Escatllar Antoni Macià, Lazaukas Tomas, Woodley Scott M, Bromley Stefan T
Departament de Ciència de Materials i Química Física & Institut de Química Teòrica i Computacional (IQTCUB), Universitat de Barcelona, E-08028 Barcelona, Spain.
Department of Chemistry, University College, London WC1H 0AJ, U.K.
ACS Earth Space Chem. 2019 Nov 21;3(11):2390-2403. doi: 10.1021/acsearthspacechem.9b00139. Epub 2019 Jul 19.
Magnesium-rich silicates are ubiquitous both terrestrially and astronomically, where they are often present as small particles. Nanosized Mg-rich silicate particles are likely to be particularly important for understanding the formation, processing, and properties of cosmic dust grains. Although astronomical observations and laboratory studies have revealed much about such silicate dust, our knowledge of this hugely important class of nanosolids largely rests on top-down comparisons with the properties of bulk silicates. Herein, we provide a foundational bottom-up study of the structure and properties of Mg-rich nanosilicates based on carefully procured atomistic models. Specifically, we employ state-of-the-art global optimization methods to search for the most stable structures of silicate nanoclusters with olivine (MgSiO) and pyroxene (MgSiO) compositions with = 1-10. To ensure the reliability of our searches, we develop a new interatomic potential that has been especially tuned for nanosilicates. Subsequently, we refine these searches and calculate a range of physicochemical properties of the most stable nanoclusters using accurate density functional theory based electronic structure calculations. We report a detailed analysis of structural and energy properties, charge distributions, and infrared vibrational spectra, where in all cases we compare our finding for nanosilicates with those of the corresponding bulk silicate crystals. For most properties considered, we find large differences with respect to the bulk limit, underlining the limitations of a top-down approach for describing these species. Overall, our work provides a new platform for an accurate and detailed understanding of nanoscale silicates.
富含镁的硅酸盐在地球和宇宙中都普遍存在，它们通常以小颗粒的形式存在。纳米级富含镁的硅酸盐颗粒对于理解宇宙尘埃颗粒的形成、加工和性质可能尤为重要。尽管天文观测和实验室研究已经揭示了许多关于此类硅酸盐尘埃的信息，但我们对这类极其重要的纳米固体的认识很大程度上依赖于与块状硅酸盐性质的自上而下的比较。在此，我们基于精心构建的原子模型，对富含镁的纳米硅酸盐的结构和性质进行了基础性的自下而上的研究。具体而言，我们采用最先进的全局优化方法来寻找具有橄榄石（MgSiO）和辉石（MgSiO）组成、n = 1 - 10的硅酸盐纳米团簇的最稳定结构。为确保搜索的可靠性，我们开发了一种专门针对纳米硅酸盐进行调整的新原子间势。随后，我们对这些搜索结果进行优化，并使用基于精确密度泛函理论的电子结构计算来计算最稳定纳米团簇的一系列物理化学性质。我们报告了对结构和能量性质、电荷分布以及红外振动光谱的详细分析，在所有情况下，我们都将纳米硅酸盐的研究结果与相应块状硅酸盐晶体的结果进行了比较。对于所考虑的大多数性质，我们发现与块状极限存在很大差异，这突出了自上而下描述这些物质方法的局限性。总体而言，我们的工作为准确详细地理解纳米级硅酸盐提供了一个新平台。
