Electronic structure study of YNbTiO$$_6$$ and CaNb$$_2$$O$$_6$$ with actinide impurities using compound-tunable embedding potential method

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 are built and centered on Y and Ca, and 20 substitutions of Y and Ca with U, Np, Pu, Am, and Cm were made in four different oxidation states for each cluster. Geometry relaxation is performed for each resulting structure, and electronic properties are analyzed by evaluating the spin density distribution and chemical shifts of X-ray emission spectra. Though the studied embedded clusters with actinides having the same oxidation state are found in general to yield similar local structure distortions, for Am, Cm and Pu in high “starting” oxidation states the electron transfer from the environment was found, resulting in decrease of their oxidation states. 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 $$^\textrm{VI}$$ case, the decrease in the oxidation state similar to that of Am $$^\textrm{VI}$$ and Cm $$^\textrm{VI}$$ in one-center clusters is observed in our calculations but in a different way, while for “starting” U $$^\textrm{III}$$ state the reverse process takes place, resulting in an increase in the oxidation state of uranium to U $$^\textrm{IV}$$ . It is known experimentally that the Nb and Ti atoms in YNbTiO $$_6$$ are statistically distributed and occupy the same Wyckoff positions. With the CTEP method, it is possible to simulate to a certain extent the effects of such random distribution on the basis of perfect crystal calculation by performing Ti $$\leftrightarrow$$ Nb substitutions in the embedded clusters. The results were compared to those obtained using the special quasirandom structures (SQS) method with structural relaxation for the single and double cell.