A₂TiO₅ (A = Dy, Gd, Er, Yb) at High Pressure

The structural evolution of lanthanide A₂TiO₅ (A = Dy, Gd, Yb, Er) at high pressure is investigated using synchrotron X-ray diffraction. The effects of A-site cation size and of the initial structure are systematically examined by varying the composition of the isostructural lanthanide titanates and the structure of dysprosium titanate polymorphs (orthorhombic, hexagonal, and cubic), respectively. All samples undergo irreversible high-pressure phase transformations, but with different onset pressures depending on the initial structure. While each individual phase exhibits different phase transformation histories, all samples commonly experience a sluggish transformation to a defect cotunnite-like (<i>Pnma</i>) phase for a certain pressure range. Orthorhombic Dy₂TiO₅ and Gd₂TiO₅ form <i>P</i>2₁<i>am</i> at pressures below 9 GPa and <i>Pnma</i> above 13 GPa. Pyrochlore-type Dy₂TiO₅ and Er₂TiO₅ as well as defect-fluorite-type Yb₂TiO₅ form <i>Pnma</i> at ∼21 GPa, followed by <i>Im</i>3<i>̅m</i>. Hexagonal Dy₂TiO₅ forms <i>Pnma</i> directly, although a small amount of remnants of hexagonal Dy₂TiO₅ is observed even at the highest pressure (∼55 GPa) reached, indicating kinetic limitations in the hexagonal Dy₂TiO₅ phase transformations at high pressure. Decompression of these materials leads to different metastable phases. Most interestingly, a high-pressure cubic X-type phase (<i>Im</i>3<i>̅m</i>) is confirmed using high-resolution transmission electron microscopy on recovered pyrochlore-type Er₂TiO₅. The kinetic constraints on this metastable phase yield a mixture of both the X-type phase and amorphous domains upon pressure release. This is the first observation of an X-type phase for an A₂BO₅ composition at high pressure

Structure and Reactivity of X‑ray Amorphous Uranyl Peroxide, U₂O₇

Recent accidents resulting in worker injury and radioactive contamination occurred due to pressurization of uranium yellowcake drums produced in the western U.S.A. The drums contained an X-ray amorphous reactive form of uranium oxide that may have contributed to the pressurization. Heating hydrated uranyl peroxides produced during <i>in situ</i> mining can produce an amorphous compound, as shown by X-ray powder diffraction of material from impacted drums. Subsequently, studtite, [(UO₂)­(O₂)­(H₂O)₂]­(H₂O)₂, was heated in the laboratory. Its thermal decomposition produced a hygroscopic anhydrous uranyl peroxide that reacts with water to release O₂ gas and form metaschoepite, a uranyl-oxide hydrate. Quantum chemical calculations indicate that the most stable U₂O₇ conformer consists of two bent (UO₂)²⁺ uranyl ions bridged by a peroxide group bidentate and parallel to each uranyl ion, and a μ₂-O atom, resulting in charge neutrality. A pair distribution function from neutron total scattering supports this structural model, as do ¹H- and ¹⁷O-nuclear magnetic resonance spectra. The reactivity of U₂O₇ in water and with water in air is higher than that of other uranium oxides, and this can be both hazardous and potentially advantageous in the nuclear fuel cycle