Electrical Conductivity and 7Li NMR Spin-Lattice Relaxation in Amorphous, Nano- and Microcrystalline Li2O-7GeO2
Differential scanning calorimetry showed that on heating Li2O-7GeO2 glass crystallized in stages. X-ray phase analysis and atomic force microscopy were used to study the structure and morphology of the phase states obtained at glass devitrification. It was shown that glass devitrified through an intermediate state in which the sample volume was occupied by nanometer-sized ordered phase nuclei with Li2Ge4O9 and Li2Ge7O15 structures surrounded by an amorphous medium. Further heating resulted in complete sample crystallization and transformation of nanometer-sized nuclei into micrometer-sized Li2Ge7O15 crystallites. It was shown that in comparison with amorphous and completely crystallized polycrystalline states, the intermediate nanocrystalline state has an increased electrical conductivity σ. Complete crystallization on heating was accompanied by sharp and irreversible decrease of σ. Charge transfer in amorphous, nano- and microcrystalline states of Li2O-7GeO2 composition was associated with motion of lithium ions which were weakly bound to the germanium-oxygen structural framework. Complex impedance spectra were studied in the glass, intermediate and polycrystalline states of Li2O-7GeO2. It was shown that the hodographs for the intermediate nanocrystalline state reflected charge transfer within the ordered nuclei and the embedding amorphous medium. The results of conductivity and impedance spectra measurements were supplemented by 7Li NMR spin-lattice relaxation studies. Comparative analysis of the data of electrical properties measurements and NMR relaxation studies gave evidence that increased conductivity of the intermediate nanocrystalline state resulted from high mobility of the Li+ ions.
KeywordsIonic conduction Lithium heptagermanate Glass Nanocrystals Impedance spectra NMR spin-lattice relaxation
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