Immiscibility and the System Lanthanum Oxide–Boric Oxide

The phase equilibrium diagram for the system La 2 O 3 ‐B 2 O 3 has been determined experimentally. The compounds La 2 O 3 ‐3B 2 O 3 and La 2 O 3 ‐B 2 O 3 melt congruently at 1141°± 5°C. and 1660°± 15°C, respectively. At 1488°± 5°C, La 2 O 3 ‐B 2 O 3 inverts from the aragonite‐type structure to a high‐temperature form. Trilanthanum borate, 3La 2 O 3 B 2 O 3 , melts incongruently at 1386°± 5°C. to give liquid and La 2 O 3 . 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 B 2 O 3 to 21.5 mole % La 2 O 3 . 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.