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The Microscopic Nature of Orbital Disorder in LaMnO$_{3}$

Bodoo Batnaran, Andrew L. Goodwin, Michael A. Hayward, Volker L. Deringer

Abstract

We present a revised atomistic picture of the order-disorder transition in the archetypal orbital-ordered perovskite material, LaMnO$_{3}$. Our study uses machine-learning-driven molecular-dynamics simulations which describe the temperature evolution of pair distribution functions in close agreement with experiment. We find the orbital-disordered phase in LaMnO$_{3}$ to comprise a mixture of differing structural distortions with and without inversion symmetry, implying a mixture of different orbital arrangements. These distortions are highly dynamic with an estimated lifetime of $\sim 40$ fs at 1,000 K, and their fluctuations converge with the timescales of conventional thermal motion in the high-$T$ phase - indicating that the electronic instability responsible for static Jahn-Teller distortions at low temperature instead drives phonon anharmonicity at high temperatures. Beyond LaMnO$_{3}$, our work opens an avenue for studying a wider range of correlated materials.

The Microscopic Nature of Orbital Disorder in LaMnO$_{3}$

Abstract

We present a revised atomistic picture of the order-disorder transition in the archetypal orbital-ordered perovskite material, LaMnO. Our study uses machine-learning-driven molecular-dynamics simulations which describe the temperature evolution of pair distribution functions in close agreement with experiment. We find the orbital-disordered phase in LaMnO to comprise a mixture of differing structural distortions with and without inversion symmetry, implying a mixture of different orbital arrangements. These distortions are highly dynamic with an estimated lifetime of fs at 1,000 K, and their fluctuations converge with the timescales of conventional thermal motion in the high- phase - indicating that the electronic instability responsible for static Jahn-Teller distortions at low temperature instead drives phonon anharmonicity at high temperatures. Beyond LaMnO, our work opens an avenue for studying a wider range of correlated materials.

Paper Structure

This paper contains 4 figures.

Figures (4)

  • Figure 1: Change in average structure of LaMnO3 with temperature. Experimental (red) and simulated (black) data are shown. (a) Reduced lattice parameters. (b) Change in tilting angle, $\varphi$, of octahedra along the (111) axis. (c) Mn--O bond lengths showing two long, two medium, and two short bonds and their evolution. Experimental data taken from Ref. Thygesen2017a.
  • Figure 2: Pair distribution functions for LaMnO3 and their evolution with temperature. Experimental (panel a) and computed (panel b) neutron (top) and X-ray (bottom) PDFs for the temperature range 500 to 1000 K are shown. Baselines are shown using dotted lines; features of particular interest are marked with arrows. As in Ref. Thygesen2017a, O' (orange) denotes the orbital-ordered low-$T$ and O (green) denotes the orbital-disordered high-$T$ phase. Panel (a) adapted from Ref. Thygesen2017a.
  • Figure 3: Evolution of long Mn--O bond orientations with temperature. We plot the percentage of parallel long bonds aligning with the $x$- or $y$-axis (green) and $z$-axis (cyan) as well as L-type (magenta). Slices in the $xy$-plane highlighting the two longest bonds are shown at relevant temperatures Stukowski2009.
  • Figure 4: Evolution of octahedral distortions in a 13,720-atom supercell with temperature and time. (a) Heatmaps with contour lines of the octahedral distortion mode components, $Q_2$ and $Q_3$, at 400 K, 500 K and 1,000 K. (b) Slices along the $xy$-plane of the supercells Stukowski2009. The amplitude of the $Q_2$ component for each octahedra and Mn--O bonds longer than 2.05 Å are colored. (c) Change in the amplitude of the $Q_2$ component with simulation time for 20 randomly selected octahedra, ordered by the initial amplitude, at each temperature.