Starace Anne K, Neal Colleen M, Cao Baopeng, Jarrold Martin F, Aguado Andrés, López José M
Department of Chemistry, Indiana University, 800 East Kirkwood Ave., Bloomington, Indiana 47405, USA.
J Chem Phys. 2009 Jul 28;131(4):044307. doi: 10.1063/1.3157263.
Heat capacities have been measured as a function of temperature for aluminum cluster anions with 35-70 atoms. Melting temperatures and latent heats are determined from peaks in the heat capacities; cohesive energies are obtained for solid clusters from the latent heats and dissociation energies determined for liquid clusters. The melting temperatures, latent heats, and cohesive energies for the aluminum cluster anions are compared to previous measurements for the corresponding cations. Density functional theory calculations have been performed to identify the global minimum energy geometries for the cluster anions. The lowest energy geometries fall into four main families: distorted decahedral fragments, fcc fragments, fcc fragments with stacking faults, and "disordered" roughly spherical structures. The comparison of the cohesive energies for the lowest energy geometries with the measured values allows us to interpret the size variation in the latent heats. Both geometric and electronic shell closings contribute to the variations in the cohesive energies (and latent heats), but structural changes appear to be mainly responsible for the large variations in the melting temperatures with cluster size. The significant charge dependence of the latent heats found for some cluster sizes indicates that the electronic structure can change substantially when the cluster melts.
已测量了含有35 - 70个原子的铝团簇阴离子的热容量随温度的变化关系。通过热容量的峰值确定了熔点温度和熔化热；从液体团簇的熔化热和解离能得到了固体团簇的结合能。将铝团簇阴离子的熔点温度、熔化热和结合能与之前对相应阳离子的测量结果进行了比较。进行了密度泛函理论计算，以确定团簇阴离子的全局最低能量几何结构。最低能量几何结构分为四个主要类别：扭曲的十面体碎片、面心立方碎片、有堆垛层错的面心立方碎片以及“无序”的大致球形结构。将最低能量几何结构的结合能与测量值进行比较，使我们能够解释熔化热中的尺寸变化。几何和电子壳层闭合都对结合能（以及熔化热）的变化有贡献，但结构变化似乎是导致熔点温度随团簇尺寸大幅变化的主要原因。对于某些团簇尺寸发现的熔化热对电荷的显著依赖性表明，团簇熔化时电子结构会发生显著变化。
