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Hyperfine Interaction in FeCl(2): Moessbauer Studies to 61 GPa

Description: Over a wide pressure range FeCl{sub 2} is rather compressible due to its layered structure. At low pressures h-FeCl{sub 2} condenses into a close-packed, hexagonal phase with a CdI{sub 2} structure similar to the other anhydrous divalent transition metal halides. Anhydrous FeCl{sub 2} was synthesized by a direct Fe-Cl{sub 2} reaction using Fe enriched to 25% {sup 57}Fe. The anhydrous material is colorless and transparent.
Date: February 1, 1999
Creator: Pasternak, M.P.; Taylor, R.D. & Xu, W.M.
Partner: UNT Libraries Government Documents Department

Electronic, magnetic and structural properties of the RFe03 antiferromagnetic-perovskites at very high pressures

Description: At ambient pressure the orthorhombic perovskites R-orthoferrites (R {triple_bond}Lu, Eu, Y, Pr, and La) exhibit very large optical gaps. These large-gap Mott insulators in which the 3d{sup 5} high-spin ferric ions carry large local moments and magnetically order at T{sub N} > 600 K, undergo a sluggish structural first-order phase transition in the 30-50 GPa range, with the exception of the LuFeO{sub 3} which undergoes an isostructural volume reduction resulting from a high to low-spin crossover. High-pressure methods to 170 GPa using Moessbauer spectroscopy, resistance, and synchrotron-based XRD in diamond anvil cells were applied. Following the quasi-isostructural volume reduction (3-5%) the new phase the magnetic-ordering temperature is drastically reduced, to {approx}100 K, the direct and super-exchange interactions are drastically weakened, and the charge-transfer gap is substantially reduced. The high-pressure (HP) phases of the La and Pr oxides, at their inception, are composed of high- and low-spin Fe{sup 3+} magnetic sublattices, the abundance of the latter increasing with pressure but HP phases of the Eu, Y, and Lu oxides consist solely of low-spin Fe{sup 3+}. Resistance and Moessbauer studies in La and Pr orthoferrites reveal the onset of a metallic state with moments starting at P > 120 GPa. Based on the magnetic and electrical data of the latter species, a Mott phase diagram was established.
Date: January 1, 2002
Creator: Pasternak, M.; Xu, W. M.; Rozenberg, G. Kh. (Gregory Kh.) & Taylor, R. D. (R. Dean)
Partner: UNT Libraries Government Documents Department

Structural response in FeCl2 (iron chloride) to pressure-induced electro-magnetic transitions

Description: High pressure (HP) synchrotron x-ray diffraction studies were carried out in FeCl{sub 2} together with resistivity (R) studies, at various temperatures and pressures to 65 GPa using diamond anvil cells. This work follows a previous HP {sup 57}Fe Mossbauer study in which two pressure-induced (PI) electronic transitions were found interpreted as: (i) quenching of the orbital-term contribution to the hyperfine field concurring with a tilting of the magnetic moment by 55 degrees and (ii) collapse of the magnetism concurring with a sharp decrease of the isomer shift (IS). The R(P,T) studies affirm that the cause the collapse of the magnetism is a PI p-d correlation breakdown, leading to an insulator-metal transition at {approx}45 GPa and is not due to a spi-Ir,crossover (S=2 {yields} S=0). The structure response to the pressure evolution of the two electronic phase transitions starting at low pressures (LP), through an intermediate phase (IP) 30-57 GPa, and culminating in a high-pressure phase (HP), P >32 GPa, can clearly be quantified. The IP-HP phases coexist through the 32-57 GPa range in which the HP abundance increases monotonically at the expense of the IP phase. At the LP-IP interface no volume change is detected, yet the c-axis increases and the a-axis shrinks by 0.21 Angstroms and 0.13 Angstroms, respectively. The fit of the equation of state of the combined LP-IP phases yields a bulk modulus K{sub 0} = 35.3(1.8) GPa. The intralayer CI-CI distances increases, but no change is observed in Fe-CI bond-length nor are there substantial changes in the interlayer spacing. The pressure-induced electronic IP-HP transition leads to a first-order structural phase transition characterized by a decrease in Fe-CI bond length and an abrupt drop in V(P) by {approx}3.5% accompanying the correlation breakdown. In this transition no symmetry change is detected,and the XRD data could be satisfactorily fitted ...
Date: January 1, 2009
Creator: Taylor, R D; Rozenberg, G Kh; Pasternak, M P; Gorodetsky, P; Xu, W M; Dubrovinsky, L S et al.
Partner: UNT Libraries Government Documents Department