Effect of pressure and quenching on superconductive La2CuO4+ delta (0< delta

Abstract

Samples of ${\mathrm{La}}_{2}$${\mathrm{CuO}}_{4+\mathrm{\ensuremath{\delta}}}$ in the range 00.1 were prepared by electrochemical insertion of oxygen in KOH; the oxygen content was determined by iodometric titration. The temperature variations of resistance R and Seeback coefficient \ensuremath{\alpha} were used to monitor as a function of the compositional parameter \ensuremath{\delta} not only the evolution of the superconductive transition temperature ${\mathit{T}}_{\mathit{c}}$, but also the diffusion-controlled spinodal phase segregation occurring in the temperature range 200T\ensuremath{\le}300 K. The data also show that the \ensuremath{\delta} interstitial oxygen atoms ${\mathrm{O}}_{\mathit{i}}$ per formula unit enter as ${\mathrm{O}}_{\mathit{i}}^{2\mathrm{\ensuremath{-}}}$ ions at least in the range 0\ensuremath{\le}0.070. Comparison of the transport properties of samples slow-cooled from room temperature to 77 K reveals an ${\mathrm{O}}_{\mathit{i}}$-atom segregation over the range 0\ensuremath{\le}0.066; this segregation disappears for \ensuremath{\delta}&gt;${\mathrm{\ensuremath{\delta}}}_{\mathit{o}}$ where the ${\mathrm{O}}_{\mathit{i}}$ atoms become ordered, and 0.066${\mathrm{\ensuremath{\delta}}}_{\mathit{o}}$0.070 was determined. Analysis of neutron-diffraction data from available literature gives a spinodal decomposition temperature ${\mathit{T}}_{\mathit{s}}$ vs \ensuremath{\delta} that falls to 200 K at ${\mathrm{\ensuremath{\delta}}}_{\mathit{s}}$(max)\ensuremath{\approxeq}0.05; the diffusion-controlled spinodal phase segregation occurs within the phase field of the ${\mathrm{O}}_{\mathit{i}}$-disordered orthorhombic structure.The oxygen-rich phase having ${\mathrm{\ensuremath{\delta}}}_{\mathit{s}}$(max)\ensuremath{\approxeq}0.05 is superconductive with a zero-resistance temperature ${\mathit{T}}_{0}$=26 K that is lower than the ${\mathit{T}}_{0}$=32 K observed for \ensuremath{\delta}&gt;${\mathrm{\ensuremath{\delta}}}_{\mathit{o}}$; ${\mathit{T}}_{0}$ falls to a minimum near \ensuremath{\delta}=0.0625 within the compositional range ${\mathrm{\ensuremath{\delta}}}_{\mathit{s}}$(max)${\mathrm{\ensuremath{\delta}}}_{\mathit{o}}$, which suggests the presence of a competitive nonsuperconductive line phase near p=1/8 holes per Cu atom as is found in the ${\mathrm{La}}_{2\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ba}}_{\mathit{x}}$${\mathrm{CuO}}_{4}$ system near x=1/8. A nonsuperconductive transition at ${\mathit{T}}_{\mathit{d}}$=47\ifmmode\pm\else\textpm\fi{}1 K was found in slow-cooled samples over the range 0${\mathrm{\ensuremath{\delta}}}_{\mathit{o}}$; it is suppressed by both high pressure and the quenching procedure. We associate it with the competitive nonsuperconductive phase stabilized near p=1/8. In the ordered-${\mathrm{O}}_{\mathit{i}}$ phase found for \ensuremath{\delta}&gt;${\mathrm{\ensuremath{\delta}}}_{\mathit{o}}$, a ``tail'' in the R-vs-T curve extends into the superconductive temperature range where \ensuremath{\alpha}=0; it is attributed to a liquid-vortex state rather than a second superconductive phase. The pressure dependences of ${\mathit{T}}_{\mathit{c}}$ and the ${\mathrm{O}}_{\mathit{i}}$-atom mobility were used to distinguish the superconductive phases found for \ensuremath{\delta}${\mathrm{\ensuremath{\delta}}}_{\mathit{o}}$ and \ensuremath{\delta}&gt;${\mathrm{\ensuremath{\delta}}}_{\mathit{o}}$. The appearance of a nonsuperconductive line phase in the interval ${\mathrm{\ensuremath{\delta}}}_{\mathit{s}}$(max)${\mathrm{\ensuremath{\delta}}}_{0}$ permits reconciliation of the data, including indirect evidence for charge fluctuations stabilized below a ${\mathit{T}}_{\mathrm{\ensuremath{\rho}}}$150 K in this interval. The indirect evidence for charge fluctuations is the same as that discussed by us elsewhere in the framework of a model developed for the system ${\mathrm{La}}_{2\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{CuO}}_{4}$.