Wide band-gap semiconductors have become an important base material for applications in opto-electronics and in high power, high temperature electronics. After doping with various Rare Earths, electro-luminescence throughout the whole visible spectrum has been observed. This very interesting application could become much more versatile if doping by implantation would be possible. We use the perturbed angular correlation technique and the probe nucleus 172Lu(172Yb) to study its behaviour after implantation into GaN. An unusually large variation of the quadrupole interaction frequency between 25 K and 700 K is observed. The results can be described excellently with a simple model if one assumes that the RE's partially filled 4f shell is responsible for this behaviour.