University of Groningen
Local excitations and magnetism in late transition metal oxides
de Graaf, Cornelis
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1998
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de Graaf, C. (1998). Local excitations and magnetism in late transition metal oxides Groningen: s.n.
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172
Summary
S UMMARY
This dissertations presents the results of a theoretical study of the behaviour
of electrons in solid state materials that contain elements from the first series of
transition metals (TM) and in addition at least one other element. The first series
of transition metal runs from scandium (Sc) to zinc (Zn); some well-known
examples are iron (Fe), nickel (Ni) and copper (Cu). Usually, the other element is
a halogen (fluorine (F), chlorine (Cl), etc.) or a chalcogen (oxygen (O), sulphur (S),
etc.) This kind of compounds —so called transition metal materials— show a
wide range of properties depending on the composition. Some TM materials
show a very intense colour, some behave as strong magnets. Furthermore, the
TM materials show extremes in the conduction of current, from true insulators
to metals. Some of the TM materials are even superconducting.
This dissertation deals with the description of the electronic structure of TM
materials, because many of the properties characteristic to TM materials are
mainly connected to the behaviour of the electrons. Roughly speaking, there are
two approaches to describe the electronic structure in solid state materials. The
first approach explores the periodicity of the solid state materials. An ideal crystal
is constructed from a small building block of a few atoms which is repeated
infinitely. By imposing periodic boundary conditions, the whole crystal can be
described theoretically at the same level of accuracy. As a consequence of this
approach, the electrons have a delocalized character. An alternative approach is
the embedded cluster method. This method cuts a small part from the crystal and
describes the electronic structure of this part of the crystal, i.e. the cluster, in a
very detailed manner. This cluster is embedded in a potential that accounts for
the rest of the crystal in a more approximate way. A first and often sufficient
approximation is to include only the electrostatic interactions in the potential,
although quantum mechanical effects can be included as well. The cluster
method leads to a very localized character of the electrons. For certain properties,
such as the conduction of electric current, this approach is not the most suitable
starting point, but for the properties of the TM materials studied in this
dissertation the local approach is a fruitful method.
Chapter 1 gives a motivation for a local approach to the electronic structure
of TM materials, followed by a description of some of the techniques to embed the
cluster in a potential that represents the surroundings of the cluster. Thereafter,
different methods are mentioned to approximate the N-electron cluster wave
function. This chapter ends with a discussion of the treatment of the large
Summary
173
electron correlation effects in the TM materials studied in this dissertation. Three
different strategies —CASSCF/CASPT2, FOCI+CEC and RASSCF— are compared.
The first part of Chapter 2 treats the electronic transitions within the d-shell
of a Ni2+ ion in bulk nickel oxide (NiO) and at the (100) surface of the same
compound. We investigated how the relative energies of the different d8 state are
influenced by electron correlation and to what extend the strategies discussed in
the first chapter account for the different electron correlation mechanisms. The
results show that the CASSCF/CASPT2 method —recently developed at the
University of Lund, Sweden— is well suited to include the most important
electron correlation effects and rather accurate excitation energies can be obtained.
In addition to the well established surface specific peak at 0.6 eV, we also
confirmed the existence of such a surface state at 2.1 eV. From the comparison of
bulk and surface, we conclude that the electron correlation effects on the d8 states
are rather similar for the two systems. The last part of this chapter is devoted to
the description of the local CT states, in which an electron is transferred from the
O-2p orbitals to the Ni-3d orbitals. These local CT states have a lower excitation
energy at the surface than in the bulk.
In Chapter 3 we apply the CASSCF/CASPT2 method to other TM oxides
than NiO, namely CoO, La2NiO4 and La2CuO4. For CoO, excitation energies have
been reported that are in conflict with the energies derived from optical
absorption experiments. The cluster calculations do not support the proposed
deviating values, but result in transition energies which are in very good
agreement with the optical absorption data. In the infrared spectra of La2NiO4 and
La 2 CuO 4 , peaks have been observed, of which the character is not clear One
interpretation states that they arise from d-d transitions on the Ni or Cu ions,
another ascribes the peaks to phonon-assisted magnon transitions. The first
hypothesis was not confirmed with our calculations. To check the second
hypothesis very large clusters are inevitable, which make the calculations too
demanding to be performed with our current computer resources.
The subject of Chapter 4 is the magnetic interaction in TM materials. After a
brief sketch of how magnetic interactions can be studied within the embedded
cluster approach, we established how sensitive the calculated magnetic coupling
parameter is to the details of the applied quantum chemical approach. An
analysis is presented of the mechanisms that determine the strength of the
magnetic interactions. An accurate estimate of the experimental coupling
parameter can be obtained, provided that all oxygens surrounding two nickel ions
are included in the cluster; that the electron correlation effects are accounted for;
and that the cluster is embedded in a potential that has included quantum
174
Summary
mechanical interactions. These observations were used to predict the strength of
the magnetic coupling between two nickel ions at the NiO (100) surface. We
predict the coupling to be weaker at the surface than in the bulk, whereas another
recent study predicts a substantially larger value. Decisive experiments will show
whether our prediction is correct.
Finally, Chapter 5 discusses a study of the processes occurring when a core
electron is ionized. As an example, we studied the character of the final high-spin
coupled states appearing in the Ni-3s XPS of NiO. Because the ionization of a Ni3s electron reduces the screening of the nuclear charge, the electrons n the ligand
orbitals are more strongly attracted by the Ni nucleus. Within some interpretations of the experimental data it is even suggested that the lowest final state
is that state in which an electron is transferred from the ligand orbitals to the Ni
orbitals. Our calculations indicate that this picture of the electronic structure is
oversimplified and that both final states have a very mixed character, neither of
the states can be identified as d8 or d9L-1.