Ab initio modeling of complex amorphous transition-metal-based ceramics.

Binary and ternary amorphous transition metal (TM) nitrides and oxides are of great interest because of their suitability for diverse applications ranging from high-temperature machining to the production of optical filters or electrochromic devices. However, understanding of bonding in, and electronic structure of, these materials represents a challenge mainly due to the d electrons in their valence band. In the present work, we report ab initio calculations of the structure and electronic structure of ZrSiN materials. We focus on the methodology needed for the interpretation and automatic analysis of the bonding structure, on the effect of the length of the calculation on the convergence of individual quantities of interest and on the electronic structure of materials. We show that the traditional form of the Wannier function center-based algorithm fails due to the presence of d electrons in the valence band. We propose a modified algorithm, which allows one to analyze bonding structure in TM-based systems. We observe an appearance of valence p states of TM atoms in the electronic spectra of such systems (not only ZrSiN but also NbO(x) and WAuO), and examine the importance of the p states for the character of the bonding as well as for facilitating the bonding analysis. The results show both the physical phenomena and the computational methodology valid for a wide range of TM-based ceramics.

Anthropogenic acidification effects in primeval forests in the Transcarpathian Mts., western Ukraine.

The precipitation chemistry, deposition, nutrient pools and composition of soils and soil water, as well as an estimate of historical deposition of sulphur (S) and inorganic nitrogen (N) for the period 1860-2008, were determined in primeval deciduous and coniferous forests at the sites Javornik and Pop Ivan, respectively. Measured S throughfall inputs of 10 kg ha(-1)year(-1) in 2008 were similar to those estimated for the period 1900-1950 at both sites. The highest estimated S inputs were in the 1980s. Measured bulk deposition of N in 2008 was lower at Pop Ivan (5.6 kg ha(-1)year(-1)) compared to Javornik (12 kg ha(-1)year(-1)). Significantly lower NO(3) deposition was both estimated and measured at Pop Ivan. Higher soil base cation concentrations were observed at well-buffered Javornik underlain by flysch (Ca pool of 2046 kg ha(-1) and base saturation of 29%) compared to Pop Ivan underlain by crystalline schist (Ca pool of 186 kg ha(-1) and base saturation of 6.5%). The soil pool of organic carbon (C) was higher at Pop Ivan (212 t ha(-1)) compared to Javornik (127 t ha(-1)). The C concentration was positively correlated with organic N in the soil (p<0.001) at both sites, but the mass average C/N ratio in the forest floor was lower at Javornik (22) than at Pop Ivan (26). High N leaching of 17 kg ha(-1)year(-1) at the 90 cm depth was measured in the soil water at Javornik, suggesting high mineralization and nitrification rates in old growth deciduous forests in the area. Despite relatively low Al concentrations in the soil water, a low soil water Bc/Al ratio (0.9) (Bc=Ca+Mg+K) was found in the upper mineral soil at Pop Ivan. This suggests that the spruce forest ecosystems in the area are vulnerable to anthropogenic acidification and to the adverse effects of Al on forest root systems.

Relationships between composition and properties of (Cr/Ti)SiN and (Cr/Ti)CN alloys: an ab initio study.

It has previously been noted that different fcc metal nitrides exhibit different superior properties, including the high hardness of TiN and the excellent corrosion and oxidation resistance of CrN. Si and C have been added into such metal nitrides in order to tailor their functional properties. Contrary to the intensively studied TiSiN and TiCN nanocomposite and solid solution systems, much less is known about the complex relationships between the composition and the properties of CrSiN and CrCN. In fact, there is a qualitative difference between cubic spin-unpolarized materials such as TiN, and spin-polarized materials such as CrN which may exhibit a cubic/orthorhombic structural transformation. In the present work, we report ab initio calculations of the properties of (Cr/Ti)SiN and (Cr/Ti)CN systems of various compositions. We specifically predict the lattice constant, bulk modulus, elastic tensor, shear modulus, Young's modulus, Poisson's ratio, magnetization, electronic structure and preference towards the cubic/orthorhombic structural transformation. Knowledge of the modeled relationships allows one to tailor the material characteristics of various ternary metal nitrides for different technological applications.