Neutron powder investigation of the tetragonal to monoclinic phase transformation in undoped zirconia

Reversed Stability of Zirconium Oxide Dimer Isomers Triggered by Electron Gain or Removal

The neutral zirconium oxide dimer and its cationic and anionic counterparts were investigated using advanced ab initio electronic structure methods and flexible basis sets. The exploration of the ground-state potential energy surfaces of (ZrO2)2, (ZrO2)2+, and (ZrO2)2- led to the identification of stable isomeric structures and their relative energies. It was found that (ZrO2)2 adopts three distinct isomeric forms resembling chair-, boat-, and scepter-like structures in its neutral, cationic, and anionic forms. The energetic ordering of these isomers in the neutral dimer is reversed upon either electron attachment or detachment. The adiabatic ionization potential of (ZrO2)2 was determined to be 9.141 eV, while the adiabatic electron affinity was found to be 1.475 eV. The vertical electron detachment energies were predicted to be 1.949, 1.852, and 1.340 eV, while the vertical ionization potentials, not previously reported in the literature, were determined to be 9.282, 9.375, and 9.594 eV. In both cases, the values correspond to the scepter, boat, and chair isomers, respectively, with the electron detachment energies showing excellent agreement with experimental data. Finally, possible scenarios of isomeric interconversion within the same charge, driven by electron attachment or detachment, were described and analyzed.

Stability of Hydroxo/Oxo/Fluoro Zirconates vs. Hafniates—A DFT Study

We performed density functional theory (DFT) calculations on binary and ternary oxo/fluoro crystals of the geochemical twin pair zirconium and hafnium to evaluate and compare their stabilities. This is the first DFT study on bulk ZrF4 or HfF4, as well as on a hypothetical ZrOF2 or HfOF2 bulk crystal. For α-MO2, β-MF4 and MOF2, we have found significantly higher cohesive energies for the respective hafnium species. This suggests a considerable gap in affinity toward fluorine and oxygen between the twin pair in the solid state. In agreement with experimental findings, this gap is slightly more pronounced for fluorine. This study is also the first to evaluate the theoretical, endothermic mono-hydroxylation of the respective fluorides or oxyfluorides to model the difference in affinity toward fluoride versus hydroxide. For these, we could also find a slight energetic preference for the hafnium compound.

Structure relations in the family of the solid solution Hf x Zr1−x O2

Hafnium Zirconium Oxide Hf x Zr1−x O2 is a potentially ferroelectric material with great perspectives in semiconductor applications, due to its compatibility with silicon technologies and its low toxicity. Despite its chemical simplicity, the solid solution Hf x Zr1−x O2 comprises a large variety of different phases. We compiled a complete list of experimentally and theoretically reported Hf x Zr1−x O2 structures. All of them are symmetrically related to the common aristotype with Fluorite type structure. The symmetry relationships between those structures have been determined and are presented in a Bärnighausen-like tree. Interestingly, not all symmetry reductions follow the conventional group-subgroup relations and involve severe atomic shifts. Further, the structures were compared to each other in detail regarding the dimensionality of atomic shifts and the accompanied lattice distortions. Finally, the information provided by the Bärnighausen-like tree was used to transform the indices of a reflection before and after a phase transition. This conversion allows the study of (dis)appearing reflections during phase transitions.

The sol-gel autocombustion as a route towards highly CO2-selective, active and long-term stable Cu/ZrO2 methanol steam reforming catalysts

The adaption of the sol-gel autocombustion method to the Cu/ZrO₂ system opens new pathways for the specific optimisation of the activity, long-term stability and CO₂ selectivity of methanol steam reforming (MSR) catalysts. Calcination of the same post-combustion precursor at 400 °C, 600 °C or 800 °C allows accessing Cu/ZrO₂ interfaces of metallic Cu with either amorphous, tetragonal or monoclinic ZrO₂, influencing the CO₂ selectivity and the MSR activity distinctly different. While the CO₂ selectivity is less affected, the impact of the post-combustion calcination temperature on the Cu and ZrO₂ catalyst morphology is more pronounced. A porous and largely amorphous ZrO₂ structure in the sample, characteristic for sol-gel autocombustion processes, is obtained at 400 °C. This directly translates into superior activity and long-term stability in MSR compared to Cu/tetragonal ZrO₂ and Cu/monoclinic ZrO₂ obtained by calcination at 600 °C and 800 °C. The morphology of the latter Cu/ZrO₂ catalysts consists of much larger, agglomerated and non-porous crystalline particles. Based on aberration-corrected electron microscopy, we attribute the beneficial catalytic properties of the Cu/amorphous ZrO₂ material partially to the enhanced sintering resistance of copper particles provided by the porous support morphology.

Extreme Biomimetics: formation of zirconium dioxide nanophase using chitinous scaffolds under hydrothermal conditions

Chitinous scaffolds isolated from the skeleton of marine sponge Aplysina cauliformis were used as a template for the in vitro formation of zirconium dioxide nanophase from ammonium zirconium(iv) carbonate (AZC) under extreme conditions (150 °C). These novel zirconia-chitin based composites were prepared for the first time using hydrothermal synthesis, and were thoroughly characterized using a plethora of analytical methods. The thermostability of the chitinous 3D matrix makes it ideal for use in the hydrothermal synthesis of monoclinic nanostructured zirconium dioxide from precursors like AZC. These zirconium-chitin composites have a high potential for use in a broad range of applications ranging from synthetic catalysis to biocompatible materials for bone and dental repair. The synthetic methods presented in this work show an attractive route for producing monoclinic zirconium dioxide on a 3D biocompatible scaffold with ease.

Size-Induced Tetragonal to Monoclinic Phase Transition in Zirconia Nanocrystals

Accurate neutron powder diffraction experiments at D20, ILL Grenoble, allowed to monitor the reconstructive tetragonal to monoclinic phase transition as a function of the size of zirconia nanoparticles. In the nanocrystals, both phases are identical to the ones generally observed in micrometric zirconia. Rietveld refinements on these samples point out an increase of the tetragonal fraction and a decrease of the lattice parameters when the size of the particle decreases. An uniaxial strain depending on the grain size is also observed. The phase transition definitely occurs above a threshold crystal size. These results are analysed within the Landau theory and they can be understood as a mechanism of size-dependent phase transition where the primary order parameter is altered by the nanoparticle size.

Molecular dynamics simulation of yttria-stabilized zirconia (YSZ) crystalline and amorphous solids

An empirically fitted atomic potential allows a classical molecular dynamics study of the static and dynamic properties of both crystalline and amorphous yttria-stabilized zirconia (YSZ) with typical dilute Y(2)O(3) concentrations (i.e. 3.0-12.0 mol% Y(2)O(3)) in the temperature range 300-1400 K. Based on the rigid ion model approximation, we find, regardless of the distinctly different geometries, that the oxygen ionic conductivity shows a maximum at ∼ 8.0 mol% Y(2)O(3), close to the experimental maximum. A lower absolute ionic conductivity is found for the high density YSZ amorphous solid, relative to crystalline YSZ, consistent with the trends observed in crystalline and stabilized amorphous thin films of YSZ reported in experiments. Different from YSZ crystals, intriguing features of mutual diffusion among the heavy cations and mobile anions are found in the amorphous phase.

Martensitic phase transition in pure zirconia: a crystal chemistry viewpoint

A crystal chemistry approach was carried out in order to decipher the mechanisms involved in phase transitions of zirconia. A detailed analysis of structures, based on structural sheets of half fluorite cell thick and described in terms of SDF and HDF (for slightly and highly distorted fluorite) sheets is proposed for the first time. This approach allowed clarifying the relationships between tetragonal, orthorhombic and monoclinic forms. This concept permits to propose simple structural relationships between the structures of the different polymorphs. These relationships and their correlated transformation pathways were confirmed using symmetry group relations. The results were compared to those obtained by the phenomenological approach of the tetragonal → monoclinic (T → M) martensitic phase transition. The different lattice correspondences are revisited and discussed. For the first time, some fine structural models were proposed to describe the pathways of atomic displacements taking place during each possible phase transitions. The T→M phase transition is the main transformation pathway. It may solely gives rise to the C-type lattice correspondence, the (100) M ||(100) T and [001] M ||[001] T relationship allowing the minimum structural changes during the transformation. The B-type variant with (100) M ||(100) T and [010] M ||[001] T may be developed from an indirect pathway involving either a ferroelastic phase transition by domain switching taking place in the T phase before the T → M phase transition operates or a two stage process involving a combined T → O and O → M successive and independent transitions. The orthorhombic phase is an alternative form to the monoclinic one and is in no way an intermediate phase.

Neutron diffraction study of the size-induced tetragonal to monoclinic phase transition in zirconia nanocrystals

Accurate neutron powder diffraction experiments at several temperatures allow one to monitor the reconstructive tetragonal to monoclinic phase transition as a function of the size of zirconia nanoparticles. The structure of the tetragonal phase observed in the nanocrystals is identical to that observed in micrometric zirconia above 1400 K. A uniaxial strain depending on grain size is observed. The phase transition occurs above a threshold crystal size. These results are analyzed within the Landau theory and can be understood as a mechanism of size-dependent phase transition where the primary order parameter is altered by the nanoparticle size.