  Topological materials have been discovered so far largely by searching for
existing compounds in crystallographic databases, but there are potentially new
topological materials with desirable features that have not been synthesized.
One of the desirable features is high tunability resulted from the band
inversion with a very small direct band gap, which can be tuned by changes in
pressure or strain to induce a topological phase transition. Using density
functional theory (DFT) calculations, we have studied the septuple layered
AB2X4 series compounds, where A=(Ge, Sn and Pb), B=(As, Sb and Bi) and X=(Se
and Te). With the DFT thermodynamic stability validated by the already reported
compounds in these series, we predict new stable Se compounds, which are not
found in crystallographic database. Among them, we find that GeBi2Se4 and
GeSb2Se4 having a small direct band gap at the Z point are very close to a
strong topological insulator, which can be tuned by a moderate pressure to
induce the band inversion. Importantly, the topological features with the small
direct band gap are well isolated in both momentum and energy windows, which
offers high tunability for studying the topological phase transition.
