot was obtained isothermally at a given temperature in the frequency range between 100 mHz and 0.1 MHz at an amplitude of 20 mV of the superimposed sinusoidal modulation voltage signal at an overall DC potential of 0 V. The real and imaginary parts of the impedance were first measured from 1 kHz up to 0.1 MHz (within 14 s) and then from 0.1 MHz down to 100 mHz (within 4 min 24 s; total measuring time: 4 min 38 s) to check for changes in the system during EIS. A 1 h interval was allowed for thermal stabilization after each temperature change. Curve fitting and resistance calculation were executed with the Zahner Thales Box and with the described equivalent circuit model.39,40 Arrhenius analysis was performed to determine the activation energies for the different processes with the resistance values obtained from the equivalent circuit model fit of the Nyquist plots. This was done by taking the reciprocal of the calculated resistance value and plotting ln(conductivity) vs the reciprocal of the reaction temperature. The activation energies were calculated with the formula EA = −Rk (R, ideal gas constant −8.3145 J/(mol K); k, slope of the line). UV−Vis−NIR Analysis. The compounds were poured in a powder cell holder, and diffuse reflectance spectra were recorded in the range between 250 and 2500 nm by using an Agilent Cary 5000 UV−Vis spectrometer equipped with an integrating sphere, DRA-2500. The CIE-Lab color coordinates were registered from 300 to 800 nm with the coupled CARY WINUV Color Software (Version 4.20(468)) using BaSO4 as a reflecting standard, D65 standard illuminate, a 10° complementary observer, a scan rate of 600 nm min−1, and a data interval of 1 nm. X-ray Powder Diffraction Analysis. Data for all new compounds were obtained by measuring in transmission geometry using a STOE Stadi P powder diffractometer with Ge(111)-monochromatized Mo Kα1 (λ = 70.93 pm) radiation. As a detector, the silicon microstrip solid state detector Mythen 1K was used. The data were obtained by measuring in 2θ steps of 0.005° from 2 to 70°. The Rietveld refinement was done by using the program TOPAS 4.2.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.6b03252.

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Article

EDX results, detailed information on the Rietveld refinement, and additional EIS values (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge funding of this research by the Austrian Research Promotion Agency (FFG) and by Durst Phototechnik Digital Technology GmbH, Lienz, Austria. 7494


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