Systematic Study On The Thermodynamic Properties Of The Rare Earth Vapor Complexes: KLnCl₄

It is well known that certain regularities exist in condensed states rareearth complexes when diverse physical chemical properties, such as stabilityconstants, separation factors,â–³_rH_m^θ,â–³_rS_m^θandâ–³_rG_m^θare plotted againstthe lanthanide atomic number or the ground electronic state of rare earth ions,known as gadolinium break, tetrad effect and inclined W effect. However, nosimilar systematic analysis has been reported yet for rare earth vapourcomplexes KLnCl₄ due to the lack of a complete set of property values for allthe rare earth elements.The rare earth vapour complexes KLnCl₄ have been investigated widelyfor potential applications as laser materials, luminous material, in preparationof anhydrous rare earth chlorides and mutual separation of rare earthelements. But the study on their thermodynamic properties has been onlycarried out for a few elements. Therefore, thermodynamic studies on the rareearth vapour complexes are required to understand the reaction mechanisms.This paper contains two parts. One is the preparation of anhydrate rareearth chlorides for chemical vapor transport reaction by excess NH₄Cl atlower temperature. In this study, we investigated some influence factors onthe chlorinated yield such as the protect atmosphere, the material loadingmethod, chlorination temperature and time. The results showed that the bestchlorination condition is that the material was loaded by layer spreadmethod and chlorinated by NH₄Cl at 340℃in air atmosphere for 40 min.More over, the chlorination mechanism of rare earth oxides chlorinated byNH₄Cl was studied. The results indicated that CeO₂ show particularity compared with the other rare earth oxides, and the main chlorination ofCeO₂ is the double decomposition reaction between CeO₂ and solid NH₄Cl,while the other rare earth oxides are chlorinated by HCl gas decomposedfrom NH₄Cl combined with the direct chlorination by NH₄Cl.The second part is thermodynamic study of vapor complexes KLnCl₄ bychemical vapor transport reaction, which is the main content of this paper. Inthis study, the reaction LnCl₃(g)+KCl(g)=KLnCl₄(g) was studied bychemical vapor transport reaction at 1073～1323 K for Ln=Sc～Eu. Theequilibrium constants and the change of Gibbs free energyΔ_rG_m at sixdifferent temperatures were calculated directly from analytical data of thechemical vapor transport experiments. The enthalpy changeΔ_rH_m^θ, entropychangeΔ_rS_m^θvalues of the reaction were derived from the equilibriumconstants by the method of least squares computation. There are someconclusions for the study as follows:(1) The equilibrium constant of forming certain metal halide vaporcomplexes decreases with temperature increasing, which showed the vaporcomplexes are more easy decomposed at higher temperature.(2) The enthalpy changeΔ_rH_m^θof the reaction is negative, whichshowed the complex reaction is exothermic. The entropy changeΔ_rS_m^θisnegative too. In addition, the change of Gibbs free energyΔ_rG_m^θof thereaction is increasing with temperature increasing which means the stable ofthe vapor complex KLnCl₄ is decreasing with temperature increasing.(3) The enthalpy changeΔ_rH_m^θand the entropy changeΔ_rS_m^θof thereaction is increasing from La to Nd firstly and then turn the contrary for Smand Eu.(4) Sc and Y, which located at the same group as the lanthanide elements,showed obvious difference from the lanthanide elements for forming vapor complexes KLnCl₄, that is the enthalpy changeΔ_rH_m^θand the entropychangeΔ_rS_m^θfor KScCl₄ are the most least, while those properties forKYCl₄ are near to that of heavy rare earth vapor complexes.(5) The results suggest that the complex reaction enhanced largely thevolatility of lanthanide chlorides but decrease that of potassium chloride, andthe enhancement factor were decreasing with temperature increasing.