On the structure distortions in Nd2 Cu O4

The present paper is concerned with one-phonon IR absorption in Nd2 Cu O4 associated with magnetic ordering which is accompanied by a multiplication of a crystallographic cell. It is shown that magnetic ordering of the << plane cross >> type results in the spin dependent phonon absorption from the Brillouin zone boundary of the paramagnetic phase and affects the phonon spectra similarly to the structure phase transition. The mechanisms of such absorption are considered from the M-point of the Brillouin zone boundary induced by magnetic ordering and from the X-point induced by an applied magnetic field. The measurements performed in a magnetic field exceeding that of the spin-reorientation phase transition made it possible to detect one-phonon absorption from the X-point of the Brillouin zone and the suppression of one-phonon absorption from the M-point and showed the absence of structure distortions in Nd2 Cu O4 at temperatures above 150 K