  We estimate the changes in lattice parameters due to iron isotope
substitution in the iron-based high temperature superconductors using a
variational calculation based on the anharmonicity of the Fe-As(Se) bond. For
BaFe2As2 and 57Fe to 54$Fe substitution we find a c-axis expansion of 1
10^{-3}{\AA} and in-plane a-axis contraction of 1 10^{-5}{\AA} at 250K in good
agreement with experimental values on (Ba,K)Fe2As2 for which a negative isotope
exponent \alpha=-\frac{d\ln T_c}{d\ln m}<0 was found [P.M. Shirage et al.,
Phys. Rev. Lett. 103, 257003 (2009)]. For FeSe in contrast we find an expansion
for both a and c lattice parameters. We discuss the relevance of these isotope
induced changes of lattice parameters to the isotope effect in light of the
well established sensitivity to the Fe-As(Se) coordination.
