D. J. Payne

Stereochemistry of post-transition metal oxides: revision of the classical lone pair model

The chemistry of post transition metals is dominated by the group oxidation state N and a lower N-2 oxidation state, which is associated with occupation of a metal s(2) lone pair, as found in compounds of Tl(I), Pb(II) and Bi(III). The preference of these cations for non-centrosymmetric coordination environments has previously been rationalised in terms of direct hybridisation of metal s and p valence orbitals, thus lowering the internal electronic energy of the N-2 ion. This explanation in terms of an on-site second-order Jahn-Teller effect remains the contemporary textbook explanation. In this tutorial review, we review recent progress in this area, based on quantum chemical calculations and X-ray spectroscopic measurements. This recent work has led to a revised model, which highlights the important role of covalent interaction with oxygen in mediating lone pair formation for metal oxides. The role of the anion p atomic orbital in chemical bonding is key to explaining why chalcogenides display a weaker preference for structural distortions in comparison to oxides and halides. The underlying chemical interactions are responsible for the unique physicochemical properties of oxides containing lone pairs and, in particular, to their application as photocatalysts (BiVO(4)), ferroelectrics (PbTiO(3)), multi-ferroics (BiFeO(3)) and p-type semiconductors (SnO). The exploration of lone pair systems remains a viable a venue for the design of functional multi-component oxide compounds.

Growth of In2O3(100) on Y-stabilized ZrO2(100) by O-plasma assisted molecular beam epitaxy

Thin films of In2O3 have been grown on Y-stabilized ZrO2(100) by oxygen plasma assisted molecular beam epitaxy with a substrate temperature of 650°C. Ordered epitaxial growth was confirmed by high resolution transmission electron microscopy. The position of the valence band onset in the x-ray photoemission spectra of the epitaxial films is found to be inconsistent with the widely quoted value of 3.75eV for the fundamental bandgap of In2O3 and suggests a revised value of 2.67eV.

The Nature of Electron Lone Pairs in BiVO4

The electronic structure of BiVO4 has been studied by x-ray photoelectron, x-ray absorption, and x-ray emission spectroscopies, in comparison with density functional theory calculations. Our results confirm both the direct band gap of 2.48 eV and that the Biu20026s electrons hybridize with Ou20022p to form antibonding “lone pair” states at the top of the valence band. The results highlight the suitability of combining s2 and d0 cations to produce photoactive ternary oxides.

Nitrogen diffusion in doped TiO2 (110) single crystals: a combined XPS and SIMS study

Rutile TiO2 (110) single crystals were doped with nitrogen by heating at 675 °C in flowing NH3 gas. This caused a red shift in the band edge and an increase in visible region absorption. Nitrogen depth profiles obtained using dynamic secondary ion mass spectrometry (SIMS) could best be fitted by assigning three distinct diffusion coefficients. X-Ray photoelectron spectroscopy (XPS) indicated the presence of two surface nitrogen states with binding energies of 395.6 eV (substitutional N) and 399.8 eV (interstitial N). Angle resolved XPS measurements allowed us to link the XPS environments to the SIMS diffusion profiles. Subsequent air annealing at 500 °C led to asymmetric diffusion of the interstitial nitrogen into the bulk, removal of substitutional nitrogen into the gas phase, a decrease in the concentration of Ti3+ and a blue shift in the band edge. These changes could be related to variation in the optical band gap, and it was found that the principle cause of band gap narrowing was substitutional rather than interstitial nitrogen.

Electronic structure of In2O3 from resonant x-ray emission spectroscopy

The valence and conduction band structures of In2O3 have been measured using a combination of valence band x-ray photoemission spectroscopy, O K-edge resonant x-ray emission spectroscopy, and O K-edge x-ray absorption spectroscopy. Excellent agreement is noted between the experimental spectra and O 2p partial density of states calculated within hybrid density functional theory. Our data are consistent with a direct band gap for In2O3.

The influence of Sn doping on the growth of In2O3 on Y-stabilized ZrO2(100) by oxygen plasma assisted molecular beam epitaxy

The influence of Sn doping on the growth of In2O3 on Y-stabilized ZrO2(100) by oxygen plasma assisted molecular beam epitaxy has been investigated over a range of substrate temperatures between 650 and 900 degrees C. The extent of dopant incorporation under a constant Sn flux decreases monotonically with increasing substrate temperature, although the n-type carrier concentration in overdoped films grown at 650 degrees C is lower than in films with a lower Sn concentration grown at 750 degrees C. The small increase in lattice parameter associated with Sn doping leads to improved matching with the substrate and suppresses breakup of the films into square islands observed in high temperature growth of undoped In2O3 on Y-stabilized ZrO2(100). Plasmon energies derived from infrared reflection spectra of Sn-doped films are found to be close to satellite energies in core level photoemission spectroscopy, but for a nominally undoped reference sample there is evidence for carrier accumulation at the surface. This influences both the In 3d core line shape and the intensity of a peak close to the Fermi energy associated with photoemission from the conduction band.

A study of the metal to nonmetal transition in Bi-doped β-PbO2 by high resolution x-ray photoemission

The influence of Bi doping on the electronic structure of β‐PbO2 has been studied by high resolution x-ray photoemission spectroscopy. Doped films were prepared on Pt substrates by electrochemical deposition from solutions of Pb(NO3)2 and Bi(NO3)3 in HNO3. Bi doping was found to lead to a lowering of the density of states at the Fermi energy in valence region x-ray photoemission and to suppression of final state screening by mobile conduction electrons in Pb 4f core level photoemission. A metal to nonmetal transition was found to occur for bulk doping levels around 5at.% Bi. There is evidence of pronounced surface segregation of Bi. In contrast to host Pb ions, core holes on surface Bi ions do not couple to the mobile conduction electrons in the metallic state. It is concluded that Bi acts as a p-type acceptor in β‐PbO2 and traps charge carriers introduced by oxygen deficiency in PbO2−x.

Role of spin-orbit coupling in the electronic structure of IrO2

The delicate interplay of electronic charge, spin, and orbital degrees of freedom is in the heart of many novel phenomena across the transition metal oxide family. Here, by combining high- resolution angle resolved photoemission spectroscopy and first principles calculations (with and without spin-orbit coupling), the electronic structure of the rutile binary iridate, IrO

Nature of the Band Gap of In2O3 Revealed by First-Principles Calculations and X-Ray Spectroscopy

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Theoretical and Experimental Study of the Electronic Structures of MoO3 and MoO2

is investigated. The detailed study of electronic bands measured on a high-quality single crystalline sample, and use of a wide range of photon energy provide a huge improvement over the previous studies. The excellent agreement between theory and experimental results shows that the single-particle DFT description of IrO