The System Bi2O,—B2O3

The phase diagram for the system Bi 2 O 3 ‐B 2 O 3 has been determined experimentally. The melting point of Bi 2 O 3 has been redetermined as 825° C with an estimated overall uncertainty of about ±3°C, and the molal heat of fusion of Bi 2 O 3 , calculated from the slope of the liquidus curve, is 2050 cal per mole. The system contains a body‐centered cubic phase of approximate composition 12Bi 2 O 3 ·B 2 O 3 , which melts incongruently at 632°C. Four congruently melting compounds exist in the system: 2Bi 2 O 3 · B 2 O 3 ·5B 2 O 3 , Bi 2 O 3 ·3B 2 O 3 , and Bi 2 O 3 ·4B 2 O 3 , with melting points, respectively, of 675°, 722°, 708°, and 715°C. The Bi 2 O 3 ·4B 2 O 3 compound exhibits a sluggish transformation at 696°C. Compositions containing up to 97.5 wt% (85 mole %) Bi 2 O 3 can be partly or totally quenched to glass. Indices of the quenched glasses are greater than 1.74. A region of liquid immiscibility extends at 709°C from almost pure B 2 O 3 to 19.0 mole % Bi 2 O 3 . The extent of immiscibility theoretically calculated agrees with the experimentally determined value when 1.20 A is used for the ionic radius of Bi 3+ .