  We propose a theory of strong spin-lattice and spin-phonon coupling in iron
pnictides and discuss its implications on superconductivity. Strong
magneto-volume effect in iron compounds has long been known as the Invar
effect. Fe pnictides also exhibit this effect, reflected in particular to the
atomic volume of Fe defined by the nearest neighbor atoms. Through the
phenomenological Landau theory, developed on the basis of the calculations by
the density functional theory (DFT) and the experimental results, we quantify
the strength of the spin-lattice interaction as it relates to the Stoner
criterion for the onset of magnetism. We suggest that the coupling between
electrons and phonons through the spin channel may be sufficiently strong to be
an important part of the superconductivity mechanism in Fe pnictides.
