Cubic anharmonicity of the Fe-As bond of the iron-pnictides estimated from Raman spectroscopy.

We study the phonon spectrum of the iron-pnictide superconductors, (Ce,Nd)O$_{1-x}$F$_{x}$FeAs, using Raman spectroscopy. Based on the temperature dependent softening of the out of plane optical Fe-mode (B$_{1g}$ at $\Gamma$) we estimate the magnitude of the cubic anharmonicity of the Fe-As bond by calculating the self-energy due to phonon-phonon interactions. This also gives an estimate of the lattice expansion or contraction due to Fe isotope substitution which may in turn influence electronic properties.