5f Delocalization of Bulk FCC Americium and the (111) Surface: A FP-LAPW Electronic Structure Study
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5f Delocalization of Bulk FCC Americium and the (111) Surface: A FP-LAPW Electronic Structure Study Da Gao and Asok K. Ray Department of Physics P. O. Box 19059 The University of Texas at Arlington Arlington, TX 76019 ABSTRACT The electronic properties of bulk fcc americium and the (111) surface have been investigated with the full-potential linearized augmented plane wave (FP-LAPW) method as implemented in the WIEN2K suite of programs. The study is carried out for the antiferromagnetic ground state of Am at different levels of theory: (1) scalar-relativity vs. fullrelativity; (2) local-density approximation (LDA) vs. generalized-gradient approximation (GGA). Our results indicate that spin orbit coupling plays an important role in determining the electronic properties of both bulk fcc americium and the (111) surface. In general, LDA is found to give a higher total energy compared to GGA results. The spin orbit coupling shows a similar effect on the surface calculations regardless of the model, GGA versus LDA. The 5f localized-delocalized transition of americium is employed to explain our results. In addition, the quantum size effects in the surface energies and the work functions of fcc (111) americium ultra thin films (UTF) are also examined. INTRODUCTION Americium, like its nearest neighbor plutonium, occupies a pivotal position in the actinide series with regard to the behavior of 5f electrons [1]. While the atomic volume of the actinides before Pu continuously decreases as a function of the increasing atomic number from Ac until Np, a sharp atomic volume increase from Pu to Am has been experimentally observed [2]. Such behavior has been explained by the localized 5f electrons in Am [3]. On the other hand, the experimentally observed 5f electron delocalization from Am II to Am III [4] has not been observed in a recent density-functional electronic calculation [5] with respect to the highpressure behavior of americium. Such controversies clearly indicate that further experimental and theoretical studies are necessary to improve our understanding of americium. Recently, Griveau et al. [6] have studied superconductivity in Am as a function of pressure and the role of a Mott-type transition. One effective way to probe the americium 5f electron properties and their roles in the chemical bonding is to study their surface properties. The unusual aspects of the bonding in bulk Am are apt to be enhanced at a surface or in a thin layer of Am adsorbed on a substrate, as a result of the reduced atomic coordination of a surface atom and the narrow bandwidth of surface states. Thus, Am surfaces and thin films may also provide valuable information about the bonding in Am. However, to the best of our knowledge, no Am surface study exists in the literature. Motivated by the above facts and our continuing interests in actinide surface chemistry and physics [7], this study has thus focused on the (111) surface of Am II with a fcc crystal structure, and a comparison of the properties of the 5f electrons properties be
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