sition has been computed,eqs.(1)and (2)can be used to calculate the mixing enthalpy of solid phase, △H8=XR△HR+X△H盘 As pointed out above the Na2B2O-K2B2O system is a complete solid solution system with minimum melting point.For such system,the assumption of S=0,G=0 is reasonable.To fulfill the condition of dT/dx=0 at minimum point:Tmn=836C,Xmin=37.5mol %K2B2O,the liquidus were fitted as: Xg=37.5-0.3922√-836-0.2496(t-836)mo1%K2B,04 for Na2B2O-rich,while for K2B20-rich: XB=37.5+2.1799√t-836+1.6579105(t-836) +3.1369103(t-836)2mol%K2B204 where t is liquidus temperature in centigrade.Since T=t+273.15,dT=dt. Thus,the solidus were calculated after eqs.(5)-(7)and shown in Fig.4. The calculated solidus was very close to onset temperature of melting peak of DTA experiment.Generally,the onset temperature of melting peak are ado- pted as solidus temperature approximately for the system with solid solubility. From the calculated solidus and measured liquidus,AHs coud be derived after eqs.(1)and (2).Considering the assumption for this system,the AFs is equal to excess Gibbs free energy of solid phase,G could be fitted by least-square method as: BGa=△H8=XaXB(38638.82-33788.59XB)J1mo1 So the molar Gibbs free energy of the solid phase was: Ga=Xx°G8+XB°G8+2 RTCXAInYA+YalnYB]+EGa (8) From this expression,it is obvious that >In the case of complete solid solution with minimum point,if >0,there often exist a miscibility gap.It was computed and shown in Fig.4. 2.3 High Temperature X-ray Diffraction As mentioned previously,the miscibility gap shown in Fig.4 could not be observed by room temperature X-ray diffraction.The reason may be due to the hydration of specimens.In order to experimentally examine the miscibi- lity gap existence,we did the high temperature X-ray diffraction of pure Na2B2O and K2B2O as well as the specimen containing 44 mol%K2B2O at 500C.The photographs of X-ray diffraction at 500C(5)show that the latter was a mixture of two phases due to the existence of the miscibility gap. Although the lattice parameters of one phase were close to those of NazB2O 159址 ， 。 立 五 ， △ “ 灵 呈 氢△ 急 ‘ 一 ‘ · 几 ， 兀 盘 ， 卫 盈 。 。 。 ， ， ， 。 ， 。 。 一 。 亿 一 一 。 一 ‘ ‘ 一 ， ‘ 一 侧才一 。 一 一 一 一 一 “ · 。 ， ， 一 。 犷， 五 ， △ 吕 · · 扭 ， △ ‘ ， 盘 一 冠 盘 △ “ 。 。 一 。 盖二 、 灵 后 〔 。 〕 盈 二 ， 召 盖 ， 盖 ， · 一 了， 。 一 · 五 一 ， 五 一 ‘ ‘ ‘ 一 ， 〔 〕 ‘