In the course of a systematic investigation of the modification of oxide layers on Si substrates upon low-temperature swift heavy ion irradiation we found that distinct phase formation and phase separation occurs at the NiO/Si interface, accompanied by the build-up of a lateral nano-scale undulated structure. Utilizing Rutherford backscattering spectroscopy and transmission electron microscopy (cross-section imaging, energy filtered imaging, energy dispersive X-ray spectroscopy) it was possible to identify the complex interface structure. The detection of NiSiO 3 , a high temperature and high pressure phase, reflects the extreme conditions in the excited ion track.