The Phase Diagram of the BaO–TiO2 System

Abstract

A method for accurately observing phase changes at high temperatures has been developed; it employs a microfurnace and uses samples of oxide mixtures weighing less than 1 mg. The samples were observed and their temperatures measured with an optical pyrometer specially adapted for this purpose and calibrated for the furnace radiation. An electrically heated resistance strip, in general of molybdenum, allows melting and crystallization of the oxide mixtures at any desired speed. The samples were placed directly on the strip and could be quenched by interruption of the current flow. The binary system, BaO–TiO₂, was investigated. The existence of four compounds, BaTi₄O₉, BaTi₂O₅, BaTiO₃, and Ba₂TiO₄, was demonstrated by liquidus curves and x‐ray diffraction. A phase change of BaTiO₃ below the solidus temperature was observed; all oxide mixtures with less than 80 mole percent TiO₂ showed this intermediate BaTiO₃ phase at about 1450°C. Single crystals of BaTi₄O₉, BaTi₂O₅, and BaTiO₃ were grown from ternary melts of BaCO₃, TiO₂, and BaCl₂. The compound, BaTi₄O₉, showed extensive polymorphism. Attack by molten BaO mixtures on molybdenum and platinum produced the compounds, BaMoO₄, Ba₃Pt₂O₇, and Ba₃Ti₂PtO₉.