A SIMPLE APPROACH TO LATTICE EFFECTS IN CONDUCTING PEROVSKITE-TYPE OXIDES

A method for parametrizing the A cation size disparity or mismatch in ATO(3) perovskites through the A cation size variance has recently bee n proposed and experimental studies are reviewed. (A is a mixture of t rivalent lanthanide and divalent alkaline-earth cations.) The metal-in sulator transition temperature in AMnO(3) perovskites and the critical temperature in A(2)CuO(4) superconductors are both found to decrease linearly with this size variance at constant doping level and average A cation radius. This result enables a pair of quadratic relationships for the mean size and size variance effects to be proposed as the res ult of changing strain energies associated with small structural disto rtions at the electronic transitions. Doping level, mean A cation radi us, and the size variance are thus the three major parameters that con trol electronic properties in oxygen stoichiometric perovskite oxides.