J. O. Ramey

Structure of zinc polyphosphate glasses

Abstract The technique of high-performance liquid chromatography (HPLC) has been used to determine the distributions of phosphate-anion chains and rings in a variety of zinc-phosphate-based glasses. The phosphate-anion distributions in simple binary zinc phosphate glasses are compared to those found in multi-cation alkali-zinc phosphate glasses. Multi-cation zinc phosphate glasses are chemically durable and can be tailored to exhibit transition temperatures sufficiently low for co-processing with a variety of plastics to form novel organic–inorganic composite materials. The intermediate range order in the multi-cation glasses is found to be similar to that measured in the binary zinc-phosphate glasses. Scanning calorimetry measurements indicate, however, that the multi-cation glasses are more resistant to crystallization than the binary glasses. The fraction of phosphorus sites with 2 (Q2), 1 (Q1) and 0 (Q0) bridging oxygen, as well as the ratio of bridging to non-bridging oxygen (BO/NBO), are obtained from the chromatograms. Structural data for the zinc phosphate glasses obtained using HPLC are compared to results reported in the literature that were deduced using more-traditional solid-state techniques such as NMR, XPS, and Raman spectroscopy.

Development of Novel Polycrystalline Ceramic Scintillators

For several decades most of the efforts to develop new scintillator materials have concentrated on high-light-yield inorganic single-crystals while polycrystalline ceramic scintillators, since their inception in the early 1980s, have received relatively little attention. Nevertheless, transparent ceramics offer a promising approach to the fabrication of relatively inexpensive scintillators via a simple mechanical compaction and annealing process that eliminates single-crystal growth. Until recently, commonly accepted concepts restricted the polycrystalline ceramic approach to materials exhibiting a cubic crystal structure. Here, we report our results on the development of two novel ceramic scintillators based on the non-cubic crystalline materials: Lu2SiO5:Ce (LSO:Ce) and LaBr3:Ce. While no evidence for texturing has been found in their ceramic microstructures, our LSO:Ce ceramics exhibit a surprisingly high level of transparency/translucency and very good scintillation characteristics. The LSO:Ce ceramic scintillation reaches a light yield level of about 86% of that of a good LSO:Ce single crystal, and its decay time is even faster than in single crystals. Research on LaBr:Ce shows that translucent ceramics of the high-light-yield rare-earth halides can also be synthesized. Our LaBr3:Ce ceramics have light yields above 42000 photons/MeV (i.e., >70%of the single-crystal light yield).

An EPR study of rare‐earth impurities in single crystals of the zircon‐structure orthophosphates ScPO4, YPO4, and LuPO4 a)

Ceramic materials based on the lanthanide orthophosphate series of compounds are known to be highly stable both chemically and physically. These characteristics have recently led to an extensive evaluation of these substances as potential primary containment media for the disposal of high‐level radioactive wastes. Since one important class of high‐level nuclear waste (i.e., reprocessed light water reactor spent fuel) contains a relatively high concentration of various lanthanides, the solid state chemical properties of the mixed rare‐earth and actinide‐doped orthophosphates are of considerable practical interest. The Kramers’ ions Ce3+, Nd3+, Dy3+, Er3+, Yb3+, and U3+ have been incorporated in single crystals of the tetragonal symmetry hosts ScPO4, YPO4, and LuPO4, and EPR spectroscopy has been used to verify the substitutional behavior of these ions and to investigate their electronic ground state properties. Principal axial spectroscopic splitting factors and hyperfine parameters were determined. These ...

Chromatographic studies of the structures of amorphous phosphates: a review

Abstract The technique of high-performance liquid chromatography (HPLC) has been used to investigate the distribution of phosphate anions in a variety of amorphous phosphate solids. In many cases, HPLC can be used to quantitatively measure the type and distribution of phosphate tetrahedral chains and rings in the solid that are between 1 and 10 nm in size. The experimental details of the HPLC technique are presented and the strengths and limitations of the technique are reviewed. The structural information obtained using HPLC is compared with structural information obtained using solid state techniques such as NMR and XPS. Several specific examples that illustrate the power and sensitivity of the HPLC technique are reviewed.

Structural differences between the glass state and ion-beam-amorphized states of lead pyrophosphate

Amorphous lead pyrophosphates (Pb2P2O7) were prepared by either thermally quenching the molten phosphate to produce a glass, by implanting O2+ or Pb3+ ions into lead pyrophosphate single crystals, or by implanting O2+ ions into lead pyrophosphate glass. The structures of the amorphous phases produced by these methods were determined using the technique of high performance liquid chromatography (HPLC) which measures the distribution of phosphate anions (i.e. the number and length of PO4 tetrahedral chains) present in each type of amorphous material. Ion channeling measurements of Pb-implanted (540 keV) lead pyrophosphate crystals showed that the amorphous layer produced via ion bombardment (1015 Pb ions/cm2) was about 200 nm thick. For all implant doses (from 1011 to 1015 ions/cm2), the structure of the damaged layer produced by ion bombardment was found to differ significantly from the structure of lead pyrophosphate glass prepared via thermal quenching. The structure of the amorphous phase produced by ion-implantation was, however, essentially independent of the implanted species since the implantation of high doses of either lead or oxygen produced virtually the same distribution of phosphate anions. The final amorphous phase formed via ion-implantation was determined to be structurally different depending on whether the implanted lead pyrophosphate surface was initially a crystal or a glass. The evolution of the structural changes that occurred during the ion-beam-induced amorphization of single crystal surfaces of lead pyrophosphate with varying doses of Pb3+ and O2+ was determined along with the thermal properties of lead pyrophosphate glass.

SrI2 scintillator for gamma ray spectroscopy

We are working to perfect the growth of divalent Eu-doped strontium iodide single crystals and to optimize the design of SrI2(Eu)-based gamma ray spectrometers. SrI2(Eu) offers a light yield in excess of 100,000 photons/MeV and light yield proportionality surpassing that of Ce-doped lanthanum bromide. Thermal and x-ray diffraction analyses of SrI2 and EuI2 indicate an excellent match in melting and crystallographic parameters, and very modest thermal expansion anisotropy. We have demonstrated energy resolution with SrI2(4-6%Eu) of 2.6% at 662 keV and 7.6% at 60 keV with small crystals, while the resolution degrades somewhat for larger sizes. Our experiments suggest that digital techniques may be useful in improving the energy resolution in large crystals impaired by light-trapping, in which scintillation light is re-absorbed and re-emitted in large and/or highly Eu2+ -doped crystals. The light yield proportionality of SrI2(Eu) is found to be superior to that of other known scintillator materials, such as LaBr3(Ce) and NaI(Tl).

EPR investigations of Fe3+ in single crystals and powders of the zircon‐structure orthophosphates LuPO4, YPO4, and ScPO4

Iron‐doped single crystals of the zircon‐structure orthophosphates LuPO4, YPO4, and ScPO4 have been grown by means of a flux technique. Corresponding powders have been prepared independently by a urea precipitation technique and electron paramagnetic resonance measurements demonstrate that the Fe3+ impurity is situated in the same substitutional site in both the single crystals and the powders for all three hosts. The sign of the crystal field parameter b02 is shown to be positive (opposite to that found for Gd3+ in these hosts). The crystal‐field splittings were found to be largest for the scandium host and smallest for the yttrium host. The EPR results show that iron can be incorporated in a substitutional crystallographic site in material prepared by either a high temperature process or by a low temperature precipitation and calcination (∼800 °C), and these findings have implications for the use of lanthanide and related orthophosphates as a host material for the isolation of nuclear wastes.

Structural properties of the amorphous phases produced by heating crystalline MgHPO4·3H2O

Abstract The crystalline phosphate MgHPO 4 ·3H 2 O (newberyite) undergoes an unusual crystalline-to-amorphous transition when heated. The structural alterations associated with this transition have been investigated using high-performance liquid chromatography (HPLC), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and X-ray diffraction (XRD). The dehydration of newberyite on heating results in amorphous XRD patterns that remain essentially unchanged over the interval between 150 and 600°C. Over the same temperature interval, however, HPLC results reveal a dramatic continuous evolution in the distribution of phosphate chains of corner-linked PO 4 tetrahedra leading to the formation of chains up to 13 PO 4 tetrahedra in length. Above 600°C, crystalline Mg 2 P 2 O 7 is formed. At each annealing temperature in the interval from 150 to 600°C, the measured distribution of phosphate anions is in quantitative agreement with theory. During the dehydration-induced crystalline-to-amorphous transition, the original faceted crystal shape is preserved even though the crystals lose up to 36% of their original weight. High-pressure DSC experiments in which the waters of hydration were retained in the specimens during heating resulted in the formation of a unique new crystalline phosphate phase that contained equal amounts of orthophosphate and pyrophosphate anions. The results of HPLC, DSC, and XRD measurements on CaHPO 4 ·2H 2 O, SrHPO 4 , and BaHPO 4 are also reported.

Exploratory Research on the Development of Novel

We report the discovery of a new family of Ce3+-activated phosphate glass scintillators that can be formed either with or without the addition of 6Li, for neutron or X-ray/gamma-ray radiation detection, respectively. Trivalent cerium can be efficiently introduced into these phosphate glasses in surprisingly high concentrations in the form of anhydrous cerium tri-chloride. Additionally, these glasses can be melted and poured at the relatively low temperatures of 1000-1050degC (i.e., substantially lower than silicate glasses), and to retain the cerium in the trivalent state it is not necessary to maintain highly reducing conditions during the synthesis process. The family of alkaline-earth-alkali phosphate glasses investigated here represents a system with two dissimilar cations - thereby offering a large range of potential compositional variations, substitutions, and combinations. In order to alter the scintillator characteristics, we have explored part of that compositional space by studying Ca-Na, Ca-Li, Ca-Cs, Ca-Rb, Ca-K and Ca-Ba-Na phosphate glasses, as well as various co-doping and post-synthesis thermal processing schemes. A series of experiments under X-ray, gamma-ray, and neutron excitations was carried out. The broad, peaking at about 354 nm, UV scintillation of these glasses is well suited for applications that use common photomultipliers with bi-alkali photo-cathodes. Pulse shape measurements show that the primary component of the scintillation in most of these glasses corresponds to 75-90% of the emitted photons, and it decays with a time constant of 30 to 40 ns, which classifies these materials as reasonably fast scintillators. Although the gamma-induced light yield of these new scintillating phosphate glasses is, thus far, only about 30% of that of commercial GS20 silicate glass, due to the generally faster scintillation, the initial amplitude of the scintillation pulse of these glasses is close to that of the above-mentioned GS20 scintillator.

{\rm Ce}^{3+}

The technique of high-performance liquid chromatography (HPLC) has been used to probe the phosphate anion distribution in a variety of metal phosphate glasses including glasses made with trivalent metal cations (Al, In, Ga, La). The composition of each glass was chosen so that the average phosphate chain length was between 2 and 4 PO{sub 4} tetrahedra. The widths of the resulting phosphate anion distributions were determined directly from an analysis of the HPLC chromatograms. Literature values for the free energy of formation of the crystalline metal-orthophosphate compounds with respect to P{sub 2}O{sub 5} and the metal oxide, were compared to the chromatogram widths. It was found that the smaller the energy of formation, the wider the distribution of phosphate chains, and the greater the ease of glass formation.