We demonstrate that the ionic conductivity of Li_1+xAl_xGe_2−x(PO₄)₃ glass-ceramic electrolytes can be tailored by adding Y₂O₃ to Li₂O–Al₂O₃–GeO₂–P₂O₅ melts. We found that the addition of 5 vol% Y₂O₃ causes noticeable changes in further glass crystallization process as the key parameters (glass crystallization temperature, viscosity and others) are affected. Small-angle neutron scattering data suggest that Y-containing glasses are suddenly crystallized upon heat treatment producing uniform Li_1+xAl_xGe_2−x(PO₄)₃ ceramics. Due to the lower resistance of intergrain interfaces, such samples demonstrate a specific ionic conductivity of 0.5 mS cm⁻¹, which is nearly 5 times higher in comparison to Y-free ceramics, prepared under similar conditions. We speculate that such an effect is caused primarily by better intergrain contact in Y-containing ceramics, along with better ionic transport in the grain bulk as evidenced by ⁷Li NMR.

Electrical conductivity in solid ionic conductors can be enhanced by faster ionic mobility resulting from optimizing the conducting pathways, increase of charge carrier concentration or improvement of crystallite interconnectivity. Here, we investigated the lithium ion mobility in two NASICON-type glass-ceramics of Li_1.5Al_0.5Ge_1.5(PO₄)₃ (LAGP) composition, prepared with and without adding Y₂O₃ (5 vol%) to the glass melt before crystallization. We applied variable-temperature ⁷Li nuclear magnetic resonance (NMR) spectroscopy, T₁ relaxation time and self-diffusion measurements as well as impedance spectroscopy to study ionic dynamics. For both materials an Arrhenius behavior of ionic mobility is obtained from various experimental approaches, thus showing a single thermally activated process in a wide temperature range with very similar activation energies of about 0.3 eV for yttrium-free and yttrium-modified LAGP ceramics. A near five-fold conductivity enhancement in the yttrium-modified sample cannot be explained by faster ionic dynamics because only minor changes of ionic mobility are registered by NMR. In conjunction with the theoretical calculations of NMR parameters and bond valence site energies, this observation suggests that the most influencing factors on ionic conductivity are an intergrain connectivity and an Li concentration enhancement, offering thus an efficient strategy for improved ionic conductors.

A wide range of technological applications of lithium tantalate (LT) is closely related to the defect chemistry. In literature, several intrinsic defect models have been proposed. Here, using a combinational approach based on DFT and solid-state NMR, we demonstrate that distribution of electric field gradients (EFGs) can be employed as a fingerprint of a specific defect configuration. Analyzing the distribution of ${}^7\mathrm{Li}$ EFGs, the FT-IR and electron spin resonance (ESR) spectra, and the ${}^7\mathrm{Li}$ spin-lattice relaxation behavior, we have found that the congruent LT samples provided by two manufacturers show rather different defect concentrations and distributions although both were grown by the Czochralski method. After thermal treatment hydrogen out-diffusion and homogeneous distribution of other defects have been observed by ESR, NMR, and FT-IR. The defect structure in one of two congruent LT crystals after annealing has been identified and proved by defect formation energy considerations, whereas the more complex defect configuration, including the presence of extrinsic defects, has been suggested for the other LT sample. The approach of searching the EFG fingerprints from DFT calculations in NMR spectra can be applied for identifying the defect clusters in other complex oxides.

One of the approaches to tackle the problem of limited electrolyte electrochemical stability, which controls the development of novel electrochemical storage devices, is the use of solid electrolytes. Here, we focus on one of the most promising among oxide-based glass-ceramic materials, Li_1+xAl_xGe_2−xP₃O₁₂, to investigate the physical and chemical mechanisms that govern the enhancement of Li-conductivity upon variation of its composition and glass crystallization conditions during a two-step heat treatment. Using X-ray and neutron diffraction, small-angle neutron scattering and Raman spectroscopy we found that the addition of 6–18 mol% yttria makes the polyphosphate chains in glass more thermally stable. This leads to sudden and uniform glass crystallization over the whole volume. This peculiar glass structure governs the further crystallization behavior and ensures the optimal organization of intergrain boundaries. The highest Li-conductivity is achieved for a sample with 12 mol% yttria annealed for 2 hours at 750 °C. In addition, nuclear magnetic resonance together with bond valence sum analysis provides further insight into atomistic mechanisms of ionic conductivity in NASICON-based lithium-conductive ceramics.

The mesocrystal system fluoroapatite—gelatine grown by double-diffusion is characterized by hierarchical composite structure on a mesoscale. In the present work we apply solid state NMR to characterize its structure on the molecular level and provide a link between the structural organisation on the mesoscale and atomistic computer simulations. Thus, we find that the individual nanocrystals are composed of crystalline fluorapatite domains covered by a thin boundary apatite-like layer. The latter is in contact with an amorphous layer, which fills the interparticle space. The amorphous layer is comprised of the organic matrix impregnated by isolated phosphate groups, Ca₃F motifs and water molecules. Our NMR data provide clear evidence for the existence of precursor complexes in the gelatine phase, which were not involved in the formation of apatite crystals, proving hence theoretical predictions on the structural pre-treatment of gelatine by ion impregnation. The interfacial interactions, which may be described as the glue holding the composite materials together, comprise hydrogen bond interactions with the apatite PO₄³⁻ groups. The reported results are in a good agreement with molecular dynamics simulations, which address the mechanisms of a growth control by collagen fibers, and with experimental observations of an amorphous cover layer in biominerals.