Phase transition in crystalline GeTe: Pitfalls of averaging effects

For the case example of GeTe, we demonstrate that the use of techniques that probe the average structure may lead to misleading conclusions regarding the nature of phase change. GeTe is a narrow band-gap semiconductor and a ferroelectric with the simplest conceivable structure that-according to previous studies-undergoes a ferroelectric-to-paraelectric displacive phase transition at ∼705 K. In this work, we provide direct experimental evidence that, contrary to the existing paradigm, the local distortion remains essentially unchanged with temperature and argue that the previous conclusion about the displacive nature of the ferroelectric-to-paraelectric transition was due to misinterpretation of Bragg diffraction, a technique that is only sensitive to the average structure and does not "see" random local distortions. The reported results have far-reaching implications for other materials exhibiting displacive phase transitions where conclusions have been reached based on results obtained using averaging techniques.