Role of Crystal Structure on the Ionic Conduction and Electrical Properties of Germanate Compounds A₂Cu₃Ge₄O₁₂ (A = Na, K)

Abstract

Crystal structure and electrical properties of A 2Cu3Ge4O12 (where A = Na, K) have been investigated by neutron diffraction and electrical impedance spectroscopy (EIS). The real and imaginary parts of the impedance, dielectric constant, and electrical modulus have been studied as a function of frequency and temperature. The crystal structures of the compounds K2Cu3Ge4O12 (KCGO) and Na2Cu3Ge4O12 (NCGO) are made of perfect 2D and quasi-2D alkali metal ion layers, respectively. The dc conductivity occurs due to small polaron hopping for both compounds with single activation energy over the entire experimentally available temperature range. The values of activation energies are 0.90(3) and 1.22(6) eV for KCGO and NCGO, respectively, as determined from dc conductivity and relaxation time analysis. The frequency dependent ac conductivity curves for both compounds follow a universal Jonscher’s power law. Soft bond valence sum analysis of neutron diffraction data reveals that the conduction pathways are confined within a plane in KCGO, whereas they form a one-dimensional channel in NCGO. The bottlenecks for ionic conduction and the role of crystal structure on them are discussed. The scaling behavior of electrical modulus indicates that the relaxation process does not change with temperature; however, a sharp decrease in the time constant has been observed. The present comprehensive study facilitates the understanding of the role of crystal structure on the microscopic mechanism of ionic conduction in both battery materials, which would be useful in designing sodium and potassium based ionic conductors. Further, the constant value of activation energy over the kHz frequency range is suitable for potential applications of the studied materials in avalanche beacons and amateur and geophysical sensors.

Structural, morphological, electrical, and dielectric properties of Na2Cu5(Si2O7)2 for ASSIBs

“…In the examination and interpretation of these plots, a brick-layer model is typically employed and has been extensively explored in prior literature, especially in the context of powder samples. [30][31][32][33] Following this model, a material is conceptualized as a three-dimensional, uniform arrangement of cubic grains, alongside identical grain boundaries, as depicted in Fig. 4(b).…”

“…An equivalent circuit akin to this has been employed in prior instances to suitably model Cole-Cole plots for numerous compounds. [30][31][32][33] We will delve into the specics of the equivalent circuit selection in subsequent discussion. Fig.…”

Li-Ion Conduction Mechanism and Relaxation Dynamics in Sorosilicate Compound Li2Cu5(Si2O7)2

“…The CPE is defined as the fractal capacitance with constant phase in a complex plane as given by C P E = 1 Q false( j ω false) p where Q is the value of the capacitance of the constant phase element and p denotes the degree of deviation from the ideal capacitor (i.e., p = 0 corresponds to a pure resistor and p = 1 corresponds to a pure capacitor). In the recent past, a similar equivalent circuit was used to fit the Cole–Cole plots of several compounds. ,− The details of the selection of the equivalent circuit are discussed later.…”