PhD Thesis Arne Lüker final version V4 - Cranfield University

Relative diel. Constant
Dielectric Constant
1200
1000
800
600
400
200
B Site Doping of PST 40/60
0
0 50 100 150 200 250 300
114
Li et al. have doped the B site of PST 40/60 with Mg 2+ [12]. They found that the
dielectric constant increases with increasing doping level up to 3 mol% Mg 2+ with a sharp
decrease afterwards. They attributed this behaviour to the generation of oxygen
vacancies.
Generally, oxygen vacancies are generated by heat treatment under non-oxidising
atmosphere. In thin films, which were annealed in ambient atmosphere, they also form
the so-called dead-layer at the interface of the bottom electrode and the ferroelectric thin
film. In the case of Mg doped PST Mg 2+ replaces Ti 4+ in the PST lattice due to a similar
ionic radii of Mg 2+ (r = 0.72 Å) and Ti 4+ (r = 0.68 Å), hence B site doping. The defect
reaction equation can therefore be written as
where
Frequency [kHz]
loss [%]
20
18
16
14
12
10
TiO
''
••
MgO ⎯⎯
→ MgTi
+ VO
+
8
6
4
2
0
0 50 100 150 200 250 300
2 O
[Eq. 6.1]
••
V O is an oxygen vacancy with two singly positive charges 3 controlled by the Mg
content. That means, according to Li et al.: “When the Mg substitutes for Ti ion in the
PST thin film system as x < 0.03, the positive charge of the [intrinsic] oxygen vacancies
is balanced by the negative charge induced by Mg in the structure. The phase formation
ability is therefore increased with increasing Mg up to 0.03. As x > 0.03, excessive Mg
3 ••
Kröger-Vink nomenclature [18]: V O : main character: chemical species, V = vacancy; subscript: site (for
example, O = regular oxygen site); superscript: charge relative to perfect lattice; •, ‘, x correspond to singly
positive, singly negative, and neutral effective charge.
O
Frequency [kHz]
Fig. 6.4: Dielectric Constant and loss vs. frequency forPST 40/60 with different Mn content