Inelastic neutron scattering study of YBaFe$_2$O$_5$

YBaFe$_2$O$_5$ belongs to a new class of oxides with the chemical formula $R$Ba$M_2$O$_5$ ($R=$~rare-earth, $M=$~transition metal), based on the perovskite structure with a doubled unit cell pyramids of five-coordinated $M$-sites. The $M$-site is mixed valent in the stoichiometric formula unit (with an average valence of +2.5). Therefore, charge and orbital ordering phenomena can exist on the $M$-site and be studied without introducing disorder. The charge ordered phase of YBaFe$_2$O$_5$ is unusual, since it does not satisfy the Anderson criterion (i.e. it is not the lowest energy electrostatic arrangement of charges), but rather orders into alternating chains of 2+/3+. This indicates that other interactions, such as electron-phonon coupling, are necessary to arrive at the chain structure. Here, we present the results of an inelastic neutron scattering study of polycrystalline YBaFe$_2$O$_5$. We find the spectrum of phonon and magnetic excitations are clearly modified at the charge- and magnetic ordering temperatures: T$_{CO}$ = 308~K and T$_N$ = 430~K, respectively.