A. K. Ivanov-Schitz

Simulation of ion transport in layered cuprates La2 − xSrxCuO4 − δ

The processes of oxygen diffusion in La2 − xSrxCuO4 − δ phases have been simulated for the first time by the molecular-dynamics method. Calculations were performed for the temperature range 300–2500 K. The behavior of the radial pair correlation functions, which characterize the degree of order of O1 ions in CuO2 layers, indicates that O2− anions form a weakly correlated subsystem within a CuO2 layer. To quantitatively confirm the conclusions about the predominantly two-dimensional character of ion transport and different mobilities of O1 and O2 particles in the cuprates under study, the pair oxygen diffusion coefficients in the La2 − xSrxCuO4 − δ lattice were calculated. It is shown that oxygen diffusion occurs through the conventional hopping mechanism mainly in CuO2 layers; correspondingly, the diffusion coefficient for equatorial ions (O1) exceeds that for apical oxygen anions (O2) by an order of magnitude. The motion of oxygen anions was traced at the microscopic level through analysis of the particle transport trajectories. It has been proven for the first time that diffusion of O1 ions in the ab plane in a nonstoichiometric LaSrCuO3.61 sample occurs through jumps to the nearest position or along CuO2 layers; in a more complicated way, it may occur through unoccupied O2 lattice sites.

Structural and transport properties of the layered cuprate Pr2CuO4

The structural and transport properties of the layered cuprate Pr2CuO4 have been studied in the temperature range 300–2100 K using molecular dynamics simulation. The first evidence is presented for a premelting effect in Pr2CuO4: disordering on one of its oxygen sites and abnormally fast oxygen diffusion at temperatures above 1700 K. We have clarified the microscopic mechanism of oxygen ion transport in this material. The large oxygen diffusion coefficient (D > 10−7 cm2/s) obtained in our simulations of the layered cuprate Pr2CuO4 suggests that it has considerable potential as a host for electrode materials with mixed ionic-electronic conductivity.

Electric-field effect on crystal growth in the Li3PO4-Li4GeO4-Li2MoO4-LiF system

We have studied the electric-field effect on crystallization processes in the Li3PO4-Li4GeO4-Li2MoO4-LiF system. In zero field, Li3+xP1−xGexO4 (x = 0.31) crystals were grown on the cathode under the conditions of this study. At low applied voltages (≤ 0.5 V), we obtained Li2MoO4, Li2GeO3, and Li1.3Mo3O8. In the range V = 0.5–1 V, crystals of Li3+xP1−xGexO4 solid solutions with x = 0.17, 0.25, 0.28, 0.29, and 0.36 were obtained. An applied electric field was shown to reduce the melting temperature of the starting mixtures and the crystallization onset temperature.

Simulation of ion transport in layered cuprate La2SrCu2O6

Oxygen diffusion in layered cuprate La2SrCu2O6 has been simulated by the molecular dynamics method in the temperature range of 300–2500 K. The lattice is found to transform at temperatures above 1550 K; this transformation is accompanied by a change in the pair correlation functions. The abrupt change in the oxygen diffusion coefficient in the range of 1500–1550 K may indicate the presence of a phase transition to the superionic state. The motion of oxygen anions could be traced at the microscopic level. It has been proven for the first time that the La2SrCu2O6 crystal lattice allows, along with displacements of O1 ions within the CuO2 layer, their migration from the crystallographic positions to the intermediate unoccupied O3 positions. The motion of O2 anions is also fairly complicated: they move not only in their layer over the O2 positions but they also jump to the neighboring layer to occupy the O1 positions. The oxygen diffusion coefficient in layered cuprate La2SrCu2O6 exceeds that in cuprates with perovskite structure and structure of the K2NiF4 type (at the same temperatures), which indicates that this material has good prospects for electrodes with mixed ionic-electronic conductivity.

Cationic transport mechanism in α-Ag1 − xCuxI(0 < x < 0.25): Molecular-dynamics simulation

The structural and transport characteristics of an Ag1 − xCuxI(0 < x < 0.25) solid solution have been simulated by the molecular-dynamics method. It is found that the cation diffusion coefficient decreases with increasing copper concentration; this correlation is in agreement with the experimentally observed decrease in ionic conductivity. It is shown that the cationic transport in disordered Ag1 − xCuxI phases is mainly due to the migration of silver cations, whereas the mobility of copper cations is much lower. Cu+ cations are found to reside in the 8c positions in a bcc cell; this finding suggests the existence of nanoscale α-CuI regions.

Molecular Dynamics Simulation of the Structure and Ion Transport in the Ce 1 – x Gd x O 2 – δ |YSZ Heterosystem

Molecular dynamics simulation has been used to develop a realistic atomistic model of two-layer Ce1 – xGdxO2 – δ|YSZ heterosystem. It is shown that Ce1 – xGdxO2 – δ and YSZ layers (about 15 and 16 Å thick, respectively) retain their crystal structure on the whole. The main structural distortions are found to occur near the Ce1 – xGdxO2 – δ|YSZ geometric interface, within a narrow interfacial region of few angstroms thick. Both the generalized diffusion characteristics of the system as a whole and the oxygen diffusion coefficients in the layers are calculated, and the diffusion activation energies are determined.

Atomic structure and mechanism of ionic conductivity of Li3.31Ge0.31P0.69O4 single crystals

The atomic structure of Li3.31Ge0.31P0.69O4 single crystals was refined based on high-precision X-ray diffraction data at 293 K. The characteristic features of the crystal structure are considered, and their influence on high ionic conductivity (Li+) of these crystals is discussed.

Atomistic Simulation of Interfaces in Materials of Solid State Ionics

The possibilities of describing correctly interfaces of different types in solids within a computer experiment using molecular statics simulation, molecular dynamics simulation, and quantum chemical calculations are discussed. Heterophase boundaries of various types, including grain boundaries and solid electrolyte‒solid electrolyte and ionic conductor‒electrode material interfaces, are considered. Specific microstructural features and mechanisms of the ion transport in real heterophase structures (cationic conductor‒metal anode and anionic conductor‒cathode) existing in solid state ionics devices (such as solid-state batteries and fuel cells) are discussed.

Computer simulation of ion transport in a new cathode material PrSrCuO4-δ

Oxygen diffusion in the new class of cuprates Pr2-xSrxCuO4-δ (x = 1) with perovskite structure has been simulated in the temperature range of 300–2100 K for the first time. A calculation has shown the presence of anisotropy of oxygen motion: the oxygen transport in PrSrCuO3.7 in the temperature range of 300–2100 K is mainly two-dimensional, with an activation energy of no more than 0.40 eV. The coefficient of thermal expansion of PrSrCuO3.7 (9.9 × 10−6 K−1 in the range 1300–2100 K) and the oxygen diffusivity in it, which exceed the corresponding values for La2-xSrxCuO4-δ, indicate that this compound is promising as an electrode material with a mixed ionic-electronic conductivity for various electrochemical devices. The results expand the previous concepts of the oxygen-ion transport in complex cuprates.

Beryl: Regeneration crystal growth and morphology of regeneration surfaces

The specific features of the regeneration growth of colored varieties of beryl (emerald and bixbite) grown under hydrothermal conditions are studied. Habits and crystallographic features of the faceting of crystals grown on seeds of different orientations are analyzed. Micromorphology of the growth surfaces of emerald and bixbite single crystals is examined using optical and atomic force microscopy. Fractal dimensions of the regeneration surfaces were evaluated for beryl single crystals.