*2.2.2.3 Compositional ordering*

The chemical formula of the solid solutions is Ba6−3*xR*8+2*x*Ti18O54, and the structural formula is [Ba4]*A*2[Ba2−<sup>3</sup>*xR*8+2*x*]*A*1Ti18O54. Here, the amount of Ba in the *A*1-sites becomes zero (2 − 3*x* = 0), that is, *x* = 2/3. This composition is special as the following sentence: the structural formula is [Ba4]*A*2[*R*8+4/3]*A*1Ti18O54 which is occupied separately by Ba in *A*2 and by *R* in *A*1 as shown in **Figure 10(b)**. This special composition is called "compositional ordering" [20, 21, 42].
