Solid-solid displacive, structural phase transformations typically undergo a discrete structural change from a parent to a product phase. Coupling electron microscopy, three-dimensional atom probe, and first-principles computations, we present the first direct evidence of a novel mechanism for a coupled diffusional-displacive transformation in titanium-molybdenum alloys wherein the displacive component in the product phase changes continuously with changing composition. These results have implications for other transformations and cannot be explained by conventional theories.

No abstract

Rare earth oxyhydrides REO x H (3−2x) , with RE = Y, Sc, or Gd and a cationic FCC lattice, are reversibly photochromic in nature. It is known that structural details and anion (O 2− :H − ) composition dictate the efficiency of the photochromic behavior. The mechanism behind the photochromism is, however, not yet understood. In this study, we use 1 H, 2 H, 17 O, and 89 Y solid-state NMR spectroscopy and density functional theory (DFT) calculations to study the various yttrium, hydrogen, and oxygen local environments, anion oxidation states, and hydride ion dynamics. DFT models of YO x H (3−2x) with both anionordered and anion-disordered sublattices are constructed for a range of compositions and show a good correlation with the experimental NMR parameters. Two-dimensional 17 O− 1 H and 89 Y− 1 H NMR correlation experiments reveal heterogeneities in the samples, which appear to consist of hydride-rich (x ≈ 0.25) and hydride-poor domains (x ≈ 1) rather than a single composition with homogeneous anion mixing. The compositional variation (as indicated by the different x values in YO x H (3−2x) ) is determined by comparing static 1 H NMR line widths with calculated 1 H− 1 H dipolar couplings of yttrium oxyhydride models. The 1D 17 O MAS spectrum demonstrates the presence of a small percentage of hydroxide (OH − ) ions. DFT modeling indicates a reaction between the protons of hydroxides and hydrides to form molecular hydrogen (H + + H − → H 2 ). 1 H MAS NMR indicates the presence of a mobile component that, based on this finding, is attributed to trapped molecular H 2 in the lattice.

No abstract