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Abstract
The composition dependence of the optical phonon modes has been investigated in ABi2B2O9 (A = Ba, Ca, Sr; B = Nb, Ta) and Bi3BTiO9 (B =Nb, Ta). The lowest under-damped modes at 293 K generally soften upon heating, except BaBi2M2O9 (M = Nb, Ta) of which A sites of pseudo-perovskite layers are occupied by large Ba ions. The substitution of A sites causes marked changes of spectra, while that of B sites influences lightly.