Jens W. Otto

X-ray and optical study of AgReO4 under pressure

Abstract The structure and optical properties of silver perrhenate (AgReO 4 ) have been studied by synchrotron radiation and optical absorption and reflectivity in a diamond anvil cell under pressure to investigate the possibility of metallization by charge transfer Ag I Re VII → Ag III Re V . The compressibility of AgReO 4 in the scheelite structure is strongly pressure dependent (the bulk modulus and the first derivative of the bulk modulus determined from a Birch-Murnaghan equation of state fit are B 0 = 30.9± 4 GPa and B ′ 0 = 30.9 ± 6), and anisotropic ( δc c 0 = 4.2% , and δa a 0 = 3.4% ). A first-order phase transition to an undetermined structure occurs at 13 ± 1 GPa. The energy gap in the scheelite phase decreases gradually and linearly by 80 ± 1 meV⧸GPa, and drops by 0.46 eV at the phase transition. The decrease in the high-pressure phase is 42 ± 1 meV⧸GPa to 48 GPa. The reflectivity remains below 5% to 52 GPa, hence AgReO 4 is not a metal up to this pressure. There is no strong evidence for charge transfer.

Structural and Electronic Transitions in AgReO4 Under Pressure

The behavior of AgReO 4 under pressure has been studied in a diamond anvil cell. The equation of state determined by synchrotron radiation shows considerable change in compressibility with increasing pressure. X-ray and Raman scattering show a first-order phase transition from the scheelite to an as yet undetermined structure at 13 GPa. Changes in both the internal and external Raman active modes suggest a change in Re coordination. Optical transmission experiments demonstrate that the energy gap in the scheelitephase decreases at the rate of 75 meV/GPa. There is a small jump in the energy gap at 13 GPa, probably related to the structural transition. AgReO 4 remains aninsulator to at least 47 GPa.

The equation of state of δ‐TaN0.8 to 72 GPa

TaCl‐type NaN0.8 synthesized by an organometallic method was studied under pressure up to 72 GPa, using a diamond anvil cell and x‐ray scattering with synchrotron radiation. No structural transition was observed, despite the existence of other TaN polymorphs of different stoichiometries and cohesion energies comparable to TaN. At 25 GPa a small kink appears at the equation of state and there is a saturation of line widths and peak heights. At the same pressure range a lattice distortion develops as it is reflected to the anomalous pressure dependence of the d‐spacing ratios. The pressure dependence of the line widths is explained as a result of deformation due to stacking faults, caused by stress‐induced differential shift of the crystallographic planes parallel to (111) plane.

Red Shift in Optical Absorption Tail and Superparamagnetism of γ- Fe 2 O 3 Nanoparticles in a Polymer Matrix

Abstract : Well defined spherical particles of gamma - Fe2O3 have been synthesized in the pores of a polymer matrix in the form of beads by an ion exchange and precipitation reaction. The particle size distribution is a gaussian with an average diameter of 80 A. The DC magnetic susceptibility and the magnetization of the nanocomposite has been measured between 4 and 300 K using a Faraday balance and a magnetometer, respectively. The magnetic measurements demonstrate that the particles are superparamagnetic with a blocking temperature T about 55 K. The optical absorption edge of the mesoscopic system is red shifted with respect to single crystal films of gamma - Fe2O3 with an absorption tail extended deeply in the gap. Although lattice distortion and existence of excitonic states in the gap can explain the absorption behavior, the red shift can successfully be explained by the quantum confinement of an electron-hole pair in a spherical well.

Powder processing of ductile materials for high pressure studies: application to intermetallic alloys

Ductile materials are difficult to powderize for use in high pressure work. The potential of different techniques (gas-atomization, mechanical alloying, ball milling and subsequent annealing) for preparing suitable powders of some aluminides is investigated. Compression of Ti 46 Al 54 and NiAl prepared by these methods yields equation of state parameters in good agreement with determinations by other methods.