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This paper focuses on the kinetics of Cr4+ formation in Cr,Ca:YAG ceramics prepared by solid-state reaction sintering. The kinetics of Cr4+ formation was studied by annealing of Cr,Ca:YAG ceramics in ambient air under different temperatures at different times, resulting in the transformation of Cr3+ to Cr4+. The activation energy (Ea) of Cr3+ oxidation determined by the Jander model was 2.7 ± 0.2 eV, which is in good correlation with the activation energy of innergrain oxygen diffusion in the YAG lattice. It is concluded that Cr3+ to Cr4+ transformation in YAG ceramics is limited by oxygen diffusion through the grain body. It was established that in Cr,Ca:YAG ceramics, the intralattice cation exchange, in which the Cr4+ ions exchange positions with the Al3+ ions, switching from "A" to "D" sites, is faster than Cr3+ to Cr4+ oxidation. In the temperature range of 900–1300 °C, the reaction enthalpy of Al3+/Cr4+ ion exchange between octahedral "A" and tetrahedral "D" lattice sites is close to zero, and this exchange ratio is thermodynamically driven by entropy. [ABSTRACT FROM AUTHOR]

In the present work we provide an analysis of the accuracy of the calculation of Cr4+ concentration in Cr4+:YAG using absorption spectroscopy. Avizonis-Grotbeck theory and Smakula–Dexter formula were used to calculate the concentration of Cr4+ ions in both octahedral and tetrahedral sites. It was shown that these approaches introduce uncertainty in the calculation of Cr4+ concentration. As a result, reported values of Cr4+ concentration differ by one order of magnitude for materials with similar absorption. We propose a new approach based on the convenient optical spectroscopy to estimate the Cr4+ concentration in Cr:YAG using survey absorption spectra. The simplified formula with the confidential range was proposed for this purpose. • The possible approach for calculation of Cr4+ concentration in Cr4+:YAG was proposed. • The modified Smakula–Dexter formula can be used for this purpose. • The actual concentration of Cr4+ can be from half to twice of the calculated value. [ABSTRACT FROM AUTHOR]

Based on a MK-40 sulfocation-exchange membrane, a hybride electrode material containing nanodispersed copper is prepared. The methods of scanning electron microscopy and X-ray diffraction (XRD) analysis reveal the formation of copper agglomerates measuring 250-470 nm and consisting of individual particles of 20-30 nm. The procedure of multistage chemical deposition of copper into the ion-exchange carrier makes it possible to obtain a continuous cluster of metal particles which determines the electron conducting properties of the resulting hybrid material. The electrochemical activity of the nanocomposite electrode is studied in the reaction of nitrate ion electroreduction. Nanodispersed copper deposited into the membrane is shown to intensify the electroreduction of nitrate ions by a factor of 1.5-2 as compared with a compact copper electrode. The electroreduction of nitrate ions on compact copper is shown to involve 6 electrons, whereas the electroreduction on the nanocomposite involves 8 electrons. The electroreduction products of nitrate ions are identified by the IR spectroscopy method. [ABSTRACT FROM AUTHOR]