EFFECT OF PRESSURE AND QUENCHING ON SUPERCONDUCTIVE LA2CUO4+DELTA (0-LESS-THAN-DELTA-LESS-THAN-0.1)

Samples of La2CuO4+8 in the range 0 < delta < 0.1 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 alpha were used to monitor as a function of the compositional parameter delta not only the evolution of the superconductive transition temperature T(c), but also the diffusion-controlled spinodal phase segregation occurring in the temperature range 200 < T less-than-or-equal-to 300 K. The data also show that the delta interstitial oxygen atoms O(i) per formula unit enter as O(i)2- ions at least in the range 0 < delta less-than-or-equal-to 0.070. Comparison of the transport properties of samples slow-cooled from room temperature to 77 K reveals an O(i)-atom segregation over the range 0 < delta less-than-or-equal-to 0.066; this segregation disappears for delta > delta0 where the O(i) atoms become ordered, and 0.066 < delta0 < 0.070 was determined. Analysis of neutron-diffraction data from available literature gives a spinodal decomposition temperature T(s) vs delta that falls to 200 K at delta(s)(max) almost-equal-to 0.05; the diffusion-controlled spinodal phase segregation occurs within the phase field' of the O(i)-disordered orthorhombic structure. The oxygen-rich phase having delta(s)(max) almost-equal-to 0.05 is superconductive with a zero-resistance temperature T0 = 26 K that is lower than the T0 = 32 K observed for delta > delta0; T0 falls to a minimum near delta = 0.0625 within the compositional range delta(s)(max) < delta < delta0 which suggests the presence of a competitive nonsuperconductive line phase near p = 1/8 holes per Cu atom as is found in the La2-xBaCuO4 system near x = 1/8. A nonsuperconductive transition at T(d) = 47 +/- 1 K was found in slow-cooled samples over the range 0 < delta < delta0; 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-O(i) phase found for delta > delta0, a ''tail'' in the R-vs-T curve extends into the superconductive temperature range where alpha = 0; it is attributed to a liquid-vortex state rather than a second superconductive phase. The pressure dependences of T(c) and the O(i)-atom mobility were used to distinguish the superconductive phases found for delta < delta0 and delta > delta0. The appearance of a nonsuperconductive line phase in the interval delta(s)(max) < delta < delta0 permits reconciliation of the data, including indirect evidence for charge fluctuations stabilized below a T(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 La2-xSrCuO4.