Electronic Structure Differences in ZrO₂ vs HfO₂

Abstract

Although ZrO2 and HfO2 are, for the most part, quite similar chemically, subtle differences in their electronic structures appear to be responsible for differing MO2/Si (M = Zr, Hf) interface stabilities. To shed light on the electronic structure differences between ZrO2 and HfO2, we have conducted joint experimental and theoretical studies. Because molecular electron affinities are a sensitive probe of electronic structure, we have measured them by conducting photoelectron spectroscopic experiments on ZrO2(-) and HfO2(-). The adiabatic electron affinity of HfO2 was determined to be 2.14 +/- 0.03 eV, and that of ZrO2 was determined to be 1.64 +/- 0.03 eV. Concurrently, advanced electronic structure calculations were conducted to determine electron affinities, vibrational frequencies, and geometries of these systems. The calculated CCSD(T) electron affinities of HfO2 and ZrO2 were found to be 2.05 and 1.62 eV, respectively. The molecular results confirm earlier predictions from solid state calculations that HfO2 is more ionic than ZrO2. The excess electron in MO2(-) occupies an sd-type hybrid orbital localized on the M atom (M = Zr, Hf). The structural parameters of ZrO2 and HfO2 and their vibrational frequencies were found to be very similar. Upon the excess electron attachment, the M-O bond length increases by ca. 0.04 A, the OMO angle increases by 2-4 degrees, and frequencies of all vibrational modes become smaller, with the stretching modes being shifted by 30-50 cm(-1) and the bending mode by 15-25 cm(-1). Together, these studies unveil significant differences in the electronic structures of ZrO2 and HfO2 but not in their structural or vibrational characteristics.

The visible spectrum of zirconium dioxide, ZrO2

“…6 The ω 2 (287 ± 2 cm −1 ) is also consistent with the 290 cm −1 value estimated from the inertial defect. 4 As expected, ω 1 and ω 2 for theX 1 A 1 state of ZrO 2 are slightly less than ω 1 (968 ± 7 cm −1 ) and ω 2 Table IV.…”

“…The vibrational frequencies and permanent dipole moment are slightly overestimated, typical of these calculations. 6,7 Although the term energy of theà 1 B 2 state is accurately predicted using TD-DFT, the values for ω 2 and ω 3 are in poor agreement with observations. The calculated T e , ω 1 , ω 2 , and ω 3 at the CASSCF/LanL2DZ level of theory are also in poor agreement with the experiment.…”

“…The electronic structure of ZrO 2 was also investigated by photoelectron spectroscopy (PES) of the anion, ZrO − 2 . 5,6 Progressions in the PES spectrum were analyzed to determine symmetric stretching frequency, ω 1 , of 887 ± 40 cm −1 for theX 1 A 1 state and an adiabatic electron affinity of 1.64 ± 0.03 eV for the neutral. The 355 nm radiation used was insufficient to probe the regions of low-lying excited states of the neutral.…”

How a Quantum Chemical Topology Analysis Enables Prediction of Electron Density Transfers in Chemical Reactions. The Degenerated Cope Rearrangement of Semibullvalene

“…However, important differences are found their respective electroaffinity. 7 In particular the electron affinities of TiO 2 and ZrO 2 dioxides are rather similar, whereas the electron affinities of HfO 2 and ZrO 2 differ substantially 7 . This fact demonstrates the nonnegligible role of f-electrons and relativistic effects in the chemistry of hafnium compounds.…”

“…5,6 Likewise, the utilization of simple metal oxides clusters has served as interesting models in understanding the nature of active species in catalysis at molecular level. 7 In particular, the activation of H 2 by metal centers is a fundamental step in nearly all metal catalytic hydrogenation reactions. [8][9][10] The reactions of molecules involving transition metal oxides and H 2 provide well-defined molecular models pertinent to mechanistic understandings of the dihydrogen in some catalytic and biochemical processes.…”