REEXAMINATION OF THE 2ND-ORDER STRUCTURAL PHASE-TRANSITION IN LA(2-X)A(X)CUO(4) (A = BA,SR)

In the high temperature tetragonal phase of La1.85Ba0.15CuO4 (LBCO) an d La1.875Sr0.125CuO4 (LSCO), the average octahedral tilts disappear at high temperature. However, we show that, locally, tilts remain at tem peratures well above the orthorhombic to tetragonal phase transition. We have used full profile refinement of the real-space atomic pair dis tribution function (PDF) obtained from pulsed neutron powder diffracti on measurements. Our modeling indicates that order-disorder models of the octahedral tilts give significantly better agreement with the data than displacive models. (C) 1998 Published by Elsevier Science B.V. A ll rights reserved.