Bond-bending modes and stability of tetrahedral semiconductors under high pressure: a puzzle of AlN

TL;DR

This study investigates the unusual positive pressure shift of bond-bending modes in AlN under high pressure, revealing discrepancies with theoretical predictions and highlighting its unique stability behavior among tetrahedral semiconductors.

Contribution

It provides the first experimental evidence of an abnormal positive pressure shift of bond-bending modes in AlN, challenging existing ab initio calculations and understanding of lattice dynamics.

Findings

Bond-bending E_2^1 mode shifts positively under pressure in AlN.

Discrepancy between experimental results and ab initio calculations.

AlN's stability behavior differs from other tetrahedral semiconductors.

Abstract

Lattice vibrations of the wurtzite-type AlN have been studied by Raman spectroscopy under high pressure up to the structural phase transition at 20 GPa. We have shown that the widely debated bond-bending E_2^1 mode of w-AlN has an abnormal positive pressure shift up to the threshold of the phase transition, whereas in many tetrahedral semiconductors the bond-bending modes soften on compression. This finding disagrees with the results of ab initio calculations, which give a "normal" negative pressure shift. Combination of high dynamical and low thermodynamical stability of AlN breaks the correlation between the mode Gruneisen parameters for the bond-bending modes and the transition pressure, which holds for CdS, InP, ZnO, ZnTe, ZnSe, ZnS, Ge, Si, GaP, GaN, SiC and BeO.