VIBRATIONAL-SPECTRA OF LN CUPRATES (LN = GD, ND, EU, SM)

IR absorption spectra have been studied for Gd2CuO4 at room temperature and liquid nitrogen temperature and are shown to be different from the spectra of other Ln cuprates (Ln = Nd, Sm and Eu). The observed spectrum of Gd cuprate is not compatible with the common D4h17 structure proposed for these classes of compound. A normal coordinate analysis has been carried out for the unit cell of Gd2CuO4 using T' structure. All the modes have been explained and a set of force constants is derived. The appearance of two of the low frequency bands at 107 and 143 cm-1 for Gd2CuO4 is an anomaly since these are not observed for the cuprates of Nd, Sm and Eu having the same structure. An attempt has been made to explain these by considering a lower symmetry. Normal coordinate analysis has also been carried out for these compounds and the outcome of the calculations has been discussed in detail.