The Effect of Zr-Doping and Crystallite Size on the Mechanical ...

I. Introduction
temperatures [11]. Upon decompression at ambient temperatures, the transformation <strong>of</strong>
OII to MI was found at 20 GPa, the cubic phase could be followed to below 9 GPa <strong>and</strong>
below 7 GPa MI transforms to a mixture <strong>of</strong> TiO2II <strong>and</strong> rutile [18]. <strong>The</strong> pressures <strong>of</strong>
phase transformations are different for compression <strong>and</strong> decompression <strong>and</strong> vary in
different studies. <strong>The</strong> reason for that is most likely that in many cases phase
transformations to the thermodynamically stable phase were hindered kinetically <strong>and</strong>
only the metastable phases were observed. <strong>The</strong> apparent pressure-temperature phase
diagram <strong>of</strong> TiO2 is presented in Figure 1, containing transformation pressures as well as
pressure ranges in which the phases were observed.
In the following chapters, the phases anatase, rutile, TiO2II <strong>and</strong> MI will play an
important role <strong>and</strong> their structures are described in more detail in the following (Figure
2). In anatase, each Ti is surrounded by an octahedron <strong>of</strong> six oxygen atoms. Four edges
per octahedron are shared, building a “zig-zag” chain parallel to a as well as b. <strong>The</strong>
chains are stacked antiparallel to the c axis. Rutile consists <strong>of</strong> TiO6 octahedra <strong>and</strong> two
opposing edges <strong>of</strong> each octahedron are shared, forming linear chains parallel to the cdirection.
In the a-b-plane, the chains are linked via corner-sharing oxygen atoms. In
TiO2II, the octahedra make up a network similar to rutile but are distorted. <strong>The</strong> structure
<strong>of</strong> MI can be seen as an even further distorted version <strong>of</strong> TiO2II, exhibiting a seven-fold
coordination <strong>of</strong> the Ti-atom.
Figure 2: Elementary cells <strong>and</strong> polyhedra <strong>of</strong> TiO2 anatase, rutile, TiO2II <strong>and</strong> MI. Oxygen atoms are shown
in red, titanium atoms <strong>and</strong> polyhedra are shown in blue.
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