Cation effects in doped La2CuO4 superconductors

The critical temperatures of (Ln(1-x)M(x))(2)CuO4 superconductors(1), in which Ln(3+) (La and other lanthanides) and M2+ (Ca, Sr, Ba) cations are randomly distributed amongst the 'type A' lattice sites, are known to depend on the doping level, x, and the mean A-site cation radius, [r(A)] (refs 2, 3). Here we show, by studying series of compositions with the same doping level and [r(A)], that the critical temperature decreases linearly with increasing A-site disorder, as quantified by the variance in the distribution of A-site cation radii. From this, we are able to show that, in the absence of disorder, the critical temperature should increase quadratically with [r(A)] for superconductors containing a single CuO2 layer. Our results therefore show that the critical temperature is very sensitive to lattice strains, as has also been shown for the metal to insulator transition temperature in the magnetoresistive (Ln(1-x)M(x))MnO3, perovskites(4).