Phase Equilibria and Fluorescence in the System Zinc Oxide‐Boric Oxide

Phase relationships in the system zinc oxide‐boric oxide have been re‐examined in detail. Two compounds exist, and , both of which exhibit low and high temperature polymorphic forms. Low temperature inverts sluggishly to the cubic high temperature at 600°C and very rapidly at 900°C. The melting point is . Low temperature is biaxial negative and inverts rapidly and reversibly to the high temperature (β) form at . It melts incongruently at 1045°. General agreement with previous investigators has been obtained on the liquidus curve and the extent of liquid immiscibility in the system. The thermal expansion and fluorescent properties of the compounds are described. The low temperature phase with 0.03 moles fluoresces orange, and the high temperature cubic phase with 0.006 moles MnO fluoresces green, under both 2537Aå and cathode ray excitation.