Many liquids can be cooled far below their equilibrium melting temperature Tm without solidification, giving origin to a metastable undercooled liquid phase. In the absence of any external driving forces, which trigger the so-called heterogeneous nucleation process, the equilibrium crystalline phase is reached through density fluctuations, leading to the spontaneous formation of a crystalline nucleus (homogeneous nucleation). The structure of this metastable phase and the local rearrangements that lead to the nucleation mechanism represent a fundamental topic in the understanding of microscopic atomic interactions. It is still poorly understood whether this metastable liquid phase can be identified as a frozen version of the equilibrium liquid above Tm, or if novel atomic configurations appear at very deep undercooling. In this report we present an investigation of the local structure of undercooled liquid Ge by means of time-resolved XAS. 

The equilibrium phase of liquid Ge (l) has several intriguing but poorly understood properties. Upon melting, Ge undergoes a semiconductor-to-metal transition, as evidenced by a jump in the conductivity by a factor of 11. At the same time, its density increases by ~ 5 % and its structure goes from an open diamond-like lattice with coordination number equal to 4 to a more compact liquid structure characterised by a coordination number between 6 and 7. This is fairly unusual as most liquid metals are more closely packed with a coordination ~ 12. A very similar phenomenology is observed for Si.

Figure 94
Fig. 94: XAS spectra recorded during a cooling cycle. 200 spectra (520 ms/spectrum) were collected at intervals of 1.2 s, but only the temperature region around crystallisation is shown. A continuous trend from very smooth spectra characteristic of the liquid to the more structured spectra typical of a crystalline state is clearly visible.

The Ge K-edge XAS measurements were performed on the dispersive XAS beamline ID24. By collecting full X-ray absorption spectra in less than a second [1], snapshots of the electronic and local structure in the metallic phase of Ge were taken at temperatures much lower than had been possible with previous static XAS investigations [2]. Upon ramping the temperature it was possible to follow the temperature evolution of the complete XAS spectrum (Figure 94), which provides information about the local atomic arrangement, through the Fourier Transform of the EXAFS oscillations and the reconstructed pair correlation function g(r). Tracking the position of the edge, physically connected with the density of states at the Fermi level, we followed the temperature evolution of the electronic structure, i.e. the appearance of the gap at the onset of the metal-semiconductor transition (Figure 95).

Figure 95
Fig. 95: (Upper panel) Measured edge shift values ( E) of the spectra shown in the inset, as a function of temperature during a heating/cooling cycle. The sample melts at T = Tm and freezes at T ~ Tf. Upon freezing the edge shifts to higher energies due to the formation of the gap at the metal-semiconductor transition; (Inset) the first (liquid Ge) and last (crystalline Ge) of a series of 500 spectra (120 ms/spectrum) collected over the near edge region; (Lower panel) Maximum amplitude of the first peak in the Fourier Transform as a function of temperature. The solid line indicates the average trend above 900 K.

By an up-to-date data analysis treatment, we find that upon a rapid freezing of the melt, and prior to the occurrence of the solidification transition, a sharpening and a slight shift of g(r) towards smaller distances occur. We interpret this phenomenon in terms of increased stability of the covalent bonds, which are continuously formed and destroyed in the liquid.

Our observations are therefore compatible with the possibility that, at high undercooling, tetrahedral configurations in liquid Ge are more probable, the melt still being in a metallic phase. This behaviour confirms the trend suggested by static XAS measurements performed at lower undercoolings.

[1] S. Pascarelli, T. Neisius and S. De Panfilis, J. Synchr. Rad. 6, 1044 (1999).
[2] A. Filipponi and A. Di Cicco, Phys. Rev. B 51, 12322 (1995).

Principal Publication and Authors
S. Pascarelli (a), S. De Panfilis (b), and T. Neisius (a), Phys. Rev. B 62, 3717 (2000).
(a) ESRF
(b) INFM, Camerino (Italy)