Phase separation and internal strains in the mixedLa0.5R0.5Ba2Cu3Oycompounds(R=rare−earthelement)

Abstract

The mixed phase ${\mathrm{La}}_{0.5}{R}_{0.5}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{y}$ (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 $\mathrm{La}\ensuremath{-}R$ 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 \ensuremath{\sim}126 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$. The in-phase vibrations of the plane oxygen atoms show a shift to a lower frequency compared with the $R{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{y}$ samples, similar to the one observed in the overdoped ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{y}(yg~6.92)$ system. Besides, the width of this phonon is considerably larger than in the ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{y}$ 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 ${\mathrm{Pr}}_{1\ensuremath{-}x}{R}_{x}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{y}$ compounds prove that the phase formation mechanism is not a pure ion-size effect.