Abstract We present a modeling framework and discuss the energetics and structural features of the surface terminations of Ca, Ba, Sr and La hexaborides using density functional theory analysis in combination with X-ray photoelectron spectroscopy. There is significant uncertainty in the literature about the nature of the surface terminations in metal hexaborides in terms of metal versus boron terminations. We show from electronic structure calculations that segregated regions of metal and boron-terminations produce the lowest energies for di-cations of CaB6, SrB6 and BaB6, while trivalent LaB6 minimizes the surface energy by arranging the metal ions in parallel rows on the surface. XPS measurements show that CaB6 and SrB6 have surfaces that are close to stoichiometric for the compound, while BaB6 has surfaces that are Ba-rich. Energetic barriers are calculated for transitions between each of the surface geometries considered. There is a substantial increase in the activation energy for the lanthanum migrations compared to the divalent cations. We also find that the boron octahedra units in these materials tend to contract or expand from their bulk values depending on the proximity to regions of high metal concentrations. These materials have many attractive features, such as low work functions, high hardness, low thermal expansion coefficients, and high melting points, among many other properties of interest for industrial applications. Promising uses of these materials also include catalytic applications for chemical dissociation reactions of various molecules such as hydrogen, water and carbon monoxide, for example, thus, the interest in determining relevant surface properties.

中文翻译：

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化学计量金属六硼化物的表面终止分析：第一性原理和 XPS 测量的见解

摘要 我们提出了一个建模框架，并使用密度泛函理论分析结合 X 射线光电子能谱来讨论 Ca、Ba、Sr 和 La 六硼化物表面末端的能量学和结构特征。关于金属六硼化物表面终端的性质，就金属与硼终端而言，文献中有很大的不确定性。我们从电子结构计算表明，金属和硼终止的隔离区域为 CaB6、SrB6 和 BaB6 的双阳离子产生最低能量，而三价 LaB6 通过在表面上平行排列金属离子来最小化表面能。XPS 测量显示 CaB6 和 SrB6 的表面接近于化合物的化学计量比，而 BaB6 的表面富含 Ba。针对所考虑的每个表面几何形状之间的过渡计算能量势垒。与二价阳离子相比，镧迁移的活化能显着增加。我们还发现，这些材料中的硼八面体单元倾向于从它们的体积值收缩或膨胀，这取决于与高金属浓度区域的接近程度。这些材料具有许多吸引人的特性，例如低功函数、高硬度、低热膨胀系数和高熔点，以及许多其他工业应用感兴趣的特性。这些材料的有前景的用途还包括用于各种分子（例如氢、水和一氧化碳）的化学解离反应的催化应用，因此，