Evidence of a Fundamental Mechanism Governing Conductivity

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C: Energy Conversion and Storage; Energy and Charge Transport

Evidence of a Fundamental Mechanism Governing Conductivity Relaxation in Room Temperature Ionic Liquids Ma#gorzata Musia#, Zaneta Wojnarowska, Shinian Cheng, Kia L. Ngai, and Marian Paluch J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b07578 • Publication Date (Web): 16 Aug 2019 Downloaded from pubs.acs.org on August 19, 2019

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The Journal of Physical Chemistry

Evidence of a Fundamental Mechanism Governing Conductivity Relaxation in Room Temperature Ionic Liquid M. Musiał1*, Z. Wojnarowska1, S. Cheng1, K. L. Ngai2, M. Paluch1* 1Institute of Physics, University of Silesia, Silesian Center for Education and Interdisciplinary Research, 75 Pulku Piechoty 1A, 41–500 Chorzow, Poland 2CNR-IPCF, Dipartimento di Fisica, Università di Pisa, Largo Bruno Pontecorvo 3,I-56127, Pisa, Italy Corresponding author: [email protected]; [email protected] ABSTRACT: In glass-forming liquids, there is a secondary -relaxation that bears a strong connection to structural -relaxation and is of fundamental importance. It is natural to inquire whether ionic conductivity relaxation in ionic conductors possesses the same property. This question has not been answered before because -relaxation had not been found or resolved in all kinds of ionic conductors. Herein, we investigate ion dynamics in supercooled and glassy states of four 1-butyl-1-methylpyrrolidinium ionic liquids with different anions differing significantly in size and strength of interactions between ionic species. Despite these differences, all four samples exhibit a prominent -relaxation with dynamics strongly connected to the primary conductivity relaxation. The general results indicate that -relaxation is indispensable as a precursor in generating the cooperative conductivity relaxation and in turn the dc-conductivity. The novel findings have an impact on the research of conductivity relaxation in the broad field of ionic conductors. 1. INTRODUCTION Among the solid state matter, amorphous solids (glasses) are fascinating materials. They constitute more than 90% of the solid matter surrounding us in everyday life, including nature and technology. It embraces various classes of materials, whether covalent, ionic, metallic, and van der Waals and the glassy state is obtained by cooling the liquid. During this process, the viscosity increases drastically to reach 1013 poise at the liquid-glass transition temperature Tg, and the structural relaxation time τα reaches long times of the order of 100 seconds. At temperatures below Tg, τα becomes exceedingly long and the structure relaxation is frozen. Then, much faster local molecular motions including the secondary relaxations become the principal source of dynamics in the glassy state.1,2,3 In the past two decades, an advance was made in the research of glass-forming materials by the discovery of a special class of secondary relaxations that have strong connections in properties to the structural dynamics responsible for glass transition. They are called the JohariGoldstein (JG) -processes with the intent to honor these two researchers for their early 1 ACS Paragon Plus Environment

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contributions to the field of secondary relaxations.4,5,6,7 The connections with structural relaxation imply JG -relaxation is fundamentally important because it is indispensable in generating cooperative structural -relaxation.8 The JG -relaxation has a broad connection to various properties of glasses, including mechanical properties, thermal stability, and crystallization. Since JG -relaxation had been found in all kinds of glass-formers including rigid and small organic molecules, inorganic, molecular, polymeric, metallic systems as well as in plastic crystals, it can be considered as a universal and intrinsic feature of supercooled liquids and glasses.6,9 Most remarkable is finding of JG -relaxation together with structural relaxation in metallic glasses, where the translational motion of atomic particles determines viscosity.6 The ionic conductors are similar in the translational ionic motion that determines dcconductivity, dc. Notwithstanding, in contrast to metallic glasses, the -relaxation is either not found or not well resolved in ionic conductors, and thus little is known on its nature.10,11 The focus in most studies is usually on frequency dispersion of conductivity relaxation M*(f), its relaxation time, (τσ=ε0εs/σdc=1/2πfmax), and associated dc-conductivity that determines electrochemical applications. Therefore, the question of the existence of a strong connection between -relaxation and conductivity relaxation remains unanswered. Despite the decades of studies,12,13,14,15,16 the experimental dielectric and nuclear magnetic resonance data did not reveal any relevant secondary process in ionically conducting systems. The only exception is the study of special room temperature ionic liquid (RTIL) composed of silicon-substituted imidazolium cation, 1-methyl-3-trimethylsilylmethylimidazolium ([Si-MIm]+) and tetrafluoroborate anion ([BF4]-).17 The conductivity relaxation (corresponding to the structural -relaxation in non-ionic glass-formers) was found to be accompanied by a single and well resolved -relaxation with relaxation time, . The strong connection between these two processes was proved by invariance of ratio (P,T)/(P,T) at different T-P thermodynamic conditions while keeping (P,T) constant. Despite the success of finding relevant -relaxation analogous to JG -relaxation in [Si-MIm][BF4], the questions remain are: whether this is peculiar to Si-substituted cation and whether this is affected by the presence of water due to hygroscopic nature of [BF4]-? Additionally, the failure to resolve the origin of -relaxation in imidazoliumRTILs10,11 can be due to delocalization of charge in the aromatic cation, which reduces the relaxation strength of -relaxation. The problem can be circumvented by replacing imidazolium with nonaromatic pyrrolidinium cation and using different types of anions in a systematic study. In this work, we employed broadband dielectric spectroscopy (BDS) to examine the ion dynamics in supercooled and glassy states of 1-butyl-1-methylpyrrolidinium BMPyr-based aprotic RTILs containing charged nitrogen atom and butyl side group in cation that provides a balance between Coulomb and van der Waals interactions.18 The four different anions, bis(fluorosulfonyl)imide (FSI), bis(trifluoromethylsulfonyl)imide (TFSI), tricyanomethanide (C(CN)3), and dicyanamide (N(CN)2), were chosen to vary the strength of electrostatic interaction between cation and anion. The chemical structures of BMPyr-based RTILs are presented in Scheme 1. Worth emphasizing is that the selected RTILs have different anion size and various physicochemical properties (such as viscosity being crucial in many industrial 2 ACS Paragon Plus Environment

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applications). This choice gives us a unique opportunity to investigate the effect of anion on the glassy dynamics in RTILs. 2. EXPERIMENTAL SECTION Materials [BMPyr][C(CN)3] (CAS number 878027-72-6; purity >98%), [BMPyr][N(CN)2] (CAS number 370865-80-8; purity >98%), [BMPyr][TFSI] (CAS number 223437-11-4; purity >99%) were purchased from Iolitec company (Germany). [BMPyr][FSI] (CAS number 1057745-51-3; purity >99.9%) was purchased from Solvionic company (France).. All examined RTILs were used without any further purification. Prior to measurements, the samples were only dried and degassed under low pressure at temperatures not exceeding 373 K. Methods BDS. The dielectric spectra of examined samples were measured using a Novocontrol Alpha Analyzer. The temperature was precisely controlled with a Quatro temperature controller using a nitrogen gas cryostat (accuracy better than 0.1 K). During the measurements, the tested samples were placed between the steel electrodes of a capacitor (10 mm diameter), with a fixed distance between the electrodes (0.08 mm) provided by quartz ring. The applied electric field was 0.1V. For the pressure dependent dielectric measurements of [BMPyr][N(CN)2], we used the capacitor, filled with the studied sample, which was next placed in the high-pressure chamber and compressed using the silicone oil. Note that during the measurement the sample was in contact with stainless steel. The pressure was measured by the Unipress setup with a resolution of 1 MPa. The temperature was controlled within 0.1 K by means of a Weiss fridge. For the high pressure dielectric measurements of [BMPyr][C(CN)3] the sample was placed in high pressure chamber and compressed at 263 K to 700 MPa. Then the temperature was decreased to 201 K. After the temperature stabilization the measurements were performed on decompression from 300 MPa. Nevertheless the cold crystallization started when the pressure was lowered to 240 MPa. To avoid oil condensation the 1-1 mixture with heptane was used. The measurements were repeated twice to prove the obtained results. DSC. Calorimetric experiments of investigated ILs were performed by a Mettler Toledo DSC1STAR System equipped with a liquid nitrogen cooling accessory and an HSS8 ceramic sensor (a heat flux sensor with 120 thermocouples). Each sample was measured in aluminum crucibles with a 40 μL volume. Prior to the measurement, the samples were annealed 15 mins at 373 K. Then, the heating from 143 to 283 K at rate 5 K·min–1 was performed. During the experiments, the flow of nitrogen was keeping at 60 mL·min–1. Enthalpy and temperature calibrations were performed using indium and zinc standards. Rheological Experiments. The viscoelastic properties of pyrrolidinium-based RTILs were measured by means of an ARES G2 Rheometer. The viscosity and shear modulus measurements in the vicinity of liquid-glass transition were performed by means of aluminum parallel plates 3 ACS Paragon Plus Environment


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geometry (diameter = 8 mm). For the oscillation frequency rheological experiments, the investigated samples were tested in the frequency range from 0.03 to 600 rad·s–1 (12 points per decade) and over the broad temperature range. The structural relaxation times τα were determined directly from the G”(ω) maximum. Since the time-temperature rule is satisfied for studied RTILs it has been employed to determine τα at higher temperatures. 3. RESULTS AND DISCUSSION The frequency dependence of capacitance and conductance of four RTILs were measured by a Novocontrol Alpha analyzer over a broad temperature range covering the supercooled liquid and glassy states. The obtained dielectric data can be presented in three equivalent formalisms: the complex conductivity, *(f), the complex permittivity, *(), and the complex electric modulus, M*(f) that are related to each other by the following equation: (1) M  ( f )  1 /   ( f )  i o /   ( f ) where o is the permittivity of a vacuum. However, only the complex electric modulus representation, M*(f)=M(f)+iM(f), is a direct way to exhibit the frequency dispersion of conductivity relaxation as well as the -relaxation of ionic materials as loss peaks.17 Hence, we analyzed the dielectric data of studied RTILs in terms of M(f). The representative spectra of [BMPyr][N(CN)2] collected at ambient pressure and temperatures above and below its calorimetric glass transition temperature, Tg (see Table 1 and Figure 1), are depicted in Figure 2A. The dielectric data show the dominant conductivity loss peak (with the maximum at frequency f=1/2σ) related to the translational ionic motions that is accompanied by a well resolved -conductivity loss peak at higher frequencies, f=1/2. The observed conductivity loss peaks of [BMPyr][N(CN)2] are asymmetric and their frequency dependence is well described by Fourier transform of the Kohlrausch-Williams-Watts (KWW) relaxation function, 𝜑(𝑡) = exp [ ― (𝑡/𝜏𝜎)𝛽𝐾𝑊𝑊]

(2)

with KWW equals to 0.57 (see Figure 2A). The M(f) spectra of other three RTILs with FSI, TFSI, and C(CN)3 anions, also exhibit the pronounced σ- and -loss peaks. The M(f) data (normalized by maximum, 𝑀𝑚𝑎𝑥) of all four RTILs having the same frequency fmax of σ-loss peak are shown in Figure 2B. The shapes of M(f)/ 𝑀𝑚𝑎𝑥 are similar and are well fitted by Fourier transforms of KWW function with slightly different KWW values given in Table 1. The broadening is indicated by the corresponding change of the value of KWW from 0.59 to 0.53. In non-ionic glass-formers, the JG -process has its relaxation time τJG(T,P) approximately equal to the primitive relaxation time, τ0(T,P), of the coupling model (CM) calculated from the structural -relaxation time τ(T,P) with tc=2 ps by relation5, 𝜏0(𝑇,𝑃) = (𝑡𝑐)1 ― 𝛽𝐾𝑊𝑊[𝜏𝛼(𝑇,𝑃)]

𝛽𝐾𝑊𝑊

(3)

The CM also applies to the dynamics of ions in ionic conductors13, by replacing 𝜏𝛼 by 𝜏𝜎 in Eq. (3). Thus, the primitive conductivity relaxation time τ0(T,P) is calculated from (P,T), instead of 4 ACS Paragon Plus Environment

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τ(T,P) in Eq. (3). Moreover, according to the CM, f0(T,P)=1/20(T,P) should be approximately equal to f(T,P)=1/2(T,P) of the observed -relaxation. This relation is verified in Figure 2B by the good agreement between f0(T,0) and f(T,0) for studied RTILs. Since (T,0) is the same for examined samples while KWW are different, an increase of f(T,0)f0(T,0) with a decrease of KWW is expected from Eq. (3). This correlation is verified by the inset of Figure 2B. The temperature dependences of  and , of four [BMPyr]-based RTILs are plotted as a function of reciprocal temperature in Figure 3. The log(T-1) in the supercooled region is non𝑑𝑙𝑜𝑔𝜏𝜎

Arrhenius that is quantified by fragility parameters 𝑚𝑝 = 𝑑𝑇𝑔/𝑇

(see Table 1). The (T) is

Arrhenius in the glassy state and exhibits a stronger dependence after crossing Tg. To verify whether or not the charge transport is controlled by viscosity in studied aprotic RTILs we have performed the mechanical measurements. The representative G”(f) data of [BMPyr][C(CN)3] are presented in Figure 3B. In the vicinity of Tg, the  were determined directly from G”(f) maximum, while at higher T the time-temperature superposition rule (TTS) was employed. As clearly seen in Figure 3A  mimics  behavior over a broad T-range that is reflected by similar mp value and the exponent in Stokes-Einstein law (s) being close to unity (see Figure 3C). Thereby, the (T) can be employed for determining the liquid-glass transition temperature by the conventional definition of Tg=T(=100 s). Such determined Tg values are close to the calorimetric TgDSC and are listed in Table 1. As can be seen by inspection of Figure 3, Tg is decreasing in the following order: [BMPyr][TFSI], [BMPyr][C(CN)3], [BMPyr][N(CN)2], and [BMPyr][FSI]. A similar trend is followed by (T). Namely, at a given temperature  is the slowest for [BMPyr][TFSI] and the fastest for [BMPyr][FSI]. Notwithstanding, the activation energy of (T) is equal to Ea≈30 kJ/mol independently on the anion type. In this context one can assume that the pyrrolidinium cation [Pyr]+ gives a source of secondary dynamics in studied RTILs. According to DFT calculations available in the literature, there are at least two possible conformational changes within the [Pyr]+: motions of non-aromatic ring and butyl side group. Nevertheless, the first type of motion can be immediately excluded as the origin of β-relaxation due to the very low activation energy (EaDFT=12 kJ/mol)18. On the other hand, to recognize the contribution of butyl chain rotation in secondary dynamics the high-pressure measurements are required. In general,  is pressure independent for intramolecular alkyl chain motions. The M(f) data of [BMPyr][N(CN)2] collected at a fixed temperature of 201 K and pressures over the range 0.1-250 MPa are shown in Figure 4A. As expected, the isothermal compression brings conductivity relaxation peak toward lower frequencies. Additionally, at highfrequency side of M(f) spectrum recorded at 250 MPa, the β-relaxation is clearly visible. Interestingly, comparison of M(f) data having the same σ-loss peak frequency however collected at various T-P conditions (250 MPa and 201 K vs. 0.1 MPa and 181 K) has revealed an exact agreement between M(f) curves on low-frequency side of fσ and limited agreement on highfrequency side of fσ being due to more significant contribution from the -loss at ambient pressure (see Figure 4B). Moreover, it can be seen that -relaxation of [BMPyr][N(CN)2] at 5 ACS Paragon Plus Environment


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ambient and high-pressures can be well described by Fourier transform of the KWW function with the same βKWW parameter like it was reported before for other ILs.19,20 Since the -relaxation appears as a shoulder of M(f) peak in the supercooled liquid state its relaxation time 𝜏(𝑃,𝑇) was determined by some fitting procedure. The one we used is the sum of two Havliliak-Negami (HN) functions to represent both - and -relaxations visible in the M(f) formalism. An example of the fitting curve is shown in Figure 4B where the - and contributions are displayed together with the exponents of two HN functions. The values of 𝜏𝜎 and 𝜏 determined at several pressures and at 201 K were obtained and entered into Figure 4C, together with the results at ambient pressure data of [BMPyr][N(CN)2]. The good agreement between experimental 𝜏(𝑃,𝑇) with 0(T) calculated from Eq. (3) is clear by inspection of the figure. Additionally, it is clearly seen that separation of -relaxation from -relaxation measured by [logτσ(T,P)-logτβ(T,P)] stays similar on elevating pressure from 0.1 MPa up to 250 MPa. Thus, one can assume that the - and -relaxations are strongly connected in [BMPyr][N(CN)2]. The link between - and -processes becomes also apparent when the pressure dependences of 𝜏𝜎(𝑃) and 𝜏(𝑃) at T=201 K are analyzed in terms of the simple volume activated equation, 𝜏(𝑃,𝑇) = 𝜏0(0,𝑇)exp

( ) 𝑃Δ𝑉

(4)

𝑅𝑇

The value of activation volume determined for 𝜏𝜎(𝑃,𝑇) and 𝜏(𝑃,𝑇) is equal to Vσ=46 cm3/g and V=27 cm3/g, respectively. Interestingly, by simple differentiating of Eq. (3) and multiplying by RT we get the direct relation between activation volume and βKWW parameter, namely V/Vσ=βKWW that is fully satisfied for [BMPyr][N(CN)2]. Importantly, the pressure sensitivity of τβ has been also confirmed for [BMPyr][C(CN)3] with the same value of activation volume (see the inset to Figure 4C). In the light of experimental facts presented above one can expect that -conductivity relaxation visible in dielectric spectra of [BMPyr][N(CN)2] and [BMPyr][C(CN)3] is strongly related to relaxation. Thus, the contribution of butyl chain rotation can be excluded as the source of glassy dynamics. Unfortunately, the same cannot be directly confirmed for the other two pyrrolidiniumbased ILs examined herein. The reason lies in the lack of a high-pressure dielectric data due to the crystallization of [BMPyr][FSI] observed during 3-hours long stabilization of T=201 K in the high-pressure chamber. Additionally, the formation of crystalline state at elevated pressure has been recently reported for pyrrolidinium-based ILs with n-alkyl side chain in cation and TFSI anion.21 Nevertheless, based on the CM-predictions, one can say that 𝜏𝜎 and 𝜏 are related also in [BMPyr][FSI], [BMPyr][TFSI]. To provide another evidence on the ionic nature of β-relaxation of JG-kind we have used a simple model to calculate the energy of interactions between ions in [BMPyr][TFSI]: 𝑞2

(5)

𝐸𝑏 = ― 4𝜋𝜀0𝜀𝑟𝑑

where q and ε0 are well known physical parameters, d describes the distance between ion pairs and εr is dielectric permittivity of a given material. The value of εr for [BMPyr][TFSI] was determined directly from dielectric measurements in the glassy state (εr≈3.7). Using the Ceff(-1/3) 6 ACS Paragon Plus Environment

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parameter, being a measure of the average interionic distance (Ceff=2.3·10-3 mol/cm3)22, we have determined d as 11.12 Å and Eb=33.8 kJ/mol for [BMPyr][TFSI], that is in a good agreement with experimental value (32.1 kJ/mol) and thereby confirms intermolecular origin of βrelaxation. 4. CONCLUSIONS In summary, we examined four pyrrolidinium-based aprotic RTILs ([BMPyr][FSI], [BMPyr][TFSI], [BMPyr][C(CN)3], [BMPyr][N(CN)2]) using BDS. The chosen liquids are good glass formers, but they exhibit cold crystallization confirmed by DSC results, with the exception of [BMPyr][N(CN)2]. Selected ILs all show prominently -relaxation so that its relation with relaxation can be examined expediently. We found that the nonaromatic chemical structure of the cation is pertinent to reveal -relaxation. Our dielectric measurements of all four RTILs with [Pyr]+ performed at ambient pressure demonstrate good agreement between - and primitive relaxation time of the CM. The high pressure data of [BMPyr][N(CN)2] and [BMPyr][C(CN)3] have revealed the pressure sensitivity of -mode. More importantly, the separation between  and -relaxation times, [logτσ(T,P)-logτβ(T,P)], is invariant to T-P-changes. Additionally, the activation energy of -process (≈30 kJ/mol) was found to be in a good agreement with the energy calculated for simple long-range Coulomb interactions between ion pairs. These results provide strong evidence on the ionic nature of β-relaxation and hence it cannot be neglected in the fundamental understanding of the ions dynamics. AUTHOR INFORMATION Corresponding Author *[email protected];

[email protected]

ORCID Zaneta Wojnarowska: 0000-0002-7790-2999 Shinian Cheng: 0000-0002-5615-8646 Małgorzata Musiał: 0000-0002-1624-6617 K.L. Ngai: 0000-0003-0599-4094 Marian Paluch: 0000-0002-7280-8557 ACKNOWLEDGMENT The authors are deeply grateful for the financial support by the National Science Centre within the framework of the Opus15 project (grant nr DEC- 2018/29/B/ST3/00889).

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Table 1 Cation Anion DSC* /K Tg BDS** /K Tg BDS mP mPRH βKWW Eaβ/kJ·mol-1 *heating

BMPyr N(CN)2 TFSI 171.0 186.8 166 180.1 89.9 84.3 N/A 98.3 0.57 0.53 29.0±0.2 32.1±0.2

C(CN)3 179.2 172.8 94.7 99.4 0.54 28.3±0.2

FSI 167.3 162.6 92 N/A 0.59 30.8±0.4

rate 5 K·min-1; see also Figure 1 ;**Tg=T(τσ=100 s)

Scheme 1. The chemical structures of studied pyrrolidinium-based RTILs. 2.0 1.5 1.0

cp/ Jg-1K-1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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No crystallization

Tg=171.0 K

[BMPyr][N(CN)2]

0.5 20

Tm=258.3 K

0 Tg=186.8 K

-20 8 4 0 -4

[BMPyr][TFSI] Tm=253.5 K

[BMPyr][FSI]

Tg=167.3 K Tm=266.7 K

10 0 -10

Tg=179.2 K

160

180

[BMPyr][C(CN)3]

200

220

T/ K

240


260

280

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Figure 1. Calorimetric experiments of [BMPyr][N(CN)2], [BMPyr][TFSI], [BMPyr][FSI], [BMPyr][C(CN)3] were performed from 143 to 283 K at heating rate 5 K·min-1.

Figure 2. A. The representative dielectric spectra of [BMPyr][N(CN)2]. The dashed line indicates the fit of KWW function to the experimental data. B. The electric modulus spectra of four pyrrolidinium-based RTILs recorded at different temperatures but all have same -loss peak frequency. Arrow of the same color as the data indicates the location of the calculated f0, which is a good agreement with the -loss peak frequency f. The inset shows values of βKWW for all four RTILs, and the anti-correlation with f0.

Figure 3. A. The relaxation map of all studied herein ILs. Solid lines are the VFT (above Tg) and Arrhenius (below Tg) fits of dielectric experimental data. B. The G”(f) data of [BMPyr][C(CN)3]. C. The test of Stokes-Einstein rule for studied RTILs.



Figure 4. The ambient and high-pressure data of [BMPyr][N(CN)2]. A. The isothermal dielectric measurements collected at 201 K and pressure range 0.1-250 MPa. B. The M”(f) peaks recorded at 201 K and parameterized using two HN functions. C. Relaxation map for [BMPyr][N(CN)2]; the inset panel presents the logτβ(P) dependence for [BMPyr][N(CN)2] and [BMPyr][C(CN)3]. REFERENCES (1) Capaccioli, S.; Paluch, M.; Prevosto, D.; Wang, L. M.; Ngai, K. L. Many-Body Nature of Relaxation Processes in Glass-Forming Systems. J. Phys. Chem. Lett. 2012, 3, 735−743. (2) Yu, H. B.; Tylinski, M.; Guiseppi-Elie, A.; Ediger, M. D.; Richert, R. Suppression of β Relaxation in Vapor-Deposited Ultrastable Glasses. Phys. Rev. Lett. 2015, 115, No. 185501. (3) Wagner, H.; Richert, R. Equilibrium and Non-Equilibrium Type β-Relaxations: D-Sorbitol versus o-Terphenyl. J. Phys. Chem. B 1999, 103, 4071−4077. (4) Johari, G. P.; Goldstein, M. Viscous Liquids and the Glass Transition. II. Secondary Relaxations in Glasses of Rigid Molecules. J. Chem. Phys. 1970, 53, 2372– 2372. (5) Ngai, K. L.; Paluch, M. Classification of secondary relaxation in glass-formers based on dynamic properties. J. Chem. Phys. 2004, 120, 857– 873. (6) Yu, H. B.; Wang, W. H.; Bai, H. Y.; Samwer, K. The β-relaxation in metallic glasses. Natl. Sci. Rev. 2014, 1, 429– 461. (7) Ngai, K. L.; Wang, L. M.; Yu, H. B. Relating ultrastable glass formation to enhanced surface diffusion via the Johari–Goldstein β-relaxation in molecular glasses. J. Phys. Chem. Lett. 2017, 8, 2739-2744. (8) Ngai, K. L.; Paluch, M. Corroborative evidences of TVγ-scaling of the α-relaxation originating from the primitive relaxation/JG β relaxation. J. Non-Cryst. Solids, 2017, 478, 1–11. (9) Tu, W.; Valenti, S.; Ngai, K. L.; Capaccioli, S.; Liu, Y. D.; Wang, L. M. Direct Evidence of Relaxation Anisotropy Resolved by High Pressure in a Rigid and Planar Glass Former. J. Phys. Chem. Lett. 2017, 8, 4341-4346. (10) Ito, N.; Richert, R. Solvation Dynamics and Electric Field Relaxation in an ImidazoliumPF6 Ionic Liquid: from Room Temperature to the Glass Transition. J. Phys. Chem. B 2007, 111, 5016–5022. 10 ACS Paragon Plus Environment

Page 10 of 12

Page 11 of 12 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(11) Russina, O.; Beiner, M.; Pappas, C.; Russina, M.; Arrighi, V.; Unruh, T.; Mullan, C. L.; Hardcare, C.; Triolo, A. Temperature Dependence of the Primary Relaxation in 1-Hexyl-3methylimidazolium bis{(trifluoromethyl)sulfonyl}imide. J. Phys. Chem. B 2009, 113, 8469– 8474. (12) Angell, C. A. Mobile Ions in Amorphous Solids. Annu. Rev. Phys. Chem. 1992, 43, 693717. (13) Leon, C.; Ngai, K. L.; Habasaki, J. Dynamics of Glassy, Crystalline and Liquid Ionic Conductors: Experiments, Theories, Simulations, Springer, New York, 2017. (14) Howell, F. S.; Bose, R. A.; Macedo, P. B.; Moynihan, C. T. Electrical relaxation in a glassforming molten salt. J. Phys. Chem. 1974, 78, 639-648. (15) Lunkenheimer, P. Dielectric Spectroscopy of Glassy Dynamics (Aachen, Shaker, 1999). (16) Wojnarowska, Z.; Ngai, K. L.; Paluch, M.; Invariance of conductivity relaxation under pressure and temperature variations at constant conductivity relaxation time in 0.4Ca(NO3)2-0.6 KNO3. Phys. Rev. E 2014, 90, No. 062315. (17) Jarosz, G.; Mierzwa, M.; Ziolo, J.; Paluch, M.; Shirota, H. Ngai, K. L. Glass transition dynamics of room-temperature ionic liquid 1-methyl-3-trimethylsilylmethylimidazolium tetrafluoroborate. J. Phys. Chem. B 2011, 115, 12709-12716. (18) Endo, T.; Hoshino, S.; Shimizu, Y.; Fujii, K.; Nishikawa, K. Comprehensive Conformational and Rotational Analyses of the Butyl Group in Cyclic Cations: DFT Calculations for Imidazolium, Pyridinium, Pyrrolidinium, and Piperidinium. J. Phys. Chem. B 2016, 120, 10336–10349. (19) Rivera-Calzada, A.; Kaminski, K.; Leon, C.; Paluch, M. Ion Dynamics under Pressure in an Ionic Liquid. J. Phys. Chem. B, 2008, 112, 3110-3114. (20) Wojnarowska, Z.; Swiety-Pospiech, A.; Grzybowska, K.; Hawelek, L.; Paluch, M.; Ngai, K. L. Fundamentals of ionic conductivity relaxation gained from study of procaine hydrochloride and procainamide hydrochloride at ambient and elevated pressure. J. Chem. Phys. 2012, 136, No. 164507. (21) Tu, W.; Szklarz, G.; Adrjanowicz, K.; Grzybowska, K.; Knapik-Kowalczuk, J.; Paluch, M. The Effect of Cation n-alkyl Side Chain Length, Temperature and Pressure on the Glass Transition Dynamics and Crystallization Tendency of [CnC1Pyrr]+[Tf2N]− Ionic Liquids Family. J. Phys. Chem. C 2019, 123, 12623-12637. (22) Ueno K.; Tokuda, H.; Watanabe. M. Ionicity in ionic liquids: correlation with ionic structure and physicochemical properties. Phys. Chem. Chem. Phys. 2010, 12, 1649–1658.



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Evidence of a Fundamental Mechanism Governing Conductivity

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