Electronic and Lattice Properties of Layered Hexagonal Compounds Under Anisotropic Compression: A First-Principles Study

Abstract

We have investigated the wurtzite and hexagonal compounds (w-and h-BN, AlN, ZnO) under various compression conditions using the first-principles molecular dynamics (FPMD) method. Applying anisotropic compression is an important approach for the investigation of novel material properties. We found remarkable changes in the internal parameters u of all calculated wurtzite compounds under uniaxial c-axis compression (P z ) within the symmetry constraint. The internal parameter u of the wurtzite structure increased as the pressure increased and finally became 0.5, resulting in a phase transformation into a hexagonal structure. Transition pressures for BN, AlN and ZnO under c-axis compression are 300-325, 15-20, 5-10 GPa, respectively. The value of the transition pressure of BN (wurtzite ! hexagonal) was found to be significantly higher than those of the other two compounds (AlN, ZnO) in which the crystal structure of BN could be broken under the large uniaxial compression. The changes in the electronic band structure and the lattice properties (lattice constant, c=a ratio, volume of the unit cell) of BN in the wurtzite-to-hexagonal transformation were also quite large and unusual. These results imply that the wurtzite-to-hexagonal transformation of BN is unlikely to occur. In contrast, the changes in the lattice properties of AlN and ZnO were small because of their low transition pressures (5-20). These lower transition pressures are mainly due to larger ionicity of AlN and ZnO.

Influence of compressive uniaxial strain on the piezoelectric response of wurtzite crystals

“…This implies the occurrence of a strain-induced phase transition: the wurtzite structure transformes into the layered hexagonal structure. One can obtain an estimate of the transition pressure using the following formula [39]:…”

“…The transition pressure is calculated to be 19.0 GPa for AlN, 43.2 GPa for GaN, 16.1 GPa for InN, 82.1 GPa for BeO, and 11.9 GPa for ZnO. Kobayashi [39] has investigated AlN and ZnO under various pressure conditions, including uniaxial c-axis pressure. He observed the same transformation with transition pressures of 20 and 10 GPa for AlN and ZnO, respectively.…”

First-Principles Study of AlBN and Related Polytypes

“…It is expected that band gaps of AlBN polytypes may be direct because that of 2H-AlN is direct. [4][5][6][7][8] Furthermore, it will be possible to control the band gap values (0 ∼ 6 eV) and properties (indirect ↔ direct switching) tuning stacking sequences and ratios of the constituents (cation and anion) in the ternary polytypes. If they are really controllable, it will be possible to apply various optical regions (infrared, visible, ultraviolet, etc.).…”

First-Principles Study of Various BN, SiC, and AlN Polytypes

“…In particular, the direct band gap is required because it is expected a higher efficiency of luminescence than the indirect band gap. From the previous studies 2,3) and the direct band gap of 2H-AlN, [20][21][22][23][24] we expect that higher AlN polytypes may have the direct band gap. Therefore, two 30H-AlN polytype structures were investigated in this study.…”