Vibrational spectra of Ln cuprates (Ln = Gd, Nd, Eu, Sm)

Abstract

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 D¹⁷_4h 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.