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The BaB2O4-BaF2-2NaF-NaB2O4 reciprocal system has been investigated by combination of experimental measurements with theoretical calculation. The Na2B2O4-2NaF binary and Na2B2O4-BaF2 pseudo-binary phase diagrams are measured by means of DTA and X-ray diffraction. The thermodynamic functions for all sub-binary systems are derived from measured phase diagrams and thermodynamic data by CAL-PHAD technique, and the sub-binary phase diagrams are calculated according to phase equilibrium principle. Then, the thermodynamic functions for sub-binary systems are extrapolated to the BaB2O4-BaF2-2NaF-Na2B2O4 reciprocal system, and the phase diagram of the reciprocal system is calculated. The calculated phase diagrams are verified by additional experiments. (C) 1996 Academic Press, Inc.