An IR and Raman study of trigonal triple molybdates M 5 I M 0.5 II Hf1.5(MoO4)6 (MI = K, Tl; MII = Ca, Sr, Ba, Pb)

The vibration spectra of triple molybdates M ₅ ^I M _0.5 ^II Hf_1.5(MoO₄)₆ (M^I = K, Tl; M^II = Ca, Sr, Ba, Pb) were examined. The internal vibration frequencies of molybdate groups were assigned. The double-charged cations affect the symmetric stretching (v₁), bending, and translation vibration frequencies in the Raman spectra. In the IR spectra, the triply degenerate stretching vibration of the MoO₄ group (～890–720 cm^?1) is split, and the splitting increases with an increase in the radius of the double-charged cation.