3D electron diffraction studies of synthetic rhabdophane (DyPO4·nH2O).

In this study, we report the results of continuous rotation electron diffraction studies of single DyPO4·nH2O (rhabdophane) nanocrystals. The diffraction patterns can be fit to a trigonal lattice (P3121) with lattice parameters a = 7.019 (5) and c = 6.417 (5) Å. However, there is also a set of diffuse background scattering features present that are associated with a disordered superstructure that is double these lattice parameters and fits with an arrangement of water molecules present in the structure pore. Pair distribution function (PDF) maps based on the diffuse background allowed the extent of the water correlation to be estimated, with 2-3 nm correlation along the c axis and ∼5 nm along the a/b axis.

Review of Rare-Earth Phosphate Materials for Nuclear Waste Sequestration Applications.

Several materials have or are currently being investigated for nuclear waste sequestration applications, including crystalline ceramic oxides, glasses, and glass-ceramic composites. Rare-earth phosphates have been investigated extensively for this application owing to the range of structures that the hydrous or anhydrous versions can adopt as well as the fact that naturally occurring rare-earth phosphates have been found to contain U or Th. The purpose of this mini-review is to discuss (generally) the properties that must be considered when identifying nuclear wasteform materials and (more specifically) the structure and properties of rare-earth phosphates with special attention being given to the resistance of these materials to radiation-induced structure damage. The last section of the mini-review contains an introduction to the development of glass-ceramic composite materials that contain rare-earth phosphate crystallites dispersed in a glass matrix. These composite materials have been suggested to be superior to using just glass or ceramic materials for nuclear waste sequestration applications owing to improved waste loading capabilities.

Effect of Annealing on Structural and Thermodynamic Properties of ThSiO4-ErPO4 Xenotime Solid Solution.

The effect of annealing on structural and thermochemical properties of a thorite-xenotime solid solution Th1-xErx(SiO4)1-x(PO4)x was assessed. The samples synthesized at low temperatures and stored at room temperature for 2 years retained their tetragonal structures. This structure was also maintained after heating to 1100 °C. During annealing, the structure lost water and exsolved some thorianite phases. The thermodynamic parameters did not change much after annealing, suggesting that xenotime was not a low-temperature metastable phase but rather a stable structure able to withstand elevated temperatures regardless of the thorium content. The solid solution exhibited subregular behavior with the Margules function W(x) = (73.1 ± 20.1) - (125.7 ± 49.8)·x.

A Structural Investigation of Hydrous and Anhydrous Rare-Earth Phosphates.

Rhabdophane- (REPO4·nH2O; RE = La to Dy), monazite- (REPO4; RE = La to Gd), and xenotime-type (REPO4 and RE'PO4·nH2O; RE = Tb to Lu and Y; RE' = Ho to Lu and Y) rare-earth phosphate materials are being considered for a number of applications including as photonic materials, for biolabeling studies, and as potential nuclear wasteforms. Structural studies of hydrous rare-earth phosphates are rather limited when compared to anhydrous rare-earth phosphates. In this study, rhabdophane- (REPO4·nH2O; RE = La, Nd, Sm, Gd, and Dy) and xenotime-type (REPO4·nH2O; RE = Y and Yb) materials were synthesized by a precipitation-based method and investigated using X-ray diffraction (XRD) and X-ray absorption near-edge spectroscopy (XANES). Examination of the powder XRD data from rhabdophane-type materials has confirmed that the rhabdophane structure crystallizes in the monoclinic crystal system rather than the hexagonal structure that has most often been reported. Materials adopting the rhabdophane- or xenotime-type structure were studied as a function of temperature to understand how the structure varies with increasing annealing temperature. Local structural information was obtained by collecting P K- and RE L1-edge XANES spectra. Examination of P K-edge XANES spectra from rhabdophane- and xenotime-type materials revealed changes in the local environment around P as a function of temperature. These changes were attributed to the removal of water from these structures as a result of high temperature annealing.