Large‐Gap Quantum Spin Hall State in Double‐Transition‐Metal Homologous Compounds of WSi2N4: A First‐Principles Study

Recently, the MoSi 2 N 4 (MoN) 4 n homologous compounds have been synthesized, extending the MA 2 Z 4 family materials to the superthick MA 2 Z 4 (MZ) n form. Herein, a first‐principles study on the double‐transition‐metal W 2 TiSi 2 N 6 and related group VI–IV M 2 M′Si 2 N 6 nanosheets is performed. Robust structural stability is affirmed in this W 2 TiSi 2 N 6 nanosheet from the energetic, mechanical, dynamical, and thermal perspectives. Unlike the WSi 2 N 4 and WSi 2 N 4 (WN) n systems, the W 2 TiSi 2 N 6 nanosheet exhibits a semimetal behavior with the top valence and bottom conduction bands touching each other at the Fermi level. With the inclusion of spin–orbit coupling, a nontrivial band gap is opened in the W 2 TiSi 2 N 6 nanosheet, which has a sizeable bulk gap of 0.11 eV. The Z 2 invariant is 1 and a pair of topologically protected edge states emerges in the gap region, confirming that W 2 TiSi 2 N 6 nanosheet is a quantum spin Hall (QSH) insulator. Through in‐plane strain engineering, the bulk gap can be further increased to 0.23 eV, which is sufficiently large for the room‐temperature QSH effect. Such a large‐gap QSH state is also present in other group VI–IV M 2 M′Si 2 N 6 systems with the combinations of MM′ = WZr, MoZr, and MoHf. This study demonstrates that the double‐metal M 2 M′Si 2 N 6 nanosheets are promising candidates to realize fascinating topological quantum states.