SYNCHROTRON X-RAY AND MOLECULAR DYNAMICS STUDIES OF

La_2-xSr_xCuO₄ WITH x≈1/8

Nobuo Ishizawa, Kenji Suzuki, Katsumi Suda, and Douglas du Boulay

Materials and Structures Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta Midori, Yokohama 226-8503 Japan (nishizaw@n.cc.titech.ac.jp)

Although the tetragonal-orthorhombic structural phase transition in the La_2–xSr_xCuO₄ high Tc super conductors has been studied and discussed extensively since their discovery, there is still some mystery about the role played by the Sr dopant in the phase transition mechanism. In our study, the electron density distribution of La_2–xSr_xCuO₄ with x≈1/8 determined at 297K using single-crystal synchrotron X-ray diffraction revealed not only the d_x2-y2 orbital hole of Cu but also the disordered nature of the La(Sr) and O constituents, even though the crystal exhibits the archetypal K₂NiF₄ structure with tetragonal I4/mmm symmetry.

Molecular dynamics (MD) calculations were applied to examine the structural disorder in more detail. By averaging related atomic positions across the MD unit cell and across all MD time steps, calculated at 2fs intervals, the Sr and La atoms were found to vibrate harmonically about slightly displaced mean atomic coordinates, with Sr displaced by 0.02Å along the c axis with respect to La. In addition, four in-plane O2 atoms localized around Sr were typically displaced by a mean of 0.15Å further along <110> vectors, and the closest apical O2 atom by 0.08Å along <001>, in order to accommodate larger radii Sr cations, as shown in the figure. The directions of these O atom displacements are incompatible with those that facilitate the collective tilting mechanism of the CuO1₄O2₂ octahedra in the low temperature orthorhombic form. The local steric hindrance identified around Sr probably impedes the cooperative deformation of the octahedral sheet and helps to explain the experimentally observed decrease in tetragonal-orthorhombic phase transition temperature with increasing Sr content in La_2-xSr_xCuO₄.

Analysis of the mean La atom positions in the MD study indicates that their equilibrium positions are slightly displaced along <110> from their ideal positions on the c axis, indicating that even in the tetragonal archetypal phase they favour a locally orthorhombic distortion.