Deviation from Vegard's law: changes in the c-axis parameter in la_2-xsr_xcuO_4-d in relation to the insulator-superconductor-metal transition

Abstract

The deviation from Vegard's law for the changes in the c parameter of the solid solutions La_2-xA'_xCuO_4-d (A=Ba,Sr,Ca) is examined. When d=0, the changes in the c parameter are controlled in a linear manner (Vegard's law) by two factors: the size effects associated with the difference in ionic radius between the A ion and La cations, as well as the number of holes created. These two effects are isolated by fixing the number of holes and varying the average size of the A' cations. Surprisingly, there is a linear dependence of the c-parameter expansion on the hole concentration for x≤0.15. The significance of these results in terms of the insulator-superconductor-metal transition La_2-xSr_xCuO₄, 0<x<0.3 is briefly discussed.