Infrared Activity of KAI(MoO4)2 and NaAl(MoO4)2

We have investigated the infrared activity of phonons in mono-clinic, pseudo-trigonal NaAl(MoO4)2 performing polarized reflectivity in the range 1200-700 cm-1 and IR absorption measurements of polycrystalline samples in the 1200-30 cm-1 region. To identify all modes predicted by group theory arguments, we compare the recorded spectra with similar measurements performed on trigonal KAl(MoO4)2. The obtained results allow us to propose the assignment of all observed bands to the respective vibrational modes. Our results suggest that the ambient temperature structure of NaAl(MoO4)2 may be described by the space group C2/c.