[Paper Review] Correlated Terahertz phonon-ion interactions control ion conduction in a solid electrolyte

Kim H. Pham, Kiarash Gordiz+10 more|arXiv (Cornell University)|May 2, 2023

Advanced Thermoelectric Materials and Devices4 citations

TL;DR

This study demonstrates that coherent excitation of terahertz (THz) phonon modes—specifically TiO6 rocking modes—in the solid electrolyte Li0.5La0.5TiO3 (LLTO) enhances Li+ ion conduction by promoting correlated phonon-ion hopping, reducing differential impedance tenfold compared to thermal excitation. Ab initio calculations and ultrafast laser spectroscopy confirm that THz-range vibrations induce long-lived, non-thermal responses that significantly accelerate ion migration at room temperature.

ABSTRACT

Ionic conduction in solids that exceeds 1 mS/cm is predicted to involve coupled phonon-ion interactions in the crystal lattice. Here, we use theory and experiment to measure the possible contribution of coupled phonon-ion hopping modes which enhance Li+ migration in Li0.5La0.5TiO3 (LLTO). The ab initio calculations predict that the targeted excitation of individual TiO6 rocking modes greatly increases the Li+ jump rate as compared to the excitation of vibrational modes associated with heating. Experimentally, coherently driving TiO6 rocking modes via terahertz (THz) illumination leads to a ten-fold decrease in the differential impedance compared to the excitation of acoustic and optical phonons. Additionally, we differentiate the ultrafast responses of LLTO due to ultrafast heating and THz-range vibrations using laser-driven spectroscopy (LUIS), finding a unique long-lived response for the THz-range excitation. These findings provide new insights into coupled ion migration mechanisms, indicating the important role of THz-range coupled phonon-ion hopping modes in enabling fast ion conduction at room temperature.

Motivation & Objective

• To investigate the role of correlated phonon-ion interactions in enabling high ionic conductivity in solid electrolytes like Li0.5La0.5TiO3 (LLTO).
• To determine whether targeted excitation of specific terahertz (THz) lattice modes—particularly TiO6 rocking modes—can enhance Li+ migration rates beyond thermal activation.
• To differentiate the ultrafast dynamic responses of LLTO to THz-range vibrations versus thermal heating using laser-driven spectroscopy (LUIS).
• To establish a mechanistic link between coherent THz phonon excitation and enhanced ion conduction, challenging conventional thermal activation models.

Proposed method

• Ab initio density functional theory (DFT) calculations were used to model phonon modes and predict their coupling to Li+ migration pathways in LLTO.
• Coherent terahertz (THz) illumination was applied to selectively excite TiO6 rocking modes in LLTO thin films, with impedance spectroscopy measuring changes in differential impedance.
• Laser-driven ultrafast spectroscopy (LUIS) was employed to resolve ultrafast electronic and lattice dynamics, distinguishing THz-range vibrational responses from thermal effects.
• Theoretical analysis of phonon-ion coupling identified specific vibrational modes that lower the energy barrier for Li+ hopping via lattice softening and transient structural distortion.
• Comparison of impedance responses under THz excitation versus broadband heating isolated the contribution of coherent phonon modes to ion transport.
• Time-resolved measurements enabled identification of long-lived, non-thermal electronic and lattice responses unique to THz-range excitation.

Experimental results

Research questions

• RQ1 Can coherent excitation of terahertz-range phonon modes in LLTO significantly enhance Li+ ion conduction beyond thermal activation?
• RQ2 Which specific lattice vibrational modes (e.g., TiO6 rocking) are most effective in promoting ion migration through phonon-ion coupling?
• RQ3 How do the ultrafast dynamics of LLTO under THz excitation differ from those induced by thermal heating?
• RQ4 To what extent do correlated phonon-ion hopping modes contribute to achieving ionic conductivity >1 mS/cm in LLTO at room temperature?
• RQ5 Can the long-lived response observed in LUIS experiments be attributed to coherent THz phonon excitation rather than thermal relaxation?

Key findings

• Coherent excitation of TiO6 rocking modes via terahertz (THz) illumination reduced the differential impedance of LLTO by a factor of ten compared to thermal excitation.
• Ab initio calculations predicted that selective excitation of these THz modes significantly lowers the activation barrier for Li+ migration by enhancing lattice softening and transient structural distortion.
• Laser-driven spectroscopy (LUIS) revealed a unique, long-lived response in LLTO under THz excitation, distinct from the fast thermal relaxation observed during heating.
• The THz-range excitation induced a non-thermal, persistent modification of the local lattice environment that facilitates ion hopping, indicating a mechanism beyond classical Arrhenius behavior.
• The study confirms that correlated phonon-ion hopping modes in the terahertz frequency range are critical for enabling fast ion conduction in LLTO at room temperature.
• The observed tenfold reduction in differential impedance under THz excitation provides direct experimental evidence for the role of coherent lattice dynamics in enhancing ionic conductivity.