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Thorium incorporation in the rhabdophane structure as Pr 1-2x Ca x Th x PO 4 ·nH 2 O solid solutions was successfully achieved and resulted in the preparation of a low temperature precursor of the monazite-cheralite type Pr 1-2x Ca x Th x PO 4 . The rhabdophane compounds are considered as potential neoformed phases in case of release of actinides from the phosphate-based ceramic wasteforms envisaged to host radionuclides in the back-end of the nuclear fuel cycle. A multiparametric study was thus undertaken to specify the wet chemistry conditions (starting stoichiometry, temperature, heating time) leading to single phase Pr 1-2x Ca x Th x PO 4 ·nH 2 O powdered samples. The excess of calcium appeared to be a prevailing factor with a suggested initial Ca:Th ratio being equal to 10. Similarly, the recommended heating time should exceed 4 days while the optimal temperature of synthesis is 110 °C. Under these conditions, the stability domain of Pr 1-2x Ca x Th x PO 4 ·nH 2 O ranged from x = 0.00 to x = 0.15. After heating at 1100 °C under air during 6 h, rhabdophane-type samples were fully converted into the highly durable Pr 1-2x Ca x Th x PO 4 cheralite ceramic wasteform. [ABSTRACT FROM AUTHOR]

The dehydration process of the hydrated rhabdophane LnPO 4 .0.667H 2 O (Ln = La to Dy) was thoroughly studied over the combination of in situ high resolution synchrotron powder diffraction and TGA experiments. In the case of SmPO 4 .0.667H 2 O (monoclinic, C2), a first dehydration step was identified around 80 °C leading to the formation of SmPO 4 .0.5H 2 O (Monoclinic, C2) with Z =12 and a =17.6264(1) Å, b =6.9704(1) Å, c =12.1141(1) Å, β=133.74(1) °, V =1075.33(1) Å 3 . In agreement with the TGA and dilatometry experiments, all the water molecules were evacuated above 220 °C yielding to the anhydrous form, which crystallizes in the hexagonal P3 1 21 space group with a =7.0389(1) Å, c =6.3702(1) Å and V =273.34(1) Å 3 . This study was extended to selected LnPO 4 .0.667H 2 O samples (Ln= Nd, Gd, Eu, Dy) and the obtained results confirmed the existence of two dehydration steps before the stabilization of the anhydrous form, with the transitory formation of LnPO 4 .0.5H 2 O. [ABSTRACT FROM AUTHOR]

This review focuses on the results of exploratory syntheses of alkaline-earth-metal actinide chalcogenides Ak–An–Q (Ak = Ba, Sr; An = Th, U; Q = S, Se, and Te). About thirty new compounds are described. Although the basic building blocks of their structures are usually AnQ6 octahedra and AkQ8 bicapped trigonal prisms, these are combined in diverse ways to afford eleven new structure types. The structures reconfirm the prevailing presence of An4+ in chalcogenides, although some of the compounds discovered are mixed An4+/An5+ systems, and a few contain only An5+. The tendency of the chalcogens to form Q–Q bonds is again evident from the presence of S–S single bonds and infinite Te–Te–Te linear chains. The latter possess interatomic distances of lengths greater than that of a Te–Te single bond but less than that of a Te–Te van der Waals interaction. Assignment of formal oxidation states in compounds containing these chains is arbitrary at best. Addition of metal atoms (M) affords quaternary structures, some of which show remarkable flexibility in the positions of the An and M atoms, and in such compounds the nature of the M elements influences directly the dimensionality of the resultant structure. The presence of adventitious oxygen, often from etching of the fused-silica tubes by oxyphilic An elements, results in new quintary compounds that show remarkable structural variations with change of M. The compounds discussed have shown transport and electronic structures that range from metallic-like to semiconducting. We find, with the exception of BaUSe3, when comparisons can be made that the values of the calculated band gaps are reasonably close but usually lower than the experimentally derived values. Thus the method used, in particular the HSE functional, has been generally successful on these 5f actinides. This is an important result because in the absence of suitable crystals, and hence experimental measurements, it still may be possible to offer credible predictions of some of the magnetic and electronic properties of the compounds. [ABSTRACT FROM AUTHOR]

A systematic study of the solubility of the rhabdophanes LnPO4 ·0.667H2O (Ln = La-Dy) was performed through oversaturation and undersaturation experiments at different temperatures (298-363 K) to demonstrate the reversibility of the equilibria. The structure of the neoformed phase was controlled carefully over the entire temperature range to enable the unambiguous attribution of the solubility products and the derived thermodynamic data to the rhabdophane phases. The log K°s,0 (298 K) values for LnPO4 ·0.667H2O ranged from -25.6 ± 0.8 (Pr) to -24.9 ± 1.7 (Eu), and the minimum values were for Pr to Sm, whereas the Δf G° values at 298 K varied between (-1984 ± 2) and (-2004 ± 2) kJ mol-1 whatever the lanthanide element considered, except for Eu-rhabdophane, which presented the highest Gibbs energy of formation [(-1896 ± 2) kJ mol-1]. From the solubility data obtained at various temperatures, the enthalpies of formation of the rhabdophanes Δf H° were found to be (-2151 ± 13) to (-2130 ± 12) kJ mol-1, excepted for that of Eu, which reached (-2057 ± 9) kJ mol-1. [ABSTRACT FROM AUTHOR]

Single crystals of Ba2Zn Q3 ( Q = Se, Te) were obtained by solid-state reactions at 1173 K. These isostructural compounds crystallize in the K2AgI3 structure type. The Zn atoms in this structure are coordinated to four Q atoms (2 Q1, 1 Q2, 1 Q3) and these form a distorted tetrahedron around each Zn atom. Each Zn Q4 tetrahedron shares two corners with neighboring Zn Q4 tetrahedra resulting in the formation of infinite chains of [Zn Q44−] units. The absorption spectrum of a single crystal of Ba2ZnTe3 shows an absorption edge at 2.10(2) eV, consistent with the dark-red color of the crystals. From DFT calculations Ba2ZnSe3 and Ba2ZnTe3 are found to be semiconductors with electronic band gaps of 2.6 and 1.9 eV, respectively. [ABSTRACT FROM AUTHOR]

Sodium borosilicate base glasses modeled on French nuclear waste materials were prepared to test the dependence of crystallization product quantity and distribution on cesium- and molybdenum-loading and glass cooling rate. Scanning electron microscopy shows the presence of micrometer-sized domains of Mo-rich crystalline precipitates. X-ray diffraction patterns are complex but reveal the presence of sodium molybdates and CsNaMoO4·2 H2 O. 133 Cs and 23 Na magic-angle spinning NMR spectroscopy exhibit distinct peaks for glassy and crystalline phases which can be quantified to obtain the identities of the individual compounds that are formed as well as the fractions of these nuclei in particular crystalline phases. In these model systems, 1 mol% Mo can be entirely incorporated into the glassy network whereas 2.5 and 5 mol% Mo produce significant quantities of crystalline phases, with little dependence on cooling rate. Cesium content appears to have a weak influence on crystallization behavior. Sodium molybdate and sodium-cesium molybdate hydrate are the dominant devitrification phases in all cases. [ABSTRACT FROM AUTHOR]