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Astron. Astrophys. 357, L13-L16 (2000)

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3. Properties of FeSi

The crystalline structure of FeSi is of cubic symmetry but with a rather unusual arrangement of the atoms in the unit cell, providing an example of coordination number 7. Each iron (silicon) atom is surrounded by 7 silicon (iron) atoms at distances between 2.28 and 2.50 Å. This arrangement seems to result from a particular distribution of the valence electrons in the bonding between the atoms (Pauling & Soldate 1948). This binary compound is not a line compound, implying that its chemical composition allows for some spread in the occupation of the atomic sites.

FeSi is also known for its unusual electronic and magnetic properties. Although it has been studied in some detail some 30 years ago (Jaccarino et al. 1967), it has recently enjoyed a renewed interest in relation with the general and topical issues of correlation effects among itinerant electrons in metals. The more recent investigations involved measurements of thermal, transport and optical properties of single crystalline FeSi. The results indicate that FeSi is a semiconductor with rather unusual features in the structure of the electronic excitation spectrum (Paschen et al. 1997and references therein). Below room temperature, the electronic conductivity decreases by more than five orders of magnitude but a non zero conductivity persists at temperatures below 0.1 K. As may be seen in Fig. 3, the conductivity at 300 K is still high enough to effectively screen the lattice excitations in the far-IR region at approximately 30 and 50 µm, respectively. If the absorption at 47.5 µm shown in Fig. 4 is indeed due to solid FeSi, its observation implies that the temperature adopted by these dust particles must be below 200 K.

[FIGURE] Fig. 3. Optical conductivity of FeSi at different temperatures (according to Degiorgi et al. 1994).

[FIGURE] Fig. 4. Upper Part: Spectrum of AFGL 4106 (taken from Molster et al. 1999) in the far infrared wavelength region [FORMULA]m. The strong bands are due to forsterite and enstatite. Lower part: Absorption efficiency [FORMULA] of FeSi grains with 0.1µm radius at [FORMULA] K. A broad distribution of grain radii would broaden the absorption bands.

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© European Southern Observatory (ESO) 2000

Online publication: May 3, 2000