EVALUATION OF SPIN ORDERING IN FE-DOPED HOLLANDITE: IMPLICATIONS FOR NUCLEAR WASTE FORMS

Hollandite (A2B8O16) is known to incorporate cesium-137 and its daughter barium-137 into its tunnel-like structure. In fact, Cs has been shown to stabilize the tunnel structure, and the localized decrease in tunnel diameter introduced by the smaller Ba atoms aid in confining Cs within the tunnel. The mineral is therefore an important constituent of a polycrystalline ceramic waste form used to immobilize nuclear waste. Previous work investigated the additional role of isovalent B-site cations (i.e., Ga³⁺, Al³⁺, and Zn²⁺) on hollandite stability; however, actual nuclear waste solutions would also include non-isovalent cations, such as Fe^{2+, 3+}. In addition to the cation ordering previously evaluated, spin ordering must be considered to capture the energetics of the Fe-doped hollandite system. Quantum-mechanical calculations were used to capture the cation and electron ordering across the Fe-doped hollandite solid solution from the Ba end member to the Cs end member. Preliminary calculations were performed for the Ga-doped system to ensure that the energetic trends in cation ordering were the same when calculated using an ultrasoft pseudopotential approach as implemented in CASTEP and a projector augmented wave approach as implemented in VASP. Spin ordering was then determined for the Fe³⁺-doped hollandite solid solution, focusing on the lowest energy configuration from the Ga-doped hollandite calculations. The stability of the structure with ferromagnetic, antiferromagnetic, and antiferrimagnetic spin systems was considered. The ferromagnetic configuration for Fe³⁺ -doped hollandite was found to be least stable, while an antiferromagnetic configuration was found to be most stable. The energy difference between the ferromagnetic system and this stable antiferromagnetic system is 0.74 eV. The energetically favorable antiferromagnetic spin configuration consists of high spin Fe heterogeneously spaced along the [010] (tunnel direction) and is 0.50 eV more favorable than Fe-doped hollandite with homogenous Fe spin (i.e., all up or all down) along the [010]. While antiferromagnetic spin state is favored, the ordering of the spin does not follow the same cation ordering preferences in Fe-doped hollandite.