Raman spectra of Pb 1–3x Bi 2x MoO 4 ( x ⩽ 0.06) and their 18 O‐substituted systems

A new band at 896 cm −1 is ascribed to a local mode, v 1 , due to a molybdate ion occupying a cation vacancy. Local modes for v 3 and v 4 were also found at ca. 800 and 397 cm −1 , respectively. The relative intensity for translational modes at 71 and 59 cm −1 varied considerably with x. This was explained in terms of a decrease in primitive cells with two Pb 2+ ions. The band decomposition was carried out for the v 1 region in 18 O‐substituted systems. The equilibrium distribution for isotopic species, [Mo 16 O 4‐n 18 O n ] 2− ( n =0‐4). was found approximately in the system with x = 0.01 after annealing, whereas the discrepancy between the observed and equilibrium distributions increased with increase in x. These observations show that a molybdate ion interacting strongly with a Bi 3+ ion and a cation vacancy is inactive for 18 O‐substitution.