| Summary: | 碩士 === 國立成功大學 === 地球科學系碩博士班 === 98 === The NaCl (B1)/ CsCl (B2) phase transformation is a reconstructive transition. The experimental observations show that some binary AB compounds with B1 phase are known to directly transform into B2 phase, or some of them transform via intermediate phases under high pressures. From computational investigation, a dense B2 phase is clearly favored at high pressures for silver halides (AgX, X= F, Cl, Br and I). In this series of AgX, the bonding character is across from ionic to covalent, which may make the high-pressure structural behavior rather complex.
The silver halides have been studied in the diamond anvil cell (DAC) by angluar dispersive X-ray diffraction technique at room temperature. The high-pressure phase transformations of AgCl, AgBr and AgI have been investigated up to the pressures of 28.0 GPa, 41.0 GPa, 47.7 GPa, respectively. In AgCl, the transformation sequence B1－KOH－TlI－B2 was observed at high pressure that is in good agreement with previous experimental study. Different from previous study, this study showed that AgCl can be transformed to B2 phase at high pressure, room temperature without heating. The pathway of transformation in AgBr was found to be B1－KOH－TlI at high pressure. Moreover, a new phase was observed in AgBr transformed to KOH-type structure from B1 phase, which was not shown in previous study. The identification for this new phase is under way. For AgI, the high pressure KOH-type structure was observed up to 27 GPa. With further compression to the pressure of ~48 GPa, AgI has not been observed to be transformed to B2 phase.
Based on current experimental results and previous computational studies, AgBr and AgI could be transformed to B2 phase at high pressure via same pathway as AgCl. The pressure of phase transition would be related to bonding character; the materials processing stronger covalent bonding will have higher transition pressures. In present study, the results show that the volume changes in B1→KOH and TlI→B2 were discontinuous, but KOH→TlI was continuous. Therefore B1→KOH and TlI→B2 would be the first-order transformation, KOH→TlI was a second-order transformation.
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