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The mixed phase La 0.5 R 0.5 Ba 2 Cu 3 O y ͑where R is yttrium or another rare earth͒ 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 Cu͑2͒ atoms remain practically unaffected, while in some compounds a new mode appears at ϳ126 cm Ϫ1 . The in-phase vibrations of the plane oxygen atoms show a shift to a lower frequency compared with the RBa 2 Cu 3 O y samples, similar to the one observed in the overdoped YBa 2 Cu 3 O y (yу6.92) system. Besides, the width of this phonon is considerably larger than in the YBa 2 Cu 3 O y 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 Pr 1Ϫx R x Ba 2 Cu 3 O y compounds prove that the phase formation mechanism is not a pure ion-size effect.

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