The Collaborative International Dictionary of English v.0.48:

Thallic \Thal"lic\, a. (Chem.)
   Of or pertaining to thallium; derived from, or containing,
   thallium; specifically, designating those compounds in which
   the element has a higher valence as contrasted with the
   thallous compounds; as, thallic oxide.
   [1913 Webster]