Search

Iron(II) oxide, wüstite, is the iron oxide with the lowest oxygen content. Under ambient conditions it is metastable for two reasons: (1) it undergoes eutectoid decomposition to Fe and Fe3O4 below ≈570° C, and (2) depending on temperature, it is thermodynamically stable only for very low oxygen partial pressures, down to below 10−20 bar. Hence, for the growth of single crystals from the melt, the growth atmosphere must contain reducing components to keep the oxygen partial pressure on the required low level. With Ar/CO2/CO mixtures this aim can be reached. It is shown experimentally and by thermodynamic calculation, that the grown crystals contain carbon inclusions. Theoretically it is shown that wüstite crystals without carbon inclusions could be grown in humid N2/H2 mixtures. First experiments are presented in this article, but a further adjustment of experimental parameters is required. [ABSTRACT FROM AUTHOR]

Planetary cores are composed mainly of Fe with minor elements such as Ni, Si, O, and S. The physical properties of Fe alloys depend on their composition. Changes in c/a ratio, center shifts, and elastic properties of Fe and Fe–Ni alloys were reported previously. However, such properties of Fe light-element alloys have not yet been extensively studied. Si is a plausible candidate as a light element in planetary cores. Therefore, we studied the electronic properties and compressional behavior of Fe-Si alloys with a hexagonal-close-packed (hcp) structure under high pressure using synchrotron Mössbauer spectroscopy (SMS) and X‑ray diffraction (XRD). Center shifts (CS) were observed at pressures of 21.4–45.3 GPa for Fe-2.8wt%Si and of 30.9–62.2 GPa for Fe-6.1wt%Si. Some of SMS and XRD measurements were performed under the same conditions using a newly developed system at the BL10XU beamline of SPring-8, which allowed simultaneous characterization of the electron information and crystal structure. Changes in the CS values were observed at 36.9 GPa in Fe-2.8wt%Si and 54.3 GPa in Fe-6.1wt%Si. The ratios of c/a in the hcp structure were measured at pressures of 21.2–49.6 GPa in Fe-2.8wt%Si and 32.9–61.4 GPa in Fe-6.1wt%Si. The c/a ratio changed at pressures of 30–45 GPa in Fe-2.8wt%Si and at 50 GPa in Fe-6.1wt%Si. Changes in the CS and c/a ratio were explained according to the electronic isostructural transition in Fe–Si alloys. In addition, the transition pressure increased with increasing Si content in metallic iron. This finding is significant as changes in seismic wave velocities due to the change in c/a ratio of Fe–Si alloys with an hcp structure might be observed if Venus has a solid inner core. [ABSTRACT FROM AUTHOR]

The crystal structure and crystal chemistry of chromian pumpellyite from basic schist in the Osayama ultramafic body, Okayama, Japan, were investigated using electron probe microanalysis (EPMA), Fourier transform infrared spectroscopy (FTIR), and single-crystal X-ray diffraction to determine the distribution of chromium between two independent octahedral sites and structural changes caused by ionic substitutions in pumpellyite, VIIW8 VIX4 VIY8 IVZ12O56-n(OH)n (Z = 1). Five chromian pumpellyite crystals (ocp1211, ocp0604, ocp1028, ocp1013, and ocp1016) with 0.52, 1.65, 1.26, 1.94, and 1.43 Cr atoms per formula unit (apfu) (EPMA data), respectively, were picked from a hand specimen for X-ray diffraction analysis. The crystal structures (space group C2/m) of ocp1211 [a = 19.1129(6), b = 5.8963(5), c = 8.818(1) Å, β = 97.449(2)°], ocp0604 [a = 19.0935(4), b = 5.900(1), c = 8.810(2) Å, β = 97.540(7)°], ocp1028 [a = 19.105(2), b = 5.9021(6), c = 8.8143(7) Å, β = 97.513(3)°], ocp1013 [a = 19.1558(6), b = 5.9125(9), c = 8.844(1) Å, β = 97.448(4)°], and ocp1016 [a = 19.174(3), b = 5.9194(8), c = 8.830(1) Å, β = 97.497(4)°] were refined using 1058, 829, 1070, 1105, and 1095 unique reflections, and calculations converged at R factors of 4.08, 5.02, 6.32, 6.92, and 7.88%, respectively. The resulting site populations at the X and Y sites are (Mg1.88Al1.51Fe0.382+ Cr0.16Mn0.052+Ni0.02)X(Al7.90Ti0.07V0.03)Y for ocp1211, (Mg1.81Al1.53Cr0.42Fe0.182+Mn0.042+Ni0.01)X(Al7.34Cr0.65V0.01)Y for ocp0604, (Al1.62Mg1.60Cr0.61Fe0.132+ Mn0.032+Ni0.01)X(Al7.36Cr0.61V0.03)Y for ocp1028, (Mg1.79Al1.33Cr0.47Fe0.332+Mn0.082+)X(Al6.66Cr1.31V0.03)Y for ocp1013, and (Mg1.94Al1.23Cr0.38Fe0.372+Mn0.082+)X(Al6.72Cr1.25V0.03)Y for ocp1016. Cr3+ ions in ocp1211 are distributed only in the X site. The distribution coefficient of Cr and Al between the X and Y sites [(Cr/Al)X/(Cr/Al)Y] is 1.66, 1.79, 3.09, and 4.54 for ocp1016, ocp1013, ocp0604, and ocp1028, respectively, indicating a stronger preference of Cr for the X site than the Y site. The a and b axes increase with increasing Cr content, whereas the c axis is almost constant. The mean Y-O distances increase linearly with increased Cr3+ content in the Y site. However, the mean X-O distances do not depend on the substitution of Cr3+ for Al3+ at the X site. The bond valence sums and the difference-Fourier synthesis indicate that hydroxyl groups are located at the O5, O7, O10, and O11 positions. FTIR spectrum shows main absorption bands at ca. 2911, 3220, 3397, and 3512 cm-1 of OH-stretching vibrations, indicating the presence of OH⋯O hydrogen bonds. [ABSTRACT FROM AUTHOR]