PhD Thesis Arne Lüker final version V4 - Cranfield University

Sol-Gel derived Ferroelectric Thin Films for Voltage Tunable Applications
doping brings out more negative charge defects and thus excessive oxygen vacancies.
The lattice distortion of the perovskite phase will increase and then the formation ability
will decrease with the increase in Mg.”
In other words: proper Mg addition (0 < x < 0.03) in Pb0.4Sr0.6MgxTi1-xO3 thin films
could be used as acceptors in the ABO3 perovskite structure. It induces a negative charge
and thus balances the positive of the oxygen vacancies. Then with the charge being
compensated, the lattice distortion ratio in the system decreases, viz. the lattice structure
of Pb0.4Sr0.6MgxTi1-xO3 becomes more perfect (cubic). According to the thermodynamic
theory, the phase formation ability of the crystal is therefore increased with increasing
Mg content up to 0.03. At the same time, more polarisation path may be provided when
the lattice structure becomes more perfect. So the dielectric constant of the film is
correspondingly increased.
However, with further Mg doping, excessive oxygen vacancies would be created in the
system. The lattice distortion ratio of the perovskite phase structure of the film would be
increased and then the phase formation ability decreased. The dielectric constant is
degraded with increasing Mg doping.
At first glance the explanation from Li et al. for the behaviour of the dielectric constant
in Mg doped PST may as well fit for Mn doped PST. But Mn is a “freakish”,
multivalence ion – it can appear as Mn 2+ (r = 0.67 Å), Mn 3+ (r = 0.61 Å) and Mn 4+ (r =
0.53 Å), all with different impacts on the charge balance of the crystal.
As mentioned before the ferroelectric and pyroelectric properties of PZT were
improved via Mn 2+ doping [14 – 16] in our Lab and it has been suggested that the
formation of oxygen vacancies is facililated by the conversion of Ti 4+ to Ti 3+
Ti 2 [Eq. 6.2].
+
+ ••
→ + O V Ti
4
3
2
By adding Mn 2+ ions (and some oxygen from e.g. the atmosphere) we may find
1
2+
••
4+
O 2 + Mn + VO
→ Mn + O
2
1 2+
1 ••
3+
O2
+ Mn + VO
→ Mn
4
2
or
X
O
1
+ O
2
X
O
[Eq. 6.3]
[Eq. 6.4].
Combining these two reactions we obtain two possible total reactions [19]:
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