Pyfer Katheryne L, Kafader Jared O, Yalamanchali Anirudh, Jarrold Martin F
Chemistry Department, Indiana University 800 E. Kirkwood Avenue, Bloomington Indiana 47405, United States.
J Phys Chem A. 2014 Jul 10;118(27):4900-6. doi: 10.1021/jp503315r. Epub 2014 Jun 30.
Heat capacities have been measured as a function of temperature for size-selected gallium cluster cations with between 60 and 183 atoms. Almost all clusters studied show a single peak in the heat capacity that is attributed to a melting transition. The peaks can be fit by a two-state model incorporating only fully solid-like and fully liquid-like species, and hence no partially melted intermediates. The exceptions are Ga90(+), which does not show a peak, and Ga80(+) and Ga81(+), which show two peaks. For the clusters with two peaks, the lower temperature peak is attributed to a structural transition. The melting temperatures for clusters with less than 50 atoms have previously been shown to be hundreds of degrees above the bulk melting point. For clusters with more than 60 atoms the melting temperatures decrease, approaching the bulk value (303 K) at around 95 atoms, and then show several small upward excursions with increasing cluster size. A plot of the latent heat against the entropy change for melting reveals two groups of clusters: the latent heats and entropy changes for clusters with less than 94 atoms are distinct from those for clusters with more than 93 atoms. This observation suggests that a significant change in the nature of the bonding or the structure of the clusters occurs at 93-94 atoms. Even though the melting temperatures are close to the bulk value for the larger clusters studied here, the latent heats and entropies of melting are still far from the bulk values.
已测量了含有60至183个原子的尺寸选择镓簇阳离子的热容随温度的变化。几乎所有研究的簇在热容中都显示出一个单一峰值，该峰值归因于熔化转变。这些峰值可以用仅包含完全固态和完全液态物种的双态模型来拟合，因此不存在部分熔化的中间体。例外情况是Ga90(+)没有显示出峰值，而Ga80(+)和Ga81(+)显示出两个峰值。对于有两个峰值的簇，较低温度的峰值归因于结构转变。先前已表明，原子数少于50的簇的熔化温度比体相熔点高出数百摄氏度。对于原子数超过60的簇，熔化温度降低，在约95个原子时接近体相值（303 K），然后随着簇尺寸的增加出现几次小的上升波动。绘制熔化潜热与熵变的关系图，发现有两组簇：原子数少于94的簇的熔化潜热和熵变与原子数超过93的簇不同。这一观察结果表明，在93 - 94个原子时，簇的键合性质或结构发生了显著变化。尽管此处研究的较大簇的熔化温度接近体相值，但熔化潜热和熵仍远非体相值。
