RAMAN-STUDY OF T'-PHASE DISTORTION IN R2CUO4 COMPOUNDS (R = ND,SM,EU,GD)

We present a Raman study of the appearance of structural distortions i n T'-phase cuprates R2-xCexCuO4 (R=Sm,Eu,Gd) upon R atomic number incr ease. Besides some anomalies in the frequencies of the allowed modes, the distortion from the T' phase is evidenced in Gd2CuO4 by the appear ance of an additional mode with B1g symmetry, B1g, that we assign to the motion of the in-plane oxygen atoms, O(1). This mode, which appear s incipiently in Eu2CuO4 and SmGdCuO4, is formally forbidden by symmet ry, and would be activated by O(1) displacement resulting in a distort ed CuO2 plane. This interpretation is in agreement with previous exper imental results either of structural or magnetic type. The appearance of weak ferromagnetism for R = Gd and beyond, for instance, is incompa tible with perfect T'-phase structure. We compare the results in pure compounds with those that are Ce doped and show that the B1g activity decreases with respect to that of the B1g mode as increasing amounts of Ce are introduced in the lattice; this is interpreted as a conseque nce of T'-phase stabilization upon doping. Temperature-dependence spec tra are also presented and discussed.