  We studied the oxygen isotope effect (OIE) on the zero point motion and the
thermal motion as well as on structural parameters in La$_2$CuO$_4$ by means of
high-resolution neutron diffraction experiments. We found a negative OIE on the
lattice parameters (-0.01%). The OIE on the isotropic thermal parameters turned
out to be positive for lanthanum, copper and negative for the oxygen atoms,
respectively. The Rietvield refinement of the anisotropic thermal parameters of
the different directions yielded the same sign of the OIE for each ion. Our
analysis shows that the OIE on isotropic thermal parameters is most pronounced
for La at 15 K (up to 100%) which we found to be originating mainly from the
thermal motion in x-direction as determined from the refinement of the
anisotropic thermal parameters.
