Large entropy change derived from orbitally assisted three-centered two-electron $\sigma$ bond formation in metallic ${\mathrm{Li}}_{0.33}{\mathrm{VS}}_2$

We discuss herein the emergence of a large entropy change in metallic ${\mathrm{Li}}_{0.33}{\mathrm{VS}}_2$ derived from orbitally assisted loose $\sigma$ bond formation. Comprehensive structural studies based on synchrotron x-ray and neutron diffraction analyses clarify the fabrication of ribbon chains at 375 K, consisting of multiple three-centered two-electron $\sigma$ bonds based on the viewpoint of local chemical bonding. Although the metallic conductivity persists down to the lowest temperature measured, an exceptionally large entropy change as a metal, as much as $\mathrm{\Delta }S=6.6{\mathrm{J}\mathrm{mol}}^{-1}{\mathrm{K}}^{-1}$, appears at the transition. The emergence of a large entropy change in a metallic state may possibly lead to different functional materials, such as a heat-storage material with a rapid thermal response.

Figure 1

(a) Magnetic susceptibility of ${\mathrm{Li}}_{0.33}{\mathrm{VS}}_2$. Magnetic susceptibility data reported by Murphy et al. are taken from Ref. [27]. The inset shows the Rietveld refinement of neutron diffraction data at 200 K. Although a small impurity peak appears (see the asterisk), magnetic Bragg peaks are absent. (b) Electrical resistivity of ${\mathrm{Li}}_{0.33}{\mathrm{VS}}_2$. Because a large change in volume occurs at the transition at 375 K, cracks are generated in the sintered body, and the electrical resistivity as a function of temperature upon cooling does not match that upon heating. Only the cooling process is shown in (b). (c) DSC of ${\mathrm{Li}}_{0.33}{\mathrm{VS}}_2$. The inset shows the differentiated DSC data. No anomaly appears around 294 K.

Figure 2

Synchrotron x-ray diffraction data at (a) 400 K and (b) 370 K. The inset of (a) shows the spectrum around 375 K. The inset of (b) shows the spectrum around 294 K. No significant change appears corresponding to the anomaly observed in the resistivity measurements.

Figure 3

(a), (b) Refined crystal structure of ${\mathrm{Li}}_{0.33}{\mathrm{VS}}_2$. $A_L$, $B_L$, and $C_L$ indicate the V1-V1, V1-V2, and interchain V2-V2 bond lengths, respectively. Ribbon chains are clearly apparent in (b). (c) V-V bond length as a function of temperature.

Figure 4

(a) Fermi surface in the high-temperature phase of ${\mathrm{Li}}_{0.33}{\mathrm{VS}}_2$. Partial density of states in (b) high- and (c) low-temperature regions. (d) Three molecular orbitals derived accompanied by the $3c\text{−}2e$ formation. Schematic view of band structures originating from $d_{xy}$, $d_{yz}$, and $d_{zx}$ orbitals at (e) high and (f) low temperatures.