As can be seen from the temperature evolution of the PL peak position, the bandgap blueshift induced by thermal lattice expansion is found to be gradually compensated by the bandgap redshift caused by electron–phonon coupling for the temperature variation from 5 K to 360 K. As the temperature is further increased, the nearly completely compensated PL peak position turns to exhibit a rapid blueshift, again at ∼360 K. Such turning behavior is consistent with an orthorhombic–tetragonal (γ–β) phase transition at this critical temperature.