Zeng Qifeng, He Zhi, San Xiaojiao, Ma Yanming, Tian Fubo, Cui Tian, Liu Bingbing, Zou Guangtian, Mao Ho-Kwang
State Key Laboratory of Superhard Materials, Jilin University, Changchun 130012, China.
Proc Natl Acad Sci U S A. 2008 Apr 1;105(13):4999-5001. doi: 10.1073/pnas.0801280105. Epub 2008 Mar 26.
There is a great interest in the behavior of diatomic molecular solids under extremely high-pressure conditions that lead to pressure-induced metallization, molecular dissociation, and formation of atomic phase. The consensus has been that the phase-transition sequence that happened in both solid bromine and iodine is from a molecular phase (phase I), to an incommensurate phase (phase V), and then to an atomic phase (phase II), with increasing pressure. However, a puzzle remains unresolved for both solids: pressure-induced X and Y bands were observed in the Raman spectra in the molecular phase at low pressures, even before the onset of phase V. Here, we suggest a phase for solid iodine in such a low-pressure range (designated as phase I') in which two different covalent intramolecular bonds coexist, based on first-principles calculations and later corroborated by x-ray diffraction experiments. The pressure dependence of the X and Y bands and other vibrational frequencies measured experimentally can be explained nicely by combining the vibrational modes of phase I and phase I'. These results help improve our understanding on the pressure-induced molecular dissociation and metallization in diatomic solids and may shed some light on the investigation of similar phenomena in solid H(2).
人们对双原子分子固体在极高压力条件下的行为有着浓厚兴趣，这种条件会导致压力诱导的金属化、分子解离以及原子相的形成。普遍的共识是，固态溴和碘中发生的相变序列是随着压力增加，从分子相（相I）转变为非公度相（相V），然后再转变为原子相（相II）。然而，对于这两种固体来说，一个难题仍然没有解决：在低压下的分子相中，甚至在相V出现之前，拉曼光谱中就观察到了压力诱导的X和Y带。在这里，基于第一性原理计算，并随后通过X射线衍射实验得到证实，我们提出在这样的低压范围内固态碘存在一个相（称为相I'），其中两种不同的共价分子内键共存。通过结合相I和相I'的振动模式，可以很好地解释实验测量的X和Y带以及其他振动频率对压力的依赖性。这些结果有助于增进我们对双原子固体中压力诱导的分子解离和金属化的理解，并可能为研究固态H₂中的类似现象提供一些启示。
