CsTmI_3生成机理及其相转变

MECHANISM OF CsTmI_3 FORMATION AND ITS PHASE-TRANSITION TEMPERATURE

一般制备三元低价稀土卤化物多采用首先制备二卤化物(REX_2)或三卤化物(REX_3),然后与相应的碱金属卤化物或稀土金属反应而制得.但REX_2与REX_3极易氧化和水解,操作困难,且不易得到纯化合物,特别是Sm^(2+)、Tm^(2+)的卤化物,往往是REX_2中混有少量REX_3或相反。

With an attempt to improve our proposed one-step method for direct synthesis ofCsTmI_3 from CsI:HgI_2:Tm (molar ratio 1: 1: 1) mixture, we have investigated themechanism of CsTmI_3 formation under this condition by using DTA and X-ray powderdiffraction methods. The results obtained indicate that there are two pathways leadingto formation of CsTmI_3 from this mixture:The first one involves the reaction betweenHgI_2 and Tm at 314℃ with formation of TmI_2, which, in turn, acts as an interme-diate for CsTmI_3 formation via its interaction with CsI at 340℃; The another path-way for CsTmI_3 formation is intermediated by CsHgI_3 (a new phase) formed from CsIand HgI_2 at 210℃ and this intermediate will convert to the final product CsTmI_3 inthe presence of Tm at 530℃. In addition,we have also studied the phase transition of CsTmI_3. It is found thatthere are three reversible endo-thermal peaks appeared at 510℃, 648℃ and 665℃ inDTA curve. The X-ray analysis indicates that the peak at 510℃ could be assignedas the phase transition of CsTmI_3 and the peak at 665℃ is responsible for its meltingpoint. The peak at 648℃ seems most likely to be another phase transtion.