Solute Effects in Metal-Rich Solid-State Phases

Program Scope: The goal is to combine experiments and theoretical calculations to elucidate the effect of small concentrations of atomic solutes on phase stability, formation pathways, and high-temperature phase transitions in solid-state inorganic materials, particularly binary glass-forming alloys. Control of low levels of solutes can be very difficult, but this is often possible for groups within the Ames Laboratory because of the specialized synthesis expertise and facilities developed here. Moreover, our time-resolved high-energy X-ray scattering techniques are especially critical for determining the dynamic evolution of glass structure and phase selection during heating.

Major Program Achievements: When a eutectic Zr₈₀Pt₂₀ liquid is quenched rapidly, the literature reports that an amorphous structure forms.  Using special techniques to reduce O solute contents, we discovered that a bcc b-Zr(Pt) phase is formed, which is in complete contrast to the universal assumption that reducing O content increases the glass forming ability in metallic liquids. A structural model comprised of a 3x3x3 stacking of the primitive bcc b-Zr structure with Pt atoms to define superlattice positions was shown using ab initio calculations to be energetically favorable over other structures. In addition, experiments show that a transition to an Fd3m (Ti₂Ni-type) Zr₆Pt₃₀ structure occurs as the O level increases. The Ti₂Ni-type structure can accommodate oxygen in three different lattice sites. Again by using first-principles calculations, the energetic preference for oxygen atoms to fill the 16c specific Wyckoff position yielded a structural model that was consistent with the available diffraction data. More recently, studies with hyper-eutectic Zr-Pt alloys show that it is possible to obtain fully amorphous structures with oxygen contents as high as 2500 ppm mass. Time-resolved X-ray diffraction of Zr₇₁Pt₂₉-Zr₇₇Pt₂₃ glasses with varied oxygen levels shows that solute additions change the structure the as-quenched glass and influence phase selection during primary crystallization. Among the Zintl-related polar intermetallics, we have found numerous phases that are stabilized by hydrogen or other small interstitials, especially those of the tetrels (Si–Pb) and triels (Ga–Tl) with alkaline-earth metal counterions. Many examples have Mn₅Si₃-type structures. Recent examples include Sr₅Tl₃, La₃In₁₁, and Yb₅Sn₄, all of which are actually hydrides. Many polyanionic clusters of Ga, In and Tl have been found that also take up transition metal atoms as interstitials in stoichiometric amounts, e.g., Rb₈Tl₁₁Pd, whereas small amounts in other clusters allow the alteration of electron counts and, therewith, stabilities.

Program Impact: A common result is the discovery that phases reported previously in the literature were, in fact, solute-stabilized and exist only as such. It is extremely important to correct such mistakes for many reasons, among them the fact that reliable experimental data is necessary to validate high-level theory for complex solid-state systems. This is also true for metastable phases such as in the Zr-Pt system, which may well lead to a deeper understanding of the relationship between quasicrystalline and crystalline systems as well as the role of solute atoms in changing short- and medium-range order of a metallic glass and, as a consequence, the crystallization pathway.

Contact Information:
Daniel Sordelet

Selected Publications