Experimental observation of phonons as spectators in FeSi electronic gap formation

The evolution of a phonon spectrum in a narrow-gap semiconductor FeSi was investigated in a wide range of temperatures (46 K [< or =] T [< or =] 297 K, P = 0.1 MPa) and pressures (0.1 MPa [< or =] P [< or =] 43 GPa, T = 297 K) using nuclear inelastic scattering of synchrotron radiation with the energy resolution 0.53 meV. Decreasing temperature as well as increasing pressure causes a strong reorganization of the phonon spectrum manifested in splitting and shifts of the phonon peaks. The phonon spectra measured under the temperature and pressure conditions corresponding to the same unit cell volume reveal nearly complete matching. On the contrary, the spectra measured under the conditions of the equal mean-square atomic displacements differ drastically. These observations suggest that the transformation of the electronic spectrum of FeSi is controlled predominantly by the change of the unit cell volume. The corresponding changes of the interatomic forces and the resulting modification of the phonon spectrum appear as the direct consequence of this transformation.