Zirconium foils (superconducting transition at
T
c
=0.7 K) were implanted at liquid helium temperatures with hydrogen, deuterium and helium. In all cases a remarkable increase of
T
c
was observed. Implantation of the inert He atoms leads only to the introduction of lattice defects, which is known to increase
T
c
of Zr. However, the implanted H and D atoms exert an additional influence on the superconducting behavior based on a change of the electronic properties and an increase of the electron-phonon coupling. Introduction of lattice defects seems to account for the
T
c
increase to 1.49 K after He implantation, whereas
T
c
is even more enhanced by implantation of the hydrogen isotopes. The maximum values for
T
c
are 3.14 K for H and 4.65 K for D implantation. The concentration necessary to produce a saturation in
T
c
for both isotopes is H(D)/Zr
0.13. The remarkably high
T
c
for the heavier isotope corresponds to an extreme inverse isotope effect. A smaller inverse isotope effect was found earlier in the Pd –H(D) system, where it could be explained by anharmonic effects.
Dedicated to Prof. Dr. W. Buckel on the occasion of his 60th birthday