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. 2023 Jan 13;16(2):813. doi: 10.3390/ma16020813

Thermoanalytical and X-ray Diffraction Studies on the Phase Transition of the Calcium-Substituted La2Mo2O9 System

Artūras Žalga 1,*, Giedrė Gaidamavičienė 1
Editor: Joan-Josep Suñol1
PMCID: PMC9862346  PMID: 36676550

Abstract

An aqueous sol-gel preparation technique was applied for the synthesis of calcium-substituted lanthanum molybdate with the initial composition of La2–xCaxMo2O9–x/2. The influence of the substitution effect, which plays a crucial role in the formation of final ceramics, was investigated. The thermal behavior tendencies of phase transition at elevated temperatures from the monoclinic crystal phase to cubic as well as reversible transformation were identified and discussed in detail. It was proved that the phase transformation in the obtained mixture significantly depends only on the impurities’ amount, while the partial substitution by calcium atoms above the value of x = 0.05 does not create a homogeneous multicomponent system for La2–xCaxMo2O9–x/2 composition.

Keywords: sol-gel synthesis, phase transition, thermal analysis, X-ray diffraction, Rietveld refinement

1. Introduction

Since the discovery of enhanced ionic conductivity for the La2Mo2O9 compound by Lacorre in 2000 [1], the efforts of application [2] for this system in different electrochemical devices have continuously increased [3]. Oxygen pumps, sensors, and solid oxide fuel cells (SOFCs) [4,5,6,7] are only a few types of equipment where lanthanum molybdenum oxide can be successfully applied. Despite a reversible phase transformation [8,9] above 540 °C from a low-temperature form α-La2Mo2O9 [10] to a high-temperature form β-La2Mo2O9 [11], its chemical stability [12] under air atmosphere in the range of temperature from 600 °C to 1000 °C creates the conditions for using this compound as a solid electrolyte of oxygen ions [13]. Moreover, the densification [14] of the corresponding ceramic could be successfully applied below the temperature of 1200 °C while creating desirable surface and crystalline properties [15,16]. The synthesis technique [17,18,19] that allows the preparation of the initial mixture of lanthanum and molybdenum oxides also plays an important role during the formation of the final ceramic at high temperatures. However, the molar ratio of initial metals remains the main factor that determines the formation of the La2Mo2O9 composition. This is the reason why the partial substitution [20,21,22] of either lanthanum [23,24,25] or molybdenum [26,27,28,29] leads to the crystallization of side phases [30,31], which significantly affects the physical properties [32,33] of the corresponding compound. This effect is directly related to both the amount of the La2Mo2O9 phase in the final ceramic mixture and the increased stabilization of the cubic phase at room temperature. Therefore, the main aim of this work was to study the dependence of the phase transition of La2Mo2O9 ceramics on the degree of calcium substitution in the corresponding system.

2. Materials and Methods

La–Ca–Mo–O tartrate gel precursor for La2–xCaxMo2O9–x/2 ceramic was prepared by an aqueous sol-gel synthesis using tartaric acid as a chelating agent that interacts as a ligand at the molecular level with the reaction mixture during both the dissolution in water and either sol or gel formation. The general scheme of this experiment is illustrated and presented in Figure 1.

Figure 1.

Figure 1

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Synthesis scheme of the La–Ca–Mo–O tartrate precursor for La2–xCaxMo2O9–x/2 ceramic.

Lanthanum (III) oxide (La2O3, 99.99% Alfa Aesar), molybdenum (VI) oxide (MoO3, 99.95% Alfa Aesar), and calcium (II) nitrate tetrahydrate (Ca(NO3)2·4H2O 99.98% Alfa Aesar) were used as starting materials and weighed before the dissolution procedure according to the desired stoichiometric ratio. It should be noted that, despite the high purity of the lanthanum (III) oxide, it was additionally heat-treated at 1000 °C for 5 h because of its tendency of the reaction with humidity and carbon dioxide from the air. In this case, even a slight deviation in the lanthanum amount from the ideal composition for La2Mo2O9 ceramic creates conditions for the formation of impurity phases such as La2Mo3O12 or La2MoO6 [34]. Nitric acid (HNO3 66% Reachem (Mississauga, Canada)), distilled water, and concentrated ammonia solution (NH3 · H2O 25% Penta (Prague, Czech Republic)) were used as solvents and reagents to regulate the pH of the solution. Tartaric acid (L–(+)–Tartaric acid (C4H6O6) (TA) ≥ 99.5% Sigma-Aldrich (Darmstadt, Germany)) was applied for escalation of solubility via coordination of starting compounds in the reaction mixture, especially during the pH changes and evaporation before sol-gel formation. The mechanism of the corresponding chemical process in the frame of the aqueous tartaric acid-assisted synthesis for the preparation of the La–Mo–O gel precursor was discussed in our previous work [35]. Finally, the obtained La−Ca−Mo−O tartrate gel precursor for La2−xCaxMo2O9−x/2 ceramics was heat-treated for 5 h at 1000 °C in the air atmosphere.

The thermal analysis of heat-treated powders was performed with TG–DSC, with a STA 6000 PerkinElmer instrument using a sample mass of about 20 mg and a heating rate of 40 °C min–1 under an airstream of 20 cm3·min–1 at ambient pressure. The heating and cooling cycle was fulfilled twice from 300 °C to 800 °C and from 800 °C to 300 °C. The sample mass, heating rate, atmosphere, and its flow rate were selected empirically during numerous tests to ensure the best signal peak efficiency and to minimize the noises and background signals, which occur because of the influence of the corundum crucible and equipment limits. The characteristics of the phase transition peak were evaluated in the ranges of temperature from 530 °C to 600 °C for heating and from 560 °C to 490 °C for the cooling regime. X-ray diffraction (XRD) patterns were recorded in air at room temperature by employing a powder X-ray diffractometer Rigaku MiniFlex II using CuKα1 radiation. XRD patterns were recorded at the standard rate of 1.5 2θ min–1. The sample was spread on the glass holder to obtain the maximum intensity of the characteristic peaks in the XRD diffractograms. The Rietveld refinements of the obtained XRD patterns were performed using X’Pert HighScore Plus version 2.0a software.

3. Results and Discussion

3.1. Thermal Analysis

In this work, thermal analysis as a powerful investigation technique was used for a detailed investigation of the crystal phase transition from the monoclinic α-phase to cubic β-phase and from the cubic β-phase to monoclinic α-phase in the La2–xCaxMo2O9–x/2 ceramic system. An example of a differential scanning calorimetry (DSC) curve for the La1.95Ca0.05Mo2O8.975 compound is presented in Figure 2. The corresponding results for other samples are presented in the Appendix A. Meanwhile, the data of the phase transition during the repeated heat treatments are collected in Table 1.

Figure 2.

Figure 2

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DSC curve of the phase transition cycles for La1.95Ca0.05Mo2O8.975 ceramic heat-treated at 1000 °C.

Table 1.

Thermoanalytical data and α↔β phase transition peak properties for La2–xCaxMo2O9–x/2 ceramic.

Initial Composition Sample Mass/mg Heating/Cooling Stages Temperature/°C Heat
Onset End Peak Position Flow/mJ Enthalpy/J·g–1
La2Mo2O9 20.181 heating stage I 556.07 579.33 563.25 195.047 9.665
stage II 555.69 578.84 563.03 204.953 10.1559
cooling stage I 540.31 523.80 533.34 –146.255 –7.2473
stage II 540.26 523.17 533.29 –147.886 –7.3281
La1.999Ca0.001Mo2O8.9995 22.158 heating stage I 558.63 580.56 567.11 191.073 8.6231
stage II 556.36 578.06 564.68 209.688 9.4632
cooling stage I 537.43 511.92 525.65 –156.176 –7.0482
stage II 537.45 511.92 526.09 –153.356 –6.9209
La1.99Ca0.01Mo2O8.995 22.153 heating stage I 562.12 585.08 569.41 194.41 8.7757
stage II 559.49 582.62 566.63 197.462 8.9134
cooling stage I 533.02 512.79 523.19 –145.764 –6.5798
stage II 532.63 511.21 522.73 –142.37 –6.4266
La1.95Ca0.05Mo2O8.975 22.146 heating stage I 565.55 587.31 573.98 183.43 8.2826
stage II 563.28 587.47 571.59 193.262 8.7266
cooling stage I 527.46 497.28 517.3 –134.667 –6.0808
stage II 527.68 498.61 518.05 –137.095 –6.1904
La1.9Ca0.1Mo2O8.95 22.183 heating stage I 567.18 589.94 575.53 182.851 8.2429
stage II 564.64 587.71 573.60 191.273 8.6225
cooling stage I 530.34 512.47 525.64 –139.347 –6.2817
stage II 530.42 513.60 525.82 –140.215 –6.3208
La1.85Ca0.15Mo2O8.925 22.189 heating stage I 566.04 589.64 574.53 181.146 8.146
stage II 563.04 588.21 572.27 189.325 8.5323
cooling stage I 530.33 515.25 525.78 –137.468 –6.1953
stage II 530.38 515.97 526.06 –137.764 –6.2086
La1.8Ca0.2Mo2O8.9 22.200 heating stage I 566.1 589.06 574.45 175.753 7.9168
stage II 563.01 585.8 571.18 183.708 8.2751
cooling stage I 532.05 515.17 526.57 –137.915 –6.2124
stage II 532.12 516.21 526.66 –133.617 –6.0188
La1.75Ca0.25Mo2O8.875 22.182 heating stage I 561.67 586.44 571.56 181.96 8.203
stage II 547.82 573.6 556.83 182.224 8.2148
cooling stage I 526.46 508.12 521.81 –115.632 –5.2128
stage II 526.48 510.01 521.98 –118.351 –5.3354
La1.7Ca0.3Mo2O8.85 22.192 heating stage I 560.67 585.41 569.80 176.441 7.9507
stage II 546.08 571.05 555.68 178.614 8.0486
cooling stage I 526.23 508.93 521.35 –111.515 –5.0250
stage II 526.17 510.26 521.13 –113.167 –5.0995
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It is seen from Table 1 that the enthalpy values of the first heating cycle are slightly lower, especially in the cases with a smaller amount of calcium ions, compared with the second one. The reversible stabilization of the cubic phase at room temperature after partial transformation from the monoclinic α-phase determines the main reason for such behavior. According to the measurement conditions, the second heating cycle corresponds to phase transition energy more precisely. Therefore, the representation of the tendency of enthalpy change of only the second heating and cooling cycles according to the substitution degree of calcium ions is shown in Figure 3 and Figure 4. The decrease in the tendency of phase transition enthalpy by increasing the calcium amount in the corresponding system is directly related to the amount of the monoclinic crystal phase of the La2Mo2O9 compound. Nevertheless, during the cooling stage, the increased enthalpy of the phase transition in the La1.9Ca0.1Mo2O8.95 sample shows that the reduction of the La2Mo2O9 phase is not the only factor that determines the energetics of the phase transition.

Figure 3.

Figure 3

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Dependency of the phase transition enthalpy values from the substitution degree by calcium in the La2–xCaxMo2O9–x/2 system under the second heating stage.

Figure 4.

Figure 4

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Dependency of the phase transition enthalpy values from the substitution degree by calcium in the La2–xCaxMo2O9–x/2 system under the second cooling stage.

This phenomenon could be explained either by the increase in the amount of the monoclinic phase or by the influence of calcium ions on the formation of side phases in the final ceramic mixture. By further increasing the concentration of calcium ions in the La2–xCaxMo2O9–x/2 system, the enthalpy of the phase transition starts to decrease, and this result is directly related to the decrease in the amount of the crystalline phase for La2Mo2O9 in the final ceramic.

Summarizing the phase transition results obtained from cooling cycles, it can be concluded that homogeneous substitution by Ca2+ ions in the La2–xCaxMo2O9–x/2 system takes place up to the value of x = 0.05. In this case, the phase transition mainly depends only on the amount of the monoclinic crystal phase in the La2Mo2O9 ceramic homogeneously substituted by Ca2+ ions. The increase in enthalpy values of the phase transition for La2–xCaxMo2O9–x/2 (x = 0.10 and 0.15) samples during the cooling stages could be explained by the side phase effect, which increases the amount of pure La2Mo2O9 compound and its monoclinic phase in the final ceramic mixture.

3.2. X-ray Diffraction

In order to prove the crystalline composition in the obtained La2–xCaxMo2O9–x/2 system, the XRD analysis of the corresponding ceramic was also performed. The XRD patterns of all samples that correspond to the data collected in Table 2 are presented in the Appendix B.

Table 2.

Crystal system, mass fraction, crystallite size, lattice parameters, and agreement indices for the La2–xCaxMo2O9–x/2 ceramic.

Initial Composition Crystal Phase Crystal System Mass Fraction/% Crystallite size/nm Unit Cell Weighted R Profile Goodness of Fit
a/pm b/pm c/pm
alpha/o beta/o gamma/o
La2Mo2O9 La2Mo2O9 monoclinic 71.4 104.75 1431.438 2145.289 2855.431 12.99106 1.29603
90.00000 90.42323 90.00000
La2Mo2O9 cubic 28.6 47.03 715.106 715.106 715.106
90.00000 90.00000 90.00000
La1.999Ca0.001Mo2O8.9995 La2Mo2O9 monoclinic 48.9 66.33 1432.093 2145.928 2857.133 10.70047 1.87511
90.00000 90.35913 90.00000
La2Mo2O9 cubic 50.4 45.56 715.357 715.357 715.357
90.00000 90.00000 90.00000
CaMoO4 tetragonal 0.7 – – – –
La1.99Ca0.01Mo2O8.995 La2Mo2O9 monoclinic 54.1 71.50 1431.437 2145.437 2856.032 10.55389 1.79591
90.00000 90.38470 90.00000
La2Mo2O9 cubic 44.2 46.61 715.103 715.103 715.103
90.00000 90.00000 90.00000
La2Mo3O12 monoclinic 1.2 41.08 1739.278 1186.510 1624.259
90.00000 107.93130 90.00000
CaMoO4 tetragonal 0.5 – – – –
La1.95Ca0.05Mo2O8.975 La2Mo2O9 monoclinic 59.1 70.52 1431.201 2145.733 2857.156 10.32976 1.76384
90.00000 90.35389 90.00000
La2Mo2O9 cubic 40.3 48.10 715.171 715.171 715.171
90.00000 90.00000 90.00000
CaMoO4 tetragonal 0.6 – – – –
La1.9Ca0.1Mo2O8.95 La2Mo2O9 monoclinic 44.5 35.83 1432.385 2140.825 2855.251 12.83825 2.41047
90.00000 90.15601 90.00000
La2Mo2O9 cubic 49.3 42.06 714.384 714.384 714.384
90.00000 90.00000 90.00000
CaMoO4 tetragonal 3.8 – – – –
La2Mo3O12 monoclinic 1.4 43.47 1719.584 1166.525 1614.533
90.00000 108.09910 90.00000
La2MoO6 tetragonal 1.0 42.52 582.792 582.792 3031.347
90.00000 90.00000 90.00000
La1.85Ca0.15Mo2O8.925 La2Mo2O9 monoclinic 76.0 66.77 1430.812 2144.216 2854.451 16.89944 2.2630
90.00000 90.36139 90.00000
La2Mo2O9 cubic 17.1 44.52 714.631 714.631 714.631
90.00000 90.00000 90.00000
CaMoO4 tetragonal 5.8 59.84 526.101 526.101 1153.607
90.00000 90.00000 90.00000
La2Mo3O12 monoclinic 1.1 42.68 1732.883 1168.940 1619.405
90.00000 107.77000 90.00000
La1.8Ca0.2Mo2O8.9 La2Mo2O9 monoclinic 56.2 45.44 1428.985 2143.602 2858.397 12.46852 2.36196
90.00000 90.31453 90.00000
La2Mo2O9 cubic 36.4 46.98 714.584 714.584 714.584
90.00000 90.00000 90.00000
CaMoO4 tetragonal 7.4 58.26 525.675 525.675 1151.621
90.00000 90.00000 90.00000
La1.75Ca0.25Mo2O8.875 La2Mo2O9 monoclinic 79.6 45.63 1430.900 2142.097 2850.290 14.09104 1.54335
90.00000 90.29116 90.00000
La2Mo2O9 cubic 12.2 39.32 714.035 714.035 714.035
90.00000 90.00000 90.00000
CaMoO4 tetragonal 6.4 48.20 523.288 523.288 1146.182
90.00000 90.00000 90.00000
La2Mo3O12 monoclinic 1.8 46.96 1732.404 1167.824 1617.912
90.00000 107.70840 90.00000
La1.7Ca0.3Mo2O8.85 La2Mo2O9 monoclinic 76.1 64.96 1430.166 2143.528 2854.548 13.81435 1.44730
90.00000 90.34066 90.00000
La2Mo2O9 cubic 13.8 45.66 714.447 714.447 714.447
90.00000 90.00000 90.00000
CaMoO4 tetragonal 9.1 67.30 523.476 523.476 1146.807
90.00000 90.00000 90.00000
La2Mo3O12 monoclinic 1.0 66.99 1733.132 1169.219 1619.159
90.00000 107.79630 90.00000
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Meanwhile, Figure 5 is consistent with XRD data, which show the formation process and trends of La1–xCaxMo2O9–x/2 and CaMoO4 crystalline phases. As it seen, the enthalpy of the phase transition for La2Mo2O9 mostly depends on the amount of the monoclinic phase in the ceramic mixture. This assumption is confirmed by the increased stabilization of the cubic phase up to 48.0% even after insignificant substitution of lanthanum by calcium ions in the La1.999Ca0.001Mo2O8.9995 system.

Figure 5.

Figure 5

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XRD patterns of the La1–xCaxMo2O9–x/2 ceramic heat-treated at a 1000 °C temperature.

Nevertheless, by a further increase in the substitution degree of lanthanum by calcium (x = 0.01 and 0.05), the amount of the monoclinic phase for the La2Mo2O9 compound slightly increases; however, the trend of phase transition enthalpy change remains in a decreasing manner as concluded from Figure 3. Considering the fact that the amount of impurity phases in the obtained ceramics is really small, this decrease in the enthalpy of phase transition is basically determined by the increase in the concentration of the mixed-phase La2–xCaxMo2O9–x/2. This statement is partially confirmed by the XRD diffractogram of the Ca1.9Ca0.1Mo2O8.95 compound, in which quite a significant amount of the crystalline side phase for the CaMoO4 was identified. It seems that this impurity phase effect reduces the amount of the La2–xCaxMo2O9–x/2 homogeneous phase in the mixture and creates conditions for the formation of pure La2Mo2O9 compound. This explains the increase in the phase transition enthalpy in La1.9Ca0.1Mo2O8.95 and La1.85Ca0.15Mo2O8.925 samples during both cooling stages (Figure 4). Meanwhile, by the further increase in the calcium substitution degree in the La2–xCaxMo2O9–x/2 system, the decrease in the phase transition enthalpy is already determined by a significant lack of the La2Mo2O9 crystalline phase. This conclusion is confirmed by the constant increase in the concentration of the crystalline phase of calcium molybdate in the final mixture of the obtained ceramics.

4. Conclusions

This study showed that the homogeneous substitution of lanthanum by calcium ions takes place up to the compound of initial composition for La1.95Ca0.05Mo2O8.975. In this case, the decrease in the phase transition enthalpy is determined by the increase in the concentration of the formation of the mixed compound for the initial composition of La2–xCaxMo2O9–x/2. Meanwhile, the influence of the monoclinic phase amount on the phase transition enthalpy remained important only in the case of the formation of a pure La2Mo2O9 compound, the amount of which significantly increases with the appearance of the CaMoO4 impurity phase in the ceramic mixture. In summary, it can be concluded that the formation of the impurity of the calcium molybdate crystal phase, which compensates for the lack of lanthanum and the excess of molybdenum in the multicomponent oxide La2–xCaxMo2O9–x/2 system, has a significant influence on the decrease in the phase transition enthalpy in the La2Mo2O9 compound. The influence of the monoclinic phase amount on the phase transition enthalpy remains an important factor only in the case of the pure lanthanum molybdate.

Appendix A

The DSC curves for the La2–xCaxMo2O9–x/2 ceramic contain details and data supporting the results presented in Table 1. For comparison, the phase transition DSC curve for the La2Mo2O9 compound is also presented in this section.

Figure A1.

Figure A1

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DSC curve of the phase transition cycles for La2Mo2O9 ceramic heat-treated at 1000 °C.

Figure A2.

Figure A2

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DSC curve of the phase transition cycles for La1.999Ca0.001Mo2O8.9995 ceramic heat-treated at 1000 °C.

Figure A3.

Figure A3

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DSC curve of the phase transition cycles for La1.99Ca0.01Mo2O8.995 ceramic heat-treated at 1000 °C.

Figure A4.

Figure A4

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DSC curve of the phase transition cycles for La1.9Ca0.1Mo2O8.95 ceramic heat-treated at 1000 °C.

Figure A5.

Figure A5

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DSC curve of the phase transition cycles for La1.85Ca0.15Mo2O8.925 ceramic heat-treated at 1000 °C.

Figure A6.

Figure A6

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DSC curve of the phase transition cycles for La1.8Ca0.2Mo2O8.9 ceramic heat-treated at 1000 °C.

Figure A7.

Figure A7

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DSC curve of the phase transition cycles for La1.75Ca0.25Mo2O8.875 ceramic heat-treated at 1000 °C.

Figure A8.

Figure A8

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DSC curve of the phase transition cycles for La1.7Ca0.3Mo2O8.85 ceramic heat-treated at 1000 °C.

Appendix B

Rietveld refinement analysis results of the corresponding XRD patterns for the La2–xCaxMo2O9–x/2 ceramic supporting the data presented in Table 2.

Figure A9.

Figure A9

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XRD pattern of the La2Mo2O9 ceramic heat-treated at 1000 °C.

Figure A10.

Figure A10

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XRD pattern of the La1.999Ca0.001Mo2O8.9995 ceramic heat-treated at 1000 °C.

Figure A11.

Figure A11

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XRD pattern of the La1.99Ca0.01Mo2O8.995 ceramic heat-treated at 1000 °C.

Figure A12.

Figure A12

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XRD pattern of the La1.95Ca0.05Mo2O8.975 ceramic heat-treated at 1000 °C.

Figure A13.

Figure A13

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XRD pattern of the La1.9Ca0.1Mo2O8.95 ceramic heat-treated at 1000 °C.

Figure A14.

Figure A14

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XRD pattern of the La1.85Ca0.15Mo2O8.925 ceramic heat-treated at 1000 °C.

Figure A15.

Figure A15

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XRD pattern of the La1.8Ca0.2Mo2O8 ceramic heat-treated at 1000 °C.

Figure A16.

Figure A16

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XRD pattern of the La1.75Ca0.25Mo2O8.875 ceramic heat-treated at 1000 °C.

Figure A17.

Figure A17

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XRD pattern of the La1.7Ca0.3Mo2O8.85 ceramic heat-treated at 1000 °C.

Author Contributions

A.Ž.: Conceptualization, Methodology, Software, Validation, Resources, Data curation, Writing—original draft, Writing—review and editing, Visualization, Supervision. G.G.: Methodology, Investigation, Resources. All authors have read and agreed to the published version of the manuscript.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data presented in this study are available on request from the corresponding author.

Conflicts of Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Funding Statement

This research received no external funding.

Footnotes

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Data Availability Statement

The data presented in this study are available on request from the corresponding author.

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