Top: In the low-temperature phase of lead oxide (α-PbO), each
lead atom (blue) forms a tetragon with four oxygen nearest neighbors
(red) on one side. Bottom: In monoclinic bismuth oxide (α-Bi2O3),
the two distinct bismuth ions (blue) both have five oxygen near
neighbors (red), creating a distorted square pyramidal coordination
geometry.
The asymmetrical crystal structures of the metal oxides α-PbO
and α-Bi2O3 were for many years explained
by the hybridization of the metals' 6s and 6p orbitals,
which are occupied by metal 6s electrons, assumed to lie
close to the Fermi energy (EF) in
the solid state. This conventional "lone-pair" model,
however, has recently been called into question on the basis of
density functional theory (DFT) calculations that suggest that
the majority of the 6s population in α-PbO is in
fact found at the bottom of the main valence band, about 10 eV
below EF. However,
definitive experimental evidence supporting this idea has been
lacking.
In this study, the investigators showed that consideration of
the relative intensities of valence-band components in oxygen K-shell
XES, obtained at ALS Beamline
7.0.1, provides a simple but incisive
experimental approach to investigating the nature of lone-pair
states in metal oxides, especially when compared with the intensities
of features in XPS obtained from an aluminum Kα x-ray source.
Both XPS and XES spectra (and the corresponding DFT calculations)
show a well-defined band (labelled III) that lies about 10 eV below
the top of the valence band. However, this band diminishes dramatically
in the XES data. Since oxygen K-shell XES is governed by a very
strict selection rule that allows only states of oxygen 2p character
to decay into the oxygen 1s core hole, the XES data directly
measures the oxygen 2p partial density of states (PDOS).
XPS (top left) and XES (bottom left) spectra
of α-PbO compared
to DFT calculations of the total density of states (DOS, top
right) and partial density of states (PDOS, bottom right) with
the contributions from the various O and Pb states indicated.
The XPS data correspond to the total DOS while XES measures the
O 2p PDOS. The spectra
are all presented on a binding energy scale referenced to the
top of the valence band. (The results for α-Bi2O3 are
similar.)
The conclusion, fully supported by DFT calculations, is that there
can be little oxygen 2p character in this band, and by
default, the associated electronic states must have dominant metal
6s character.
These findings confirm that the structural distortions in α-PbO
and α-Bi2O3 should not be attributed
to the direct hybridization of metal orbitals close to EF,
resulting in purely metal-based 6s-6p lone pairs.
Instead, the dominant contribution to the metal 6s PDOS
is found at the bottom rather than the top of the valence band,
and indirect mixing
between 6s and 6p states is mediated by hybridization
with oxygen 2p states at the top of the valence band.
It follows that qualitative textbook explanations of structural distortions
in many heavy post-transition-metal compounds should be revised,
with important implications for understanding the structural physics
of, for example, magnetic ferroelectric materials such as BiMnO3
and BiFeO3. The results are also of general significance in relation
to the electronic structures of ternary lead and bismuth oxides,
including the many high-temperature superconducting phases that contain
these heavy cations. It has in the past been assumed that the 6s states
lie close to EF and therefore contribute significantly
to the states responsible for conduction in metallic phases. The
present findings demonstrate that this viewpoint is not correct.
Again, this will have an impact on our understanding and tuning
of the physical properties of these and related materials.
Research conducted by D.J. Payne and R.G. Egdell (Oxford University);
A. Walsh and G.W. Watson (Trinity College); J. Guo (ALS); and P.-A.
Glans, T. Learmonth, and K.E. Smith (Boston University).
Research funding: U.K. Engineering and Physical Sciences Research
Council; Higher Education Authority (Ireland); U.S. Department
of Energy, Office of Basic Energy Sciences (BES). Operation of
the ALS is supported by BES.
Publication about this research: D.J. Payne, R.G. Egdell, A. Walsh,
G.W. Watson, J. Guo, P.-A. Glans, T. Learmonth, and K.E. Smith, "Electronic
origins of structural distortions in post-transition metal oxides:
Experimental and theoretical evidence for a revision of the lone
pair model," Phys. Rev. Lett. 96,
157403 (2006). |