Abstract: With an attempt to improve our proposed one-step method for direct synthesis of CsTmI_3 from CsI:HgI_2:Tm (molar ratio 1: 1: 1) mixture, we have investigated the mechanism of CsTmI_3 formation under this condition by using DTA and X-ray powder diffraction methods. The results obtained indicate that there are two pathways leading to formation of CsTmI_3 from this mixture:The first one involves the reaction between HgI_2 and Tm at 314 ℃ with formation of TmI_2, which, in turn, acts as an interme-diate for CsTmI_3 formation is intermediated by CsHgI_3 (a new phase) formed from CsI and HgI_2 at 210 ℃ and this intermediate will convert to the final product CsTmI_3 in the presence of Tm at 530 ℃.
In addition, we have also studied the phase transition of CsTmI_3. It is found that there are three reversible endo-thermal peaks appeared at 510 ℃, 648 ℃ and 665 ℃ in DTA curve. The X-ray analysis indicated that the peak at 510 ℃ could be assigned at the phase transition of CsTmI_3 and the peak at 665 ℃ is responsible for its melting point. The peak at 648 ℃ seems most likely to be another phase transtion.