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

IR absorption spectra have been studied for Gd2CuO4 at room temperatur e and liquid nitrogen temperature and are shown to be different from t he spectra of other Ln cuprates (Ln = Nd, Sm and Eu). The observed spe ctrum of Gd cuprate is not compatible with the common D4h17 structure proposed for these classes of compound. A normal coordinate analysis h as been carried out for the unit cell of Gd2CuO4 using T' structure. A ll the modes have been explained and a set of force constants is deriv ed. 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 cupr ates of Nd, Sm and Eu having the same structure. An attempt has been m ade to explain these by considering a lower symmetry. Normal coordinat e analysis has also been carried out for these compounds and the outco me of the calculations has been discussed in detail.