J.C. Mikkelsen

The Raman spectra and structure of pure vitreous P2O5

We report the polarized Raman spectra of pure anhydrous vitreous P2O5 and conclude that this glass is a 3-connected network in which each P atom is surrounded almost tetrahedrally by three bridging O atoms and one double-bonded O terminator. Vibrational calculations on an anchored fragment are used to determine atomic motions in the higher frequency modes; these calculations confirm that the dominant Raman modes involve in-phase compression of neighboring PO bonds.

Optical properties and electronic structure of crossroads material MnTe

Abstract Combining an energy level model for the rocksalt manganese chalcogenides with a measurement of 1.3 eV for the optical gap of NiAs-structured MnTe, it is argued that MnTe has extensive energetic p-d overlap and that such overlap is characteristic of NiAs-structured materials.

Ion dynamics and sublattice melting in the superionic conductor PbF2

Abstract The F 19 relaxation rates in PbF 2 exhibit anamolous changes as the temperature is increased into the superionic phase. These changes confirm the concept of a continuous melting of the fluorine sublattice and show that the fluorine motion in the superionic phase is highly correlated.

Anisotropic conductivity in a channel-structured superionic conductor: Li2Ti3O7

Abstract From measurements on single crystals of Li2Ti3O7, the conductivity is determined to be predominately ionic with an anisotropy of σ b σ a ≈4 and σ c σ b ≈7 . This anisotropy is significant but is not sufficient to classify this channel-structured ramsdellite material as a one-dimensional conductor. A conduction path through the ramsdellite crystal structure consistent with the determined anisotropy is presented.

Raman studies of the thermal oxide of silicon

Abstract We report the Raman spectra of silicon oxide films that have been formed by the wet oxidation of Si wafers at ∼ 10 atmospheres of steam and at temperatures near 800° C. Apart from small features due to differing water content, the Raman spectra are identical to those of melt-quenched fused silica that has been annealed at the film growth temperature. We detect no growth anisotropy and conclude that the gross structures are the same, that numerous properties of steam oxide films can be safely inferred from measurements on appropriately annealed samples of bulk fused silica, and vice-versa.

EXAFS investigation of superionic conduction in CuI

Abstract EXAFS results on the three solid phases of CuI support an excluded volume model for the CuI pair correlation function. The Cu ion density in the superionic α-phase peaks at the tetrahedral locations but is also substantial in the octahedral voids. The indicated Cu ion conduction path is in 〈111〉 directions through the faces shared by the tetrahedra and octahedra in the FCC iodine lattice with an energy barrier of roughly 0.16 eV in the superionic phase.

Nonlinear X-ray production of defect spins in vitreous SiO2: The roles of creation and activation

Abstract The dependence of spin concentration on X-ray dose is shown to be nonlinear in distinctive ways for the three most common defect structures in vitreous SiO 2 . We fit the data with a model invoking two concurrent simple processes: creation of new defects and activation of preexisting ones. The resultant fit parameters show dependencies on hydroxyl concentration and fictive temperature that shed new light on the processes, the defects, their origins and the radiation hardness of v -SiO 2 .

Light scattering from correlated ion fluctuations in ionic conductors

Abstract Inelastic light scattering spectra of CuI and AgI in the α and melt phases are reported and shown to involve two depolarized Lorentzian components centered at zero frequency. The narrow component is interpreted in terms of ionic motions and we propose that the observed depolarized scattering is caused by correlated configurations of the mobile ions.

EXAFS investigation of mobile-ion density: Cul and Cu2Se contrasted

Abstract The extended x-ray absorption fine structure (EXAFS) has been measured in the superionic phases of the Cu and Ag halides and chalcogenides. Cul and Cu 2 Se, both of which have an FCC anion lattice in the α phase, are discussed as representative examples. In both cases the excluded volume model provides an accurate description of the near neighbor cation-anion pair correlation function. It is found, however, that a significant fraction of the Cu ions occupy the octahedral sites in α-Cul but not for α-Cu 2 Se. For Cu 2 Se the Cu ions are predominately confined to the tetrahedral regions of the Se FCC lattice. The lack of octahedral occupation may result from larger Cu-Cu interactions for octahedral ions in the denser superionic material, Cu 2 Se.

Superionic behavior in CuCl and CuBr at elevated temperatures

Abstract EXAFS data on several phases of CuCl and CuBr are analyzed in terms of the various structural models that have been proposed. The one favored is the excluded volume model which has previously given good results on superionic AgI and CuI. It is determined that the Cu ion density peaks at the tetrahedral locations in all phases of CuCl and CuBr and that it spreads substantially toward and through the tetrahedral faces as the temperature is increased. This behavior is qualitatively identical to that in AgI and CuI, and therefore properly regarded as superionic.