(A=Ge, Sn, Pb; B=As, Sb, Bi; X=Se, Te) Compounds*

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.