High-pressure X-ray powder diffraction has been performed on corundumtype aFe,O, (hematite) in the pressure range up to 65 GPa using synchrotron radiation and a diamond anvil cell. A structural phase transformation has been observed with a transition pressure of about 55 GPa. The high-pressure form of Fe,O, is orthorhombic. The lattice parameters at 60GPa are a = 4.59(3)& b = 4.97( 3)& c = 6.68( 5)& Z = 4. The crystal structure is most probably a GdFe0,-type perovskite with space group Pbnm. The volume per formula unit in the orthorhombic phase is 10% smaller than in aFe,O,. It is suggested that the volume change is due to a combination of a valence change and a high-spin/low-spin transition. The bulk modulus, E,, and its pressure derivative, E& evaluated at zero pressure for aFe,O, are E, = 230(5)GPa and EA = 3.5(6).

Ni K-edge x-ray-absorption spectroscopy ͑XAS͒ measurements on a series of perovskite-layer-based nickelate compounds, with varying formal Ni valence, are presented. Spectral features that vary systematically with the Ni configuration are identified for these Ruddlesden-Popper phase and Sr substituted Ln 2Ϫx Sr x NiO 4 ͑LnϭLa and Nd͒ materials. Selected polarized XAS results on the La 2Ϫx Sr x NiO 4 system emphasize that a collapse of the Ni 4p -4p energy separation accompanies the substantial contraction of the apical Ni-O bond length in this series. In view of this, it is suggested that a displacement of the apical oxygen ͑O ab ͒ could play an important role in the small polarons previously proposed to form in this system. The polarized XAS results also provide evidence for in-plane character for the Ni-site component of the doped holes in this system. The XAS evidence supporting an increasing Ni d 7 ground-state admixture in these materials, with increasing formal Ni valence, is discussed. This discussion is couched in terms of simplified Ni d configuration modeling, which emphasizes the close energetic proximity of the Ni d 8 state to O p states in these systems.

No abstract

No abstract

The so-called 'valence transition' in EuPd,Si2 has been observed using energydispersive synchrotron x-ray powder diffraction a1 elevated pressures and is centred at 8 kbar. The estimated increase in valence at the lransition is 0.2, and is associated with crossover of 41 and 6d/s bands.