Isomorphic A‐Site Substitution on Sillenite‐Type Compounds

Investigations of A‐site substitution in sillenite‐type compounds showed that only a substituent ion with a lone‐electron pair can be successfully incorporated onto the A‐site in place of Bi ³⁺ . This is because the stability of sillenite structural polyhedra is conditioned by the presence of a stereochemically active lone‐electron pair. Accordingly, the substitutional experiments combined with an X‐ray diffraction analysis, energy‐ and wavelength‐dispersive microanalysis, and Rietveld refinement showed that Nd ³⁺ and La ³⁺ ions, which do not possess a lone‐electron pair, cannot be incorporated into the sillenite‐type structure above the detection limits of the applied analytical techniques. In contrast, the A‐site substitution of Pb ²⁺ , Tl ⁺ , and Sb ³⁺ ions, which do possess a lone‐electron pair, was experimentally confirmed. Te ⁴⁺ also possesses a lone‐electron pair, but the substitutional experiments showed that this ion did not incorporate into the sillenite structure.