David McKeown - Academia.edu (original) (raw)
Papers by David McKeown
Physical Review B, 1992
X-ray-absorption near-edge structure (XANES) data were collected for Zn in sphalerite (ZnS), and ... more X-ray-absorption near-edge structure (XANES) data were collected for Zn in sphalerite (ZnS), and for Cu and Fe in chalcopyrite (CuFeS~), where all three cations are in nearly identical coordination environments. The data have similar features, except near the edge, where the edge maximum decreases in amplitude, while a pre-edge feature appears and increases in amplitude from Zn to Cu to Fe. The pre-edge feature was previously assigned to a 1s-to-3d atomic transition for Cu and Fe in the chalcopyrite structure. XANES calculations were performed for all three edges. The multipleand single-scattering contributions to the calculated edge spectra were scaled down to better fit the experimental data. The conclusions from the calculations indicate that the pre-edge feature in the experimental Cuand Fe-edge data for chalcopyrite can be due to interference effects from the atomic structure surrounding the absorber, but cannot exclude the possibility that the pre-edge feature is due to atomic bound-state transitions of the absorber.
Physical Review B, 1993
The normal modes of vibration and their frequencies are calculated for benitoite, a mineral whose... more The normal modes of vibration and their frequencies are calculated for benitoite, a mineral whose crystal structure (space group Ds~) consists of three-membered silicate rings (Sis09) linked by Ba + and Ti + ions. Factor-group analysis dictates that certain normal modes involve the motion of only the ring atoms. On the assumption that mode mixings and splittings due to interring interactions are small, the normal frequencies of the isolated ring of |3 h, symmetry are determined by fitting to suitable averages of selected frequencies in the Raman spectra. A valence force potential consisting of only central interactions between nearest neighbors and bond-bending interactions centered at the silicon atoms is used. This potential is then extended to the full crystal structure by including interactions involving the Ba + and Ti + ions. The frequencies obtained are in excellent agreement with the infrared and Raman spectra, requiring only minor adjustment of the force constants obtained for the isolated ring. The identification of normal modes characteristic of three-membered silicate rings may prove to be a valuable guide in the interpretation of the infrared and Raman spectra of amorphous silicates, potentially leading to new information on the ring statistics of these materials.
MRS Proceedings, 1992
ABSTRACTThe normal modes of vibration and their frequencies are calculated for benitoite, a miner... more ABSTRACTThe normal modes of vibration and their frequencies are calculated for benitoite, a mineral whose crystal structure (space group D23h) consists of three-membered silicate rings (Si3O9) linked by Ba+2 and Ti+4 ions. The calculation unambiguously assigns the normal modes to the lines in the Raman and infrared spectra and determines the atomic interactions. On the assumption that mode mixings and splittings due to inter-ring interactions are small, the normal frequencies of the isolated ring of C3h, symmetry are determined. The identification of normal modes characteristic of three-membered silicate rings may prove to be a valuable guide in the interpretation of the infrared and Raman spectra of amorphous silicates, potentially leading to new information on the ring statistics of these materials.
Review of Scientific Instruments, 1990
The design and performance of a high-precision optical reflectometer are described. This instrume... more The design and performance of a high-precision optical reflectometer are described. This instrument has been optimized for measuring the specular reflectivity of thin films and multilayer structures of interest in semiconductor technology. Its design emphasizes high spectral and spatial resolution, photometric accuracy, and stray light rejection. Use of a spectrometer drive linear in wavenumber (energy) and a flexible data acquisition system facilitates data analysis. The performance of the reflectometer is demonstrated using a set of specimens consisting of silicon-dioxide layers on silicon substrates for which the oxide thicknesses had been determined by ellipsometry. Excellent agreement is obtained between the thicknesses derived from the reflectivity spectra and those determined ellipsometrically.
American Mineralogist, 2010
We use the density functional perturbation theory to determine for the first time the pressure ev... more We use the density functional perturbation theory to determine for the first time the pressure evolution of the Raman intensities for a mineral, the two high-pressure structures of MgSiO 3 perovskite and post-perovskite. At high pressures, the Raman powder spectra reveals three main peaks for the perovskite structure and one main peak for the post-perovskite structure. Due to the large differences in the spectra of the two phases Raman spectroscopy can be used as a good experimental indication of the phase transition.
Journal of Nuclear Materials, 2017
Mo-containing high-level nuclear waste borosilicate glasses were investigated as part of an effor... more Mo-containing high-level nuclear waste borosilicate glasses were investigated as part of an effort to improve Mo loading while avoiding yellow phase crystallization. Previous work showed that additions of vanadium decrease yellow phase formation and increases Mo solubility. X-ray absorption spectroscopy (XAS) and Raman spectroscopy were used to characterize Mo environments in HLW borosilicate glasses and to investigate possible structural relationships between Mo and V. Mo XAS spectra for the glasses indicate isolated tetrahedral Mo 6+ O 4 with MoO distances near 1.75 Å. V XANES indicate tetrahedral V 5+ O 4 as the dominant species. Raman spectra show composition dependent trends, where MoO symmetrical stretch mode frequencies ( 1) are sensitive to the mix of alkali and alkaline earth cations, decreasing by up to
This work was conducted under a quality assurance program compliant with 10 CFR 830 Subpart A, NQ... more This work was conducted under a quality assurance program compliant with 10 CFR 830 Subpart A, NQA-1 (2000), and DOE Order 414.1C. This program is supplemented by a Quality Assurance Project Plan for ORP work that is conducted at VSL [1-21]. Test and procedure requirements by which the testing activities are planned and controlled are defined in this plan. The program is supported by VSL standard operating procedures that were used for this work [1-22]. Exceptions are the X-ray absorption spectroscopy measurements and the tin Mössbauer spectroscopy measurements, which were performed at outside facilities and are of research quality. The requirements of DOE/RW-0333P are not applicable to this work.
Physical Review B, 1988
X-ray-absorption fine-structure (XAFS) measurements of GeC14, GeH3Cl, and GeH4 were made. We expe... more X-ray-absorption fine-structure (XAFS) measurements of GeC14, GeH3Cl, and GeH4 were made. We experimentally isolate the single-and multiple-scattering terms in the XAFS of GeC14 by cornparison of the spectra of the three compounds. The multiple-scattering (MS) amplitude is comparable to the single-scattering (SS) amplitude only within 15 eV of the edge. Beyond 40 eV the MS to SS amplitude ratio is less than 0.06. Calculations are in qualitative agreement with experiment. o ] Our results suggest that XAFS data in the range 1 & k & 3 A can be analyzed in a SS picture in many cases.
American Mineralogist, 2005
Raman spectra were collected for crystalline albite from 25 °C to above the 1118 °C melting tempe... more Raman spectra were collected for crystalline albite from 25 °C to above the 1118 °C melting temperature, where vibrational assignments for the crystal spectra were determined by lattice dynamics (LD). The Raman spectra and associated vibrational assignments are reported for triclinic albite (NaAlSi 3 O 8) at 25 °C and monoclinic albite at 1060 °C. The 25 °C calculations determined that localized TO stretch and O-TO bend modes are above 900 cm-1 (where T = Si,Al), while motions from the aluminosilicate tetrahedral cage mixed with Na displacements occur in modes as high as 814 cm-1. Vibrational modes for the most prominent peaks in the spectrum, between 350 and 550 cm-1 , are dominated by four-membered tetrahedral ring deformations. For completeness, calculated infrared mode frequencies and their atomic displacements are reported for the 25 °C structure and compared with normal mode calculation results and observed infrared mode frequencies presented by von Stengel (1977). At higher temperatures, modes above 550 cm-1 broaden and shift to lower frequencies by 15 to 27 cm-1 ; modes below 550 cm-1 broaden, but experience little, if any frequency shifts. Albite melted sluggishly, was completely liquid at 1320 °C, and remained amorphous upon cooling to room temperature. At frequencies above 550 cm-1 , the crystalline albite peaks, and possibly their vibrational assignments, can be correlated to Raman bands for albite glass. Spectral differences below 550 cm-1 between crystal and glass correspond to changes of average tetrahedral ring type upon melting, as shown by Taylor and Brown (1979).
American Mineralogist, 2014
Chromium K-edge X-ray absorption spectra were collected to characterize Cr in forsterite (Mg 2 Si... more Chromium K-edge X-ray absorption spectra were collected to characterize Cr in forsterite (Mg 2 SiO 4) as well as sulfides within the MAC 88136 EL3 chondrite to determine Cr valence and to see whether forsterite within this meteorite can be used as a Cr 2+-silicate standard. Spectra were measured on several areas within a nearly pure 100 × 200 µm forsterite grain containing 0.13 wt% Cr. XANES findings indicate highly reduced Cr 2+ species, with no clear evidence of Cr 3+ or Cr 6+. EXAFS data indicate an average 2.02 Å Cr-O nearest-neighbor distance, consistent with Cr-O distances found in square-planar Cr 2+ O 4 sites observed in synthetic crystalline silicates, and an average 2.69 Å Cr-Si second-nearest neighbor distance, consistent with Cr 2+ substituting for Mg 2+ in the forsterite M(1) site. Nearest-neighbor Debye-Waller factor and coordination number parameters indicate Cr 2+ is likely entering forsterite in disordered sites that are possible intermediates between M(1) and squareplanar Cr 2+ O 4 configurations. Preliminary Cr XAS measurements on sulfides within this meteorite also indicate Cr 2+ in CrS 6 octahedra.
American Mineralogist, 2001
X-ray absorption data were collected for a series of varnish and dendrite Mn oxide coatings on ro... more X-ray absorption data were collected for a series of varnish and dendrite Mn oxide coatings on rock substrates containing a wide variety of mineralogies exposed to a variety of environments. Near-edge spectra of the coatings indicate that the Mn-oxide phases present have Mn valences between 3+ and 4+, with average Mn valences for the varnishes closer to 4+ than those for the dendrites. Mn EXAFS data and analyses indicate that Mn-oxide structure types for the varnishes range, perhaps continuously, from large tunnel phases, similar to todorokite and romanechite, to layer phases, i.e., birnessite-family. Similar results were found for the dendrite samples, except that the variety of Mn-oxide phases is somewhat larger than those found for the varnishes. No correlations were found between Mn-oxide structure-type within these coatings and the corresponding substrate petrology.
Le Journal de Physique Colloques, 1986
X-ray absorption spectroscopic studies of cation environments in oxide glasses are selectively re... more X-ray absorption spectroscopic studies of cation environments in oxide glasses are selectively reviewed. New results are presented on K and Yb environments in silicate glasses and on Fe in silicate melts at temperatures up to 1173 " K. INTRODUCTION Recent investigations of oxide glasses and crystalline model compounds by x-ray absorption spectroscopy (XAS and the acronyms EXAFS for extended fine structure and NE)(AFS or XANES for near-edge structure) have provided useful and sometimes unique information on the structural environments of network-forming and network-modifying cations. Synchrotron-based XAS is well suited for studying the local structural environment and bonding of cations in amorphous materials that cannot be probed directly by standard spectroscopic or scattering methods (e.g. Na, Mg, K, Ca, Zr) or which are present in small concentrations (e.g. transition metals, rare earth and actinide elements a t 100-2000 ppm). T o date, several dozen XAS studies of cations in oxide glasses have reported information on local coordination environments of network formers such as Si, Al, Ga, and Ge and of network modifiers such as Na, K, Ca, Ti, V, Fe, Zn, Zr, Pb, and U. This work has provided new insights about oxide glass structure and structure-property-composition relationships which shed light on processes such as homogeneous nucleation, viscous flow, cation diffusion, and corrosion behavior of glasses. This paper presents a brief overview of recent XAS studies of silicate and oxide glasses which is selective because of space limitations. Also reported are new results on the structural environments of K and Yb in silicate glasses under ambient conditions and of Fe in silicate melts at temperatures up t o 1173" K.
Journal of Non-Crystalline Solids, 1984
ABSTRACT Polarized Raman spectra have been obtained for a series of sodium aluminosilicate glasse... more ABSTRACT Polarized Raman spectra have been obtained for a series of sodium aluminosilicate glasses and selected minerals, in order to investigate changes in the glass structure with composition. Particular attention is paid to characterization of the Al coordination, and to test Lacy's hypothesis concerning the role of “triclusters” in Al-rich glasses (Al/Na = R > 1.0). A comparison of five glasses ranging from nearly sodium trisilicate composition glass (R = 0.03) to albite composition glass (R = 1), and beyond to a glass of R = 1.61, leads to the following observations with increasing R: the polarized bands previously assigned to T-O (non-bridging) vibrations (950, 1110 cm−1) decrease in intensity, while those assigned to T-O (bridging) vibrations (485, 980 cm−1) either remain the same or increase in intensity. The narrow band at 495 cm−1 and a somewhat wider one at 595 cm−1, may be related to Raman active “defects” that have been observed in vitreous SiO2 at essentially the same frequencies. Our observations indicate a general increase in tetrahedral polymerization and random incorporation of Al into tetrahedral sites. Comparison of the Raman spectra of mullite (which contains (Si, Al)3O10 triclusters) with sillimanite (which contains no triclusters) shows that the bands at 418 and 1200 cm−1 in mullite are absent for sillimanite. Assuming that these “extra” bands are due to the triclusters in mullite, we have to conclude that similar triclusters do not exist in any of the glasses we examined. If these “extra” bands in mullite are not due to triclusters, we can draw no such conclusion. Our study indicates that observed changes in physical properties of sodium aluminosilicate glasses as R increases are not due to an Al coordination change.
Journal of Non-Crystalline Solids, 1985
ABSTRACT Photoelectron total yield Al K-XANES and EXAFS spectra were collected for a series of Na... more ABSTRACT Photoelectron total yield Al K-XANES and EXAFS spectra were collected for a series of Na2OAl2O3SiO2 minerals and glasses. The purpose of this study is to determine if EXAFS can accurately characterize these known Al environments in terms of AlO distances and coordination numbers, and to characterize the unknown Al environment in the glasses. Al XANES spectra were collected for corundum, sillimanite, albite, jadeite and nepheline, as well as jadeite glass, nepheline glass and a 75 mol% silica glass series having R ( = Al/Na) ranging from 0.28 to 1.61. Al EXAFS spectra were collected for corundum, sillimanite, jadeite and albite, as well as jadeite glass, albite glass and a 75 mol% silica glass with an R of 1.61.Al XANES spectra for our model compounds have two edge maxima at 1568 and 1572 eV for minerals having octahedral Al, while one edge maximum is present at 1566 eV for minerals with tetrahedral Al. The glasses all have a single edge maximum near 1566 eV, indicating tetrahedral-like Al sites in the glasses. The results of our Al EXAFS refinements of the model compounds are in agreement to within ±0.03 Å for AlO distances and to within ±23% for coordination numbers of recent crystal structure refinements in the literature. The EXAFS results for the glasses clearly indicate Al in tetrahedral sites, yielding AlO distances near 1.77 Å, well within the range of average AlO distances for aluminate tetrahedra of aluminosilicate minerals. Our XANES and EXAFS results indicate tetrahedral Al throughout our 75 mol% silica glass series, which may be consistent with the existence of tricluster units occurring in the Al-rich glasses in the sodium aluminosilicate system.
Journal of Non-Crystalline Solids, 1985
Photoelectron total-yield Na XANES and EXAFS spectra were gathered to characterize the Na coordin... more Photoelectron total-yield Na XANES and EXAFS spectra were gathered to characterize the Na coordination for a collection of Na2OAl2O3SiO2 glasses and minerals. Included in the collection are the crystalline model compounds albite, jadeite and nepheline as well as glasses at these compositions, Na2Si2O5 glass, and a series of glasses containing 75 mol% silica with R (= Al/Na) values ranging from 0.03 to 1.61. High resolution Na K-XANES spectra are reported for the first time and are of good enough quality for the specimens studied to make qualitative comparisons of Na edge features possible. Due to the variable quality of the Na EXAFS data, only Na2Si2O5 glass, three out of five of the glasses in the glass series, nepheline and nepheline glass were worthy of analysis; these samples are the more soda-rich materials used for this study. Comparisons of Na XANES and EXAFS spectra for the glass series, nepheline, nepheline glass and Na2Si2O5 glass indicate little if any change in the Na environment among these materials. Nepheline glass and crystal show reversal of intensity of the 1055 eV and 1058 eV Na edge features. The similarity of the Na coordination environment is also apparent in the EXAFS spectra, which yield NaO distances ranging from 2.56 to 2.62 A. This overall similarity of the Na coordination for samples in this study is not surprising since Na in crystalline compounds within this system is usually coordinated by seven to ten oxygens. The physical property variations for our glass series are not explained by the differences in the Na environments depicted by our Na XANES and EXAFS results. The Na EXAFS for nepheline appears to sense only the closest set of oxygens of those in the entire known Na coordination, and this seems to be true for nepheline glass as well, using high temperature crystal structure studies of nepheline for distance comparisons.
American Mineralogist, 2007
The silica polymorph moganite is commonly intergrown with quartz in microcrystalline silica varie... more The silica polymorph moganite is commonly intergrown with quartz in microcrystalline silica varieties that are less than ~100 Ma in age. Synchrotron X-ray diffraction suggests that a displacive phase transition occurs when moganite is heated above ~570 K, with an increase in symmetry from I2/a to Imab. In the present study, we employed hard-mode Raman spectroscopy to conÞ rm the existence of the α-β moganite transformation and to offer complementary insight into the transition mechanism. Our analysis of the displacement of the 501 ∆cm-1 symmetric stretching-bending vibration (B 3g mode) with changing temperature strongly supports the existence of a monoclinic-to-orthorhombic phase transition between 570 and 590 K. Between 593 and 723 K, however, the mode remained Þ xed at 496 ∆cm-1. This behavior was repeated on cooling, but with a hysteresis of over 100 K. We offer three hypotheses that may explain this observation: (1) the intergrowth of nanoscale quartz lamellae within moganite may exert a strain that inhibits the transition; (2) the transition may exhibit a martensitic character marked by the coexistence of αand β-moganite over a Þ nite temperature interval; and (3) the αand β-moganite transition may occur via an intermediate phase.
Journal of Non-Crystalline Solids, 2011
This article appeared in a journal published by Elsevier. The attached copy is furnished to the a... more This article appeared in a journal published by Elsevier. The attached copy is furnished to the author for internal non-commercial research and education use, including for instruction at the authors institution and sharing with colleagues. Other uses, including reproduction and distribution, or selling or licensing copies, or posting to personal, institutional or third party websites are prohibited. In most cases authors are permitted to post their version of the article (e.g. in Word or Tex form) to their personal website or institutional repository. Authors requiring further information regarding Elsevier's archiving and manuscript policies are encouraged to visit: http://www.elsevier.com/copyright
Journal of Non-Crystalline Solids, 2022
Journal of Nuclear Materials, 2014
High-level waste (HLW) glasses containing bismuth, phosphorus, and chromium were investigated usi... more High-level waste (HLW) glasses containing bismuth, phosphorus, and chromium were investigated using Raman and X-ray absorption spectroscopy (XAS). The novel and practically important occurrence of foaming on cooling of these melts is associated with P and Cr from the HLW. In response, glasses were synthesized where Bi 2 O 3 and P 2 O 5 contents were varied independently. Relationships between P and Cr were found, where as P 2 O 5-content increases, chromate Cr-O stretch Raman modes diminish intensity, while Cr XAS shows that Cr reduces, from 50% Cr 6+ + 50% Cr 3+ to nearly 20% Cr 6+ + 80% Cr 3+ , explaining the chromate mode behavior. In the most P 2 O 5-rich glass, the chromate Cr-O distance increases by approximately 0.10 Å, which may indicate bonding between CrO 4 and PO 4 tetrahedra, similar to that in chromophosphates. The presence of chromo-phosphate domains in HLW melts can be linked to oxygen generation as a source of the foaming.
Physical Review B, 1992
X-ray-absorption near-edge structure (XANES) data were collected for Zn in sphalerite (ZnS), and ... more X-ray-absorption near-edge structure (XANES) data were collected for Zn in sphalerite (ZnS), and for Cu and Fe in chalcopyrite (CuFeS~), where all three cations are in nearly identical coordination environments. The data have similar features, except near the edge, where the edge maximum decreases in amplitude, while a pre-edge feature appears and increases in amplitude from Zn to Cu to Fe. The pre-edge feature was previously assigned to a 1s-to-3d atomic transition for Cu and Fe in the chalcopyrite structure. XANES calculations were performed for all three edges. The multipleand single-scattering contributions to the calculated edge spectra were scaled down to better fit the experimental data. The conclusions from the calculations indicate that the pre-edge feature in the experimental Cuand Fe-edge data for chalcopyrite can be due to interference effects from the atomic structure surrounding the absorber, but cannot exclude the possibility that the pre-edge feature is due to atomic bound-state transitions of the absorber.
Physical Review B, 1993
The normal modes of vibration and their frequencies are calculated for benitoite, a mineral whose... more The normal modes of vibration and their frequencies are calculated for benitoite, a mineral whose crystal structure (space group Ds~) consists of three-membered silicate rings (Sis09) linked by Ba + and Ti + ions. Factor-group analysis dictates that certain normal modes involve the motion of only the ring atoms. On the assumption that mode mixings and splittings due to interring interactions are small, the normal frequencies of the isolated ring of |3 h, symmetry are determined by fitting to suitable averages of selected frequencies in the Raman spectra. A valence force potential consisting of only central interactions between nearest neighbors and bond-bending interactions centered at the silicon atoms is used. This potential is then extended to the full crystal structure by including interactions involving the Ba + and Ti + ions. The frequencies obtained are in excellent agreement with the infrared and Raman spectra, requiring only minor adjustment of the force constants obtained for the isolated ring. The identification of normal modes characteristic of three-membered silicate rings may prove to be a valuable guide in the interpretation of the infrared and Raman spectra of amorphous silicates, potentially leading to new information on the ring statistics of these materials.
MRS Proceedings, 1992
ABSTRACTThe normal modes of vibration and their frequencies are calculated for benitoite, a miner... more ABSTRACTThe normal modes of vibration and their frequencies are calculated for benitoite, a mineral whose crystal structure (space group D23h) consists of three-membered silicate rings (Si3O9) linked by Ba+2 and Ti+4 ions. The calculation unambiguously assigns the normal modes to the lines in the Raman and infrared spectra and determines the atomic interactions. On the assumption that mode mixings and splittings due to inter-ring interactions are small, the normal frequencies of the isolated ring of C3h, symmetry are determined. The identification of normal modes characteristic of three-membered silicate rings may prove to be a valuable guide in the interpretation of the infrared and Raman spectra of amorphous silicates, potentially leading to new information on the ring statistics of these materials.
Review of Scientific Instruments, 1990
The design and performance of a high-precision optical reflectometer are described. This instrume... more The design and performance of a high-precision optical reflectometer are described. This instrument has been optimized for measuring the specular reflectivity of thin films and multilayer structures of interest in semiconductor technology. Its design emphasizes high spectral and spatial resolution, photometric accuracy, and stray light rejection. Use of a spectrometer drive linear in wavenumber (energy) and a flexible data acquisition system facilitates data analysis. The performance of the reflectometer is demonstrated using a set of specimens consisting of silicon-dioxide layers on silicon substrates for which the oxide thicknesses had been determined by ellipsometry. Excellent agreement is obtained between the thicknesses derived from the reflectivity spectra and those determined ellipsometrically.
American Mineralogist, 2010
We use the density functional perturbation theory to determine for the first time the pressure ev... more We use the density functional perturbation theory to determine for the first time the pressure evolution of the Raman intensities for a mineral, the two high-pressure structures of MgSiO 3 perovskite and post-perovskite. At high pressures, the Raman powder spectra reveals three main peaks for the perovskite structure and one main peak for the post-perovskite structure. Due to the large differences in the spectra of the two phases Raman spectroscopy can be used as a good experimental indication of the phase transition.
Journal of Nuclear Materials, 2017
Mo-containing high-level nuclear waste borosilicate glasses were investigated as part of an effor... more Mo-containing high-level nuclear waste borosilicate glasses were investigated as part of an effort to improve Mo loading while avoiding yellow phase crystallization. Previous work showed that additions of vanadium decrease yellow phase formation and increases Mo solubility. X-ray absorption spectroscopy (XAS) and Raman spectroscopy were used to characterize Mo environments in HLW borosilicate glasses and to investigate possible structural relationships between Mo and V. Mo XAS spectra for the glasses indicate isolated tetrahedral Mo 6+ O 4 with MoO distances near 1.75 Å. V XANES indicate tetrahedral V 5+ O 4 as the dominant species. Raman spectra show composition dependent trends, where MoO symmetrical stretch mode frequencies ( 1) are sensitive to the mix of alkali and alkaline earth cations, decreasing by up to
This work was conducted under a quality assurance program compliant with 10 CFR 830 Subpart A, NQ... more This work was conducted under a quality assurance program compliant with 10 CFR 830 Subpart A, NQA-1 (2000), and DOE Order 414.1C. This program is supplemented by a Quality Assurance Project Plan for ORP work that is conducted at VSL [1-21]. Test and procedure requirements by which the testing activities are planned and controlled are defined in this plan. The program is supported by VSL standard operating procedures that were used for this work [1-22]. Exceptions are the X-ray absorption spectroscopy measurements and the tin Mössbauer spectroscopy measurements, which were performed at outside facilities and are of research quality. The requirements of DOE/RW-0333P are not applicable to this work.
Physical Review B, 1988
X-ray-absorption fine-structure (XAFS) measurements of GeC14, GeH3Cl, and GeH4 were made. We expe... more X-ray-absorption fine-structure (XAFS) measurements of GeC14, GeH3Cl, and GeH4 were made. We experimentally isolate the single-and multiple-scattering terms in the XAFS of GeC14 by cornparison of the spectra of the three compounds. The multiple-scattering (MS) amplitude is comparable to the single-scattering (SS) amplitude only within 15 eV of the edge. Beyond 40 eV the MS to SS amplitude ratio is less than 0.06. Calculations are in qualitative agreement with experiment. o ] Our results suggest that XAFS data in the range 1 & k & 3 A can be analyzed in a SS picture in many cases.
American Mineralogist, 2005
Raman spectra were collected for crystalline albite from 25 °C to above the 1118 °C melting tempe... more Raman spectra were collected for crystalline albite from 25 °C to above the 1118 °C melting temperature, where vibrational assignments for the crystal spectra were determined by lattice dynamics (LD). The Raman spectra and associated vibrational assignments are reported for triclinic albite (NaAlSi 3 O 8) at 25 °C and monoclinic albite at 1060 °C. The 25 °C calculations determined that localized TO stretch and O-TO bend modes are above 900 cm-1 (where T = Si,Al), while motions from the aluminosilicate tetrahedral cage mixed with Na displacements occur in modes as high as 814 cm-1. Vibrational modes for the most prominent peaks in the spectrum, between 350 and 550 cm-1 , are dominated by four-membered tetrahedral ring deformations. For completeness, calculated infrared mode frequencies and their atomic displacements are reported for the 25 °C structure and compared with normal mode calculation results and observed infrared mode frequencies presented by von Stengel (1977). At higher temperatures, modes above 550 cm-1 broaden and shift to lower frequencies by 15 to 27 cm-1 ; modes below 550 cm-1 broaden, but experience little, if any frequency shifts. Albite melted sluggishly, was completely liquid at 1320 °C, and remained amorphous upon cooling to room temperature. At frequencies above 550 cm-1 , the crystalline albite peaks, and possibly their vibrational assignments, can be correlated to Raman bands for albite glass. Spectral differences below 550 cm-1 between crystal and glass correspond to changes of average tetrahedral ring type upon melting, as shown by Taylor and Brown (1979).
American Mineralogist, 2014
Chromium K-edge X-ray absorption spectra were collected to characterize Cr in forsterite (Mg 2 Si... more Chromium K-edge X-ray absorption spectra were collected to characterize Cr in forsterite (Mg 2 SiO 4) as well as sulfides within the MAC 88136 EL3 chondrite to determine Cr valence and to see whether forsterite within this meteorite can be used as a Cr 2+-silicate standard. Spectra were measured on several areas within a nearly pure 100 × 200 µm forsterite grain containing 0.13 wt% Cr. XANES findings indicate highly reduced Cr 2+ species, with no clear evidence of Cr 3+ or Cr 6+. EXAFS data indicate an average 2.02 Å Cr-O nearest-neighbor distance, consistent with Cr-O distances found in square-planar Cr 2+ O 4 sites observed in synthetic crystalline silicates, and an average 2.69 Å Cr-Si second-nearest neighbor distance, consistent with Cr 2+ substituting for Mg 2+ in the forsterite M(1) site. Nearest-neighbor Debye-Waller factor and coordination number parameters indicate Cr 2+ is likely entering forsterite in disordered sites that are possible intermediates between M(1) and squareplanar Cr 2+ O 4 configurations. Preliminary Cr XAS measurements on sulfides within this meteorite also indicate Cr 2+ in CrS 6 octahedra.
American Mineralogist, 2001
X-ray absorption data were collected for a series of varnish and dendrite Mn oxide coatings on ro... more X-ray absorption data were collected for a series of varnish and dendrite Mn oxide coatings on rock substrates containing a wide variety of mineralogies exposed to a variety of environments. Near-edge spectra of the coatings indicate that the Mn-oxide phases present have Mn valences between 3+ and 4+, with average Mn valences for the varnishes closer to 4+ than those for the dendrites. Mn EXAFS data and analyses indicate that Mn-oxide structure types for the varnishes range, perhaps continuously, from large tunnel phases, similar to todorokite and romanechite, to layer phases, i.e., birnessite-family. Similar results were found for the dendrite samples, except that the variety of Mn-oxide phases is somewhat larger than those found for the varnishes. No correlations were found between Mn-oxide structure-type within these coatings and the corresponding substrate petrology.
Le Journal de Physique Colloques, 1986
X-ray absorption spectroscopic studies of cation environments in oxide glasses are selectively re... more X-ray absorption spectroscopic studies of cation environments in oxide glasses are selectively reviewed. New results are presented on K and Yb environments in silicate glasses and on Fe in silicate melts at temperatures up to 1173 " K. INTRODUCTION Recent investigations of oxide glasses and crystalline model compounds by x-ray absorption spectroscopy (XAS and the acronyms EXAFS for extended fine structure and NE)(AFS or XANES for near-edge structure) have provided useful and sometimes unique information on the structural environments of network-forming and network-modifying cations. Synchrotron-based XAS is well suited for studying the local structural environment and bonding of cations in amorphous materials that cannot be probed directly by standard spectroscopic or scattering methods (e.g. Na, Mg, K, Ca, Zr) or which are present in small concentrations (e.g. transition metals, rare earth and actinide elements a t 100-2000 ppm). T o date, several dozen XAS studies of cations in oxide glasses have reported information on local coordination environments of network formers such as Si, Al, Ga, and Ge and of network modifiers such as Na, K, Ca, Ti, V, Fe, Zn, Zr, Pb, and U. This work has provided new insights about oxide glass structure and structure-property-composition relationships which shed light on processes such as homogeneous nucleation, viscous flow, cation diffusion, and corrosion behavior of glasses. This paper presents a brief overview of recent XAS studies of silicate and oxide glasses which is selective because of space limitations. Also reported are new results on the structural environments of K and Yb in silicate glasses under ambient conditions and of Fe in silicate melts at temperatures up t o 1173" K.
Journal of Non-Crystalline Solids, 1984
ABSTRACT Polarized Raman spectra have been obtained for a series of sodium aluminosilicate glasse... more ABSTRACT Polarized Raman spectra have been obtained for a series of sodium aluminosilicate glasses and selected minerals, in order to investigate changes in the glass structure with composition. Particular attention is paid to characterization of the Al coordination, and to test Lacy's hypothesis concerning the role of “triclusters” in Al-rich glasses (Al/Na = R > 1.0). A comparison of five glasses ranging from nearly sodium trisilicate composition glass (R = 0.03) to albite composition glass (R = 1), and beyond to a glass of R = 1.61, leads to the following observations with increasing R: the polarized bands previously assigned to T-O (non-bridging) vibrations (950, 1110 cm−1) decrease in intensity, while those assigned to T-O (bridging) vibrations (485, 980 cm−1) either remain the same or increase in intensity. The narrow band at 495 cm−1 and a somewhat wider one at 595 cm−1, may be related to Raman active “defects” that have been observed in vitreous SiO2 at essentially the same frequencies. Our observations indicate a general increase in tetrahedral polymerization and random incorporation of Al into tetrahedral sites. Comparison of the Raman spectra of mullite (which contains (Si, Al)3O10 triclusters) with sillimanite (which contains no triclusters) shows that the bands at 418 and 1200 cm−1 in mullite are absent for sillimanite. Assuming that these “extra” bands are due to the triclusters in mullite, we have to conclude that similar triclusters do not exist in any of the glasses we examined. If these “extra” bands in mullite are not due to triclusters, we can draw no such conclusion. Our study indicates that observed changes in physical properties of sodium aluminosilicate glasses as R increases are not due to an Al coordination change.
Journal of Non-Crystalline Solids, 1985
ABSTRACT Photoelectron total yield Al K-XANES and EXAFS spectra were collected for a series of Na... more ABSTRACT Photoelectron total yield Al K-XANES and EXAFS spectra were collected for a series of Na2OAl2O3SiO2 minerals and glasses. The purpose of this study is to determine if EXAFS can accurately characterize these known Al environments in terms of AlO distances and coordination numbers, and to characterize the unknown Al environment in the glasses. Al XANES spectra were collected for corundum, sillimanite, albite, jadeite and nepheline, as well as jadeite glass, nepheline glass and a 75 mol% silica glass series having R ( = Al/Na) ranging from 0.28 to 1.61. Al EXAFS spectra were collected for corundum, sillimanite, jadeite and albite, as well as jadeite glass, albite glass and a 75 mol% silica glass with an R of 1.61.Al XANES spectra for our model compounds have two edge maxima at 1568 and 1572 eV for minerals having octahedral Al, while one edge maximum is present at 1566 eV for minerals with tetrahedral Al. The glasses all have a single edge maximum near 1566 eV, indicating tetrahedral-like Al sites in the glasses. The results of our Al EXAFS refinements of the model compounds are in agreement to within ±0.03 Å for AlO distances and to within ±23% for coordination numbers of recent crystal structure refinements in the literature. The EXAFS results for the glasses clearly indicate Al in tetrahedral sites, yielding AlO distances near 1.77 Å, well within the range of average AlO distances for aluminate tetrahedra of aluminosilicate minerals. Our XANES and EXAFS results indicate tetrahedral Al throughout our 75 mol% silica glass series, which may be consistent with the existence of tricluster units occurring in the Al-rich glasses in the sodium aluminosilicate system.
Journal of Non-Crystalline Solids, 1985
Photoelectron total-yield Na XANES and EXAFS spectra were gathered to characterize the Na coordin... more Photoelectron total-yield Na XANES and EXAFS spectra were gathered to characterize the Na coordination for a collection of Na2OAl2O3SiO2 glasses and minerals. Included in the collection are the crystalline model compounds albite, jadeite and nepheline as well as glasses at these compositions, Na2Si2O5 glass, and a series of glasses containing 75 mol% silica with R (= Al/Na) values ranging from 0.03 to 1.61. High resolution Na K-XANES spectra are reported for the first time and are of good enough quality for the specimens studied to make qualitative comparisons of Na edge features possible. Due to the variable quality of the Na EXAFS data, only Na2Si2O5 glass, three out of five of the glasses in the glass series, nepheline and nepheline glass were worthy of analysis; these samples are the more soda-rich materials used for this study. Comparisons of Na XANES and EXAFS spectra for the glass series, nepheline, nepheline glass and Na2Si2O5 glass indicate little if any change in the Na environment among these materials. Nepheline glass and crystal show reversal of intensity of the 1055 eV and 1058 eV Na edge features. The similarity of the Na coordination environment is also apparent in the EXAFS spectra, which yield NaO distances ranging from 2.56 to 2.62 A. This overall similarity of the Na coordination for samples in this study is not surprising since Na in crystalline compounds within this system is usually coordinated by seven to ten oxygens. The physical property variations for our glass series are not explained by the differences in the Na environments depicted by our Na XANES and EXAFS results. The Na EXAFS for nepheline appears to sense only the closest set of oxygens of those in the entire known Na coordination, and this seems to be true for nepheline glass as well, using high temperature crystal structure studies of nepheline for distance comparisons.
American Mineralogist, 2007
The silica polymorph moganite is commonly intergrown with quartz in microcrystalline silica varie... more The silica polymorph moganite is commonly intergrown with quartz in microcrystalline silica varieties that are less than ~100 Ma in age. Synchrotron X-ray diffraction suggests that a displacive phase transition occurs when moganite is heated above ~570 K, with an increase in symmetry from I2/a to Imab. In the present study, we employed hard-mode Raman spectroscopy to conÞ rm the existence of the α-β moganite transformation and to offer complementary insight into the transition mechanism. Our analysis of the displacement of the 501 ∆cm-1 symmetric stretching-bending vibration (B 3g mode) with changing temperature strongly supports the existence of a monoclinic-to-orthorhombic phase transition between 570 and 590 K. Between 593 and 723 K, however, the mode remained Þ xed at 496 ∆cm-1. This behavior was repeated on cooling, but with a hysteresis of over 100 K. We offer three hypotheses that may explain this observation: (1) the intergrowth of nanoscale quartz lamellae within moganite may exert a strain that inhibits the transition; (2) the transition may exhibit a martensitic character marked by the coexistence of αand β-moganite over a Þ nite temperature interval; and (3) the αand β-moganite transition may occur via an intermediate phase.
Journal of Non-Crystalline Solids, 2011
This article appeared in a journal published by Elsevier. The attached copy is furnished to the a... more This article appeared in a journal published by Elsevier. The attached copy is furnished to the author for internal non-commercial research and education use, including for instruction at the authors institution and sharing with colleagues. Other uses, including reproduction and distribution, or selling or licensing copies, or posting to personal, institutional or third party websites are prohibited. In most cases authors are permitted to post their version of the article (e.g. in Word or Tex form) to their personal website or institutional repository. Authors requiring further information regarding Elsevier's archiving and manuscript policies are encouraged to visit: http://www.elsevier.com/copyright
Journal of Non-Crystalline Solids, 2022
Journal of Nuclear Materials, 2014
High-level waste (HLW) glasses containing bismuth, phosphorus, and chromium were investigated usi... more High-level waste (HLW) glasses containing bismuth, phosphorus, and chromium were investigated using Raman and X-ray absorption spectroscopy (XAS). The novel and practically important occurrence of foaming on cooling of these melts is associated with P and Cr from the HLW. In response, glasses were synthesized where Bi 2 O 3 and P 2 O 5 contents were varied independently. Relationships between P and Cr were found, where as P 2 O 5-content increases, chromate Cr-O stretch Raman modes diminish intensity, while Cr XAS shows that Cr reduces, from 50% Cr 6+ + 50% Cr 3+ to nearly 20% Cr 6+ + 80% Cr 3+ , explaining the chromate mode behavior. In the most P 2 O 5-rich glass, the chromate Cr-O distance increases by approximately 0.10 Å, which may indicate bonding between CrO 4 and PO 4 tetrahedra, similar to that in chromophosphates. The presence of chromo-phosphate domains in HLW melts can be linked to oxygen generation as a source of the foaming.