Effect of preparation conditions on the polymorphism and transport

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Effect of preparation conditions on the polymorphism and transport properties of La6-xMoO12-# (0Sm>Gd.13 Hence, the lan-

thanum tungstates with higher ionic conductivity have attracted more attention. These materials crystallize in a cubic fluorite-related superstructure, with 43 space group.19 Furthermore, independent investigations showed that the lanthanum tungstate (LWO) with La/W=6 ratio is not a single phase. 20 A solid solution was found for compositions with La/W ratio between 5.3 and 5.7, and outside these compositional range phase segregations were detected. In the last few years, different doping strategies on both Laand W-sites of LWO have been investigated in order to improve the conducting properties of these materials. Particularly interesting are those with Mo-doping on the Wsite, with the goal of increasing the electronic conductivity due to the higher reducibility of Mo6+ with respect to W6+. From the results of literature21-23, it is clear that Mo-doping increases the hydrogen fluxes compared to undoped lanthanum tungstates, with the best result obtained by Chen et al.22 for La5.6W0.6Mo0.4O11.4-. The replacement of tungsten by molybdenum in La5.5W1xMoxO12- yields materials with different crystal structure: cubic for x≤0.4 and rhombohedral for 0.4≤x≤0.8.24 The composition with x=1, La5.5MoO11.25, was a mixture of phases. On the other hand, La6MoO12- was initially reported to be cubic in 1972 and isostructural to La6WO12. Later, Cros et al. 25 studied the La2O3-MoO3 phase diagram and the compound La6MoO12 (LMO) was reported to be rhombohedral. More recently, a cubic fluorite La5.5MoO11.25 compound was prepared from mechanically activated oxide mixture at 900 ºC. However, this material turned out to be metastable transforming to rhombohedral at higher firing

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temperatures. The electrical properties of the rhombohedral polymorph were reported in this previous work, however, the cubic polymorph was not investigated.26 This work presents a systematic study of the La6-xMoO12- (0x0.8) series. The evolution of the crystalline phase has been analyzed as a function of the synthetic conditions. In particular, the temperature and the cooling rate have a significant influence on the stabilization of different polymorphic structures. Laboratory X-ray powder diffraction (XRPD), transmission electron microscopy (TEM) and Xray photoelectron spectroscopy (XPS) have been used to investigate the crystal structure. The phase stability of the different polymorphs was evaluated after annealing the samples at different temperatures and atmospheres. Finally, electrochemical impedance spectroscopy under dry and wet gases and as function of the oxygen partial pressure allowed establishing the nature of the conductivity. 2. Experimental 2.1. Synthesis Materials with composition La6-xMoO12- (x=0, 0.4, 0.6 and 0.8) were prepared by the freeze-drying precursor method, following a procedure similar to that reported elsewhere for La6-xWO12- series.27 Starting materials used as reagents were: La2O3 (99.99%, Aldrich) and MoO3 (99.9%, Aldrich). Precursor solutions were prepared by dissolving separately La2O3 and MoO3 in diluted nitric acid and ammonia, respectively. Ethylenediamminetetraacetic acid solution (EDTA, 99.99%, Aldrich) was added as a complexing agent in a EDTA:metal molar ratio of 1:1. The different cation solutions were mixed in stoichiometric amounts under stirring, obtaining transparent solutions with pH=7 and concentration 0.1 mol L-1. The solutions were frozen into liquid nitrogen and then freeze-dried for 2 days in a Scanvac Coolsafe freeze-dryer. The dried precursor powders were rapidly fired at 300 ºC to prevent rehydration and afterwards at 800 ºC for 1 h to remove the organic compounds. In order to optimize the sintering temperature, the powders were compacted into pellets. Different pellets with 10 and 1 mm of diameter and thickness, respectively, were prepared to obtain approximately 2.5 g of final product. The pellets were placed onto platinum plates and sintered between 1000 and 1500 ºC for 1 h and then cooled at different rates, 0.5, 2.5, 5, 50 ºC min-1 and air quenching (the samples were directly removed from the furnace at 1500 ºC). Samples obtained at low cooling rates were prepared in a conventional furnace, while those obtained at fast heating rate were prepared in an elevator furnace. Finally, the pellets were ground in an agate mortar for further characterization. The synthesis process was repeated several times for the different compositions of the series, obtaining reproducible results. 2.2. Structural and thermal characterization All samples were analyzed by laboratory X-ray powder diffraction (XRPD). The patterns were collected on a PANalytical X´Pert Pro MPD diffractometer equipped with monochromatic CuKα1 radiation and Anton Paar HTK1200 camera for high temperature measurements. The acquisition time was approximately of 4 h over the 10 to 120º (2) angular range, with 0.017º step size. The phase identifica-

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tion and analysis were performed using the X'Pert HighScore Plus and GSAS programs.28,29 The crystal structure of the materials was also investigated by selected area electron diffraction (SAED) and high resolution transmission electron microscopy (HRTEM). The specimens were prepared by grinding the powders in acetone and a few drops of the resulting suspension were deposited onto a holey carbon-coated copper grid. The measurements were carried out on a Jeol JEM 2100 electron microscope operating at 200 kV. TEM images were analyzed using Digital Micrograph™ software.30 The microstructure of the ceramics was observed by scanning electron microscopy (SEM) (Jeol JSM-6490LV), combined with energy dispersive spectroscopy (EDS) (Oxford Instruments). The grain size of the dense pellets was estimated from the SEM micrographs, using the linear intercept method.31 X-ray photoelectron spectroscopy (XPS) were recorded on a Physical Electronics PHI-5700 using a MgKα source (1253.6 eV) (model 04–548 Dual Anode X-ray Source). A Multipak-V9.3 software package was used for data analysis. A Shirley-type background was subtracted from the signals and the recorded spectra were always fitted using Gaussian –Lorentzian curves in order to determine more accurately the binding energy of the different element core levels.32 The XPS spectra were collected on powder samples, which were obtained by grinding the pellets in an agate mortar before the analysis to minimize a possible surface carbonation. Thermogravimetric and Differential Thermal analysis (TGDTA) curves were recorded in a SDT-Q600 equipment (TA Instruments) at a heating/cooling rate of 5 ºC·min-1 under wet air (~2% H2O) from RT to 950 ºC. 2.3. Electrical characterization Impedance spectra were acquired using a Solartron 1260 FRA in dry (using a zeolite desiccant) and wet (bubbling through H2O) gases (N2, O2 and 5% H2-Ar) in the 0.01 Hz to 1 MHz frequency range with an ac perturbation of 100 mV. The spectra were recorded on cooling from 800 to 200 °C with a dwell time of 30 min at each measurement temperature. The data were simulated by equivalent circuit models using the ZView program.33 Pt current collectors were made by coating the pellets surfaces with Pt-ink (METALOR® 6082) and then fired at 800 ºC for 1 h in air. Conductivity data were also acquired as a function of the oxygen partial pressure pO2 to identify the different charge carriers. The samples were reduced in a closed tube furnace under dry 5% H2-Ar for 12 h at 800 °C. After that, the flushing was switched off and the oxygen partial pressure was continuously monitored with an YSZ sensor. The impedance spectra were acquired at intervals of 30 min during the reoxidation process. Each isothermal measurement took more than two days to complete. 3. Results and discussion 3.1. Phase formation XRPD patterns for La5.4MoO11.1 samples calcined at different temperatures are shown in Fig. 1a. All samples were heated in air between 800 and 1500 ºC for 1 h and then cooled down to room temperature at 5 ºC min-1. The sam-

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ples fired at 800 ºC can be indexed as a cubic fluorite-type structure, nevertheless, broad diffraction peaks are observed due to the nanocrystalline nature of the powders with an average crystal size of 10 nm, estimated using the Scherrer´s equation. Different crystalline compounds are identified between 1000 and 1300 ºC, including the rhombohedral La6MoO12-type (PDF No 00-34-1220), hexagonal La4MoO9-type (PDF No 00-023-1144) and La2O3 (PDF No 01-073-2141). Above 1400 ºC, all samples are a mixture of La6MoO12 and La4MoO9-type phases, and the content of the second phase decreases with increasing temperature. It should be noted that these results are identical to that reported by Amsif et al.24 Hence, no single phase materials could be achieved under these synthetic conditions. □

(a)

∆ La2O3 □ La6MoO12 ○ La4MoO9

□

□ ○ ○ □ ○

○

□□

1500 ºC

○ □○

1400 ºC

○ ○

1200 ºC

○ □○

1100 ºC

□ ○ ○

□1000 ºC

○ ○ ∆ ○ ∆

□○

○ ∆

∆

○

□

○ ○□ ∆ □ ○∆

800 ºC

20

30

40

50

60

2/º (b)

○ ○

○ cubic □ La6MoO12‐type ∆ La4MoO9‐type

○

quenching

□□

50 ºC min

□□

5 ºC min

∆ ∆

2.5 ºC min

∆ ∆

0.5 ºC min

□ □

□

-1

simplicity, these samples are hereafter labelled as La6xMo_Y, where Y represents the cooling rate. As can be observed in Fig. 1b, the quenched sample, La5.4Mo_Q, crystallizes with a simple cubic fluorite-type structure and isostructural to that reported for rare-earth molybdates (Ln6xMoO12, Ln=Sm, Gd, Dy, Ho and Er) with smaller Ln ionic radius.26 At a cooling rate of 50 ºC min-1, the rhombohedral La6MoO12-type polymorph is stabilized, which is similar to that previously reported by Amsif et al.24 However, under our synthetic conditions La5.4Mo_50 is a pure compound, while the same composition reported by Amsif et al. was a mixture of phases as a consequence of the different cooling rate used.24 For slower cooling rates, 5 and 2.5 ºC min-1 a mixture of two different polymorphs, La6MoO12 and La4MoO9-type phases, are identified. Finally, the slowest cooling rate, 0.5 ºCmin-1, leads to a single compound with a similar pattern to that of La4MoO9 (PDF No 00-023-1144). The same behavior was observed for all compositions of La6-xMoO12- series (Fig. S1, supporting information). Hence, it is very clear that the cooling rate has significant influence on the stabilization of different polymorphic phases in lanthanum molybdates. This stabilization may be attributed to the ordering of oxygen vacancies, similarly to related tungstates,34 as no reversible phase transition was detected by DTA and HT-XRPD analysis (see below). In addition, the cubic fluorite polymorph is only obtained when the samples are cooled down quickly, otherwise, the rhombohedral symmetry is stabilized. It is also important to note that any phase can be transformed into the other two when is heated again to 1500 ºC and then cooled at its proper cooling rate. On the other hand, La6MoO12 is not a single compound; La2O3 segregation is observed in all the synthetic conditions used (Figure S1). In addition, La5.6MoO11.4 pellets shows a little fraction of La2O3 and suffers a complete breakdown after exposure to air due to hydration and carbonation. Hence, in this work, only the single phase materials with composition La5.4Mo_Q, La5.4Mo_50 and La5.4Mo_0.5 will be further characterized.

□ □

□ ∆ ∆

∆

∆ □

-1

-1

∆ ∆

∆ 20

30

40

-1

50

60

2/º Figure 1. a) XRPD patterns for La5.4MoO11.1 heated at different temperatures and cooled at 5 ºC min-1 at RT; and (b) heated at 1500 ºC for 1 h and cooled down to RT at different rates.

The influence of the cooling rate on the phase formation was also investigated. For this purpose, the samples were heated at 1500 ºC for 1 h and then cooled down at different rates: 0.5, 2.5, 5.0, 50 ºC min-1 and quenching in air. For

3.2. Structural analysis by XPRD. The polymorph with cubic symmetry, La5.4Mo_Q, was analyzed by the Rietveld method using a fluorite-type structure with Fm3m space group as starting structural model. The usual parameters, such as histogram scale factors, background and peak shape coefficients were fitted. Lanthanum and molybdenum cations were set to occupy the same crystallographic position and their contents were fixed to the nominal stoichiometry. For different atoms located on the same crystallographic site, their corresponding isotropic thermal factors were constrained to be equal. The cell parameters are listed in Table 1 and Figure 2a shows the corresponding Rietveld refinement plot. The agreement factors are relatively good, Rwp=6.9% and RF=3.2%, confirming the accuracy of the proposed structural model. It is also important to note that there are not superstructure reflections, unlike lanthanum tungstates, La5.4WO11.1, where a 222 cubic superstructure is observed.35

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(a)

(b)

La5.4 5 Mo_Q

(c)

La5.44Mo_50

* 25

30

35

40

45

35

55

75

95

*

La5..4Mo_0.5

* *

*

50

25

15

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115 15

35

30

55

2/º

35

75

40

95

45

25

50 0

115 15

2/º

35

55

30

2/º

35

75 5

* 40

45

95

50 0

115

Figure 2. Rietvveld plot for (a)) cubic La5.4Moo_Q and Le Baiil plots for rhom mbohedral (b) L La5.4Mo_50 and (c) La5.4Mo_0..5. [Observed ddata (open circles), ccalculated patteern (continuous line), and differrence curve (boottom)]. Asteriskks show the supperstructure reflections.

Table 1. Bassic unit cell parameters foor the as-preppared samples deterrmined by XRP PD data. composition

V/Z(Å Å3)

a(Å)

c(Å)

Rwp (%) RF (%)

as-prepared

5.6676(1)

-

45.51(1)

6.87

3.15

800ºC / 48h (air)

5.6729(2)

-

45.644(1)

8.75

2.02

800ºC / 48h (5%H H2-Ar)

5.6671(2)

-

45.500(1)

7.53

2.15

La5.4Mo_Q

La5.4Mo_50 as prepared

3.9914(1)

9.8715(2) 45.400(1)

10.35

-

800ºC / 48h (air)

3.9934(1)

9.8747(4) 45.466(1)

10.17

-

800ºC / 48h (5%H H2-Ar)

3.9922(1)

9.8814(4) 45.466(1)

8.48

-

as prepared

4.0996(1)

9.5204(2) 46.199(1)

9.39

-

800 ºC / 48h (air)

4.1089(2)

9.4933(6) 46.277(2)

10.17

-

800 ºC / 48h (5%H H2-Ar)

4.1083(2)

9.5007(6) 46.299(2)

8.71

-

proocess (Fig. 3 aand 4). The SA AED patterns of the quenchhed sam mple, La5.4Moo_Q, can be indexed as a simple cubbic fluuorite structurre, in good aagreement witth the unit cell c parrameters deterrmined by XR RPD (Fig. 3a). Nevertheless,, in som me zone axes the SAEDs shhow faint sateellite reflectionns, whhich are ascribbed to structuure modulatioon due to minnor varriations in thee oxygen sublattice (Fig. 3bb). The HRTE EM im mages further confirm c the ccubic structuree of this sampple (Fiig. 3c).

La5.4Mo_0.5

s In the case off La5.4Mo_50 aand La5.4Mo_00.5, no initial structural models are available in the literatuure. Thereforee, the M and Laa5.4Mo_0.5 arre inXRPD patterrns of La5.4Mo_50 dexed using thhe Dicvol soft ftware to determ mine the basicc unit cell from the m most intense ddiffraction peaaks and to carrry out the structural analysis by electron diffrraction. Both polymorphs are inndexed in hexxagonal cells. The obtainedd cell parameters weere used for thhe Le Bail refiinement; the reesults are given in T Table 1 and the t corresponding plots aree displayed in Figg. 2b and 2c. As can be observed, o the most intense reflecttions are adeqquately fitted, confirming c thaat the basic unit celll has been corrrected determ mined; howeveer, the low intensity peaks are nnot indexed. T These small ppeaks msif et al.244 in were previoously observved by Am La5.5W0.4Mo0..6O12- and theey were attribuuted to supersstructure reflectionns as confirmeed below by eelectron diffraaction analysis. 3.3. Structural Analysis byy TEM. ED) and the ccorreSelected areaa electron difffraction (SAE sponding highh-resolution T TEM images confirm the presence of differrent polymorpphs as a functtion of the coooling

Figgure 3. (a) andd (b) SAED pattterns and (c) H HRTEM imagee of cubbic La5.4Mo_Q in different zzone axes. Thee arrows indiccate difffuse scattering due to structuure modulation.. The SAEDs are inddexed by considdering a cubic flluorite-type struucture.

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Figure 4. (a)-(cc) SAED patterrns and (d) - (e) HTEM image of rhombohedrral La5.4Mo_0.5 revealing a 5a 5ac superstruucture relative to the basic single uniit cell determineed by XRPD daata.

For the sam mple cooled down at the t slowest rate, La5.4Mo_0.5, the structure iis far more coomplex. Some zone axes reveal thhe presence off a superstructture along diffferent directions (Fig 4a, 4b and 4c). In additioon, the SAED D patterns show soome diffuse sscattering alonng those partiicular directions, whhich can be induced i by disordering or lessordering of soome atomic arrrangements liikely oxygen vvariations. The SA AEDs patternss can be indeexed considerring a 5a5ac supperstructure frrom the basiic cell param meters determine byy XRPD (a=44.10 Å and c= 9.52 Å). The HRTEM imaages show clearly a superrstructure andd the interatomic ddistances are in good aggreement withh the XRPD data and a the propoosed 5a5ac supercell (Fig. 4d and 4e). The sample cooled c down at a 50 ºCmin-1, La5.4Mo_50,, also exhibits superrstructure refleections, whichh are reminisceent of those reportedd by Amsif et e al.24 In thiis case a 7a 7ac superstructuree is observed ((Fig. S2). Thee HRTEM imaage in the [001] zones-axis showss clearly this ssuperstructure (Fig. S2d and S2e).. N Note that ann accurate struuctural determ mination by uusing powder diffraaction techniquues seems to bbe complicatedd due to the large uunit cell volume of these ccompounds, aabove 6000 Å3. hermal characcterization. 3.4. Phase staability and th Bearing in mind m the polym morphism of thhese materialss, the stability studies are requirred to ensuree that all sam mples u maintain theirr structure at iintermediate ttemperatures under both oxidizinng and reducinng atmospherres. Thereforee, the stability of thhe samples waas evaluated bby XRPD afteer an-

neaaling the pow wders between 800 and 10000 ºC for 48 h in airr and 5%H2-Arr (Fig. S3). At 800 ºC in staatic air and 5% %H2-Ar, all saamples retain the t oriiginal structuree without any evidence of secondary phasses (Fiig. S3a and S S3b). In addittion, no variaation in the cell c parrameters was detected witth respect to the as-preparred com mpounds, eveen under reduucing atmosphhere, confirmiing thee stability of thhe samples (Table 1). At higher annnealing tempperatures, 10000 ºC, whhile Laa5.4Mo_50 andd La5.4Mo_0.55 samples doo not show any a struuctural transfo formation, La55.4Mo_Q suffeers a partial deegraddation after 488 h, leading to a mixture of cubic and rhombohhedral polymoorphs (Fig. S33c). Hence, it can be concluuded that the diffeerent polymorpphs of La5.4M MoO12- are struucs for potential appliccaturrally stable, annd therefore suitable tioons at temperattures below 8000 ºC. High temperatuure XRPD stuudies were performed in air mosphere andd no phase ttransformationn was observved atm bettween RT andd 1000 ºC (Fig. S4). The eevolution of the t unit cell volumee for La5.4Mo__0.5 and La5.4M Mo_50 follow ws a neaarly linear deependence witth the temperrature (Fig. S5). Hoowever, La5.4Mo_Q show ws an anom malous expaansioon/shrinkage bbetween 200 annd 400 ºC, whhich is reproduuciblle on heating and cooling, and might be attributed to oxyygen-water uuptake/releasee. It is also observed thhat Laa5.4Mo_0.5 exhhibits the largeest unit cell vvolume, althouugh thee cation compoosition is the ssame for all saamples. This fa fact is rrelated to the ddifferent crysttal structure off the materialss, a sim milar behaviouur is observed for - and  polymorphs of Laa2Mo2O9 and L La2W2O9 oxidee ion conductoors. 36-38

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The thermal expansion coefficients, determined by the HTXRPD data, take values of 10.5·10-6 K-1 for La5.4Mo_Q and 12.0·10-6 K-1 for La5.4Mo_0.5. These are similar to those reported for lanthanum tungstates, i.e. 11·10-6 K-1 for La5.4WO11.1 39,40 and, therefore, they are thermomechanically compatible with conventional electrodes, such as La0.8Sr0.2MnO3- cathodes. 100.0 La5.4Mo_Q

99.9

La5.4Mo_50 La5.4Mo_0.5

99.8 TG (%)

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

La5.4MoO11.1·0.16H2O

99.7 La5.4MoO11.1·0.19H2O

99.6 La5.4MoO11.1·0.25H2O

99.5 0

250

500 T(ºC)

750

1000

Figure 5. Thermogravimetric curves for La5.4Mo_Q, La5.4Mo_50 and La5.4Mo_0.5 performed under humidified air from room temperature to 1000 °C on cooling. The number of water molecules per mole of lanthanum molybdate are shown over the curves.

The water uptake was monitored by thermogravimetric analysis as a function of the temperature to indirectly determine the concentration of oxygen vacancies available for hydration. The thermogravimetric curves, collected under humidified air, were reproducible on both heating and cooling cycles. For the sake of comparison, only the curves taken on cooling are shown in Figure 5. All samples show the typical behaviour of a proton conducting material with a weight increase upon cooling due to water uptake and the formation of protonic defects, according to the exothermic hydration of oxygen vacancies: O v •• → 2OH• (1) H O g The water uptake starts approximately at 650 ºC, a temperature similar to that of lanthanum tungstates.27 Therefore, these materials are expected to be predominantly proton conductors below 650 ºC. As can be seen, the highest water uptake is for La5.4Mo_0.5, with 0.25 mol(H2O)/mol(LMO), and the lowest one for La5.4Mo_50, with 0.16 mol(H2O)/mol(LMO). 3.5. XPS analysis The samples were analyzed by X-ray photoelectron spectroscopy in order to obtain insights on the superficial atomic concentration as well as the possible different oxidation states of the molybdenum. Table S1, (supporting information) includes the estimated atomic concentration of each sample, as well as the La/Mo atomic ratios. As can be observed, the superficial La/Mo ratio is similar for all sam-

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ples, about 4.8, but is lower than the nominal one 5.4, indicating that the material surface is slightly lanthanum deficient, although the limited precision of this technique for cation quantification should be considered. The O 1s core level was decomposed into three different contributions at binding energies (BE) of 528.5, 530.8 and 532.1 eV (Fig. 6a). The relative intensity of these components varied for each sample. The main contribution (OI), located at 530.8 eV, is close to that reported for MoO3, 530.9 eV 41 and for La2O3 oxide, 530.5 eV 42 and hence, this is ascribed to the lanthanum molybdenum mixed oxide. Moreover, the band at 532.1 eV (OIII) is assigned to adsorbed oxygen or OH−groups.43 This contribution is more important for those samples prepared at lower cooling rate and could be related to superficial OH-species due to water uptake. A third contribution at lower binding energies (OII), 528.5 eV is also observed for La5.4Mo_50 and La5.4Mo_0.5 samples, which becomes more important as the cooling rate is slower. This contribution, which is not observed for the cubic La5.4Mo_Q sample, is also detected in the Mo 3d and La 3d signals, as discussed below, and might be related to the different crystal symmetry of the samples.44 The Mo 3d core level consist of two doublets: Mo 3d5/2 and Mo 3d3/2 separated by 3.16 eV, with similar FWHM and area ratio Mo 3d5/2/Mo 3d3/2=1.5 (Fig. 6b). The Mo 3d5/2 signal was considered to identify the molybdenum oxidation states. All of them showed a main contribution at 232.4 eV, assigned to Mo6+ species.45-47 A small contribution is also discernible at higher BE, about 233.4 eV, which cannot be assigned to molybdenum in lower oxidation states. This contribution is not observed for the cubic polymorph and becomes more important for those samples prepared at lower cooling rates, indicating that this is possibly related to Mo6+ species in different chemical environments. Lanthanum spectra show strong satellite peaks, their intensity and energy separation depends on the ligand atom due to a charge transfer from the valence band of the ligand atom to the 4f orbital of the core ionized lanthanum atom.42 This fact makes the La 3d spectrum difficult to decompose. In the present work, only the La 3d5/2 component is studied (Fig. 6c). Dashed lines correspond to satellite peaks. The main peak is located at 834.8 eV and is assigned to La3+ of the La-Mo-O mixed oxide.45,48Again, a second contribution, centered at 833.4 eV, is observed for those samples prepared at slower cooling rate and assigned to a new chemical environment for the La3+ in the rhombohedral polymorphs. The XPS spectra of the cubic polymorph, previously reduced in 5% H2-Ar at 800 ºC for 24 h, were also acquired to study the possible reduction of molybdenum. The Mo3d core level is similar to the oxidized sample, indicating that only a small and undetectable fraction of Mo6+ is reduced to lower oxidation states (Fig. S6). This result is in good agreement with the nearly constant unit cell volume of the samples treated in air and hydrogen (Table 1).

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(a)

OIII

O 1s

OI OII

(b)

MoI

La5.4Mo_0.5

(c)

Mo 3d

La 3d5/2

LaI LaII

MoII

La5.4Mo_0.5

La5.4Mo_0.5

La5.4Mo_50

La5.4Mo_50

La5.4Mo1_50

La5.4Mo_Q

La5.4Mo_Q

La5.4Mo_Q 535

533

531

BE(eV)

529

527

237

235

233

BE(eV)

842

231

840

838

836

834

832

830

BE(eV)

Figure 6. XPS spectra of (a) O 1s, (b) Mo 3d and (c) La 3d5/2 core levels for La5.4Mo_0.5, La5.4Mo_50 and La5.4Mo_Q samples.

proton contribution to the overall conductivity in the low temperature range (T