Vibrational spectra of 1:2 ordered perovskites

The infrared and Raman spectra of some perovskites with general formula A 3 BB′ 2 O 9 are reported, viz. Sr 3 MgNb 2 O 9 , Sr 3 CaNb 2 O 9 , Pb 3 MgNb 2 O 9 , and Ba 3 Gd 2 WO 9 . The interpretation is by far not as simple as for 1:1 ordered perovskites. Only a rough assignment of the internal modes of the niobate and tungstate octahedra can be made. The spectra of the niobates are very sensitive to the degree of order between the divalent metal ions and the Nb 5+ ions among the smaller cation sublattice. This is evaluated qualitatively.