3.4 Important Crystal Structures
51
Fig. 3.25 Fluorite crystal
structure, the cations are
depicted as red spheres, the
anions as blue spheres
Fig. 3.26 Hexagonal unit
cell of delafossite CuGaO 2 .
Oxygen atoms are bonded
to the Cu in a dumbbell
(‘DB’) configuration. In
the edge-sharing (‘ES’)
layer the Ga atoms are
octahedrally configured as
GaO 6
O
a
2uc
c
ES
DB
Ga
Cu
ES
3.4.8 Fluorite Structure
Named after the minerale fluorite (CaF 2 , space group 225, Fm3m), this structure for binary ionic
compounds occurs when the cation valence is twice the anion valence, e.g. for (cubic) ZrO 2 (zirconia)
or HfO 2 . The lattice is fcc with a triatomic base. At (0,0,0) is the cation (e.g. Zr
4+ ), the anions (e.g. O
2− )
are at (1/4, 1/4, 1/4) a (as in the zincblende structure) and (3/4, 3/4, 3/4) a (Fig. 3.25). The anion
atom positions are on a simple cubic lattice with lattice constant a/2. Zirconia can crystallize in various
phases [223], the most prominent being the monoclinic, tetragonal and cubic phases. The cubic phase
can be extrinsically stabilized using yttrium [224, 225] (YSZ, yttria-stabilized zirconia). Hafnium
oxide has the remarkable property that the HfO 2 /Si interface is stable and allows the fabrication of
transistor gate oxides with high dielectric constant (see Sect. 24.5.5).
3.4.9 Delafossite Structure
The I–III–O 2 materials crystallize in the trigonal delafossite (CuFeO 2 , space group 166, R ¯
3m) structure
(Fig. 3.26). This structure is also called caswellsilverite (NaCrS 2 ). In Table 3.5 the lattice parameters of
some delafossite compounds are given. The (Cu,Ag) (Al,Ga,In)O 2 materials are transparent conductive
oxides (TCO). We note that Pt and Pd as group-I component create metal-like compounds because of
the d
9 configuration as opposed to the d
10 configuration of Cu and Ag.
51
Fig. 3.25 Fluorite crystal
structure, the cations are
depicted as red spheres, the
anions as blue spheres
Fig. 3.26 Hexagonal unit
cell of delafossite CuGaO 2 .
Oxygen atoms are bonded
to the Cu in a dumbbell
(‘DB’) configuration. In
the edge-sharing (‘ES’)
layer the Ga atoms are
octahedrally configured as
GaO 6
O
a
2uc
c
ES
DB
Ga
Cu
ES
3.4.8 Fluorite Structure
Named after the minerale fluorite (CaF 2 , space group 225, Fm3m), this structure for binary ionic
compounds occurs when the cation valence is twice the anion valence, e.g. for (cubic) ZrO 2 (zirconia)
or HfO 2 . The lattice is fcc with a triatomic base. At (0,0,0) is the cation (e.g. Zr
4+ ), the anions (e.g. O
2− )
are at (1/4, 1/4, 1/4) a (as in the zincblende structure) and (3/4, 3/4, 3/4) a (Fig. 3.25). The anion
atom positions are on a simple cubic lattice with lattice constant a/2. Zirconia can crystallize in various
phases [223], the most prominent being the monoclinic, tetragonal and cubic phases. The cubic phase
can be extrinsically stabilized using yttrium [224, 225] (YSZ, yttria-stabilized zirconia). Hafnium
oxide has the remarkable property that the HfO 2 /Si interface is stable and allows the fabrication of
transistor gate oxides with high dielectric constant (see Sect. 24.5.5).
3.4.9 Delafossite Structure
The I–III–O 2 materials crystallize in the trigonal delafossite (CuFeO 2 , space group 166, R ¯
3m) structure
(Fig. 3.26). This structure is also called caswellsilverite (NaCrS 2 ). In Table 3.5 the lattice parameters of
some delafossite compounds are given. The (Cu,Ag) (Al,Ga,In)O 2 materials are transparent conductive
oxides (TCO). We note that Pt and Pd as group-I component create metal-like compounds because of
the d
9 configuration as opposed to the d
10 configuration of Cu and Ag.