Pressure dependence of electronic structures and spin states of iron-chalcogenide Fe 1.01 Se superconductors up to ∼66 GPa has been investigated with x-ray emission spectra and x-ray absorption spectra with partial-fluorescence yield. The intensity of the pre-edge peak at energy of ∼7112.7 eV of the Fe K-edge x-ray absorption spectrum of Fe 1.01 Se decreases progressively with pressure up to ∼10 GPa. A new prepeak at energy of ∼7113.7 eV develops for pressure above ∼13 GPa, indicating formation of a new phase. The experimental and the calculated Fe K-edge absorption spectra of Fe 1.01 Se using the FDMNES code agree satisfactorily. The larger compression accompanied by significant distortion around the Fe atoms along the c axis in Fe 1.01 Se upon applying pressure suppresses the Fe 3d-Se 4p and Fe 4p-Se 4d hybridization. The applied pressure suppresses the nearest-neighbor ferromagnetic superexchange interaction and enhances spin fluctuations on the Fe sites in Fe 1.01 Se. A discontinuous variation of the integrated absolute difference values of the Kβ emission line was observed, originating from a phase transition of Fe 1.01 Se for a pressure >12 GPa. Fe 1.01 Se shows a small net magnetic moment of Fe 2+ at ambient pressure, probably arising from strong Fe-Fe spin fluctuations. The satellite line Kβ was reduced in intensity upon applying pressure and became absent for pressure >52 GPa, indicating a continuous reduction of the spin moment of Fe in Fe 1.01 Se superconductors. The experimental results provide insight into the spin state of Fe 1.01 Se superconductors under pressure.

Raman spectra of several compositions of (Mg, Fe, Ca)SiO 3 pyroxenes were collected at ambient conditions. More than 10 Raman vibrational modes were observed for these pyroxenes in the wavenumber range between 200 and 1200 cm -1 . In general, these pyroxenes are characterized by (1) the Si-O stretching modes above 800 cm -1 ; (2) the Si-O bending modes between 500 and 760 cm -1 ;(3) SiO 4 rotation and metal-oxygen translation modes below 500 cm -1 . For a constant Ca content, frequencies of the Raman modes in the enstatite-ferrosilite (opx) and diopside-hedenbergite (cpx) series generally decrease with an increase in Fe content. This phenomenon is attributed to an increase in both the bonding lengths and the reduced mass as Fe 2+ is substituted for Mg. However, two modes at ~900 cm -1 in the enstatite-ferrosilite series increase in frequencies as Fe content increases. A possible explanation is to the shortening in the Si-O-Si bridging bonding bonds when the M2 sites are preferentially occupied by the iron cation. The effect of Fe substituting for Mg on the frequency shift in cpx is less profound than opx because the larger M2 was occupied by calcium and the substitution of iron and magnesium in the M1 site results in a less significant change in the bond length. The major-element composition of the (Mg, Fe, Ca)-pyroxenes, especially the orthopyroxene series, can be semi-quantitatively determined on the basis of the peak positions of their characteristic Raman modes. EXPERIMENTAL METHODSChemical compositions of all natural single-crystals of pyroxene (Table 1) were analyzed by the electron microprobe. In each sample, several different spots were analyzed to check the homogeneity of the sample. We also examined series of synthetic orthopyroxene with compositions (En 97.5 , En 80 , En 75 , En 70 , En 60 , En 50 , En 40 , En 35 , En 30 , En 25 , En 17 , and En 10 ) that were produced by H. Yang. See Yang and Ghose (1994) for details. The chemical composition of all the specimens in this study is plotted in Ca-Mg-Fe pyroxene diagram (Fig. 1). Strictly speaking, wollastonite is not a pyroxene. However, it is also included in this study because it is one of the end-members in the MgSiO 3 -FeSiO 3 -CaSiO 3 composition diagram.Raman spectra were excited using the 514 nm line of an Argon-Ion laser. We used a Raman spectrometer of the Renishaw Company, which contains a micro-objective that focuses the size of the laser beam to about 5 µm at the surface of the sample. A charge-coupled device is used as a detector to collect the signal in an 180° geometry. The position of the Raman peaks was determined by the PeakFit program. Typical acquisition time was about 10 minutes. Normally, the intensity of the modes varies with the orientation of the single crystal under investigation whereas the wavenumber of the modes remains the same. Therefore, in each sample, several orientations are attempted and an average in the peak position was taken for each vibrational mode. The spectral resolution for each mode is on the order of ±1 cm -1

Understanding the alloying effects of nickel and light element(s) on the physical properties of iron under core conditions is crucial for interpreting and constraining geophysical and geochemical models. We have studied two alloys, Fe0.92Ni0.08 and Fe0.85Si0.15, with nuclear resonant inelastic x‐ray scattering up to 106 GPa and 70 GPa, respectively. The sound velocities of the alloys are obtained from the measured partial phonon density of states for 57Fe incorporated in the alloys. Addition of Ni slightly decreases the compression wave velocity and shear wave velocity of Fe under high pressures. Silicon alloyed with Fe increases the compressional wave velocity and shear wave velocity under high pressures, which provides a better match to seismological data of the Earth's core.

The transition from body-centered cubic to hexagonal close-packed phase in iron has been studied in a diamond anvil cell with synchrotron radiation. The hexagonal close-packed phase, when it first appears, has a ratio of lattice parameters that is significantly larger than normal. This is attributed to a displacive mechanism that causes a distortion of the hexagonal close-packed structure in a body-centered cubic matrix. The hexagonal close-packed phase adjacent to a boundary with the body-centered cubic phase is stretched in the c direction and compressed in the a direction when it first forms.

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