Nb2O5

from The Collaborative International Dictionary of English v.0.48

Columbic \Co*lum"bic\, a. [From {Columbium}.] (Chem.)
   Pertaining to, or containing, columbium or niobium; niobic.
   [1913 Webster]

   {Columbic acid} (Chem.), a weak acid derived from columbic or
      niobic oxide, {Nb2O5}; -- called also {niobic acid}.
      [1913 Webster]