Phase equilibria in the CaO-La 2 O 3 -Nb 2 O 5 system are of great significance for the utilization and application of rare earth and Nb resources. In the present work, the phase equilibria in high w(Nb 2 O 5 ) corner of the CaO-La 2 O 3 -Nb 2 O 5 system were studied by high-temperature equilibrium and quenching technology. Based on the continuity of phase diagram, a "Concavity and Convexity Rule" was proposed to predict the reasonable range of the unknown liquidus line. Finally, the isothermal sections and liquidus projection of the CaO-La 2 O 3 -Nb 2 O 5 phase diagram were constructed, which include the primary phase fields of CaNb 2 O 6 , LaNbO 4 , Ca 2 Nb 2 O 7 , (Ca, Nb, La)O, and related liquidus lines at 1823, 1773, and 1673 K. Moreover, the subsolidus phase field of (Ca, Nb, La)O-LaNbO 4 -Ca 2 Nb 2 O 7 with peritectic nonvariant reaction and CaNb 2 O 6 -LaNbO 4 -Ca 2 Nb 2 O 7 with eutectic nonvariant reaction were determined, respectively.
The adjacent relation of primary phase fields and corresponding invariant reactions of the system CaO‐SiO2‐Nb2O5‐La2O3 are of great importance for the study on its phase diagram. In the present work, the phase equilibrium in the high w(La2O3) region of CaO‐SiO2‐Nb2O5‐La2O3 system was studied by thermodynamic equilibrium experiment. The adjacent relation of primary phase fields was determined and represented in the form of adjacent tetrahedrons. The Alkemade Rule applicable to quaternary phase diagram was deduced, which can be used to infer the liquidus temperature trend on univariant curves. The rule was then used to determine the possible temperature range of invariant reactions corresponding to the adjacent tetrahedron in CaO‐SiO2‐Nb2O5‐La2O3 system, and the result was shown in the form of Schairer diagram. Finally, the reaction types of five invariant points were determined according to the Lever Rule for quaternary phase diagram, including:
① L1+CaO·3SiO2·2La2O3→CaO·SiO2+SiO2+La2O3·Nb2O5,
② L2→CaO·SiO2+La2O3·Nb2O5+SiO2+CaO·Nb2O5,
③ L3+CaO·3SiO2·2La2O3+2CaO·Nb2O5→10CaO·6SiO2·Nb2O5+La2O3·Nb2O5,
④ L4+10CaO·6SiO2·Nb2O5→CaO·SiO2+2CaO·Nb2O5+La2O3·Nb2O5,
⑤ L5+2CaO·Nb2O5→CaO·Nb2O5+La2O3·Nb2O5+CaO·SiO2.
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