X-Ray Diffraction and μ-Raman Investigation of the Monoclinic-Orthorhombic Phase Transition in Th_1−xU_x(C₂O₄)₂·2H₂O Solid Solutions

Abstract

A complete Th(1-x)U(x)(C(2)O(4))(2).2H(2)O solid solution was prepared by mild hydrothermal synthesis from a mixture of hydrochloric solutions containing cations and oxalic acid. The crystal structure has been solved from twinned single crystals for x = 0, 0.5, and 1 with monoclinic symmetry, space group C2/c, leading to unit cell parameters of a approximately 10.5 A, b approximately 8.5 A, and c approximately 9.6 A. The crystal structure consists of a two-dimensional arrangement of actinide centers connected through bis-bidentate oxalate ions forming squares. The actinide metal is coordinated by eight oxygen atoms from four oxalate entities and two water oxygen atoms forming a bicapped square antiprism. The connection between the layers is assumed by hydrogen bonds between the water molecules and the oxygen of oxalate of an adjacent layer. Under these conditions, the unit cell contains two independent oxalate ions. From high-temperature mu-Raman and X-ray diffraction studies, the compounds were found to undergo a transition to an orthorhombic form (space group Ccca). The major differences in the structural arrangement concern the symmetry of uranium, which decreases from C2 to D2, leading to a unique oxalate group. Consequently, the nu(s)(C-O) double band observed in the Raman spectra recorded at room temperature turned into a singlet. This transformation was then used to make the phase transition temperature more precise as a function of the uranium content of the sample.

From uranium(IV) oxalate to sintered UO 2 : Consequences of the powders' thermal history on the microstructure

“…Indeed, the unit cell parameters of U(C 2 O 4 ) 2 ·2H 2 O (monoclinic structure, space group C2/c) were estimated to a = 10.5174(5) Å, b = 8.7085(6)Å, c = 9.2433(7)Å and ˇ = 90.398(3) • Å through Rietveld refinement and agreed well with the values reported in the literature [21]. The characteristic PXRD lines of this compound were then conserved up to 60 • C: even if a phase transition toward an orthorhombic variety was reported around 40 • C [21], the conditions of acquisition used for HT measurements hinder the detection of the slight pattern modifications associated. From 90 • C, the appearance of additional peaks marked the progressive dehydration of the initial compound, first leading to U(C 2 O 4 ) 2 ·H 2 O (see red patterns on Fig.…”

“…Monoclinic uranium(IV) oxalate dihydrate, U(C 2 O 4 ) 2 ·2H 2 O was further obtained through a protocol derived from that described by Clavier et al [21]. Hydrochloric solution of uranium(IV) (0.3 M) was poured, without any stirring, into a large excess of oxalic acid, in order to ensure the quantitative precipitation of uranium(IV).…”

“…In this study, the preparation of UO 2 powders was undertaken from U(C 2 O 4 ) 2 ·2H 2 O [21,22] in order to study the influence of several precursor-related parameters over the sintering process and the subsequent pellet microstructure. In first place, the variation of the powder characteristics during the conversion step will be detailed, both at the microscopic (crystal structure, carbon content) and macroscopic (morphology, specific surface area) levels.…”

Synthesis and characterization of Th1−xLnxO2−x/2 mixed-oxides

“…90 elements with oxalic acid was chosen to prepare the starting precursors of expected solid solutions since their use usually led to an improvement of the final sample homogeneity [41][42][43][44][45][46] and to the optimization of some properties of interest, such as sintering capability or chemical durability [45][46][47][48][49][50][51][52][53].…”

Europium Luminescence as a Structural Probe: Structure‐Dependent Changes in Eu3+‐Substituted Th(C2O4)2·xH2O (x = 6, 2, and 0)

“…The structural features of the TOH and TOD have been analyzed and explained previously. [33][34][35] The structural analyses of the TOH and TOD phases indicate that both have eightfold-coordinated Th 4+ ions with four oxalate groups connected to the Th 4+ ions in a bidentate fashion. However, the arrangements of the water molecules around the Th 4+ ions in these two structures are distinctly different.…”