This article reviews the current status of lattice-dynamical calculations in crystals, using density-functional perturbation theory, with emphasis on the plane-wave pseudopotential method. Several specialized topics are treated, including the implementation for metals, the calculation of the response to macroscopic electric fields and their relevance to long-wavelength vibrations in polar materials, the response to strain deformations, and higher-order responses. The success of this methodology is demonstrated with a number of applications existing in the literature.

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Phonons and related crystal properties from density-functional perturbation theory

In this review the phenomenon of proton conductivity in materials and the elements of proton conduction mechanismsproton transfer, structural reorganization and diffusional motion of extended moietiesare discussed with special emphasis on proton chemistry. This is characterized by a strong proton localization within the valence electron density of electronegative species (e.g., oxygen, nitrogen) and self-localization effects due to solvent interactions which allows for significant proton diffusivities only when assisted by the dynamics of the proton environment in Grotthuss and vehicle type mechanisms. In systems with high proton density, proton/proton interactions lead to proton ordering below first-order phase transition rather than to coherent proton transfers along extended hydrogen-bond chains as is frequently suggested in textbooks of physical chemistry. There is no indication for significant proton tunneling in fast proton conduction phenomena for which almost barrierless proton transfer is suggest...

Proton Conductivity: Materials and Applications

Interpretation of europium(III) spectra

During the past half-century, the nuclear fuel cycle has generated approximately 1400 metric tons of plutonium and substantial quantities of the “minor” actinides, such as Np, Am, and Cm. The successful disposition of these actinides has an important impact on the strategy for developing advanced nuclear fuel cycles, weapons proliferation, and the geologic disposal of high-level radioactive waste. During the last decade, there has been substantial interest in the use of the isometric pyrochlore structure-type, A2B2O7, for the immobilization of actinides. Most of the interest has focused on titanate-pyrochlore because of its chemical durability; however, these compositions experience a radiation-induced transition from the crystalline-to-aperiodic state due to radiation damage from the alpha-decay of actinides. Depending on the actinide concentration, the titanate pyrochlore will become amorphous in less than 1000 years of storage. Recently, systematic ion beam irradiations of a variety of pyrochlore compo...

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Nuclear waste disposal—pyrochlore (A2B2O7): Nuclear waste form for the immobilization of plutonium and “minor” actinides

The 5d level positions of the trivalent lanthanides in inorganic compounds

Crystal Structure Refinements of Zircon-Type MVO4 (M = Sc, Y, Ce, Pr, Nd, Tb, Ho, Er, Tm, Yb, Lu)

Experimental measurements and theoretical calculations are reported which may resolve some of the remaining difficulties associated with photoemission from anion vacancies in thermochemically reduced MgO crystals. The intensities of the 2.3- and 3.2-eV luminescence bands are found to be strongly influenced by both the concentrations of ${\mathrm{H}}^{\ensuremath{-}}$ ions and anion vacancies present, and also by the intensity of the \ensuremath{\sim}5.0-eV exciting light. Theoretical calculations predict that for a $^{1}A_{1g}$ relaxation of the surrounding ions the $^{1}T_{1u}$ and $^{3}T_{1u}$ electronic states of the $F$ center are almost degenerate and \ensuremath{\sim}0.05 eV above the $^{3}A_{1g}$ state. The $^{1}T_{1u}\ensuremath{\rightarrow}^{1}A_{1g}$ and the $^{3}T_{1u}\ensuremath{\rightarrow}^{1}A_{1g}$ transition energies are predicted to be 2.2 and 2.9 eV, respectively.

Luminescence from oxygen vacancies in MgO crystals thermochemically reduced at high temperatures

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.

Monazite and other lanthanide orthophosphates as alternate actinide waste forms

Radiation effects in a natural, metamict zirconolite from Sri Lanka that has received an alpha-decay dose greater than 1026 alpha decays/m3 have been studied using x-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), electron paramagnetic resonance spectroscopy (EPR), extended x-ray absorption fine structure spectroscopy (EXAFS), and x-ray absorption near edge structure spectroscopy (XANES). The same techniques were applied to the sample annealed between 1000°and 1100°C. The heat of recrystallization was measured by differential thermal analysis (DTA) with values of 40 to 50J/g. In contrast to previous work [A. E. Ringwood, Am. Scientist 70, 201 (1982); Mineralog. Mag. 49, 159 (1985)], these results demonstrate that there are fundamental differences at the atomic level between the annealed, crystalline, and the natural, fully damaged zirconolite. We suggest that the most likely structure for the fully damaged state is that of a random, three-dimensional network with no atomic periodicity extending beyond the first coordination sphere. Even within the first coordination sphere, there is a reduction in coordination number and an associated decrease in bond length for principal cations (Ca and Ti). Despite this structural modification and the great age of the specimen (∼550 million years), the natural zirconolite shows only minor signs of geochemical alteration. The metamict structure is readily annealed, initially to a disordered, flourite-type structure, and finally, at higher temperatures, to a highly twinned, monoclinic zirconolite structure.

M. M. Abraham

Papers

Crystal Structure Refinements of Zircon-Type MVO4 (M = Sc, Y, Ce, Pr, Nd, Tb, Ho, Er, Tm, Yb, Lu)

Luminescence from oxygen vacancies in MgO crystals thermochemically reduced at high temperatures

Monazite and other lanthanide orthophosphates as alternate actinide waste forms

Alpha-recoil damage in zirconolite (CaZrTi 2 O 7 )

Structural investigations of several LnVO4 compounds