while the lattice parameters of the other phase were greater than those of Na2B2Os but smaller than those of K2B2O.Since the lattice parameters of Na2B2Os were a=1,1921nm,c=0.6418nm and those of K2B2Os were a=1.275nm, c=0.733nm.Our high tem perature X-ray diffraction resultss indicated that one of the two phases in mixture was rich in Na2B2O,while the other phase was rich in K2B2O4.Furthermore,the Na:B2Os-rich phase was with smaller solubility,while the KaB2O.rich phase with greater solubility.These behaviors were in agreement with the calculated miscibility gap shown in Fig.4. Although the miscibility gap of solid solution phase in the Na2B2O4- K2B2O,binary system could not be observed in DTA and room temperature X-ray diffraction experiments,it could be theoretically predicted by thermo- dynamical analysis and confirmed by high temperature X-ray diffraction.This is an excellent example to show that the combination of limited experiments and thermodynamical calculation is a powerful way to construct accurate phase diagrams. 3 Rare Earth Chloride Systems Dealing with binary phase diagrams involving rare earth chlorides,not a few papers have been published.But,there are few references related on the ternary phase diagrams of rare earth chloride systems.Obviously,for so large number of possible ternary rare earth chloride systems,it is not only time and money consuming,but also very tedious to measure their phase diagrams one by one. In order to critical assess the binary phase diagrams published in references, lots of binary phase diagrams involving rare earth chlorides have been opti- mized by combination of limited experiments and computer-assisted calcula- tiont10-12).The assessed binary phase diagrams and thermodynamic data with selfconsistency are a better basis for constructing multicomponent phase diagrams. Taking the PrCl,-CaCla-MgCl2 ternary phase diagram as an example,on the basis of critical assessement of its three subsystems,using Li-Qiao modelt1s) or Hillert model(14)as well as taking MgCla as an asymmetric component,the PrCl3-CaCl2-MgCla phase diagram has been calculated.The good agreement between the calculated results and some critical measured data,including ternary eutectic point as well as compositions and temperatures of diflecting points on liquidus in some vertical sections (see Fig.6)showed that the constructed phase diagram is reliable.By combining computer calculation and limited experiments,a series of ternary phase diagrams involving rare earth chloride are being constructed in our group and will be published. 160。 。 ， 二 。 。 。 ， 。 。 一 〕 ， ， 一 ， 五 五 了 。 了 一 了 ， 五 一 了 。 五 了 ， · ， · ， ， 几 ， 五 ， 一 一 幻 五 一 一 ， ， 一 “ 〕 。 一 一 一 扭 ， 月 尹 ， 犷 五 ·