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

In order to prove the crystalline composition in the obtained La_2–xCa_xMo₂O_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–xCa_xMo₂O_9–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 La_1.999Ca_0.001Mo₂O_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 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 La_2–xCa_xMo₂O_9–x/2. This statement is partially confirmed by the XRD diffractogram of the Ca_1.9Ca_0.1Mo₂O_8.95 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 La_2–xCa_xMo₂O_9–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 La_1.9Ca_0.1Mo₂O_8.95 and La_1.85Ca_0.15Mo₂O_8.925 samples during both cooling stages (Figure 4). Meanwhile, by the further increase in the calcium substitution degree in the La_2–xCa_xMo₂O_9–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.