Phase separation in fully oxygenated ${\mathrm{Y}}_{1-y}{\mathrm{Ca}}_y{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_x$ compounds

A micro-Raman spectroscopic study has been carried out on fully oxygenated ${\mathrm{Y}}_{1-y}{\mathrm{Ca}}_y{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_x$ (YBCO) compounds with $y=0.0–0.2$ and oxygen concentration varying with Ca in the range ($x\sim 6.960$ and 6.995). The spectral profiles of the $4A_g$ and $1B_{1_g}$-like symmetry phonons show minor modifications with Ca, but two modes that appear close in energy to the in-phase and the out-of-phase phonons gain intensity with the Ca substitution. The changes in the intensity of these two satellite modes with the amount of Ca are correlated with modifications in the dimpling of the $\mathrm{Cu}{\mathrm{O}}_2$ planes, as detected by extended x-ray absorption fine structure, and can be attributed to the formation of superstructures and to phase separation phenomena. The evolution of two bands in the in-phase phonon, associated in pure YBCO with local structural distortions and the coexistence of optimal and oxygen overdoped phases, remains completely independent of the Ca concentration. It appears that the Ca atoms have the tendency to be surrounded by Y atoms forming an independent nanophase of the type ${\mathrm{Y}}_{0.5}{\mathrm{Ca}}_{0.5}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_x$ with the additional carriers from Ca not contributing to an excess doping of the system.

Figure 1

Typical spectra from separate microcrystallites of ${\mathrm{Y}}_{1-y}{\mathrm{Ca}}_y{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_x$ compounds for selected Ca concentrations in the $y\left(zz\right)\overline{y}$ scattering geometry. A new mode appears at $402{\mathrm{cm}}^{-1}$.

Figure 2

The corresponding spectra of the same microcrystallite of each selected Ca concentration of Fig. 1 in the $y\left(xx\right)\overline{y}$ scattering geometry. A strong mode appears at $322{\mathrm{cm}}^{-1}$ with increasing intensity with Ca.

Figure 3

Comparison of the spectra from $\mathrm{Y}{\mathrm{Ba}}_2{\mathrm{Cu}}_3$ (Refs. 16, 18) ${\mathrm{O}}_{6.89}$ obtained in the $y\left(xz\right)\overline{y}$ scattering polarization with those of ${\mathrm{Y}}_{0.83}{\mathrm{Ca}}_{0.17}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6.954}$ in the $y\left(xz\right)\overline{y}$, $y\left(xx\right)\overline{y}$, and $z\left(xx\right)\overline{z}$ polarizations.

Figure 4

The dependence of the energy and width (measured in ${\mathrm{cm}}^{-1}$) of the Ba and the Cu(2) phonons on the oxygen concentration of $\mathrm{Y}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_x$ (left side), and on the Ca content of ${\mathrm{Y}}_{1-y}{\mathrm{Ca}}_y{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_x$ (right side).

Figure 5

The dependence of the relative intensity of the Cu(2) and Ba $A_g$-symmetry phonons on the oxygen concentration of $\mathrm{Y}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_x$ (left side), and on the Ca content of ${\mathrm{Y}}_{1-x}{\mathrm{Ca}}_x{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_y$ (right side) for the two scattering polarizations. In the $xx$ polarization the Ba line disappears completely for $x=6.3$, i.e., at the orthorhombic-to-tetragonal transition.

Figure 6

The dependence of the in-phase phonon energy and width (measured in ${\mathrm{cm}}^{-1}$) on the oxygen content of $\mathrm{Y}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_x$ (left side) and the Ca doping of ${\mathrm{Y}}_{1-y}{\mathrm{Ca}}_y{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_x$ (right side).

Figure 7

The dependence of the apex phonon energy and width (measured in ${\mathrm{cm}}^{-1}$) on the oxygen content of $\mathrm{Y}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_x$ (left side), and the Ca doping of ${\mathrm{Y}}_{1-x}{\mathrm{Ca}}_x{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_y$ (right side). Inset: dependence of the apex phonon energy on the $\mathrm{Cu}\left(2\right)\text{−}{\mathrm{O}}_{\text{apex}}$ interatomic distance.

Figure 8

Typical spectra of ${\mathrm{Y}}_{0.83}{\mathrm{Ca}}_{0.17}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ at various scattering geometries to prove the $B_{1g}$-like symmetry character of the $322{\mathrm{cm}}^{-1}$ mode.

Figure 9

Typical $y\left(xx\right)\overline{y}$ spectra at RT and high pressures for the ${\mathrm{Y}}_{0.83}{\mathrm{Ca}}_{0.17}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ compound. It is clear that the $322{\mathrm{cm}}^{-1}$ mode tends to merge with the $B_{1g}$-like phonon at high pressures.

Figure 10

Dependence of the relative intensity of the $322{\mathrm{cm}}^{-1}$ mode with respect to the $B_{1g}$-like band and the $402{\mathrm{cm}}^{-1}$ with respect the in-phase phonon on the Ca concentration. The $\mathrm{Cu}{\mathrm{O}}_2$ plane dimpling (measured by EXAFS) is also shown for comparison.

Figure 11

Comparison of the spectra from $\mathrm{Y}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6.220}$, $\mathrm{Y}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6.438}$, and ${\mathrm{Y}}_{0.83}{\mathrm{Ca}}_{0.17}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6.954}$ obtained in the $y\left(zz\right)\overline{y}$ polarization. Inset: the relative intensity of the $402{\mathrm{cm}}^{-1}$ mode to the apex phonon for $\mathrm{Y}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_x$ and ${\mathrm{Y}}_{1-y}{\mathrm{Ca}}_y{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_x$.

Figure 12

Quantities versus Ca content. Top panel: $\mathrm{Cu}{\mathrm{O}}_2$ plane dimpling as measured by neutron scattering (Ref. 8) and EXAFS (Ref. 10). The straight line is the expected reduction of the dimpling due to the reduced charge of ${\mathrm{Ca}}^{+2}$ substituting for ${\mathrm{Y}}^{+3}$ (see text). Middle panel: Oxygen content—different symbols correspond to different sets of measurements. Bottom panel: relative intensity of the Raman $428{\mathrm{cm}}^{-1}$ line with respect to the total in-phase band. Note the scaling with the oxygen content and not with Ca or the amount of dimpling.

Figure 13

The dependence of the relative intensity of the in-phase to the apex phonons on the oxygen concentration of $\mathrm{Y}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_x$ (left side), and on the Ca content of ${\mathrm{Y}}_{1-y}{\mathrm{Ca}}_y{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_x$ (right side). Straight lines are best linear fits.

Figure 14

Comparison of the dependence of several interatomic distances on the amount of Ca (Ref. 8) or La (Ref. 15) substitution for Y. It is seen that in the case of Ca substitution, only the distance between the Cu to the oxygen of the chains is modified, affecting also the $b$ axis (not presented).