First-principles study on lattice instabilities and structural phase transitions in Ba doped La2CuO4

• Investigating the lattice instability by obtaining the phonon dispersion in whole Brillouin zone from first-principles.
• Describing the structural phase transitions by doping and pressure, based on soft phonon modes.
• Showing that the “x = 1/8 puzzle” may be answered without invoking strong correlations.

We present linear-response density-functional theory calculations for the high-Tc superconductor La2−xBaxCuO4 to study the doping dependence of phonon dispersion. Using the virtual crystal approximation, the doping range up to x = 0.20 is investigated. We find unstable phonon modes that soften around high-symmetry points of the Brillouin zone. These branches are analyzed as a function of doping and pressure. The structural distortions related to these phonons are in accordance with the observed phase transitions from the high-temperature tetragonal (HTT) phase to the low-temperature orthorhombic (LTO) and the low-temperature tetragonal (LTT) phases. The calculated tilting angles of the CuO6 octahedra corresponding to these phases are in good agreement with experiment. Our findings confirm that strong electron correlations have only minor impact on the structural properties of high-Tc cuprates.