Double Perovskite Crystalline Structures
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When two different cations (e.g. B and B’) are introduced into the B site in perovskite structures, double perovskite structures with the formula A2BB’O6 are formed. Depending on differences in size and of charges of the B-site cations, the double perovskite structures can be categorized by three types of arrangements: random; rocksalt; and layered structures [3].

Lead magnesium niobate [Pb(Mgm/nNbn-m/n)O3, PMN] at room temperature has a complex cubic ABO3 perovskite structure, where Pb2+ sits on A-sites while Mg2+ and Nb5+ share the B-sites (namely, the octahedral center in the perovskite structures). Therefore, the B-site cations adopt a doubled perovskite unit cell, A(B'1/2B"1/2)O3-type 1:1 structure with a face centered arrangement of the two different (B' and B") cation positions. [1] For simplicity, A-site cation and oxygen are not shown in the complex cubic perovskite structure in Figure 3530a, while  the {111} B planes are highlighted in green and red. We can see every other {111} plane belongs to B' (e.g. Mg2+ in PMN) sublattice (in green) but the remaining {111} planes belong to B" (e.g. Nb5+ in PMN) sublattice (in red).

Double perovskite crystalline structures

Figure 3530a. Double perovskite crystalline structures.

The double perovskite crystalline structures can be described by an F-centred 2a0 x 2a0 x 2a0 superlattice (where a0 is the lattice parameter of the ideal perovskite structures). As results, extra diffraction spots present besides the "standard" electron diffraction spots from the ideal perovskite crystalline structures. As an example, Figure 3530b (a) shows the dark field image revealing B-site ordered domains (bright areas) in PMN materials and (b) presents such extra diffraction spots as well as the standard diffraction spots in the selected area diffraction pattern  from the <l 1 0> zone axis of the area in Figure 3530b (a).

Extra electron diffraction spots from perovskite crystalline structures

Figure 3530b. (a) The dark field image revealing B-site ordered domains (bright areas) in
PMN materials and (b) The extra diffraction spots as well as the standard diffraction
spots from the <l 1 0> zone axis of the area in figure (a). [2]

Figure 3530c shows the double perovskite-rocksalt oxide La2CuSnO6 (LCSO) with Cu2+ and Sn4+ ions determined by X-ray analysis. The La–La distance around the Cu ion is slightly shorter than that around the Sn ion, because the CuO6 octahedrons are slightly distorted by the Jahn–Teller effect. The alternation of CuO6 and SnO6 octahedral layers and the buckling of the CuO2 and SnO2 sheets induce a monoclinic superstructure with the lattice parameters of a = 0.8510, b = 0.7815, c = 0.7817 nm (corresponding to 2a0 x 2a0 x 2a0), and β = 91.151°. Note there are four non-equivalent La atoms with different atomic coordinates and Debye–Waller factors (MLa1 = 0.0056 nm2, MLa2 = 0.005 nm2, MLa3 = 0.0042 nm2, and MLa4 = 0.0081 nm2) in a unit cell [4].

double perovskite-rocksalt oxides La2CuSnO6 (LCSO) determined by X-ray analysis

Figure 3530c. The double perovskite-rocksalt oxides La2CuSnO6 (LCSO) determined by X-ray analysis: (a) b-axis and (b) c-axis projections. [5]



[1] G. A. Smolenskii and A. I. Agranovskaya, Soviet Physics - Technical Physics. 3, 1380 (1958).
[2] A. D. Hilton, D. J. Barber, C. A. Randall, and T. R. Shrout, Journal of Materials Science, 25, 3461(1990).
[3] M.T. Anderson, K.B. Greenwood, G.A. Taylor, K.R. Poeppelmeier, Prog. Solid State Chem. 22 (1993) 197.
[4] M.T. Anderson, K.R. Poeppelmeier, Chem. Mater. 3 (1991) 476.
[5] M. Haruta, H. Kurata, H. Komatsu, Y. Shimakawa, S. Isoda, Effects of electron channeling in HAADF-STEM intensity in La2CuSnO6, Ultramicroscopy 109 (2009) 361–367.



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