In situ high pressure (0-1 GPa) Mössbauer spectroscopy has been used to demonstrate for the first time the enhanced compressibility $\kappa$ of the intercrystalline (IC) regions of nanocrystalline iron compared with that of bulk crystalline $\alpha$-iron (${\kappa }_{\mathrm{IC}}\approx 6.6{\kappa }_{\mathrm{Fe}}$). The component of the Mössbauer spectra due to IC regions exhibits a pressure-induced reversible change of the center shift that is about an order of magnitude larger than that of crystalline bulk $\alpha$-iron. The effect saturates at pressures above 600 MPa. This change of center shift was removed on pressure annealing the samples, indicating transformation of the interfacial region to a more dense, less compressible structure.

• Received 17 February 1995

DOI:https://doi.org/10.1103/PhysRevLett.75.3760