Fig. 4. (Color online) (a) Structure of tetragonal KNbO
3 perovskite, the vertical arrow shows displacement of Nb ion with respect to O ions within the NbO
2 monolayer. (b) Structure of tetragonal KNbO
2S perovskite, the sulfur ion is shown as the bigger yellow sphere atop of Nb ion that has been shown as a small green sphere. Displacement of the Nb ion with respect to the O ions within the NbO
2 monolayer is three times larger than that of Nb in KNbO
3. Relaxed structures of PbTiO
3-xS
x : (c) two unit cells of
x = 0, (d) one unit cell of
x = 0.5, (e) two unit cells of
x = 1, and (f) one unit cell of
x = 2. The view is of the
a-
c plane. All dimensions and ionic radii are to scale, except the Ti ions, which are enlarged for clarity. (a) and (b) adapted with permission from Ref. [
44]. Copyright 2014 AIP Publishing LLC. (c) and (d) adapted with permission from Ref. [
42]. Copyright 2014 American Physical Society.
