RELATION BETWEEN STRUCTURE AND DOPING IN LA2-XSRXCUO4+DELTA - A NEUTRON-DIFFRACTION STUDY ON SINGLE-CRYSTALS

We present the results of neutron-diffraction experiments on a series of La2-xSrCuO4+delta single crystals at temperatures between 540 K and 22 K. There is an intrinsic misfit between the size of the LaO9 polyhedra and the CuO6 octahedra, which seems to cause the structural transition from the high-temperature tetragonal (HTT) to the low-temperature orthorhombic (LTO) phase. Within this misfit model it is possible to explain almost the entire phenomenology of the HTT-LTO transition. The electronic structure is related to this phase transition via the length of the Cu-O bonds. One of the effects of the transition on the structure is to lengthen these Cu-O bonds and hence to alter the electronic band structure. The introduction of additional charge by doping leads to a decrease of the equilibrium Cu-O distances, and hence to a reduction of the misfit. In this way the, charge-carrier concentration determines the structural transition. We always observe strongly anisotropic thermal parameters, which confirm the instability of the La2-xSrCuO4 structure against a rotation of the CuO6 octahedra around any axis. In the case of an undoped crystal (x=0.0), we find a good agreement between the experimental mean square displacements and the results of a lattice dynamics model which is based on the known phonon dispersion curves. In a doped (x=0.13) and superconducting crystal, the anharmonic effects related to the structural transition lead to significant changes in the temperature dependence of the thermal parameters.