Electronic and structural properties of the high-pressure phase of Fe 2 O 3 were determined by combining the methods of Mössbauer spectroscopy, x-ray diffraction, and electrical resistance, R͑P, T͒, to 80 GPa. Because of a first-order phase transition taking place in the 50 -75 GPa range and accompanied by a volume decrease of ϳ10%, a breakdown of the electronic d-d correlation occurred, leading to a Mott transition, a metallic and a nonmagnetic single Fe 31 electronic state. The highpressure structure is of the distorted Rh 2 O 3 -II type. The accommodation of the denser phase within this six-coordinated structure is attributable to the metallic state. [S0031-9007(99)09286-8] PACS numbers: 62.50. + p, 71.30. + h, 76.80. + y More than half of the binary transition metal (TM) compounds and the majority of the undoped TM oxides are magnetic insulators that order antiferromagnetically below a certain Néel temperature (T N ). The localization of carriers in those so-called Mott insulators results from strong on-site Coulomb repulsion within the 3d band. Its insulating nature [1] is commensurate to an effective energy-gap U, far exceeding the d-band width W, splitting the d band into upper (empty) and lower (filled) subbands. In view of the "Coulomb" nature of the gap, it is expected that in ionic insulators, such as the binary TM oxides, very high pressures will be needed to reach the W͞U ¿ 1 regime. In this regime the d-electron correlation collapses, giving rise to an insulating-metal transition concurring with a magnetic moment breakdown, a phenomenon often called the Mott transition. Indeed, as shown by Cohen et al. [2] for the case of binary TM oxides, the Mott transition could be attained only at pressures close to 200 GPa. Pasternak et al. experimentally observed a pressure-induced Mott transition in the highly covalent NiI 2 , [3] using the combined methods of 129 I Mössbauer spectroscopy (MS), x-ray diffraction (XRD), and electrical conductivity in diamond anvil cells (DAC's). The isostructural transition was second order and quasi-isochoric. The question then arises whether in the case of a first-order phase transition resulting in a reduction in volume, such a TM-oxide Mott insulator might undergo a Mott transition.In this Letter we report the observation of such a transition attributable to a huge volume reduction, the first-order phase transition in a-Fe 2 O 3 (hematite) at 50 GPa. As will be shown, this work is important not only to the elucidation of such a basic phenomenon as the Mott transition, but also to ultimately untangling the baffling nature of the high-pressure phase of hematite.Fe 2 O 3 , a wide-gap antiferromagnetic insulator (T N 956 K [4]) can be regarded as archetypal of a Mott insulator. Motivated primarily by its importance in the earth sciences, high-pressure studies of this corundumtype mineral have been extensively performed since the early days of high-pressure physics. As early as in the mid-1960s shock-wave experiments were performed by McQueen and Marsh [5] who reached energy...

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A miniature piston-cylinder diamond-anvil cell (DAC) was constructed and tested for pressure operation at and beyond 100 GPa. Its advantages compared to other piston-cylinder DACs are its compactness (22-mm diam by 21-mm high), self-contained force generator, and simple way of operation. Tungsten carbide backing plates are used for supporting the anvils; one with a hemispherical shape allowing for parallelism alignment, and one with a flat circular shape allowing for lateral alignment of the anvils’ culets. The force is generated by six M3 Allen screws and is conveyed to the piston via force rings. Pressures to 130 GPa were achieved with beveled culets having 300-μm flats and Re gaskets. Design features, mode of operation, and performance are described. The latter has been demonstrated for the particular case of Mössbauer spectroscopy in La57FeO3.

A method for calculating the doubledifferential cross sections of the inelastic x-my scattering (Ixs) associated with low-Z-atom K-level excitation of solids is proposed. We have included both the first-and the second-order contributions of oneelectron perturbxion theory. The secondader contribution is shown to be signiticmt bnly far comparatively amall initial photon energy ( R q c 2 keV). The fist-order term is proved to be determined by p partial local densities of electron states (p wos) whereas the secondader term is determined by s was. An expression is proposed which enables us to determine the p and s LbOs from expedmental LXP spectrn mensured for different initial photon energies and transferred momenta. Using available experimental 1ks specw and x-ray absorption spectra we have determined the E* and a ' p LDOS for graphite crystal. The resulB obtained are compared with the theoretid n* and a* LDOS calculated using the full multiple-scnttering method. The calculated and experimenol mos are in =%onable agreement. both as regards peak positions and absolute values.