Intrinsic metastability of orthorhombic HfTiO4 in thin film hafnia-titania

Abstract

Orthorhombic (o) HfTiO4 is crystallized when sputter deposited hafnia-titania nanolaminates with ultrathin layers and bilayer (HfO2)0.5(TiO2)0.5 composition are annealed between 573 and 1173 K. However, o-HfTiO4 demixes after annealing at 1273 K, a result not predicted from bulk thermodynamics. X-ray diffraction and Raman microscopy are used here to study structural changes as o-HfTiO4 demixes upon long-term annealing at 1273 K into Ti-doped monoclinic HfO2 and Hf-doped rutile TiO2. We conclude that o-HfTiO4 crystallized at low temperature is intrinsically metastable. A space group symmetry analysis shows that demixing can be accomplished by a continuous phase transition chain.

Addendum to “Phase selection and transition in Hf-rich hafnia-titania nanolaminates” (on SiO2) [J. Appl. Phys. 109, 123523 (2011)]: Hafnon formation

“…1,3 The aforementioned double peak vicinal to the rutile standard, labeled A and B, as well as a lower angle peak, labeled C, were also observed after 24 h at 1273 K. Peak A is attributed to the (110) planes of TiO 2 :Hf as before, 1-3 but peak B is now attributed to hafnon (200) planes. 8 Peak C is attributed to hafnon (101) planes.…”

“…Post-deposition air annealing was carried out first at 1173 K for up to 192 h (denoted stage I). Previous results on the same architecture 1 showed that annealing at 1173 K for 1 h did not produce a double XRD peak vicinal to (110) rutile. Here, we wanted to determine whether long term annealing at 1173 K would produce it.…”

“…1,2 X-ray diffraction (XRD) studies of nanolaminates with (HfO 2 ) x -(TiO 2 ) 1Àx with 0.51 x 0.66 HfO 2 mole fraction that were annealed in air at 1173 K for !24 h showed a persistent feature: a double peak at slightly lower angle than the (110) rutile TiO 2 standard peak position. Our initial (incorrect) hypothesis 1,2 was that rutile crystallized in structures containing two different amounts of incorporated Hf. We carried out the following experiment to identify the source of this double peak.…”

“…[1][2][3] The as-grown film consists of nanocrystalline orthorhombic (o) HfTiO 4 . (o-HfTiO 4 is intrinsically unstable and demixes upon annealing above 973 K. 1,3 ). Post-deposition air annealing was carried out first at 1173 K for up to 192 h (denoted stage I).…”

Ti-catalyzed HfSiO4 formation in HfTiO4 films on SiO2 studied by Z-contrast scanning electron microscopy

“…We previously reported that orthorhombic (o) HfTiO 4 in sputter deposited films on fused SiO 2 substrates demixes upon annealing to form monoclinic (m) HfO 2 and rutile (r) TiO 2 . 6,7 When demixing occurs at high temperature (1273 K), crystalline hafnon (HfSiO 4 ) with a zircon archetype lattice is formed, 8 indicating a reaction between the film and the substrate. This last result is surprising because HfSiO 4 does not form in a HfO 2 -on-SiO 2 film subjected to a similar annealing regimen.…”