Structural and electronic properties of Fe₂SiO₄‐fayalite at ultrahigh pressures: Amorphization and gap closure

Abstract

X ray diffraction and infrared spectroscopic measurements to pressures of 45–50 GPa, electrical resistivity and optical absorption to 70–80 GPa, and reflectance measurements to 225 GPa are presented for Fe2SiO4 fayalite at 300 K. Diffraction results document that Fe2SiO4 fayalite becomes amorphous on static loading to pressures in excess of 39 (±3) GPa, a pressure identical to that at which the “mixed phase” regime of fayalite commences under shock compression. As with more polymerized silicates, the high‐pressure amorphization of metastable fayalite is associated with the SiO4 tetrahedron becoming unstable relative to higher coordinations of silicon. Infrared absorption spectroscopy reveals the pressure‐induced change in coordination through a decline in intensity of the tetrahedral asymmetric stretching vibration, accompanied by an increase in amplitude at frequencies characteristic of SiO6 vibrations. The coordination change is not quenchable to zero pressure, and infrared spectra of amorphous fayalite quenched from pressure document that the local structure of the sample is similar to that of crystalline Fe2SiO4. This represents the first example of the static synthesis of a glass without fusion in a silicate containing isolated tetrahedra. The electrical resistivity of initially crystalline Fe2SiO4 drops from approximately 2×105 ohm m at zero pressure to 10−3 ohm m at 79 GPa, with the pressure dependence of the resistivity decreasing at approximately the pressure of the coordination change (and amorphization). The electrical properties are quantitatively consistent with previous measurements, both under static and under shock wave loading. Optical transmission experiments demonstrate that in accord with the rapidly changing electron‐transport properties of Fe2SiO4, a strong absorption edge decreases in energy from the ultraviolet to infrared energies of about 3500 cm−1 (0.4 eV) on compression to 70 GPa. However, the reflectance of Fe2SiO4 is less than 3.1% between 780 and 2100 nm at all pressures to 225 GPa, indicating that amorphous fayalite does not metallize to at least this pressure. We interpret our resistivity and optical results in terms of increased interactions between iron ions with increasing pressure in Fe2SiO4 and an approach toward a Fe3++Fe1+ metallic configuration at high pressures. When combined with previous observations of amorphization at low temperatures and high pressures, our results suggest that transition to an amorphous phase is likely to be a general phenomenon in metastable silicates when the distortion of SiO4 tetrahedra becomes a major mechanism of compression.

Compression and amorphization of (Mg,Fe)2SiO4 olivines: An X-ray diffraction study up to 70 GPa

“…As in the present study, only diffraction rings of amorphous phases were detected at these pressures by Richard and Richet (1990) with the same film technique used by Williams et al (1990). A salient feature of the volumes calculated from our unit cell parameters (Fig.…”

“…For fayalite, the positions of the diffraction peaks tend to converge at around 26 GPa to form a broad peak that has been followed up to 52 GPa. This behavior is associated with the amorphization that takes place in this pressure range as described by Richard and Richet (1990), Williams et al (1990), Guyot and Reynard (1992) from TEM, X-ray diffraction or infrared spectroscopy.…”

“…This compressional anisotropy could lead to an hexagonal symmetry at very high pressures. For fayalite, however, the pressure-induced transformation that has been observed at room temperature is amorphization (Richard and Richet 1990;Williams et al 1990). Observations of quenched samples by transmission electron microscopy (TEM) indicate that amorphization also develops for Mg-bearing olivines at much higher pressures than for fayalite (Guyot and Reynard 1992;Bohifd et al in prep.…”

First-principles studies of the electronic structure and magnetism in fayalites: M2SiO4 (M=Fe and Co)

“…These behaviors could stem from strong correlations among the crystal structure and the electronic or magnetic properties. At room temperature and pressure, fayalite is an insulator [3,4].Recently, the electronic structure has been studied by experimental investigation for better understanding of its properties [4]. In particular, the application of ab initio techniques to the study of systems of geophysical interest has expanded…”

“…These behaviors could stem from strong correlations among the crystal structure and the electronic or magnetic properties. At room temperature and pressure, fayalite is an insulator [3,4].…”