  Non-invasive local probes are needed to characterize bulk defects in binary
and ternary chalcogenides. These defects contribute to the non-ideal behavior
of topological insulators. We have studied bulk electronic properties via
$^{125}$Te NMR in Bi$_2$Te$_3$, Sb$_2$Te$_3$, Bi$_{0.5}$Sb$_{1.5}$Te$_3$,
Bi$_2$Te$_2$Se and Bi$_2$Te$_2$S. A distribution of defects gives rise to
asymmetry in the powder lineshapes. We show how the Knight shift, line shape
and spin-lattice relaxation report on carrier density, spin-orbit coupling and
phase separation in the bulk. The present study confirms that the ordered
ternary compound Bi$_2$Te$_2$Se is the best TI candidate material at the
present time. Our results, which are in good agreement with transport and ARPES
studies, help establish the NMR probe as a valuable method to characterize the
bulk properties of these materials.
