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Uranium trioxide (UO3) is known to adopt a variety of crystalline and amorphous phases. Here we applied the Perdew-Burke-Ernzerhof functional + U formalism to predict structural, electronic, and elastic properties of five experimentally determined UO3 polymorphs, in addition to their relative stability. The simulations reveal that the methodology is well-suited to describe the different polymorphs. We found better agreement with experiment for simpler phases where all bonds are similar (α- and δ-), while some differences are seen for those with more complex bonding (β-, γ-, and η-), which we address in terms of the disorder and defective nature of the experimental samples. Our calculations also predict the presence of uranyl bonds to affect the elastic and electronic properties. Phases containing uranyl bonds tend to have smaller band gaps and bulk moduli under 100 GPa contrary to those without uranyl bonds, which have larger band gaps and bulk moduli greater than 150 GPa. In line with experimental observations, we predict the most thermodynamically stable polymorph as γ-UO3, the least stable as α-UO3, and the most stable at high pressure as η-UO3. [ABSTRACT FROM AUTHOR]

A combination of experimental and computational techniques has been employed to explore the crystal structure and thermoelectric properties of A-site-deficient perovskite La 1/3 NbO 3 ceramics. Crystallographic data from X-ray and electron diffraction confirmed that the room temperature structure is orthorhombic with Cmmm as a space group. Atomically resolved imaging and analysis showed that there are two distinct A sites: one is occupied with La and vacancies, and the second site is fully unoccupied. The diffuse superstructure reflections observed through diffraction techniques are shown to originate from La vacancy ordering. La 1/3 NbO 3 ceramics sintered in air showed promising high-temperature thermoelectric properties with a high Seebeck coefficient of S 1 = -650 to -700 μV/K and a low and temperature-stable thermal conductivity of k = 2-2.2 W/m·K in the temperature range of 300-1000 K. First-principles electronic structure calculations are used to link the temperature dependence of the Seebeck coefficient measured experimentally to the evolution of the density of states with temperature and indicate possible avenues for further optimization through electron doping and control of the A-site occupancies. Moreover, lattice thermal conductivity calculations give insights into the dependence of the thermal conductivity on specific crystallographic directions of the material, which could be exploited via nanostructuring to create high-efficiency compound thermoelectrics.

U 2 O 5 is the boundary composition between the fluorite and the layered structures of the UO 2→3 system and the least studied oxide in the group. δ-U 2 O 5 is the only layered structure proposed so far experimentally, although evidence of fluorite-based phases has also been reported. Our DFT work explores possible structures of U 2 O 5 stoichiometry by starting from existing M 2 O 5 structures (where M is an actinide or transition metal) and replacing the M ions with uranium ions. For all structures, we predicted structural and electronic properties including bulk moduli and band gaps. The majority of structures were found to be less stable than δ-U 2 O 5 . U 2 O 5 in the R-Nb 2 O 5 structure was found to be a competitive structure in terms of stability, whereas U 2 O 5 in the Np 2 O 5 structure was found to be the most stable overall. Indeed, by including the vibrational contribution to the free energy using the frequencies obtained from the optimized unit cells we predict that Np 2 O 5 structured U 2 O 5 is the most thermodynamically stable under ambient conditions. δ-U 2 O 5 only becomes more stable at high temperatures and/or pressures. This suggests that a low-temperature synthesis route should be tested and so potentially opens a new avenue of research for pentavalent uranium oxides.