Modeling of the Phase Evolution in Mg_1−xAl_xB₂ (0 < x < 0.5) and Its Experimental Signatures

Abstract

USA andersson @Ianl.gov RECEIVED DATE CORRESPONDING AUTHOR FOOTNOTEDespite the chemical and structural simplicity of MgB2' at 39 K this compound has the highest known Tc of any binary compound. Electron doping by substituting Al for Mg leads to decreasing Tc and the observed concentration dependent rate of decrease has been proposed to arise from the non-ideal character of MgB2-AIB2 solid solutions, which derives from the existence of an ordered M~.sAlo.sB2 compound. Heterogeneous nano-scale structure patterns in solid solutions have emerged as an important concept for complex materials, ranging from actinide alloys and oxides to high-temperature cuprate superconductors and mallganite-based materials exhibiting colossal magnetoresistivity. In this work we investigate the formation of structural heterogeneities in Mg 1 . ,AI.B2' which take the form of nano-scale AI-AI and AI-Mg domains of different geometry and size, using molecular statics/dynamics simulations and in particular we study the corresponding signatures in diffraction experiments. In order to undertake this task we first derive appropriate 1 Mg-AI-B semi-empirical potentials within the Modified Embedded Atom Method formalism.These potentials are also applied to explore the equilibrium Mg 1 _ x Al x B 2 phase diagram for 0 < x < 0.5. Additionally, density functional theory calculations were utilized to study the influence of heterogeneities on the electronic structure and charge distribution in Mg 1 _ x AI.B 2 • KEYWORDS

Intrinsic Nanoscience of δ Pu–Ga Alloys: Local Structure and Speciation, Collective Behavior, Nanoscale Heterogeneity, and Aging Mechanisms

“…The most incisive example is the demonstration within the data presented here that a crystalline material can contain tens of percents of a second ordered structure that is clearly visible in local structure measurements but that may not (or may) display a diffraction pattern accompanying that of the host, indicating the importance of the morphology of its domains on the nanometer scale. Motivated by this finding, we discovered via modeling ,,− , that this is not a difficult feat. However, insofar as the concept of nanoscale heterogeneity and its origin in the collective behavior of defects in a host lattice is relatively new, a number of issues remain.…”

Nanoscale heterogeneity, premartensitic nucleation, and a new plutonium structure in metastableδfcc Pu-Ga alloys

“…The material is therefore heterogeneous-containing multiple ordered structures-on the nanometer scale at or below the diffraction limit. Addressing the putative contradiction in this concept, that a material could contain tens of percents of a second ordered structure that would not give an unambiguous signal in diffraction, delayed detailed reporting of these results [20,21] while we undertook an extensive evaluation of local order and heterogeneity [38][39][40][41]43,44], ultimately finding that this type of behavior was easily produced because of the limitations of crystallography. Arguing this attribute of the structure by claiming that, for example, scattering, photoemission, microscopy, and ultrasound data contradict it ignores the fact that all experimental methods are limited to their intrinsic length and timescales.…”

“…A corollary to this type of distribution would be that, if these probabilities are different in different directions, small tetragonal or orthorhombic distortions will occur [23,65]. We have shown that, if differently oriented domains of chemically ordered structures are coherent at their interfaces, then the expected splitting of the diffraction peaks will not occur because the strain is distributed over all of the atoms, and the distortion is averaged out over the entire crystal [43,44]. Breaking this registry at the interfaces, such as might occur during the initiation of the martensitic transformation or from radiation damage, allows the domains to relax and exhibit their lower symmetries while concomitantly stabilizing the dislocations or other types of defects [43].…”

“…This result is not just an oddity, it is profound in its implications for the arrangement of the Ga atoms in δ Pu. Molecular dynamics-type modeling, in which the atom positions are relaxed for a given set of relative potentials for the different atom pairs [42][43][44] using the measured Ga/Pu-Pu distances in which the relative bond strengths of the various Ga/Pu-Pu bonds are varied, shows that this effect cannot be duplicated in structures when the Ga atoms are randomly distributed throughout the lattice until the Ga-Pu bond strengths are many hundreds of times as great as the Pu-Pu ones. In this modeling, as the Ga concentration increases the peaks in the Ga partial g(r) always become much broader, especially at higher R, so that this behavior of the spectra cannot be achieved in any physically realistic scenario.…”

“…The results were analyzed in some detail to gain insight into the nature of the microstructure, homogeneity, and phases in these intriguing binary alloys of plutonium. This set of local structure measurements probing the element specific environments below the diffraction limit [20,[38][39][40][41][42][43][44] demonstrates that, far from being a random solid solution, the behavior of the Ga atoms is totally dissimilar to the Pu. The Ga atoms that rigorously avoid bonding to each other even at relatively high concentrations nevertheless interact over longer distances so that they are loosely organized into a quasi-intermetallic.…”