  By means of ternary chemical potential phase diagram and phonon spectrum
calculations, we propose that $M$Tl$_4$Te$_3$ ($M$ = Cd, Hg), the derivatives
of Tl$_5$Te$_3$, are thermodynamically and dynamically stable in the body
centered tetragonal crystal structure with $I$4/$mcm$ symmetry. Our electronic
structures calculations confirm that a robust $s$-$p$ band inversion occurs
near the Fermi level in $M$Tl$_4$Te$_3$, and a topological band gap about 0.13
eV in CdTl$_4$Te$_3$ is induced by the spin-orbit coupling. These results
suggest that $M$Tl$_4$Te$_3$ are a new class of large band gap 3D strong
topological insulators that are stable and synthesizable in experiments, which
could be used to design efficient spin torque equipment and spin device.
