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It is suggested to use the ellipsometric technique to measure the electronic properties of static highly compressed matter. The information derived from this technique is important for the theoretical and phenomenological research for the equation of state. This diagnostic was found to be useful and sensitive for measuring pressure induced phase transitions. In particular, high-pressure ellipsometry with a diamond anvil cell was used to detect the α<=>[variant_greek_epsilon] phase transition in iron. The polarization of the light reflected from an iron foil was analyzed in the pressure domain of 0–24 GPa and the α<=>[variant_greek_epsilon] transition in compression and decompression was detected. These results are in good agreement with Mössbauer spectroscopy, x-ray diffraction, and resistivity measurements. [ABSTRACT FROM AUTHOR]

Focuses on iron phase transformation by the Mössbauer effect of iron in a diamond anvil cell at hydrostatic pressures. Occurrence of nuisance background counts; Reason for a small quadrupole splitting; Hysteresis width at the midpoints.

Structural aspects of magnetic/electronic transitions in strongly correlated systems in a regime of very high static density are the main issues of this article. To achieve this objective, we have carried out series of X-ray diffraction (XRD) measurements up to 120 GPa using diamond anvil cells (DACs) to probe structural features specifically related to pressure-induced (PI) magnetic/electronic phenomena in transition metal (TM) compounds. The types of phenomena are the Mott transition (MT), high-spin (HS) to low-spin (LS) transition, valence transformations, the quenching of the orbital term, and Verwey transition. In all these cases, the electronic transition may induce or be a consequence of structural alterations. These studies provide essential information concerning: (i) structural alterations attributed to the valence transformation in some TM compounds; (ii) mechanisms and precursors responsible for the PI MT; features of the structural transformation specifically attributed to the MT for different types of the electronic transitions (Mott-Hubbard and Charge-Transfer); (iii) the mechanism of the PI degradation of the magnetic state due to HS-LS transition and their influence on the structural properties of material; (iv) mechanism of Verwey transition in magnetite; (v) structure of new phases at high pressure. As example cases we present recent results in some Fe-oxides, halides, and sulfides. [ABSTRACT FROM AUTHOR]

Temperature-dependent 57Fe Mössbauer spectroscopy to 40 GPa shows that Fe3O4 magnetite undergoes a coordination crossover (CC), whereby charge density is shifted from octahedral to tetrahedral sites and the spinel structure thus changes from inverse to normal with increasing pressure and decreasing temperature. A precursor to the CC is a d-charge decoupling within the octahedral sites at the inverse-spinel phase. The CC transition takes place almost exactly at the Verwey transition temperature (TV=122 K) at ambient pressure. While TV decreases with pressure the CC-transition temperature increases with pressure, reaching 300 K at 10 GPa. The d electron localization mechanism proposed by Verwey and later by Mott for TTV. [ABSTRACT FROM AUTHOR]