William S. Hammack

Pressure‐induced amorphization of wollastonite (CaSiO3) at room temperature

Energy dispersive x‐ray diffraction and Raman measurements show that the mineral wollastonite (CaSiO3) becomes amorphous at a pressure of 25.6 GPa at 300 K. A consideration of the high pressure/high temperature behavior of CaSiO3 shows that the amorphization occurs because the phase transition wollastonite→perovskite CaSiO3 is kinetically impeded. We suggest that the amorphous phase may be viewed as a defective long‐period modulated wollastonite phase.

Pressure-induced Amorphization of R-Al5Li3Cu: A Structural Relation Among Amorphous Metals, Quasi-Crystals, and Curved Space

A central question in the study of amorphous materials is the extent to which they are ordered. When the crystalline intermetallic R-Al5Li3Cu is compressed to 23.2 gigapascals at ambient temperature, an amorphous phase is produced whose order can be described as defects in a curved-space crystal. This result supports a structural relation between quasi-crystals and amorphous metals based on icosahedral ordering. This result also shows that a metallic crystal can be made amorphous by compression.

Pressure-induced disordering in α-SrZrF6: Implications for the role of the countercation in glassy SrZrF6

Reversible pressure‐induced changes in the Raman and x‐ray scattering of α‐SrZrF6 are reported. Above a pressure of ∼10.0 GPa there is a precipitous decline in the Raman intensity; this is a phenomenon associated with the formation of an amorphous state. Energy dispersive x‐ray diffraction (EDXD) measurements show that the amorphization is not complete—some residual ordering persists. The results suggest that glassy SrZrF6, prepared at ambient pressure, (1) contains kinked chains of ZrF8 dodecahedra and (2) the medium range order of the glass is determined by the way ZrF8 units are arranged around the strontium countercation.

Effect of pressure on the charge-transfer band of the [Fe(CN)6]4−·dimethyl viologen ion pair

The effect of pressure on the optical charge-transfer (CT) band of the ion pair (Fe(CN)/sub 6/)/sup 4 -//center dot/DMV/sup 2 +/ (DMV = dimethyl viologen) has been studied in aqueous solution. The most significant effects of pressure were twofold: (1) In the pressure range 0.001 to 10.0 kbar (1 kbar = 986.92 atm = 0.1 GPa) the CT band maximum shifted to higher energy by /approximately/1700 cm/sup -1/. (2) In this same pressure range the integrated intensity of the CT band increased by /approximately/35%. There were no changes in the peaks full-width at half maximum (FWHM). Qualitatively, these phenomena are in accord with a vibronic coupling model used to describe mixed-valence metal compounds and also with a general theory of absorption spectra of ions in solution, as well as with Mullikens theory of charge transfer. We also noted that the oscillator strength of the CT band decreased by over a factor of 2 at ambient pressure with increasing concentration of the ion pair (from 1.0 to 50 mM). This change correlates with the change of the ionic strength over the same concentration range. 28 refs., 3 figs.

Effect of pressure-induced freezing on the energy of the intervalence transfer electronic absorption band of binuclear mixed-valence complexes

Abstract The Marcus continuum model for the solvent reorientation contribution to the energetics of outer-sphere electron transfer has often been used to analyze the energy of the intervalence transfer (IT) electronic absorption band for a binuclear mixed-valence transition-metal complex. E op , the energy required to transfer an electron optically in a mixed-valence complex, was measured as a function of pressure for two binuclear mixed-valence complexes in different solvents which freeze at 25°C under pressures

Pressure-induced transformations of beta -BaZr2F10 and its relationship to glassy BaZr2F10

X-ray and Raman measurements show that beta -BaZr2F10 becomes disordered at pressures greater than 3.7 GPa at ambient temperature. This transformation occurs via a largely displacive mechanism because the disordering is reversible. Comparisons of glassy BaZr2F10 and the high-pressure states of beta -BaZr2F10 provide structural insight into the medium-range order of ionic glasses. Specifically the authors believe that layers of tightly packed pentagonal bipyramidal units are present in the glass.

Pressure‐induced crystalline‐to‐noncrystalline transformations of barium fluorozirconates: A probe of the medium range order of noncrystalline solids

The reversible pressure‐induced transformation of crystalline barium fluorozirconates to noncrystalline solids is reported. The transformation is observed by in situ high pressure Raman spectroscopy. Since the reported crystal‐to‐noncrystalline transformations occur reversibly, the medium range order (MRO) of the noncrystalline solid formed can be determined; there are very few experimental methods for determining the MRO of amorphous materials. Specifically, it is reported that crystalline β‐BaZr2F10 becomes noncrystalline at 35–45 kb, α‐BaZrF6 at 65–75 kb, and β‐BaZrF6 at 100–120 kb. The medium range order for the noncrystalline phases formed at high pressures is as follows: those formed from β‐BaZrF6 crystals consist of kinked chains of zirconium fluoride; noncrystalline materials formed from crystals of α‐BaZrF6 contain chains which are connected in many directions forming a ‘‘net’’ of zirconium fluoride polyhedra; and crystals of β‐BaZr2F10 form a solid composed of distorted layers of zirconium fluor...