Structures and Phase Transitions

The properties of iron under extreme conditions are intensively studied [1] due to its relevance for geophysical models of the earth’s interior. From a more general point of view, the behaviour of bcc iron under compression is interesting since almost half of the elements contain the bcc structure in their phase diagram. The phonon dispersion of bcc elements was studied in the past in search of dynamical precursor effects of lattice instabilities. Phonon dispersion curves exist for example for alkaline metals (Li, Na and K [2]) at low temperature as well as for transition metals (Zr, Ti, Hf and Sc) at high temperatures [3]. The lattice dynamics of bcc metals under reduced volume, however, is almost completely unknown. Similarly, the behaviour of the magnon dispersion under pressure is unknown for any of the ferromagnetic transition metals. We have measured both the phonon and magnon dispersion of Fe at room temperature up to 10 GPa, i.e. over almost the entire stability range of the bcc-phase (which transforms to hcp iron at ~11 GPa) [4]. The measurements were carried out at the 1T1 spectrometer similar to previous experiments on Ge and Zn [5]. Typical scans through phonon and magnon branches are shown in Fig. 1. The position of the frequencies could be determined with a statistical error of < 1% in almost all cases and the ambient pressure frequencies agree within ~ 2% with those of published values. Figure 2 shows the phonon dispersion along highsymmetry directions, 55 phonon mode frequencies have been determined at 0 and 9.8 GPa. All measured frequencies increase by 5-10% for a volume reduction corresponding to 5%. Of particular interest is the T1-mode with wave vector along [110] and [1-10] polarisation which increases completely regularly at the zone boundary (N-point). This observation is important for the transition mechanism to the hcp-phase at ~10 GPa. The latter can be obtained from the bccphase by displacements which involve the T1(N) phonon (Burgers-mechanism). Ab initio calculations on the bcc-hcp transition in Ba predict a substantial softening of the T1(N) phonon on approaching the transition pressure [6]. In iron, such dynamical precursor effects due to the Burgers mechanism seem to be absent. Our results are in agreement with spin-polarised fullpotential total energy calculations [7] which predict a slight increase of the T1(N) phonon frequency in the 0-10 GPa range. This scenario of the bcc-hcp transition is therefore different from that in Ba under pressure [6], and different from the situation found in all group 3 and 4 transition elements where there is a pronounced softening of the T1(N) frequency at ambient pressure and elevated temperature as the hcp phase is approached [3]. Iron seems to be an exceptional case in the sense that no dynamical precursor effects are visible on the T1(N) mode. The critical ingredient for this behaviour is the presence of ferromagnetism which stabilises the bcc-phase with respect to the non-magnetic fcc-phase. The effect of pressure is essentially to broaden the flat d-bands and thereby to decrease the density of states at the Fermi level D(EF) below the stability limit for ferromagnetism [7]. The bcc-hcp instability is hence not primarily due to phonon softening but due to the effect of pressure on the magnetism of iron.