M. M. Abraham

Growth of High‐Purity and Doped Alkaline Earth Oxides: I. MgO and CaO

A new variation of the carbon arc‐fusion technique is reported which makes possible the growth of large, high‐purity MgO and CaO single crystals which are free of microbubbles. A technique has also been developed for impurity doping. We have successfully doped MgO with Na, D, Ce, Eu, Gd, Dy, and Yb; and CaO with Li, Na, Ce, and Nd. The presence of these dopants has been ascertained by chemical analyses, optical, or EPR measurements. EPR signals showed minimal effects from strains in these crystals.

Ion implantation and thermal annealing of α‐Al2O3 single crystals

The effects of ion implantation and post‐implantation thermal annealing of α‐Al2O3 have been characterized using ion scattering‐channeling techniques, and correlated with electron paramagnetic resonance (EPR) and microhardness measurements. Although most of the work was done on 52Cr implanted specimens, preliminary results have been obtained also for implanted 90Zr and 48Ti. For Cr implantation, the Al2O3 lattice damage saturates at relatively low doses, but the near‐surface region never becomes amorphous. A preferential annealing behavior begins in the Al sublattice after ∼800 °C annealing, and in the oxygen sublattice, only after 1000 °C annealing. Lattice location measurements show that after annealing to 1500 °C, Cr is greater than 95% substitutional in the Al sublattice. Above 1500 °C, implanted Cr atoms redistribute by substitutional diffusion processes. EPR measurements show that part, if not all, of the implanted Cr is trivalent and substitutional after annealing to 1600 °C. Microhardness measurem...

Analytical electron microscopic studies and positron lifetime measurements in Al‐doped MgO crystals

MgO crystals intentionally doped with Al were characterized by analytical electron microscopic examinations and positron lifetime measurements. Large spinel (MgO Al2O3) precipitates were observed in samples with high contents of Al. A well‐defined crystallographic relationship between the precipitates and the matrix was found. The characteristics of positron lifetime spectra appear to depend on the valence state of the different impurities in the MgO lattice suggesting that positrons are trapped by vacancy impurity complexes.

ELECTRON SPIN RESONANCE OF IRRADIATED QUARTZ: ATOMIC HYDROGEN

An electron spin resonance spectrum observed in irradiated synthetic quartz crystals is attributed to atomic hydrogen in interstitial sites. The quartz specimens were irradiated with 1.7 MeV electrons at ∼300°K (1018 electrons cm−2) and then with 60Co γ‐rays (∼107 R) at ∼80°K and measurements were made without warming. The hydrogen spectra had a hyperfine constant A = 521.3 Oe (1453.1 Mc/sec) when the applied field was along the crystal threefold axis. An anisotropy of the hyperfine interaction of the order of 0.75 Oe was detected. The g value was 2.0021±0.0005 and evidence for a small variation in this value with rotation was inferred. There are three sites for the atomic hydrogen in the unit cell which are related by the three fold symmetry operator of the crystal and which are otherwise equivalent. Additional hyperfine interactions with 29Si nuclei were also observed enabling a tentative model of the sites to be proposed. The width of the line was 0.1 Oe and was attributed mainly to field inhomogeneity...

Structural investigations of several LnVO4 compounds

Refinements of the crystallographic structures of SmVO4, EuVO4, GdVO4 and DyVO4 have been carried out by means of three dimensional single-crystal X-ray diffractometry. These orthovanadate compounds crystallize in the tetragonal space group 141/amd (Z = 4) with respective lattice constants of a = 7.2647(9), 7.2358(7), 7.2122(7), 7.1429(8) A and c = 6.384(1), 6.362(1), 6.346(2), 6.300(2) A. Anisotropic refinements of the structural data using a full-matrix least-squares program yielded final reliability (R) factors of 0.026, 0.049, 0.042 and 0.041 based on 266, 273, 261 and 220 unique reflections for Ln = Sm, Eu, Gd and Dy, respectively. The lanthamide (Ln) atoms are eight-coordinated to oxygen atoms with two unique LnO bond lengths. These non-equivalent vertices form two bisphenoidal sets. A distorted triangulated dodecahedron is produced by connecting the oxygen vertices. The orthovanadate group in each compound is a distorted tetrahedron. Important crystal data, parameters, bond lengths and angles are tabulated.

Structural properties of lead-iron phosphate glasses☆

The effects of iron on the structure and chemical stability of several lead-iron phosphate glasses ((1 − x) Pb(PO3)2 + (x) Fe2O3, O < x < 0.25) were investigated using Mossbauer, electron paramagnetic resonance, Raman, and infrared spectroscopies. Dilatometer measurements were used to determine the glass transition temperature, softening point, and thermal expansion coefficient of glasses where the fusion temperature, soak time, or iron concentration had been varied. The major conclusion of this study is that iron improves the chemical durability of these glasses by strengthening the cross bonding between the polyphosphate chains in the glass structure.

EPR Investigations of Er3+, Yb3+, and Gd3+ in Zircon‐Structure Silicates

The EPR spectra of Er3+, Yb3+, and Gd3+ have been observed in the host single crystals ZrSiO4, HfSiO4, and ThSiO4 which were grown by a flux‐gradient technique. For most systems, spectra were observed which exhibited both tetragonal and orthorhombic symmetry. The tetragonal spectra were characterized by a principal axis system which coincided with the principal crystallographic axes. The observed orthorhombic spectra were of two types. For Gd3+ in ZrSiO4 and HfSiO4, orthorhombic spectra due to eight inequivalent sites were observed and had principal axis systems which were not simply related to the host crystallographic axes. All other orthorhombic spectra resulted from four inequivalent sites whose z and x axes lay in {110} planes with equal angles between the crystal tetragonal symmetry axis (c axis) and each z axis. For both types of orthorhombic spectra, the corresponding EPR transitions from the different sites were super‐imposed with the applied magnetic field parallel to the c axis. Spin‐Hamiltonia...

Monazite and Other Lanthanide Orthophosphates as Alternate Actinide Waste Forms

The recent results of leach tests performed under hydrothermal conditions have shown that borosilicate glasses are physically and chemically unstable at elevated temperatures and pressures (1). These results emphasize the need for investigations of alternate high level waste forms which offer the potential of both increased resistance to leaching and stability under various possible metamorphic geological conditions. Although many of the previous (and present) studies of high level waste forms emphasize the containment of fission products, in fact, the dominant long term (i.e., after 300 to 500 year) radiation hazard is posed by the α-emitting actinides. Accordingly, the object of the present investigation is to evaluate an alternate means of actinide containment which is potentially superior to encapsulation in borosilicate glass. The mineral monazite, which is a mixed lanthanide-actinide orthophosphate [i.e., (Ce, La, Nd, Th, U)PO4] of varying composition, is a crystalline material with established chemical and physical stability when subjected to natural geological conditions over extremely long time periods. Additionally, since monazite is a natural source of the actinides Th and U, it has been subjected to radiation damage (including α-particle damage) during its natural lifetime, and the continued maintenance of its physical and chemical integrity is evidence of its intrinsic resistance to radiation. In view of these obviously desirable characteristics, a series of investigations of the physical and chemical properties of both natural monazite and the related, synthetically produced lanthanide orthophosphates has been undertaken. These investigations have included the usual applied studies such as leach testing under various conditions and the consolidation of precipitated orthophosphate powders by hot pressing as well as more fundamental determinations of the valence states and site symmetry of the actinide impurities. The details of this work are presented in the following discussion.

Crystal field analysis of Tm3+ and Yb3+ in YPO4 and LuPO4

The optical and near infrared absorption of dilute Tm3+ and Yb3+ impurities in YPO4 and LuPO4 single crystals have been measured at liquid helium and nitrogen temperatures. For Tm3+, the spectral region from 5 000 to 38 000 cm−1 was examined and Zeeman spectra were obtained in the visible region. The observed transitions were assigned and fit to a semiempirical Hamiltonian with adjustable parameters via a least‐squares procedure. Satisfactory fits and good agreement between the calculated and measured g values were obtained. For Yb3+, there are more parameters than experimental levels, so ζ, B20, B40, and B44 were adjusted, while B60 and B64 were fixed at the values found for Tm3+. Energy levels and Zeeman splittings calculated with these parameters are in good agreement with the measured quantities.

Trapped-hole centers in alkaline-earth oxides

Abstract Alkaline-earth oxides frequently exhibit a visible coloration when stimulated with photons or ionizing radiation. The defects responsible for this coloration are often due to paramagnetic hole centers, which absorb over a broad spectrum in the visible and near-visible region. As a result, the defects can be detrimental to the use of these materials as hosts for tunable lasers. In this article, we present our current understanding of trapped-hole (or V-type) centers in the alkaline-earth oxides with emphasis on experimental research using optical and magnetic resonance techniques. Discussions are focused on the production mechanisms, the structure of both intrinsic and impurity-compensated defects, and their thermal stability.