Strong coupling between local polar displacements and a commensurate octahedral tilting is proposed to explain the onset of classic ferroelectric behavior in tetragonal tungsten bronzelike dielectrics Ba2LaxNd1−xNb3Ti2O15. The ferroelectric phase transition is associated with a discontinuous non-lock-in transformation of an incommensurate tilted structure to a commensurate superstructure. In a manner reminiscent of perovskitelike oxides, the driving force for commensurate tilting increases as the average ionic radius of the rare-earth ion decreases; no classical ferroelectric transition is observed for compositions with x>0.75, which remain incommensurate and exhibit only relaxor behavior below room temperature.

Uranium (U) is a ubiquitous element in the Earth’s crust at ~2 ppm. In anoxic environments, soluble hexavalent uranium (U(VI)) is reduced and immobilized. The underlying reduction mechanism is unknown but likely of critical importance to explain the geochemical behavior of U. Here, we tackle the mechanism of reduction of U(VI) by the mixed-valence iron oxide, magnetite. Through high-end spectroscopic and microscopic tools, we demonstrate that the reduction proceeds first through surface-associated U(VI) to form pentavalent U, U(V). U(V) persists on the surface of magnetite and is further reduced to tetravalent UO₂ as nanocrystals (~1–2 nm) with random orientations inside nanowires. Through nanoparticle re-orientation and coalescence, the nanowires collapse into ordered UO₂ nanoclusters. This work provides evidence for a transient U nanowire structure that may have implications for uranium isotope fractionation as well as for the molecular-scale understanding of nuclear waste temporal evolution and the reductive remediation of uranium contamination.

The structure and dielectric properties of a new family of tetragonal tungsten bronze (TTB) ceramics with the general formula, ${\mathrm{Ba}}_{2}M{\mathrm{Ti}}_2{\mathrm{Nb}}_3{\mathrm{O}}_{15}$ where $M={\mathrm{Bi}}^{3+},{\mathrm{La}}^{3+},{\mathrm{Nd}}^{3+},{\mathrm{Sm}}^{3+},{\mathrm{Gd}}^{3+}$ , have been investigated. Hereafter the compositions will be referred to by the abbreviation $\mathrm{B}M{\mathrm{TNO}}_{15}$ where $M=\mathrm{B}\left({\mathrm{Bi}}^{3+}\right)$ , $\mathrm{L}\left({\mathrm{La}}^{3+}\right)$ , $\mathrm{N}\left({\mathrm{Nd}}^{3+}\right)$ , $\mathrm{S}\left({\mathrm{Sm}}^{3+}\right)$ or $\mathrm{G}\left({\mathrm{Gd}}^{3+}\right)$ . ${\mathrm{BLTNO}}_{15}$ [permittivity maximum $\left(T_m\right)\sim -80°\mathrm{C}$ ] and ${\mathrm{BBTNO}}_{15}$ $(T_m\sim -100°\mathrm{C})$ exhibited relaxorlike dielectric behavior. In contrast, ${\mathrm{BNTNO}}_{15}$ ($T_m\sim 165°\mathrm{C}$ ), ${\mathrm{BSTNO}}_{15}$ $(T_m\sim 250°\mathrm{C})$ , and ${\mathrm{BGTNO}}_{15}$ $(T_m\sim 320°\mathrm{C})$ were classic ferroelectrics. $T_m$ increased with decreasing radius of the $M$ ion. Room temperature x-ray powder diffraction (XRD) patterns of all the compounds indexed on a prototype $P4∕mbm$ (or $P4bm$ ) space group with lattice parameters $a\approx 12.4\mathrm{\AA }$ and $c\approx 4\mathrm{\AA }$ . However, electron diffraction revealed that the relaxor phases, ${\mathrm{BLTNO}}_{15}$ and ${\mathrm{BBTNO}}_{15}$ , exhibited an incommensurate modulation, whereas the classic ferroelectric ${\mathrm{BNTNO}}_{15}$ , ${\mathrm{BSTNO}}_{15}$ , and ${\mathrm{BGTNO}}_{15}$ featured an orthorhombic superstructure with lattice parameters $a\approx b\approx \surd 2a_{\mathrm{TTB}}$ and $c\approx 2c_{\mathrm{TTB}}$ .