Topological behaviour of ternary non-symmorphic crystals KZnX (X = P, As, Sb) under pressure and strain: a first principles study

Abstract

An ab initio study on the impact of hydrostatic pressure and strain on the electronic properties of an unexplored class of ternary Zintl phases KZnX (X = P, As, Sb) is reported. Density functional theory (DFT) based studies revealed that all the three materials are direct band gap semiconductors under ambient conditions. We have theoretically demonstrated that KZnX can be driven into different metallic phases under pressure. In contrast, by applying strain the compounds can be realized as topological insulators. This is confirmed by the observed non-trivial topological character in the electronic band structure of the present ternary systems. For the precise determination of low energy band topology, the Tran Blaha modified Becke-Johnson (TBmBJ) exchange potential was used by incorporating spin-orbit coupling. The concomitant change of electronic band shapes as a function of pressure indicates a semi-metallic nature in KZnX (X = P, As, Sb) at 30 GPa, 21 GPa and 11 GPa respectively. Based on an analysis of the parity eigenvalues, we anticipate that a band inversion occurs between the Zn-s and X-p states, thus demonstrating a weak topological behaviour in semi-metallic states. Also, a weak non-trivial topologically insulating phase is realized in strained KZnAs (18%) and KZnSb (10%) which appears to be due to overlapping of the Zn-s and X-p orbitals. The calculated surface spectral functions further validate the non-triviality of strained KZnX (X = As, Sb), whereas strained KZnP is found to be a trivial insulator. We confirm the topological behaviour of these materials by calculating topological surface states and defining a Z topological invariant. Our work based on sophisticated first-principles calculations highlights that both pressure and strain can trigger topological phases in non-symmorphic trivial band insulators even with a weak spin orbit interaction. This study paves the way for realizing semi-metallic and topological insulating states in non-symmorphic ternary semiconductors, which have not been experimentally demonstrated so far.

Ab initio study on anisotropic thermoelectric transport in ternary pnictide KZnP

“…The intermediate ZnP layer is similar to graphene with breaking sublattice symmetry, and the adjacent ZnP layers are AB stacked intercalated by K in between. The optimized lattice constants are a∼4.10 Å and c∼10.27 Å using the PBE functional, which is consistent with the previous calculations [25] and experimental results [26].…”

“…The Γ-point hosts a direct band gap of about 0.85 (1.57) eV based on the PBE (HSE06) calculations. Compared to the band gap of 2.0 eV calculated from the TBmBJ potential [25], the band gap predicted using HSE06 functional (1.57 eV) is underestimated. Interestingly, KZnP exhibits a 'flat-and-dispersive' band character that the valence band is nearly flat along the c axis direction but highly dispersive in the ab plane, while the conduction band is significantly dispersive along all directions.…”

“…Here we take KZnP as an example. The formation energy per unit cell of KZnP with the symmetry group of P6 3 /mmc is −0.46 eV, which is 58 meV lower than that of K 4 ZnP 2 with the symmetry group of m R3 [25]. The intermediate ZnP layer is similar to graphene with breaking sublattice symmetry, and the adjacent ZnP layers are AB stacked intercalated by K in between.…”

First principle investigations of the structural, electronic, and phase stability in 2D layered ZnSb

“…Similarly, the compound KZnSb (p6 3 /mmc,n o 194), ICSD No.12161 is also stable in hexagonal phase at 0 K with the atoms K, Zn and Sb at Wyckoff position 2a, 2d, and 2c respectively. LiZnSb and KZnSb shows direct semiconducting behavior in all the computational methods including GGA; [45][46][47] however, the calculated band structure for NaZnSb depends on the method used. It shows metallic behavior within local density approximation (LDA), and generalized gradient approximation (GGA) 48,49 but it is predicted to be a semiconductor with a narrow, direct band gap within full potential linear augmented plane wave (FPLAPW) method with the modified Becke-Johnson potential(MBJ), and Cambridge Serial Total Energy Package (CASTEP) program within GGA.…”

The Zintl phase compounds AEIn2As2 (AE = Ca, Sr, Ba): topological phase transition under pressure

“…And the introduction of pressure often leads to interesting and important potential properties, such as thermoelectric performance control, 33 structural phase transition, 29,34–37 metal–insulator phase transitions with band gap changes 38–41 and topological phase transitions with band inversion. 29–32,42–47…”