Infrared absorption spectra of double anhydrous sulfates of lithium and rare earths

Infrared absorption spectra of double sulfates of LiLn(SO4)2 (Ln=La,Pr,Nd,Eu,Tm,Yb,Lu) and of some isotope-exchanged (6Li) analogs have been studied. In the series of double sulfates of lanthanides and lithium three structural types are realized. The first group comprises salts of La and Pr, the second - salts of Nd and Eu, and the third group consists of the salts of Tm,Yb, and Lu. Infrared spectra of the lanthanide compounds of the beginning of the series have the more complex nature and the vibrational frequency in the region of 900-1400 cm-1 is considerably higher than that of the lanthanide double sulfates in the middle and at the end of the series. The assignment of the frequencies to vibrations of anion and cation sublattices of the crystal has been performed. The results of the group-theoretical analysis of vibrations confirm the data of X-ray structural investigations