Ac impedance of A₄C₆₀fullerides under pressure

Abstract

Three A 4 C 60 compounds, with A = Li, Na and K, have been studied by impedance spectroscopy between 100 and 293 K at pressures up to 2 GPa. The results are in very good agreement with earlier dc resistance studies and with data from the literature. For all three materials the measured conductivity can be fitted by a sum of at least two Arrhenius terms. The band gaps derived from the resistance data, 0.3 eV for Na 4 C 60 and 0.5 eV for K 4 C 60 , are in excellent agreement with data measured by other methods. For Li 4 C 60 , our results disagree with a recent suggestion that the conductivity is dominated by ionic conduction. Although a certain ionic component probably exists we suggest that electronic transport dominates in our samples at and below room temperature because the derived 'activation energy' decreases under pressure, the derived 'activation volume' is negative, and we observe neither a significant electrode blocking capacitance nor any significant metal transport under dc conditions. changes in conductivity magnitude might be due to changes in the sample geometry there remained possibilities that the results were modified by deintercalation due to heating, ionic electrotransport, or both.We present here new data for the resistivity of A 4 C 60 under pressure for A = Li, Na and K, measured by ac impedance spectroscopy. Using an ac method in a mechanically very stable experimental set-up proved to yield very stable and repeatable results, and the derived transport parameters (excitation energies and/or band gaps) were very similar to those found earlier. This confirms that the large changes in magnitude observed earlier were experimental artifacts caused mainly by deformation of the samples. To investigate ionic transport in Li 4 C 60 further we also carried out a simple dc experiment, driving a small dc current through a sample sandwiched between lithium electrodes. After 50 h, no change was detected in the relative weights of the electrodes, showing that most of the current was carried by electrons rather than Li + ions.

Electronic and ionic conductivities in superionicLi4C60

“…In our view 12 , above the 125 K onset of Li + ionic diffusion to about ambient temperatures the DC conductivity arises mainly from the ionic conductivity, σ ion involving all Li ions. (Sundqvist et al 18 raised, however some doubts about this). On the other hand, we suggest that the microwave conductivity is the electronic conductivity σ el at 10 GHz associated with a small concentration of charged defects.…”

“…14]. The electronic properties under pressure and calorimetric measurements of Li, Na and K doped fullerides were carried out in detail by the group of Sundqvist et al [15][16][17][18][19] . The ionic conductivity depends also on other factors than the barrier between sites; counter-intuitively, the DC conductivity of Li 4 C 60 polymer increases under pressure.…”

Electronic and ionic conductivities in superionic Li$_4$C$_{60}$

“…In our view 12 , above the 125 K onset of Li + ionic diffusion to about ambient temperatures the DC conductivity arises mainly from the ionic conductivity, σ ion involving all Li ions. (Sundqvist et al 18 raised, however some doubts about this). On the other hand, we suggest that the microwave conductivity is the electronic conductivity σ el at 10 GHz associated with a small concentration of charged defects.…”

A First-Principles Investigation of Lithium and Sodium Ion Diffusion in C60 Molecular Solids

“…In the solid-state, C 60 molecules form a face centered cubic (FCC) molecular crystal, and solid-state C 60 finds use as components in photovoltaics, supercapacitors, photomasks, and lubricants . When intercalated with alkali and alkali earth ions, solid state C 60 is known to exhibit composition-dependent superconductivity, tunable optical properties, room temperature high ZT thermoelectricity, and notably, superionic conductivity. − However, the high ionic conductivity of lithium-doped crystalline C 60 is a point of contention and pertinent investigations have shown that electronic and mixed electronic–ionic contributions need to be considered as well. , Thus, despite past valuable experimental and simulation investigations, fundamental questions still remain on the extent of ionic conductivity and the underlying diffusive pathways that dictate ionic motion in C 60 solids . Unraveling such questions can shed further light on the utility of C 60 structures as potential components of alkali ion batteries, especially since there have been past efforts in integrating them both as electrode materials as well as interphase materials for ensuring the interfacial integrity of electrodes in lithium-ion batteries (LIB).…”