The high-pressure structural evolution of Ln2B2O7 pyrochlore oxides and uranyl peroxide nanoclusters [electronic resource]
Keywords: Geological and Environmental Sciences.

This dissertation examines the in situ structural evolution of two groups of materials under high pressure (0-50 GPa): (i) Ln2B2O7 (Ln= Nd, Sm, Eu, Gd, Dy, Er, Yb, Y; B=Hf, Sn) pyrochlore oxides, and (ii) uranyl peroxide nanoclusters U60 and U24Py12. Ln2B2O7 pyrochlore oxides (Fd-3m), where Ln is a trivalent lanthanide and B is a tetravalent transition metal, undergo symmetry-lowering phase transitions at high pressures in excess of 20 GPa and often transform to multi-scale crystalline structures upon decompression from > 50 GPa. Pyrochlore oxides are candidate materials for applications in nuclear waste disposal due to their ability to incorporate radionuclides of varying sizes into the structure and their resistance to amorphization under extreme environments including high pressure, high temperature, and high radiation fields. Lanthanide titanate and zirconate compounds have been extensively studied at high pressure, and their structural behavior under compression is well characterized. The specifics of titanate and zirconate behavior under pressure such as the onset pressure of the phase transitions, extent of the phase transformations vs. pressure, and final phase adopted by the material upon decompression depend on the ratio of cation ionic radii: rLn/rB. The stability field for the formation of Ln2B2O7 pyrochlore oxides is typically between 1.46 < rLn/rB < 1.78, though materials with rLn/rB outside this range can sometimes be synthesized as pyrochlore as well. Pyrochlore oxides with smaller radius ratios (rLn/rB < 1.6) typically undergo a pressure-induced phase transition to a cotunnite-like (Pnma) structure, with the onset of this transition at approximately ~25 GPa. After decompression from pressures as high as 50 GPa, these materials typically adopt a multi-scale structure that is a long-range ordered defect-fluorite (Fm-3m), and short-range (< 10 Å) best described by an orthorhombic, weberite-type structure. The material tessellates such that these two structures coexist over different length scales. I systematically examined lanthanide hafnate and stannate pyrochlores in order to understand the governing parameters of their structural behavior at high pressure, and to compare this behavior to that of the well-characterized zirconate and titanate systems. I examined a suite of hafnate pyrochlores (Ln2Hf2O7; Ln=Sm, Eu, Gd, Dy, Y, Yb) at high pressure in order to systematically probe the evolution of the structure while maintaining the same B-site cation (Sn and Hf) and varying the A-site cation such that the radius ratio, rLn/rB, crosses the pyrochlore stability field, which is rA/rB < 1.46. Hafnates, regardless of radius ratio (1.38 < rLn/rHf < 1.52) transform to a cotunnite-like phase (Pnma) with the pressure of onset between 18-25 GPa. This transition is not complete at 50 GPa, and upon decompression, the cotunnite-like phase transforms to either a defect-rich pyrochlore structure (Sm2Hf2O7, Fd-3m), or a multi-scale defect-fluorite + weberite structure (all other compositions). Determinations of the bulk modulus, B0, indicate an inverse relationship between rLn/rHf and B0. This relationship is typically linear in analogous titanate and zirconate pyrochlores, but in hafnates, a discontinuity exists at rLn/rHf = 1.445. Dy2Hf2O7 (rLn/rHf=1.45) has a B0 of 303 GPa, and Y2Hf2O7 (rLn/rHf=1.44) has a B0 of 398 GPa. This discontinuity is attributed to the difference in initial structure type: Dy2Hf2O7 is a partially ordered pyrochlore, while Y2Hf2O7 is a disordered defect-fluorite. Similarly, I examined stannate pyrochlores, Ln2Sn2O7 (Ln=Nd, Gd, Er), in order to probe the effect of changing Ln3+ cation size, and hence rLn/rSn, in a covalently bonded pyrochlore. Titanates and zirconates possess ionic metal-oxygen bonds, and though Sn4+ is comparable in size to Ti4+ and Zr4+, the < Sn-O> bond in pyrochlore is covalent. Previous computational studies on stannates have argued that the covalency of the < Sn-O> bond is an important parameter that may determine stannate pyrochlore structural behavior at high pressure and under ion irradiation, rather than the ratio of cation size. However, the stannates studied here show behavior consistent with rA/rB ratio being the dominant parameter that determines their structural evolution at high pressure. Stannates undergo a phase transition to the cotunnite-like (Pnma) high pressure phase with an onset pressure of ~28-30 GPa; this pressure is constant regardless of the radius ratio, which varies in the stannates studied between 1.46-1.61. When decompressed from 50 GPa, Nd2Sn2O7 transforms to a multi-scale defect-fluorite + weberite structure also observed in hafnates, zirconates, and some titanates. Gd2Sn2O7 and Er2Sn2O7 transform to a defect-rich pyrochlore structure, similar to Sm2Hf2O7, which was also observed in a previous study on Eu2Sn2O7. Like titanates and zirconates, the stannates show a linear inverse trend of B0 with rLn/rSn, which varies between 111 GPa (Nd2Sn2O7) and 251 GPa (Er2Sn2O7). The range of B0 observed in stannates is comparable to pyrochlore zirconates. Overall, the behavior of hafnate and stannate pyrochlore under pressure is consistent with the behavior of titanate and zirconate pyrochlore and suggests that the cation radius ratio, rLn/rB, is a predictive parameter for their structural behavior at high pressure. Uranyl peroxide nanoclusters are actinide-based polyoxometalates with a cage-like topology. The backbone of these clusters is the uranyl UO22+ cation, which forms hexagonal bipyramids with peroxide and hydroxide to form uranyl peroxide-hydroxide monomers: UO2(O2)2(OH)2. These monomers connect through edge-shared interactions which are inherently bent to an angle less than 180 ° and form nano-sized cage clusters as extended structures. The clusters contain anywhere between 20-124 uranyl peroxide-hydroxide monomers. Nanoclusters can consist solely of these monomers bonded together, as in the case of cluster U60: [UO2(O2)(OH)]6060- in water-or they can incorporate other ligands into their structural backbone, such as in the case of cluster U24Py12: [(UO2)24(O2)24(HP2O7)6(H2P2O7)6]30- which includes twelve pyrophosphate groups in its structure. When clusters are allowed to stand in solutions with sufficient cations for charge balance, they crystallize as complex structures with multiple length-scales. On the short-range (< 2.5 A), the crystalline materials consist of uranyl peroxide-hydroxide or uranyl peroxide-pyrophosphate monomers. The clusters themselves have a specific point group symmetry and are approximately ~2 nm in size, and the crystalline materials containing clusters, for example Li68K12(OH)20[UO2(O2)(OH)]60(H2O)310 (Fm-3) and Na8[(UO2)24(O2)24(P2O7)12] (P42/mnm), have unit cell lengths of several nm and form mm-sized crystals. The conditions that favor uranyl peroxide nanocluster formation in nature are areas where the ionizing radiation from spent nuclear fuel interacts with water. In these conditions, hydrogen peroxide will form via radiolysis of water, and clusters are likely to self-assemble in the resulting solutions regardless of the pH. These conditions are likely present at contaminated nuclear sites such as the Hanford site and at the Fukushima Daiichi reactors where groundwater is in contact with melted nuclear core. I studied the crystalline forms of U60 and U24Py12 at high pressure in situ in order to determine how these complex, multi-scale structures respond to stress. In the case of U60, the crystalline material Li68K12(OH)20[UO2(O2)(OH)]60(H2O)310 (Fm-3) undergoes a phase transition to a tetragonal structure at P< 2 GPa. The material persists as a crystalline structure until 9 GPa; at pressures above 9 GPa the material becomes amorphous. The crystalline U60-bearing material has a bulk modulus of 25 GPa. The amorphous material contains intact U60 nanoclusters to at least 17.4 GPa, which have not broken down, decomposed, or lost their point group symmetry as a result of pressure. In the case of U24Py12, the crystalline material, Na8[(UO2)24(O2)24(P2O7)12] (P42/mnm) retains its long-range order until pressures as high as 50 GPa. However, the conformation of the nanocluster likely changes reversibly at much lower pressures < 2.5 GPa. The pyrophosphate groups in the cluster structure make corner-shared bonds with the uranyl peroxide monomers; the additional degree of freedom (compared to edge-shared bonds in U60) likely allows the U24Py12 cluster to undergo conformational changes more easily. Samples of the crystalline U24Py12-bearing material, Na8[(UO2)24(O2)24(P2O7)12] (P42/mnm) compressed to ~30 GPa and recovered to ambient conditions indicate that some intact nanoclusters likely still persist at this pressure, but some are partially decomposed. The difference in behavior of these two nanoclusters under pressure likely results from (i) differing topology, (ii) differing ligands and hence degrees of freedom in their structural backbone, and (iii) differing solubilities. U60 is highly soluble in aqueous solvents including the methanol:ethanol mixture used as a pressure-transmitting medium in high pressure experiments; this may have allowed the clusters to persist at high pressure after the loss of long-range periodicity. U24Py12 is not as soluble in methanol:ethanol, but the corner-shared P2O7 groups likely allow the cluster molecules to distort more easily without losing long-range periodicity. The results of this work emphasize the variety of behaviors exhibited by complex materials: ternary lanthanide transition-metal oxides and actinide polyoxometallates, under extreme pressure.