Reexamination of the isomer shift has led us to propose that the phonons in a lattice change the electron density at the site of the resonant nucleus, thereby causing a change in the energy of the emitted $\gamma$ photon in the recoilless emission of radiation. The new shift is highly temperature dependent and is significantly smaller than the well-known isomer shift. Experimental evidence is presented.

• Received 21 August 1969

DOI:https://doi.org/10.1103/PhysRevB.1.955