  In the present work, we develop a systematic examination of the existence,
stability and dynamical properties of a discrete breather at the interface
between a diatomic and a monoatomic granular chain. We remarkably find that
such an "interface breather" is more robust than its bulk diatomic counterpart
throughout the gap of the linear spectrum. The latter linear spectral gap needs
to exist for the breather state to arise and the relevant spectral conditions
are discussed. We illustrate the minimal excitation conditions under which such
an interface breather can be "nucleated" and analyze its apparently weak
interaction with regular highly nonlinear solitary waveforms.
