Zigzag chain order of LiVSe2 developing away from the vanadium trimer phase transition boundary

The phenomenon of self-assembly of constituent elements to form molecules at\nlow temperatures appears ubiquitously in transition metal compounds with\norbital degrees of freedom. Recent progress in local structure studies using\nsynchrotron radiation x-rays is shifting the interest in structural studies in\nsuch molecule-forming systems from the low-temperature ordered phase to the\nshort-range order that appears like a precursor at high temperatures. In this\nstudy, we discuss both experimentally and theoretically the relationship\nbetween the trimer structure that appears in the layered LiV$X_2$ ($X$ = O, S,\nSe) system with a two-dimensional triangular lattice of vanadium and the zigzag\nchain-like local structure that appears near the phase transition boundary\nwhere molecular formation occurs. The vanadium trimerization that persistently\nappears in both low-temperature phases of LiVO$_2$ and LiVS$_2$ disappears in\nLiVSe$_2$, and a regular triangular lattice is thought to be realized in\nLiVSe$_2$, but this study reveals that the zigzag chain local distortion\nappears with a finite correlation length. This zigzag chain state local\ndistortions are similar to the motif of local distortions in the\nhigh-temperature phase of LiVS$_2$, indicating that the local distortions are\npersistent away from the trimer phase transition boundary. On the other hand,\nit is concluded that the zigzag chain order appearing in LiVSe$_2$ is more\nstable than that in LiVS$_2$ in terms of the temperature variation of atomic\ndisplacement and correlation length. The zigzag chain order is considered to be\ncompetitive with the trimer order appearing in the LiV$X_2$ system. In this\npaper, we discuss the similarities and differences between the parameters that\nstabilize these electronic phases and the local distortions that appear in\nother molecular formation systems.\n