A 23Na Magic Angle Spinning Nuclear Magnetic Resonance, XANES, and High-Temperature X-ray Diffraction Study of NaUO3, Na4UO5, and Na2U2O7

Polymorphism and phase transitions in Na2U2O7 from density functional perturbation theory

Polymorphism and phase transitions in sodium diuranate, Na₂U₂O₇, are investigated with density functional perturbation theory (DFPT). Thermal properties of crystalline α-, β- and γ-Na₂U₂O₇ polymorphs are predicted from DFPT phonon calculations, i.e., the first time for the high-temperature γ-Na₂U₂O₇ phase (R3̄m symmetry). The standard molar isochoric heat capacities predicted within the quasi-harmonic approximation are for P2₁/a α-Na₂U₂O₇ and C2/m β-Na₂U₂O₇, respectively. Gibbs free energy calculations reveal that α-Na₂U₂O₇ (P2₁/a) and β-Na₂U₂O₇ (C2/m) are almost energetically degenerate at low temperature, with β-Na₂U₂O₇ becoming slightly more stable than α-Na₂U₂O₇ as temperature increases. These findings are consistent with XRD data showing a mixture of α and β phases after cooling of γ-Na₂U₂O₇ to room temperature and the observation of a sluggish α → β phase transition above ca. 600 K. A recently observed α-Na₂U₂O₇ structure with P2₁ symmetry is also shown to be metastable at low temperature. Based on Gibbs free energy, no direct β → γ solid-solid phase transition is predicted at high temperature, although some experiments reported the existence of such phase transition around 1348 K. This, along with recent experiments, suggests the occurrence of a multi-step process consisting of initial β-phase decomposition, followed by recrystallization into γ-phase as temperature increases.

In-Plane Cation Ordering and Sodium Displacements in Layered Honeycomb Oxides with Tetravalent Lanthanides: Na2LnO3 (Ln = Ce, Pr, and Tb)

The detailed structural characterization of "213" honeycomb systems is a key concern in a wide range of fundamental areas, such as frustrated magnetism, and technical applications, such as cathode materials, catalysts, and thermoelectric materials. Na₂LnO₃ (Ln = Ce, Pr, and Tb) are an intriguing series of "213" honeycomb systems because they host tetravalent lanthanides. "213" honeycomb materials have been reported to adopt either a cation-disordered R3̅m subcell, a cation-ordered trigonal (P3₁12), or monoclinic (C2/c or C2/m) supercell. On the basis of analysis of the average (synchrotron diffraction) and local [pair distribution function (PDF) and solid-state NMR] structure probes, cation ordering in the honeycomb layer of Na₂LnO₃ materials has been confirmed. Through rationalization of the ²³Na chemical shifts and quadrupolar coupling constants, the local environment of Na atoms was probed with no observed evidence of cation disorder. Through these studies, it is shown that the Na₂LnO₃ materials adopt a C2/c supercell derived from symmetry-breaking displacements of intralayered Na atoms from the ideal crystallographic position (in C2/m). The Na displacement is validated using distortion index parameters from diffraction data and atomic displacement parameters from PDF data. The C2/c supercell is faulted, as evidenced by the increased breadth of the superstructure diffraction peaks. DIFFaX simulations and structural considerations with a two-phase approach were employed to derive a suitable faulting model.

Magic angle spinning NMR spectroscopy: a versatile technique for structural and dynamic analysis of solid-phase systems

Magic Angle Spinning (MAS) NMR spectroscopy is a powerful method for analysis of a broad range of systems, including inorganic materials, pharmaceuticals, and biomacromolecules. The recent developments in MAS NMR instrumentation and methodologies opened new vistas to atomic-level characterization of a plethora of chemical environments previously inaccessible to analysis, with unprecedented sensitivity and resolution.