High-pressure polymorphic transformation of rutile to alpha-PbO2-type TiO2 at {011}R twin boundaries

Formation of contact and multiple cyclic cassiterite twins in SnO2-based ceramics co-doped with cobalt and niobium oxides

Contact and multiple cyclic twins of cassiterite commonly form in SnO₂-based ceramics when SnO₂ is sintered with small additions of cobalt and niobium oxides (dual doping). In this work, it is shown that the formation of twins is a two-stage process that starts with epitaxial growth of SnO₂ on CoNb₂O₆ and Co₄Nb₂O₉ seeds (twin nucleation stage) and continues with the fast growth of (101) twin contacts (twin growth stage). Both secondary phases form below the temperature of enhanced densification and SnO₂ grain growth; CoNb₂O₆ forms at ∼700°C and Co₄Nb₂O₉ at ∼900°C. They are structurally related to the rutile-type cassiterite and can thus trigger oriented (epitaxial) growth (local recrystallization) of SnO₂ domains in different orientations on a single seed particle. While oriented growth of cassiterite on columbite-type CoNb₂O₆ grains can only result in the formation of contact twins, the Co₄Nb₂O₉ grains with a structure comparable with that of corundum represent suitable sites for the nucleation of contact and multiple cyclic twins with coplanar or alternating morphology. The twin nucleation stage is followed by fast densification accompanied by significant SnO₂ grain growth above 1300°C. The twin nuclei coarsen to large twinned grains as a result of the preferential and fast growth of the low-energy (101) twin contacts. The solid-state diffusion processes during densification and SnO₂ grain growth are controlled by the formation of point defects and result in the dissolution of the twin nuclei and the incorporation of Nb⁵⁺ and Co²⁺ ions into the SnO₂ matrix in the form of a solid solution. In this process, the twin nuclei are erased and their role in the formation of twins is shown only by irregular segregation of Co and Nb to the twin boundaries and inside the cassiterite grains, and Co,Nb-enrichment in the cyclic twin cores.

Stabilizing γ-MgH2 at Nanotwins in Mechanically Constrained Nanoparticles

Reversible hydrogen uptake and the metal/dielectric transition make the Mg/MgH₂ system a prime candidate for solid-state hydrogen storage and dynamic plasmonics. However, high dehydrogenation temperatures and slow dehydrogenation hamper broad applicability. One promising strategy to improve dehydrogenation is the formation of metastable γ-MgH₂ . A nanoparticle (NP) design, where γ-MgH₂ forms intrinsically during hydrogenation is presented and a formation mechanism based on transmission electron microscopy results is proposed. Volume expansion during hydrogenation causes compressive stress within the confined, anisotropic NPs, leading to plastic deformation of β-MgH₂ via (301)_β twinning. It is proposed that these twins nucleate γ-MgH₂ nanolamellas, which are stabilized by residual compressive stress. Understanding this mechanism is a crucial step toward cycle-stable, Mg-based dynamic plasmonic and hydrogen-storage materials with improved dehydrogenation. It is envisioned that a more general design of confined NPs utilizes the inherent volume expansion to reform γ-MgH₂ during each rehydrogenation.

Polymorphism of nanocrystalline TiO2 prepared in a stagnation flame: formation of the TiO2-II phase

A metastable "high-pressure" phase known as α-PbO₂-type TiO₂ or TiO₂-II is prepared via a single-step synthesis using a laminar premixed stagnation flame. Three other TiO₂ polymorphs, namely anatase, rutile and TiO₂-B phases, can also be obtained by tuning the oxygen/fuel ratio. TiO₂-II is observed as a mixture with rutile under oxygen-lean flame conditions. To the best of our knowledge, this is the first time that this phase has been identified in flame-synthesised TiO₂. The formation of TiO₂-II in an atmospheric pressure flame cannot be explained thermodynamically and is hypothesised to be kinetically driven through the oxidation and solid-state transformation of a sub-oxide TiO_2-x intermediate. In this scenario, rutile is nucleated from the metastable TiO₂-II phase instead of directly from a molten/amorphous state. Mixtures containing three-phase heterojunctions of anatase, rutile, and TiO₂-II nanoparticles as prepared here in slightly oxygen-lean flames might be important in photocatalysis due to enhanced electron-hole separation.

Shock synthesis and characterization of titanium dioxide with α-PbO2 structure

The phase transformation behavior of anatase and rutile titanium dioxide with particle sizes of 60 nm and 150 nm under shock compression have been investigated. To increase the shock pressure and reduce the shock temperature, copper powder and a small amount of paraffin were mixed with the TiO₂ powder. The shock recovered samples were characterized by x-ray diffraction, Raman spectroscopy, and transmission electron microscope. The results indicate that both anatase and rutile TiO₂ can transform to α-PbO₂ phase TiO₂ through shock-induced phase transition. The transformation rate of α-PbO₂ phase TiO₂ for anatase TiO₂ under shock compression is 100% and pure α-PbO₂ phase TiO₂ can be obtained, while the transformation rate for rutile TiO₂ is over 90%. The influence of the particle size on the yield of α-PbO₂ phase TiO₂ is not noticeable. The thermal stability of the recovered pure α-PbO₂ phase TiO₂ was characterized by high temperature x-ray diffraction, thermogravimetric analysis and differential scanning calorimetry. The results show that α-PbO₂ phase TiO₂ transforms to rutile TiO₂ when heated to temperature higher than 560 °C. The mechanisms of the phase transition of TiO₂ under shock compression are discussed.