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Topological materials have been discovered so far largely by searching forexisting compounds in crystallographic databases, but there are potentially newtopological materials with desirable features that have not been synthesized.One of the desirable features is high tunability resulted from the bandinversion with a very small direct band gap, which can be tuned by changes inpressure or strain to induce a topological phase transition. Using densityfunctional theory (DFT) calculations, we have studied the septuple layeredAB2X4 series compounds, where A=(Ge, Sn and Pb), B=(As, Sb and Bi) and X=(Seand Te). With the DFT thermodynamic stability validated by the already reportedcompounds in these series, we predict new stable Se compounds, which are notfound in crystallographic database. Among them, we find that GeBi2Se4 andGeSb2Se4 having a small direct band gap at the Z point are very close to astrong topological insulator, which can be tuned by a moderate pressure toinduce the band inversion. Importantly, the topological features with the smalldirect band gap are well isolated in both momentum and energy windows, whichoffers high tunability for studying the topological phase transition.

Highly Tunable Band Inversion in AB2X4 (A=Ge, Sn, Pb; B=As, Sb, Bi; X=Se, Te) Compounds