Structural origins of the low-temperature orthorhombic-to-tetragonal phase transition in high-Tc cuprates.*

The La(2-x)Ba(x)CuO4 (LBCO) high-temperature superconductor exhibits a structural phase transition from a low-temperature orthorhombic (LTO) phase to a low-temperature tetragonal (LTT) phase near x = 1/8 doping, which is correlated with a suppression of superconductivity and the concomitant formation of charge density waves. Here we study La2MgO4, which is a structural analog of LBCO. Since La2MgO4 does not possess the strong electronic correlations that give rise to, e.g., the superconductivity observed in the cuprates, we use it to help disentangle structural and electronic mechanisms of the LTO-LTT phase transition.

 

Using density-functional theory (DFT), we explore the energy landscape associated with the order parameters of the phase transition. We find that LTT becomes energetically more favourable than LTO above a critical magnitude of the order parameter. Our synthesis and characterisation of La2MgO4 using high-resolution diffraction confirms that this system also exhibits the LTO-LTT phase transition observed in LBCO. The critical octahedral tilt angle at which this phase transition is observed is in excellent agreement with the DFT calculations. Our work provides valuable insight into the origins of the complex structural behaviour observed for this family of compounds.