Evidence for two mechanisms of temperature-dependent “forbidden” resonant diffraction in ZnO

Abstract

The resonance spectrum of glide-plane forbidden x-ray diffraction have been measured for different temperatures in hexagonal ZnO near the zinc K-edge. This diffraction appears exclusively owing to anisotropy of the Zn atomic scattering factor induced either by thermal motion or by a temperature-independent dipole-quadrupole amplitude. The results provide the first evidence for a complex line shape in the spectrum of thermal motion induced scattering, and the first observation of a dramatic change in resonance spectrum with temperature.The measurements are in excellent agreement with a phenomenological model, based on a combination of constant and a temperature-dependent amplitudes. This model has provided a means of extracting amplitude spectra of each component, and their relative phase. We find that the strongest spectral feature of the room-temperature resonance spectrum arises from a sudden phase change between the two resonant processes. These results are of great importance for better understanding of the temperature effect on the excited electronic states in crystals like hybridization of different orbitals, etc.