Immiscibility and the System Lanthanum Oxide–Boric Oxide

The phase equilibrium diagram for the system La2O3-B2O3 has been determined experimentally. The compounds La2O3-3B2O3and La2O3-B2O3 melt congruently at 1141°± 5°C. and 1660°± 15°C, respectively. At 1488°± 5°C, La2O3-B2O3 inverts from the aragonite-type structure to a high-temperature form. Trilanthanum borate, 3La2O3 B2O3, melts incongruently at 1386°± 5°C. to give liquid and La2O3. No solid solutions exist in the system. A region of liquid immiscibility exists in the system and extends at 1136°± 5°C. from almost pure B2O3 to 21.5 mole % La2O3. The experimental value for the extent of immiscibility agrees with that calculated from theoretical considerations. A second method for estimating immiscibility in the system is demonstrated, which requires experimentally only the determination of the index of refraction of the modifier-rich liquid. Principles governing immiscibility are discussed.