Infrared activity of KAl(MoO4)(2) and NaAl(MoO4)(2)

We have investigated the infrared activity of phonons in monoclinic, pseudo -trigonal NaAl(MoO4)(2) performing polarized reflectivity in the range 1200 -700 cm(-1) and IR absorption measurements of polycrystalline samples in th e 1200-30 cm(-1) region. To identify all modes predicted by group theory ar guments, we compare the recorded spectra with similar measurements performe d on trigonal KAl(MoO4)(2). The obtained results allow us to propose the as signment of all observed bands to the respective vibrational modes. Our res ults suggest that the ambient temperature structure of NaAl(MoO4)(2) may be described by the space group C2/c. (C) 1999 Academic Press.