Science and Technology

Mixing and non-stoichiometry in Fe-Ni-Cr-Zn-O spinel compounds: Density functional theory calculations

Mixing and non-stoichiometry in Fe-Ni-Cr-Zn-O spinel compounds: Density functional theory calculations

D.A. Andersson and C.R. Stanek
Materials Science and Technology Division
Los Alamos National Laboratory

Density functional theory (DFT) calculations have been employed to better understand the thermodynamic properties of AB2O4 (where A=Fe2+, Ni2+ or Zn2+, and B=Fe3+ or Cr3+) spinel oxides (the structure of which is shown in Figure 1). In particular, spinel pseudo-binary solid solutions have been investigated, e.g. Fe3O4-NiFe2O4, and reactions leading to the formation of non-stoichiometric spinel compounds. The transition metal spinel compounds are of fundamental interest due to their complex chemical, structural, magnetic and charge ordering characteristics, which originate from the versatility of the unfilled d electron orbitals. This gives rise to a rich set of properties that make these materials attractive for a range of technological applications, including spintronics and other emerging technologies. The transition metal spinel oxides are also important from a “classical” materials science or chemistry perspective, since they are formed as corrosion products on stainless steels and during internal oxidation, especially in high temperature applications. One example of the importance of spinel corrosion products is the formation of so-called CRUD on the cladding surface of nuclear fuels, which is also the application that primarily motivated the present study. CRUD is an acronym for Chalk River Unidentified Deposit and refers to corrosion products that deposit on internal light water reactor (LWR) components, specifically on the upper parts of fuel rods where sub-cooled nucleate boiling occurs. The porous CRUD further attracts boron-containing precipitates, which, due to its neutron absorbing properties, shift the power distribution along the rod axis and leads to the CRUD Induced Power Shift (CIPS). This phenomenon is of increasing importance as nuclear plant operators are targeting higher power uprates and longer operating cycles.

The DFT results obtained in this study enable a better understanding of both the formation of protective films on corrosion sources and the chemistry of the CRUD that is deposited on the fuel pins. From this knowledge strategies could be developed to improve materials performance. Addition of Zn to the reactor coolant has already been demonstrated to generate such effects. Development of thermodynamic models of the Fe-Ni-Cr-Zn-O system also benefit from the DFT data presented in this work. Finally, the basic thermodynamic, magnetic and electronic properties of the spinel compounds may impact applications of spinels as functional materials.

Click on image for a larger view

The primitive tetragonal and cubic unit cells of spinel, where red atoms are oxygen, green are A and blue are B cations. The atomic structure repeats according to the shading of the 1/8 cubes shown.