Stabilizing metastable tetragonal HfO<sub>2</sub> using a non-hydrolytic solution-phase route: ligand exchange as a means of controlling particle size

Waetzig, Gregory R.; Depner, Sean W.; Asayesh‐Ardakani, Hasti; Cultrara, Nicholas D.; Shahbazian‐Yassar, Reza; Banerjee, Sarbajit

doi:10.1039/c6sc01601d

Cited by 39 publications

(47 citation statements)

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“…DFT calculations of surface energies by Ramprasad and co-workers19 suggest that the surface energies of {110} and {001} planes are substantially higher for the monoclinic phase (1.38 and 1.51 J m −2 , respectively) as compared to the values for the tetragonal phase (1.08 and 1.21 J m −2 , respectively). In other words, as also observed for ZrO 2 (refs 9, 42), the tetragonal phase of HfO 2 has a substantially lower surface energy and thus . This term is furthermore expected to be strongly size-dependent as per:…”

Section: Resultsmentioning

confidence: 70%

“…In contrast, the higher temperature tetragonal (stable above ∼1700 °C) and cubic (>2600 °C) phases have κ -values of 75 and 29, respectively 7 , and band gaps which reach a maximum value of 6 eV in the tetragonal phase 8 . There is great interest, therefore, in trapping the tetragonal phase of HfO 2 at room temperature, which has thus far only been possible for extremely small crystallites that are <3.6 nm in size, or within defective matrices 9 10 11 12 .…”

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confidence: 99%

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Real-time atomistic observation of structural phase transformations in individual hafnia nanorods

et al. 2017

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High-temperature phases of hafnium dioxide have exceptionally high dielectric constants and large bandgaps, but quenching them to room temperature remains a challenge. Scaling the bulk form to nanocrystals, while successful in stabilizing the tetragonal phase of isomorphous ZrO2, has produced nanorods with a twinned version of the room temperature monoclinic phase in HfO2. Here we use in situ heating in a scanning transmission electron microscope to observe the transformation of an HfO2 nanorod from monoclinic to tetragonal, with a transformation temperature suppressed by over 1000°C from bulk. When the nanorod is annealed, we observe with atomic-scale resolution the transformation from twinned-monoclinic to tetragonal, starting at a twin boundary and propagating via coherent transformation dislocation; the nanorod is reduced to hafnium on cooling. Unlike the bulk displacive transition, nanoscale size-confinement enables us to manipulate the transformation mechanism, and we observe discrete nucleation events and sigmoidal nucleation and growth kinetics.

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Section: Resultsmentioning

confidence: 70%

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confidence: 99%

Real-time atomistic observation of structural phase transformations in individual hafnia nanorods

et al. 2017

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“…There has been interest in stabilizing different structures in zirconia (ZrO 2 ) and hafnia (HfO 2 ) in an attempt to avoid their room-temperature monoclinic structures. Zirconia typically has additions of calcia (CaO), magnesia (MgO), lanthana (La 2 O 3 ) and yttria (Y 2 O 3 ) to stabilize the tetragonal and/or cubic phase to avoid the destructive monoclinic to tetragonal displacive phase transition (Kelly & Denry, 2008); while HfO 2 has additions of silica (SiO 2 ), germania (Ge 2 O 3 ), stannic oxide (SnO 2 ), titania (TiO 2 ) and ceria (CeO 2 ) to stabilize its tetragonal phase (Waetzig et al, 2016;Fischer & Kersch, 2008) to take advantage of its superior dielectric properties (Fischer & Kersch, 2008), superior bandgap and hardness (Waetzig et al, 2016). Thus, the determination of other crystal structures containing ZrO 2 and/or HfO 2 are of interest for hightemperature structural and electronic applications.…”

Section: Introductionmentioning

confidence: 99%

Crystal structure solution for the A ₆ B ₂O₁₇ (A = Zr, Hf; B = Nb, Ta) superstructure

McCormack

Kriven

2019

Acta Crystallogr B Struct Sci Cryst Eng Mater

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crystal structure solutions have been solved using synchrotron X-ray powder diffraction and neutron powder diffraction in conjunction with simulated annealing, charge flipping and Rietveld refinement. These structures have been shown to be isomorphous with the Zr 6 Nb 2 O 17 superstructure, leading to the classification of the A 6 B 2 O 17 (A = Zr, Hf; B = Nb, Ta) orthorhombic compound family with symmetry Ima2 (No. 46). The asymmetrical structural units of cation-centred oxygen polyhedra used to build the structure are as follows: (i) one set of symmetry-equivalent sixcoordinated polyhedra, (ii) three sets of symmetry-equivalent seven-coordinated polyhedra and (iii) one set of symmetry-equivalent eight-coordinated polyhedra. The potential for cation order and disorder was discussed in terms of cation atomic number contrast in X-ray and neutron powder diffraction as well as the bond valence method. In addition, the structural mechanisms for experimentally observed compositional variations within the solid solution range can be attributed to the addition or removal of a set of symmetryequivalent seven-coordinated polyhedra accompanied by corresponding oxygen tilts within the A 6 B 2 O 17 structure.

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“…The binary Hf–O phase diagram has several variants other than the monoclinic and tetragonal polymorphs noted above; for instance, a cubic polymorph is stable above 2700 °C and an orthorhombic polymorph, stabilized by substitutional Y or Al doping on the cation lattice, shows a pronounced ferroelectric distortion with an experimentally demonstrated saturation polarization approaching 16 µC cm −2 . However, the binary Hf–O energy landscape appears to be much more sparsely populated as compared to the energy landscapes of binary vanadium oxides VO 2 and V 2 O 5 , which are characterized by a multitude of energetically proximate polymorphs .…”

Section: Introductionmentioning

confidence: 97%

Stabilization of a Metastable Tunnel‐Structured Orthorhombic Phase of VO₂ upon Iridium Doping

Braham

Andrews

Alivio

et al. 2018

Physica Status Solidi (a)

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Metastable compounds accessible through kinetic stabilization or under conditions of constrained equilibrium represent a richly varied landscape of structures, properties, and function that are oftentimes entirely inaccessible in thermodynamic minima. The multiple redox states of vanadium and the ability to modulate the connectivity of vanadium and oxygen atoms yields a rich diversity of structures for binary vanadium oxides. Here, it is demonstrated that an orthorhombic quasi‐1D polymorph of VO2, characterized by extended tunnels with a rectangular cross‐section, can be stabilized through the substitutional doping of iridium on the vanadium sublattice. The obtained structure is considerably distorted from a previously reported paramontroseite mineral phase. The metastable phase is obtained in nanoplatelet form and is stable with respect to the energetically proximate monoclinic/rutile thermodynamic minima up to a temperature of 350 °C. The open framework structure and the accessibility of multiple redox states at the vanadium center suggests that the polymorph could potentially serve as an intercalation host. Beyond the solubility limit of iridium on the vanadium sublattice (1.28–3.15 at.% depending on the precursor concentration), metallic Ir nanocrystals are found to be homogeneously dispersed on the surfaces of the nanoplatelets indicating a strong interaction between the metal nanocrystals and oxide lattice.

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Stabilizing metastable tetragonal HfO₂ using a non-hydrolytic solution-phase route: ligand exchange as a means of controlling particle size

Abstract: A non-hydrolytic condensation route allows for precise control over the size distribution of HfO2 nanocrystals and enables the stabilization of the tetragonal phase of HfO2.

Cited by 39 publications

References 49 publications

Real-time atomistic observation of structural phase transformations in individual hafnia nanorods

Real-time atomistic observation of structural phase transformations in individual hafnia nanorods

Crystal structure solution for the A ₆ B ₂O₁₇ (A = Zr, Hf; B = Nb, Ta) superstructure

Stabilization of a Metastable Tunnel‐Structured Orthorhombic Phase of VO₂ upon Iridium Doping

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Stabilizing metastable tetragonal HfO2 using a non-hydrolytic solution-phase route: ligand exchange as a means of controlling particle size

Abstract: A non-hydrolytic condensation route allows for precise control over the size distribution of HfO2 nanocrystals and enables the stabilization of the tetragonal phase of HfO2.

Cited by 39 publications

References 49 publications

Real-time atomistic observation of structural phase transformations in individual hafnia nanorods

Real-time atomistic observation of structural phase transformations in individual hafnia nanorods

Crystal structure solution for the A 6 B 2O17 (A = Zr, Hf; B = Nb, Ta) superstructure

Stabilization of a Metastable Tunnel‐Structured Orthorhombic Phase of VO2 upon Iridium Doping

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Stabilizing metastable tetragonal HfO₂ using a non-hydrolytic solution-phase route: ligand exchange as a means of controlling particle size

Crystal structure solution for the A ₆ B ₂O₁₇ (A = Zr, Hf; B = Nb, Ta) superstructure

Stabilization of a Metastable Tunnel‐Structured Orthorhombic Phase of VO₂ upon Iridium Doping