John K. Vassiliou

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.

Nanometer-Scale Iron Oxide Magnetic Particles: Synthesis and Magnetic Properties

Nanometer-scale iron oxide magnetic particles have been formed in the porous network of a cross-linked polymer matrix by ion exchange and subsequent hydrolysis. The oxide particles are uniform, well-dispersed and spherical with a diameter ranging between 30 and 1200 A depending on the synthesis conditions. The DC magnetic susceptibility, measured between 4 and 300 K, continuously increases with decreasing temperature and tends to saturate at low temperatures. Composites containing iron oxide particles with an average diameter of 80 A exhibit superparamagnetism while those on the order of 1000 A undergo an antiferromagnetic-type transition at 33 K. The magnetic susceptibility is critically dependent upon the particle size and the strength of the magnetic field.

The influence of yield strength on the non-hydrostatic compression of elastically anisotropic polycrystals

Abstract The compression behavior of a foil of Ni3A1 in a pressure medium of NaCl has been studied by energy-dispersive X-ray diffraction in a diamond anvil cell. Increasingly non-hydrostatic conditions lead to a region of incompressibility starting at around 3 GPa and terminated by a discontinuity in the compressibility at 7 GPa. Strong line broadening indicating plastic deformation starts at 3 GPa and continues to above 7 GPa. Direct compression of a foil between the diamonds results in a strong divergence in lattice parameters calculated from different lattice spacings and a volume incompressibility terminated by a discontinuity in the compressibility. The divergence in lattice spacings calculated from multiple orders of reflections can be explained from the introduction of stacking faults. The trend with pressure of the divergence agrees with that of the line profiles suggesting that these are to a large extent due to stacking faults. Comparison with earlier results for Cu3Au in identical experiments ...

Elastic and Plastic Deformation of NaCl and Ni3Al Polycrystals during Compression in a Multi-Anvil Apparatus.

Abstract The compression behaviour in a multi-anvil apparatus of pure NaCl and of a foil of Ni3Al embedded in a pressure medium of NaCl has been studied by energy-dispersive X-ray diffraction. At ambient temperature, the pressure and stresses, determined from line positions of NaCl, were constant throughout the sample chamber. Line positions and line widths of NaCl reflections were reversible on pressure release. A saturation of microstrains observed in NaCl at 2 GPa is thus attributed to brittle fracture setting in at uniaxial stresses of around 0.3 GPa. Ni3Al polycrystals, in contrast, undergo extensive (ductile) plastic deformation above 4 GPa. The compression behaviour of both Ni3Al and NaCl is identical to that previously determined in a diamond anvil cell. While a multi-anvil device thus has the advantage, compared with a diamond anvil cell, of constant pressure and stress throughout the sample chamber, microstrains in poly-crystalline samples arise in both devices. Samples in a multi-anvil apparatus thus need to be mixed with a pressure medium and to consist of essentially single crystals just as in a diamond anvil cell. Annealing experiments at high pressures confirm that the release of the uniaxial stress component in the pressure medium does not cause a release of microstrains in the embedded sample if the latter has been plastically deformed. Annealing for the purpose of attaining hydrostatic conditions in compression studies thus has to be carried out with care.

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.