Filming ultrafast roaming-mediated isomerization of bismuth triiodide in solution

Roaming reaction, defined as a reaction yielding products via reorientational motion in the long-range region (3 – 8 Å) of the potential, is a relatively recently proposed reaction pathway and is now regarded as a universal mechanism that can explain the unimolecular dissociation and isomerization of various molecules. The structural movements of the partially dissociated fragments originating from the frustrated bond fission at the onset of roaming, however, have been explored mostly via theoretical simulations and rarely observed experimentally. Here, we report an investigation of the structural dynamics during a roaming-mediated isomerization reaction of bismuth triiodide (BiI3) in acetonitrile solution using femtosecond time-resolved x-ray liquidography. Structural analysis of the data visualizes the atomic movements during the roaming-mediated isomerization process including the opening of the Bi-Ib-Ic angle and the closing of Ia-Bi-Ib-Ic dihedral angle, each by ~40°, as well as the shortening of the Ib···Ic distance, following the frustrated bond fission.

Colossal barocaloric effects in the complex hydride Li$$_{2}$$B$$_{12}$$H$$_{12}$$

Traditional refrigeration technologies based on compression cycles of greenhouse gases pose serious threats to the environment and cannot be downscaled to electronic device dimensions. Solid-state cooling exploits the thermal response of caloric materials to changes in the applied external fields (i.e., magnetic, electric and/or mechanical stress) and represents a promising alternative to current refrigeration methods. However, most of the caloric materials known to date present relatively small adiabatic temperature changes ($$|\Delta T| \sim 1$$ | Δ T | ∼ 1 to 10 K) and/or limiting irreversibility issues resulting from significant phase-transition hysteresis. Here, we predict by using molecular dynamics simulations the existence of colossal barocaloric effects induced by pressure (isothermal entropy changes of $$|\Delta S| \sim 100$$ | Δ S | ∼ 100 J K$$^{-1}$$ - 1 kg$$^{-1}$$ - 1 ) in the energy material Li$$_{2}$$ 2 B$$_{12}$$ 12 H$$_{12}$$ 12 . Specifically, we estimate $$|\Delta S| = 367$$ | Δ S | = 367 J K$$^{-1}$$ - 1 kg$$^{-1}$$ - 1 and $$|\Delta T| = 43$$ | Δ T | = 43 K for a small pressure shift of P = 0.1 GPa at $$T = 480$$ T = 480 K. The disclosed colossal barocaloric effects are originated by a fairly reversible order–disorder phase transformation involving coexistence of Li$$^{+}$$ + diffusion and (BH)$$_{12}^{-2}$$ 12 - 2 reorientational motion at high temperatures.

Colossal Barocaloric Effects in The Complex Hydride Li2B12H12

Traditional refrigeration technologies based on compression cycles of greenhouse gases pose serious threats to the environment and cannot be downscaled to electronic device dimensions. Solid-state cooling exploits the thermal response of caloric materials to external fields and represents a promising alternative to current refrigeration methods. However, most of the caloric materials known to date present relatively small adiabatic temperature changes (|ΔT| ~ 1 K) and/or limiting irreversibility issues resulting from significant phase-transition hysteresis. Here, we predict the existence of colossal barocaloric effects (isothermal entropy changes of |ΔS| ~ 100 JK-1 kg-1) in the energy material Li2B12H12 by means of molecular dynamics simulations. Specifically, we estimate |ΔS| = 367 JK-1 kg-1 and |ΔT| = 23 K for an applied pressure of P = 0.1 GPa at T = 480 K. The disclosed colossal barocaloric effects are originated by an fairly reversible order-disorder phase transformation involving coexistence of Li+ diffusion and (BH)12-2 reorientational motion at high temperatures.

Comparative Molecular Dynamics Study of the Roles of Anion– Cation and Cation–Cation Correlation in Cation Diffusion in Li2B12H12 and LiCB11H12

Complex hydrides are potential candidates for the solid electrolyte of all-solid-state batteries owing to their high ionic conductivities, in which icosahedral anion reorientational motion plays an essential role in high cation diffusion. Herein, we report molecular dynamics (MD) simulations based on a refined force field and first-principles calculations of the two complex hydride systems Li₂B₁₂H₁₂ and LiCB₁₁H₁₂ to investigate their structures, order–disorder phase-transition behavior, anion reorientational motion, and cation conductivities. For both systems, force-field-based MD successfully reproduced the structural and dynamical behavior reported in experiments. Remarkably, it showed an entropy-driven order–disorder phase transition associated with high anion reorientational motion. Furthermore, we obtained comparative insights into the cation around the anion, cation site occupancy in the interstitial space provided by anions, cation diffusion route, role of cation vacancies, anion reorientation, and effect of cation–cation correlation on cation diffusion. We also determined the factors that activate anion reorientational motion leading to a low to high conductivity phase transition. These findings are of fundamental importance in fast ion-conducting solids to diminish the transition temperature for practical applications.<b></b>

Comparative Molecular Dynamics Study of the Roles of Anion– Cation and Cation–Cation Correlation in Cation Diffusion in Li2B12H12 and LiCB11H12

Complex hydrides are potential candidates for the solid electrolyte of all-solid-state batteries owing to their high ionic conductivities, in which icosahedral anion reorientational motion plays an essential role in high cation diffusion. Herein, we report molecular dynamics (MD) simulations based on a refined force field and first-principles calculations of the two complex hydride systems Li₂B₁₂H₁₂ and LiCB₁₁H₁₂ to investigate their structures, order–disorder phase-transition behavior, anion reorientational motion, and cation conductivities. For both systems, force-field-based MD successfully reproduced the structural and dynamical behavior reported in experiments. Remarkably, it showed an entropy-driven order–disorder phase transition associated with high anion reorientational motion. Furthermore, we obtained comparative insights into the cation around the anion, cation site occupancy in the interstitial space provided by anions, cation diffusion route, role of cation vacancies, anion reorientation, and effect of cation–cation correlation on cation diffusion. We also determined the factors that activate anion reorientational motion leading to a low to high conductivity phase transition. These findings are of fundamental importance in fast ion-conducting solids to diminish the transition temperature for practical applications.<b></b>

Anion and Cation Dynamics in Polyhydroborate Salts: NMR Studies

Polyhydroborate salts represent the important class of energy materials attracting significant recent attention. Some of these salts exhibit promising hydrogen storage properties and/or high ionic conductivities favorable for applications as solid electrolytes in batteries. Two basic types of thermally activated atomic jump motion are known to exist in these materials: the reorientational (rotational) motion of complex anions and the translational diffusion of cations or complex anions. The present paper reviews recent progress in nuclear magnetic resonance (NMR) studies of both reorientational and diffusive jump motion in polyhydroborate salts. The emphasis is put on sodium and lithium closo-borates exhibiting high ionic conductivity and on borohydride-based systems showing extremely fast reorientational motion down to low temperatures. For these systems, we discuss the effects of order–disorder phase transitions on the parameters of reorientations and diffusive jumps, as well as the mechanism of low-temperature rotational tunneling.

Ion hydration: linking self-diffusion and reorientational motion to water structure

A link between water dynamics and the “water structure” has been established through the combination of the extended jump model, transition state theory and the Kirkwood-Buff theory.