  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.
