"The emission or absorption of a quantum by a nucleus bound in a crystal leads in general to an alteration of the vibrational state of the crystal lattice, which takes up the recoil momentum. On account of the quantization of the internal energy, the crystal can absorb the recoil energy only in discrete amounts. With decreasing temperature the probability of excitation of the internal states decreases more and more, so that for soft gamma rays for a part in the quantum transitions the crystal as a whole takes up the recoil momentum. The quanta emitted or absorbed in this situation undergo practically no energy loss because of the large mass of the crystal, and ideally satisfy the resonance condition".
Rudolf Mossbauer
http://motls.blogspot.com/2009/01/rudolf-mossbauer-80th-birthday.html