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

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.


Introduction
Since the discovery of enhanced ionic conductivity for the La 2 Mo 2 O 9 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-La 2 Mo 2 O 9 [10] to a high-temperature form β-La 2 Mo 2 O 9 [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 La 2 Mo 2 O 9 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 La 2 Mo 2 O 9 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 La 2 Mo 2 O 9 ceramics on the degree of calcium substitution in the corresponding system.

Materials and Methods
La-Ca-Mo-O tartrate gel precursor for La 2-x Ca x Mo 2 O 9-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.

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. 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 cm 3 ·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 Lanthanum (III) oxide (La 2 O 3 , 99.99% Alfa Aesar), molybdenum (VI) oxide (MoO 3 , 99.95% Alfa Aesar), and calcium (II) nitrate tetrahydrate (Ca(NO 3 ) 2 ·4H 2 O 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 La 2 Mo 2 O 9 ceramic creates conditions for the formation of impurity phases such as La 2 Mo 3 O 12 or La 2 MoO 6 [34]. Nitric acid (HNO 3 66% Reachem (Mississauga, Canada)), distilled water, and concentrated ammonia solution (NH 3 · H 2 O 25% Penta (Prague, Czech Republic)) were used as solvents and reagents to regulate the pH of the solution. Tartaric acid (L-(+)-Tartaric acid (C 4 H 6 O 6 ) (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 La 2−x Ca x Mo 2 O 9−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 cm 3 ·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 CuKα 1 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.

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 La 2-x Ca x Mo 2 O 9-x/2 ceramic system. An example of a differential scanning calorimetry (DSC) curve for the La 1.95 Ca 0.05 Mo 2 O 8.975 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. 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. 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 Figures 3 and 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. 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 Figures 3 and 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 La 2 Mo 2 O 9 compound. Nevertheless, during the cooling stage, the increased enthalpy of the phase transition in the La 1.9 Ca 0.1 Mo 2 O 8.95 sample shows that the reduction of the La 2 Mo 2 O 9 phase is not the only factor that determines the energetics of the phase transition.    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. 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 La 2-x Ca x Mo 2 O 9-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 La 2 Mo 2 O 9 in the final ceramic.
Summarizing the phase transition results obtained from cooling cycles, it can be concluded that homogeneous substitution by Ca 2+ ions in the La 2-x Ca x Mo 2 O 9-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 La 2 Mo 2 O 9 ceramic homogeneously substituted by Ca 2+ ions. The increase in enthalpy values of the phase transition for La 2-x Ca x Mo 2 O 9-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 La 2 Mo 2 O 9 compound and its monoclinic phase in the final ceramic mixture.

X-ray Diffraction
In order to prove the crystalline composition in the obtained La 2-x Ca x Mo 2 O 9-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.  Meanwhile, Figure 5 is consistent with XRD data, which show the formation process and trends of La 1-x Ca x Mo 2 O 9-x/2 and CaMoO 4 crystalline phases. As it seen, the enthalpy of the phase transition for La 2 Mo 2 O 9 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 La 1.999 Ca 0.001 Mo 2 O 8.9995 system.
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 La 2 Mo 2 O 9 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 mixedphase La 2-x Ca x Mo 2 O 9-x/2 . This statement is partially confirmed by the XRD diffractogram of the Ca 1.9 Ca 0.1 Mo 2 O 8.95 compound, in which quite a significant amount of the crystalline side phase for the CaMoO 4 was identified. It seems that this impurity phase effect reduces the amount of the

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

Conclusions
This study showed that the homogeneous substitution of lanthanum by calcium ions takes place up to the compound of initial composition for La 1.95 Ca 0.05 Mo 2 O 8.975 . 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 La 2-x Ca x Mo 2 O 9-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 La 2 Mo 2 O 9 compound, the amount of which significantly increases with the appearance of the CaMoO 4 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 La 2-x Ca x Mo 2 O 9-x/2 system, has a significant influence on the decrease in the phase transition enthalpy in the La 2 Mo 2 O 9 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.

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.

Appendix A
The DSC curves for the La 2-x Ca x Mo 2 O 9-x/2 ceramic contain details and data supporting the results presented in Table 1. For comparison, the phase transition DSC curve for the La 2 Mo 2 O 9 compound is also presented in this section. 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.

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.

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.

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.

Appendix B
Rietveld refinement analysis results of the corresponding XRD patterns for the La 2-x Ca x Mo 2 O 9-x/2 ceramic supporting the data presented in Table 2.

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.