Association

A. Appendix
A.1. Properties of EuO-related phases and cubic oxide substrates
EuO is metastable and easily reacts to thermodynamically competing compounds. In Tab. A.1,
we provide an overview of the most common Eu reaction products which are possibly related
to EuO during synthesis.
One objective of this thesis is the investigation of single-crystalline EuO epitaxially integrated
on different cubic substrates providing biaxial strain, m = a S−a F
a
. In order to allow for a gradation
of available substrates, in Tab. A.2, we compile lattice parameters and strain with respect
F
to single-crystalline EuO for different temperatures: at liquid helium, liquid nitrogen, room
temperature, and during MBE synthesis of EuO. For the substrates used in this thesis, Fig. A.1
illustrates the cubic cells of EuO relative to underlying oxide substrates with an indication of
biaxial strain, m, and interface reactivity.
For lattice-matched EuO growth, one may consider SrO (a = 5.161 Å, m =+0.4%) as well
as Sr x Ba 1−x O, in which the cubic lattice parameter a is tunable by composition x (Lettieri
et al. (2003)), 24,217 but these two are not commercially available due to their high reactivity
in air and with humidity. The cubic ITO substrate allows for coherent EuO growth, yet tends
to reduce its lattice constant for lower doping (e. g. 6% Sn). In this case, ITO would provide a
small compressive strain with respect to EuO, if integrated epitaxially on cubic planes. Also
the ITO crystals or films need to be synthesized on-site.
For moderate tensile strain, comparable to Si (001), CeO 2 (a = 5.412 Å, m =+5.2%) would be
an option.
Among the commercially available substrates, cubic CaO (a = 4.815 Å, m = −6.3%) would
be suited for a moderate compressive strain provided to EuO (001). CaO is chemically reactive
with water and also carbon, which destroys the surface crystal structure immediately.
Thus, we estimate its thermodynamic stability: CaO has a Gibbs free energy of formation of
−636 kJ/mol which is larger than for EuO (Δ f G(EuO) ≈−500 kJ/mol). This predicts the thermodynamic
stability of CaO in contact with EuO synthesis. However, during oxygen-limited
EuO synthesis at elevated temperatures, the reaction CaO + 3EuO −→ Eu 3 O 4 + Ca results
in a Gibbs free energy balance near zero. Thus, EuO may reduce CaO to propel its own oxidation,
however the energy gain for that reaction is small which renders the reaction in the
reversible regime (see Fig. 2.2). In conclusion, CaO is promising by means of its compressive
strain, but chemically challenging due to its air-sensitivity and comparable thermodynamic
stability with EuO.
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