Characterization of metastable tetragonal hafnia

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

“…Hunter et al used this expression to predict a critical size for stabilization of tetragonal HfO 2 of d = 3.6 nm. 16 In the absence of well-defined, monodisperse, and phase-pure nanocrystals, a comprehensive evaluation of this formalism has not been possible thus far. Another notable complication for HfO 2 nanocrystals prepared by aqueous methods is the almost 20–30% diminution of surface energy as a result of surface hydroxylation, which in this case further dilutes the influence of the surface energy contribution.…”

“… 20 , 21 Estimates of the critical size required to stabilize the tetragonal phase of HfO 2 vary widely from about 2 to 10 nm but it is clear that this value is substantially smaller than the critical size for ZrO 2 . 16 , 20 , 22 Several methods for the preparation of nanometer-sized particles of HfO 2 report the stabilization of at least some fraction of tetragonal HfO 2 ; for instance, signatures of the tetragonal phase have been identified in particles obtained by the thermal decomposition of pure Hf(OH) 4 , 16 the oxidation of metallic Hf nanocrystals, 23 and a ligand-mediated reaction at the interface of water and oil phases. 24 However, the polydispersity and relatively poor crystallinity of the particles obtained by these methods implies that the obtained samples almost always contain only minor proportions of tetragonal phases and a clear delineation of a size-dependent phase diagram has thus far not been possible.…”

Characterization of Hafnia Powder Prepared from an Oxychloride Sol–Gel

“…Although in many respects similar to ZrO 2 , the HfO 2 system has not been characterized extensively. Starting materials, temperature, heating rate, heating environment, and pH are all factors affecting the crystallization process in HfO 2 and HfO 2 ‐containing materials 2–12 . Much information available in the literature appears contradictory, both for bulk and thin‐film samples and have not yet been adequately explained as Shukla and Seal 13 demonstrate.…”

“…After high‐temperature annealing, the Hf‐containing layer in thin films often fully or partially crystallizes. The result is widely reported to be the m ‐HfO 2 phase, which is the expected, stable phase, but some researchers have observed the t ‐HfO 2 and o ‐HfO 2 phases 4,28–32,12 . Recent work by MacLaren et al 33 has shown that a thin film that was deposited in the amorphous state ended as crystalline m ‐HfO 2 after a postdeposition anneal, retaining a small volume fraction of a more symmetric t ‐ or c ‐phase.…”

“…When the particle reaches the critical size of 30 nm, a transformation to m ‐ZrO 2 is more thermodynamically favored after which the particle continues to grow. Although some debate remains as to the magnitude of the critical particle size (e.g., Hunter et al 12 calculate a critical particle size of 10 nm from experimental data), Garvie's theory was based on the observation of high‐temperature t ‐ZrO 2 formation with the assumption that t ‐ZrO 2 particles must be spherical for the stabilization to be caused only by the surface energy effect. However, if the particles grow within a matrix—amorphous or crystalline—the stress and strain induced by the surrounding matrix may also cause the particles to crystallize into and remain in the t ‐ZrO 2 phase.…”