Phase separation and internal strains in the mixed La0.5R0.5Ba2Cu3Oy compounds (R = rare-earth element)

The mixed phase La0.5R0.5Ba2Cu3Oy (where R is yttrium or another rare earth) has been prepared using a variation of the solid-state reaction technique. X-ray diffraction and Raman measurements have been carried out to study the effect of the mixed rare-earth substitution at the site of the Y atom. The x-ray-diffraction measurements show characteristic changes in the interatomic distances, which are indicative of strains in the unit cell. A strain-relaxation mechanism is proposed, attributed to the separation of phases. In the micro-Raman spectra, an increase of the A(g) mode frequency of the apex oxygen with increasing average La-R ionic radius is observed, the mode frequencies corresponding to the Ba and the Cu(2) atoms remain practically unaffected, while in some compounds a new mode appears at similar to 126 cm(-1). The in-phase vibrations of the plane oxygen atoms show a shift to a lower frequency compared with the RBa2Cu3Oy samples, similar to the one observed in the overdoped YBa2Cu3Oy (y greater than or equal to 6.92) system. Besides, the width of this phonon is considerably larger than lin the YBa2Cu3Oy compounds, attributable to the existence of phases with underdoped, optimally doped, and overdoped oxygen concentration. As concerns the changes induced in the B-1g Raman active mode of the out-of-phase vibrations of the plane oxygen atoms, they are indicative of phases rich in either La, R, or an intermediate phase. Differences observed from the Pr1-xRxBa2Cu3Oy compounds prove that the phase formation mechanism is not a pure ion-size effect. [S0163-1829(98)03635-5].