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A.1. EuO-related phases and cubic oxide substrates 129
(001)
+ =
O 2-
Zr 4+
Y 3+
ZrO 2 (monoclinic) Y 2 O 3
(cubic, simplified)
Yttria-stabilized zirconia
(cubic, Y 2 O 3 : 8–20%)
Figure A.2.: Structure and (001) plane of yttria-stabilized zirconia (YSZ).
In the following, we describe the crystal structure of substrates used in this thesis. Yttriastabilized
zirconia (YSZ) is used as a seed for coherent EuO growth of reference quality. Pure
zirconium dioxide (ZrO 2 ) undergoes a phase transformation from monoclinic (stable at the
room temperature) to tetragonal (at about 1000 ◦ C) and then to cubic (at about 2370 ◦ C). A
stabilization of the desired cubic phase of zirconia can be accomplished by substitution of
some of the Zr 4+ ions (ionic radius of 0.82 Å, too small for ideal cubic lattice) in the crystal
lattice with slightly larger ions, e. g. those of Y 3+ (ionic radius of 0.96 Å). The result is cubic
yttria-stabilized zirconia (YSZ). Doping with Y 3+ ions produces oxygen vacancies, as three
O 2− ions are bound to Y 3+ rather than four O 2− ions in case of Zr 4+ . This permits YSZ to conduct
O 2− ions (and thus conduct an electrical current), provided there is a sufficient vacancy
site mobility, a property that increases with temperature. 227
(a)
(b)
(c)
(001)
c
a
b
O 2-
La 3+
Al 3+
LaAlO 3
(rhombohedral)
rhombohedral unit cell
cubic unit cells
LaAlO 3 (cubic)
Figure A.3.: Rhombohedral structure, conversion, and pseudocubic structure with (001) plane of
LaAlO 3 (LAO).
Pseudocubic LaAlO 2 (LAO) is used for epitaxial EuO/LAO (100) heterostructures with tensile
biaxial strain. The transformation between rhombohedral and cubic structure for LAO
is illustrated in Fig. A.3b. 218 The LAO (100) cubic plane is parallel to the (0¯12) rhomb plane,
and [01¯1] cubic ‖ [100] rhomb and [011] cubic ‖ [121] rhomb . The LAO substrates are cut along
(100) cubic planes. We will use the cubic indexing for LAO in this work. LAO is rhombohedral
(pseudocubic) at room temperature, and undergoes a structural displacement transition at
∼540 ◦ C. 164