High resolution synchrotron x-ray powder diffraction combined with micro-Raman spectroscopy are used to investigate the effect of Pr substitution for Y in optimally doped or overdoped polycrystalline YBa2Cu3Oy (Y123) compounds. The spectral analysis of the Raman-active B1g-symmetry mode indicates a phase separation into coexisting nanoscopic environments consisting of pure Y123 and Pr123 and a mixed (Y–Pr)123 phase of almost the nominal amount of Pr. The Y123 phase disappears at x≈0.6 where superconductivity is suppressed, while the formation of the pure Pr123 phase is correlated with the increase of local lattice distortions at the Cu and Ba sites, the presence of crystal defects and the increase of microstrains, as obtained by analyzing the anisotropic XRD peak broadening. The comparison of the Ba Ag-symmetry phonon shift for the Y1−xPrxBa2Cu3Oy and YBa2−zPrzCu3Oy compounds as well as lattice dynamic calculations proves that, when Pr substitutes for Y, it also occupies an amount of Ba sites. The data from Pr, La or Ca substitution for Y indicate that loss of superconductivity is correlated with the substitution of Pr, La for Ba and Ca for Y, though the underlying effects may not be the hole filling by these occupancies.

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Combined synchrotron angle-dispersive powder diffraction and micro-Raman spectroscopy are used to investigate pressure-induced lattice instabilities that are accompanied by superconducting Tc anomalies in YBa2Cu4O8 and optimally doped YBa2Cu3O 7−δ , in comparison with the nonsuperconducting PrBa2Cu3O6.92. In the first two superconducting systems there is a clear anomaly and hysteresis in the evolution of the lattice parameters and increasing lattice disorder with pressure, which starts at ≈ 3.7 GPa. On the contrary, in the Pr-compound the lattice parameters follow very well the expected equation of state (EOS) up to 7 GPa. The micro-Raman data of the superconducting compounds show that the energy and width of the Ag phonons exhibit anomalies over the same pressure range where the lattice parameters deviate from the EOS and the average Cu2-O pl bond length exhibits a strong contraction that correlates with the non-linear pressure dependence of Tc. The anomalous Raman behavior is not observed for the non superconducting Pr compound, clearly indicating a connection with the charge carriers. It appears that the cuprates close to optimal doping are at the edge of lattice instability.PACS numbers: 61. 05.cp, 74.25.Kc, IntroductionIt is well accepted that structural and electronic inhomogeneities constitute intrinsic properties of cuprate superconductors [1][2][3]. To this context the study of any lattice distortions induced by application of either internal chemical or external hydrostatic pressure [4][5][6] that modify the transition temperature (T c ) is important for understanding the role of lattice effects in the high T c superconductivity. Lattice instabilities in hydrostically compressed YBa 2 Cu 3 O y (Y123) and YBa 2 Cu 4 O 8 (Y124) cuprates where T c dependence on pressure shows saturation or non-linear behavior [7,8], manifest themselves in the Raman phonon frequencies as a deviation from an expected linear behavior at a critical pressure range 2.5-6 GPa [9,10]. Recent structural investigations, using synchrotron angle-dispersive powder diffraction and dense sampling on optimally doped Y123 superconductor, have revealed in the pressure range 3.7 GPa