A new problem in the correlation of nuclear‐spin relaxation and ionic conductivity in superionic glasses

An explanation of the differences in diffusivity of the components of the metallic glass Pd43Cu27Ni10P20

Bartsch et al. [Phys. Rev. Lett. 104, 195901 (2010)] reported measurements of the diffusivities of different components of the multi-component bulk metallic glass Pd43Cu27Ni10P20. The diffusion of the largest Pd and the smallest P was found to be drastically different. The Stokes-Einstein relation breaks down when considering the P constituent atom, while the relation is obeyed by the Pd atom over 14 orders of magnitude of change in Pd diffusivity. This difference in behavior of Pd and P poses a problem challenging for explanation. With the assist of a recent finding in metallic glasses that the β-relaxation and the diffusion of the smallest component are closely related processes by Yu et al. [Phys. Rev. Lett. 109, 095508 (2012)], we use the Coupling Model to explain the observed difference between P and Pd quantitatively. The same model also explains the correlation between property of the β-relaxation with fragility found in the family of (CexLa1-x)68Al10Cu20Co2 with 0 ≤ x ≤ 1.

Origin of activation energy in a superionic conductor

The characteristics of cation diffusion with many-body effects are discussed using Ag β-alumina as an example of a superionic conductor. Polarized Raman spectra of Ag β-alumina have been measured at room temperature. The interatomic potentials were determined by a non-linear least square fitting between the phonon eigenvalues from the Raman observations and a dynamical matrix calculation based on a rigid-ion model. The obtained potential parameters for the model crystal of Ag β-alumina successfully reproduce the macroscopic properties with respect to the heat capacity, isothermal compressibility and self-diffusion constant. A molecular dynamics (MD) calculation has been carried out using the model crystal of Ag β-alumina to understand the many-body effects for the fast ionic diffusion. It was found that the Ag-Ag repulsion by excess Ag defects significantly reduced the cost of the energy difference of the occupancy between the stable and metastable sites. It is possible for the system to take various configurations of the mobile ions through defects easily, and then the fast ionic diffusion will appear. On the other hand, the Ag-Ag repulsion changes the dynamics of the Ag ions from a random hopping to a cooperative motion. In the cooperative motion, the ionic transport becomes difficult due to the additional energy required for the structural relaxation of the surrounding Ag ions. We propose a new insight into the superionic conduction, that is, the activation energy for the ionic transport is composed of two kinds of elements: a 'static' activation energy and a 'dynamic' one. The static activation energy is the cost of the averaged energy difference in the various structural configurations in the equilibrium state. The dynamic activation energy is the additional energy required for the structural relaxation induced by the jump process.

A New Type of Cation-Conducting Rubbery Solid Electrolyte: The Ionic Rubber

We show that the conductivity maximum, well known for salt-in-polymer electrolytes is not the maximum conductivity in the system when the salts used are chosen for their resistance to crystallization at high concentration. The polymer + salt system, with increasing salt content, goes through a transition zone characterized by a maximum in glass transition temperature. Beyond this point additional salt serves to plasticize the salt-crosslinked polymer system, establishing thereby a region of increasing conductivity in which the conductivity can reach very large values. We show that “polymer in-salt” solutions constitute a new type of rubbery solid electrolyte in which the conductivity is dominated by cation motion. Such systems are true hybrids of the previously distinct “superionic glass” and “polymer electrolyte” types of amorphous solid conductors.

Conductivity Vs Nmr Correlation Times, and Decoupled Cation Motion in Polymer-In-Salt Electrolytes

We report 7Li spin lattice relaxation times at 12.82 MHz over a range of temperatures for liquid electrolytes of various salt:polymer ratios, and compare the NMR correlation time obtained at the temperature of the T1 minimum with the conductivity relaxation time at the same temperature. We find that at low salt contents the two relaxation times have the same value, but beyond the “salt-inpolymer to polymer-in-salt transition” zone at the Tg maximum, the two times increasingly separate. This is taken as evidence for the onset of cation-matrix mobility decoupling, which maximizes at the pure salt extreme.

Comparison of Dynamics of Ions in Ionically Conducting Materials and Dynamics of Glass-Forming Substances: Remarkable Similarities

We consider many of the fundamental dynamic properties of ionically conducting glasses, crystals and melts and show there are analogues in the dynamic properties of glass-forming substances. These similarities suggest the dynamics of these two classes of complex systems are governed by the same physics. We also show within each class, the evolution of dynamics from short time to long times are principally governed by the stretch exponent of the Kohlrasuch function, which determines either the primary relaxation of glass-formers or the conductivity relaxation of ionic conductors.

Characterization of borate glasses by W-band pulse electron-nuclear double resonance spectroscopy

(100-x) mol % B(2)O(3) x mol % Me(2)O (Me = Li,Na,K) glasses, exposed to gamma-(60)Co irradiation to produce paramagnetic states, were characterized by W-band (95 GHz) pulse electron-nuclear double resonance (ENDOR) spectroscopy in order to characterize local structures occurring in the range of compositions between x=16 and x=25 at which the "boron oxide" anomaly occurs. The high resolution of nuclear frequencies allowed resolving the (7)Li and (11)B ENDOR lines. In the samples with x=16 and x=20 glasses, (11)B hyperfine couplings of 16, 24, and 36 MHz were observed and attributed to the tetraborate, triborate, and boron oxygen hole center (BOHC) structures, respectively. The x=25 samples showed hyperfine couplings of 15 MHz for the tetraborate and 36 MHz for BOHC. Density functional theory (DFT) calculations predicted for these structures negative hyperfine couplings, which were confirmed by W-band ENDOR. This suggests that a spin polarization mechanism accounts for the negative hyperfine structure splitting.

Ultraslow Li diffusion in spinel-type structured Li4Ti5O12 - a comparison of results from solid state NMR and impedance spectroscopy

The cubic spinel oxides Li(1+x)Ti(2-x)O(4) (0 < or =x< or = 1/3) are promising anode materials for lithium-ion rechargeable batteries. The end member of the Li-Ti-O series, Li(4)Ti(5)O(12), can accommodate Li ions up to the composition Li(7)Ti(5)O(12). Whereas a number of studies focus on the electrochemical behaviour of Li insertion into and Li diffusion in the Li intercalated material, only few investigations about low-temperature Li dynamics in the non-intercalated host material Li(4)Ti(5)O(12) have been reported so far. In the present paper, Li diffusion in pure-phase microcrystalline Li(4)Ti(5)O(12) with an average particle size in the microm range was probed by (7)Li solid state NMR spectroscopy using spin-alignment echo (SAE) and spin-lattice relaxation (SLR) measurements. Between T = 295 K and 400 K extremely slow Li jump rates tau(-1) ranging from 1 s(-1) to about 2200 s(-1) were directly measured by recording the decay of spin-alignment echoes as a function of mixing time and constant evolution time. The results point out the slow Li diffusion in non-intercalated Li(4)Ti(5)O(12) x tau(-1) (1/T) follows Arrhenius behaviour with an activation energy E(ASAE) of about 0.86 eV. Interestingly, E(ASAE) is comparable to activation energies deduced from conductivity measurements (0.94(1) eV) and from SLR measurements in the rotating frame (0.74(2) eV) rather than from those performed in the laboratory frame, E(A)(low-T) = 0.26(1) eV at low T.

Glass structure and ion dynamics of lead–cadmium fluorgermanate glasses

Glass structure and fluorine motion dynamics are investigated in lead-cadmium fluorgermanate glasses by means of differential scanning calorimetry, Raman scattering, x-ray absorption (EXAFS), electrical conductivity (EC), and (19)F nuclear magnetic resonance (NMR) techniques. Glasses with composition 60PbGeO(3)-xPbF(2)-yCdF(2) (in mol %), with x+y=40 and x=10, 20, 30, 40, are studied. Addition of metal fluorides to the base PbGeO(3) glass leads to a decrease of the glass transition temperature (T(g)) and to an enhancement of the ionic conductivity properties. Raman and EXAFS data analysis suggest that metagermanate chains form the basic structural feature of these glasses. The NMR study leads to the conclusion that the F-F distances are similar to those found in pure crystalline phases. Experimental results suggest the existence of a heterogeneous glass structure at the molecular scale, which can be described by fluorine rich regions permeating the metagermanate chains. The temperature dependence of the NMR line shapes and relaxation times exhibits the qualitative and quantitative features associated with the high fluorine mobility in these systems.

Multi-nuclear and multi-dimensional nuclear magnetic resonance investigation of silver iodide-silver phosphate fast ion conducting glasses

The results of a multi-nuclear nuclear magnetic resonance (NMR) study of (AgI)x(Ag2O)y(P2O5)1-x-y glasses are reported. Using the two-dimensional variable-angle correlation spectroscopy experiment, the isotropic and anisotropic chemical shift interactions of phosphorus were determined as a function of silver iodide and silver oxide composition. From these measurements we determine the average conformation of the phosphate groups. In addition, the 109Ag NMR spectra were recorded, as a function of both composition and temperature. At high silver oxide concentrations, interaction between the silver and phosphate groups has been detected, but in glasses in the series the (AgPO3)x(AgI)1-x 31P NMR is strikingly independent on the silver iodide composition. The temperature dependence of the silver NMR linewidths in these systems shows clearly the silver mobility, and at lower temperatures no evidence for multiple distinct silver binding sites was observed. The silver chemical shift is strongly dependent on both composition and temperature. The former effect is interpreted in terms of the influence on the chemical shift of binding to oxygen versus iodine.