  The topological effects of phonons have been extensively studied in various
materials, particularly in the wide-bandgap semiconductor GaN, which has the
potential to improve heat dissipation in power electronics due to its
intrinsic, topologically-protected, non-dissipative phonon surface states.
Nevertheless, the phase transition of the Weyl phonons in nitrides and their
composite alloys has yet to be elucidated. To unveil the microscale origin,
topological phonon properties in AlGaN alloys are investigated using the
virtual crystal approximation (VCA) and special quasi-random structure (SQS)
approaches in this work. It is found that phase transitions in Weyl phonons are
evidently present in AlGaN alloys and nitride single crystals. Under strain
states, both GaN and AlN show a more prominent phase transition of Weyl phonons
when subjected to biaxial compressive and uniaxial tensile strains. And it has
been observed that the zz components in the self-term and the transverse 1NN
force constants (FCs) are the most influential during the phase transition. The
nonlinear Weyl phonon transition in AlGaN alloys, as modeled by the VCA, is
reflected in the normalized self-term and first-nearest-neighbor (1NN) FCs,
which vary in a nonlinear fashion with an increasing magnitude. This nonlinear
phenomenon is also confirmed in the SQS modeling, where the unfolded phonon
dispersions are consistent with those in the VCA modeling. With increased
branches, hundreds of Weyl phonons are present accompanied by significant
disorders in normalized FCs, which mainly occur for N atoms in self-terms and
for all components in normalized 1NN FCs.
