Tilting and Distortion in Rutile-Related Mixed Metal Ternary Uranium Oxides: A Structural, Spectroscopic, and Theoretical Investigation

Abstract

A systematic investigation examining the origins of structural distortions in rutile-related ternary uranium AUO4 oxides using a combination of high-resolution structural and spectroscopic measurements supported by ab initio calculations is presented. The structures of β-CdUO4, MnUO4, CoUO4, and MgUO4 are determined at high precision by using a combination of neutron powder diffraction (NPD) and synchrotron X-ray powder diffraction (S-XRD) or single crystal X-ray diffraction. The structure of β-CdUO4 is best described by space group Cmmm whereas MnUO4, CoUO4, and MgUO4 are described by the lower symmetry Ibmm space group and are isostructural with the previously reported β-NiUO4 [Murphy et al. Inorg. Chem. 2018, 57, 13847]. X-ray absorption spectroscopy (XAS) analysis shows all five oxides contain hexavalent uranium. The difference in space group can be understood on the basis of size mismatch between the A 2+ and U6+ cations whereby unsatisfactory matching results in structural distortions manifested through tilting of the AO6 polyhedra, leading to a change in symmetry from Cmmm to Ibmm. Such tilts are absent in the Cmmm structure. Heating the Ibmm AUO4 oxides results in reduction of the tilt angle. This is demonstrated for MnUO4 where in situ S-XRD measurements reveal a second-order phase transition to Cmmm near T = 200 °C. Based on the extrapolation of variable temperature in situ S-XRD data, CoUO4 is predicted to undergo a continuous phase transition to Cmmm at ∼1475 °C. Comparison of the measured and computed data highlights inadequacies in the DFT+U approach, and the conducted analysis should guide future improvements in computational methods. The results of this investigation are discussed in the context of the wider AUO4 family of oxides.

Electrode and Electrolyte Materials From Atomistic Simulations: Properties of LixFEPO4 Electrode and Zircon-Based Ionic Conductors

“…In our past studies of Nd 2−x Zr 2+x O 7+x/2 compound, with the PBEsol exchange-correlation functional we got perfect match to the lattice parameter in case of the pyrochlore phase (Finkeldei et al, 2017). In the most recent contribution we computed the MUO 4 compounds with M = Ni, Fe, Co, Cd, and found that only by using the correct projectors for estimation of occupation of d orbitals, e.g., Wannier functions, we could reproduce experimentally seen structural distortions (Murphy et al, 2021). The problem arises from the fact that with the standard DFT+U approach, when using atomic orbitals as projectors, the total occupancy of the d or f states of interest is much higher than the actual one (Kick et al, 2019;Murphy et al, 2021).…”

“…In the most recent contribution we computed the MUO 4 compounds with M = Ni, Fe, Co, Cd, and found that only by using the correct projectors for estimation of occupation of d orbitals, e.g., Wannier functions, we could reproduce experimentally seen structural distortions (Murphy et al, 2021). The problem arises from the fact that with the standard DFT+U approach, when using atomic orbitals as projectors, the total occupancy of the d or f states of interest is much higher than the actual one (Kick et al, 2019;Murphy et al, 2021). As illustrated in Table 3, in our case it gives ∼1.3 excess electrons for Fe(III) and ∼0.5 for Fe(II).…”

Incorporation of iodine into uranium oxyhydroxide phases

“…7,8 However, there are at present no studies that have examined the potential retention of anionic fission species into UOH phases for instance radiolytic iodine, despite the renewed focus these materials have recently received [23][24][25][26] coupled with the broader reinvigorated interest in actinide and actinide-iodine chemical science. [27][28][29][30][31][32][33][34][35][36][37] In the present investigation, we describe the synthesis and characterisation of a novel UOH structure which successfully intercalates IO (0.45 g, 1 mmol), KIO 3 (0.07 g, 0.3 mmol) and RbIO 3 (0.08 g, 0.3 mmol) were mixed and dissolved in 2 ml deionised water and dilute HNO 3 was added to reduce the pH to 5 which was checked via pH paper. The reagents were placed and sealed in a Teflon lined stainless steel autoclave and then transferred into a box furnace.…”

The Role of Acidity in the Synthesis of Novel Uranyl Selenate and Selenite Compounds and Their Structures

“…Many of these studies were performed on what can be described as an explorative basis which left little understanding regarding chemical conditions which dictate synthesis control and outcome. However, more contemporary investigations have utilized systematic approaches, which better enable improved synthesis control and increased understanding and prediction of potential reaction outcomes [17,[26][27][28][29][30][31][32][33]. Saliently, many of these more recent studies have highlighted the unique ability and promising application of uranium derived compounds in societally important materials such as radiation detectors, scintillators to proton conductors [34][35][36].…”

“…We have recently embarked on a systematic investigation into the structure-property relations and effect of synthesis on novel SNF related actinide phase formation under variable conditions [14,17,[29][30][31]37,38]. In light of the identification of the effect of acid on controlling the polymerization of iodate units in uranyl iodates [14] (60 wt.%) was added to the reaction mixtures to reduce the pH to below 1 confirmed using pH paper.…”