Electronic structure study of YNbTiO$_6$ vs. CaNb$_2$O$_6$ with U, Pu and minor actinide substitutions using compound-tunable embedding potential method

TL;DR

This study uses the compound-tunable embedding potential method to analyze actinide substitutions in niobate crystals, revealing oxidation state changes and structural effects through electronic and geometric analysis.

Contribution

It introduces a detailed computational approach combining one-center and multi-center models to investigate actinide doping effects in YNbTiO$_6$ and CaNb$_2$O$_6$.

Findings

Oxidation state changes depend on initial states and actinide type.

Electron transfer effects vary with actinide and oxidation state.

Structural distortions are similar across different actinide substitutions.

Abstract

The compound-tunable embedding potential (CTEP) method is applied to study actinide substitutions in the niobate crystals YNbTiO$_6$ and CaNb$_2$O$_6$. Two one-center clusters centered on Ca and Y are built and 20 substitutions of Ca and Y with U, Np, Pu, Am, and Cm in four different oxidation states were made for each cluster. Geometry relaxation is performed for each resulting structure, and electronic properties are analyzed by evaluating the spin density distribution and X-ray emission spectra chemical shifts. Though the studied embedded clusters with actinides having the same oxidation state are found in general to yield similar local structure distortions, for Am and Cm in high "starting" oxidation states the electron transfer from the environment was found, resulting in decrease of their oxidation states, while for "starting" U$^{\rm III}$ state the electron transfer goes in the opposite direction, resulting in increase of its oxidation state to U$^{\rm IV}$. The U substitutions are additionally studied with the use of multi-center models, which can provide both more structural and electronic relaxation and also include charge-compensating vacancies. For "starting" U$^{\rm VI}$ case, the decrease in oxidation state similar to that of Am$^{\rm VI}$ and Cm$^{\rm VI}$ in one-center clusters is observed in our calculations but in a different way. Since the really synthesized YNbTiO$_6$ structures can not be considered as perfect (periodic) crystals because the Nb and Ti atoms are statistically distributed within them occupying the same Wyckoff positions, different Ti $\leftrightarrow$ Nb substitutions are studied and corresponding structural changes are estimated.