Chromium-doped hexagonal barium titanate is studied with first-principles density functional theory. The results are compared with experimental data available from electron paramagnetic resonance, x-ray diffraction, and optical absorption spectra. The probable site for the impurity atom occupancy in the lattice, their probable charge states, and the role of oxygen vacancies in their stabilization are investigated. Defect formation energy is used to analyze the role of electronic-and ionic-compensation mechanisms in stabilizing the point defect. Various atomic positions for the oxygen vacancy surrounding the impurity atom are taken into consideration in order to compare with some of the conclusions derived from experiments. Our results on the substitutional site preference and the location of oxygen vacancy in the next-neighbor surrounding of the impurity Cr are in good agreement with experiments.

We have studied the defect properties of substitutionally doped chromium in hexagonal barium titanate using the density functional theory together with the generalized gradient approximation restricted to exclusively electronic compensation of the charged defect. The supercell used in our studies is large enough to mimic a chromium concentration of about 2 mol%, which corresponds to an amount typically applied in experiments. Chromium is found to prefer the Ti sites inside the face-sharing oxygen octahedra compared with the other non-equivalent Ti sites within the exclusively corner-sharing octahedra of the hexagonal host lattice. Analysis of formation energy, derived from the total energies of defective supercells in various charge states and systematic post-processing, shows that the charge state of -1 is stable for wide range of Fermi energy within the band gap. The charge state of 0 is only stable for a very low electron concentration, i.e. for highly oxidizing conditions. (C) 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

DTA, XRD and sintering investigations of the system BaSnO3−BaGeO3,prepared by a mixed-oxide method, are described herein. The melting temperature ofthis system is about 1270 ± 5 °C. We find a partial solubility of BaGeO3 into BaSnO3 ofthe order of 6−7 mol%. Up to 50 mol% BaGeO3, the calcined powders (1150 °C) aswell as the once-sintered samples consist of BaSnO3 and orthorhombic BaGeO3 at roomtemperature. A gradual appearance of hexagonal BaGeO3 can be observed in calcinedpowders and once-sintered ceramics with a BaGeO3 content above 50 mol%. Aftersintering at ³ 1200 °C for more than 1 h all ceramic bodies consist of BaSnO3 andorthorhombic BaGeO3. The addition of BaGeO3 leads to a considerable reduction of the sintering temperature and to a strong densification. Sintering at 1180 °C for 10 h and anaddition of only 1 mol% BaGeO3 leads to dense ceramic bodies with cubic-like grains.

Since the first investigations of perovskite type oxynitrides with the generalised composition ABO3−xNx about twenty years ago, these compounds have become of growing interest. The incorporation of nitride ions in the perovskite lattice results in distinct changes in the electronic structure leading to unusual physical properties. In this article we report on new synthesis techniques, different analytical methods, progress in the structural characterisation by comprehensive diffraction techniques and local spectroscopic methods like XAS and NMR as well as state of the art theoretical investigations. Various physical characteristics like electrical and thermal transport parameters and dielectric properties are described. The thermal and chemical stability of oxynitride perovskites are investigated and their applications in different photocatalytic reactions are discussed.

Violet prismatic crystals of {[Cu(tn)2]3[Pt(CN)4]2}[Pt(CN)4] (tn = 1,3-diaminopropane) were crystallized from the water–methanol solution containing CuCl2·2H2O, tn and K2[Pt(CN)4]·3H2O. Prepared complex was characterized using elemental analysis, infrared and UV–Vis spectroscopy, magnetic measurement and thermal analysis. X-ray analysis revealed an ionic character of the complex containing mononuclear square planar [Pt(CN)4]2− complex anions and penta-nuclear [Cu(tn)2-Pt(CN)4-Cu(tn)2-Pt(CN)4-Cu(tn)2]2+ complex cations. The inner Cu(II) atom of the complex cation is hexa-coordinated, whereas two crystallographically equivalent peripheral Cu(II) atoms are penta-coordinated in the shape of a deformed square pyramid. Four v(CN) absorption bands observed in the IR spectrum are in agreement with the higher number of crystallographically different cyano groups and a broad highly asymmetric band observed in the reflectance UV–Vis spectrum is consistent with the presence of both hexa- and penta-coordinated Cu(II) atoms in the structure. The temperature dependence of the inverse susceptibility suggests the presence of a weak antiferromagnetic exchange coupling between Cu(II) ions. The complex is stable up to 210 °C when its two-stage thermal decomposition starts.

We here report about oxygen-nitrogen substitution in the anionic sublattice of BaTiO3 by calcination in a NH3 flow at 950 degrees C. The resulting oxynitride BaTi(O,N)(3) is shown to possess cubic structure by powder XRD and to contain 0.57 mass percent of nitrogen by hot gas extraction analysis. Solid-state N-14-NMR is used to characterize the electronic surroundings of the N3- anions in the oxynitride. Analysis of N-14-NMR line shapes as well as comparison of line positions to those of TiN and BaTaO2N implies that the incorporated nitrogen anions do indeed occupy the oxygen sites in the BaTi(O,N)3 lattice. The study illustrates the usefulness of solid-state NMR as a method for tracking ion substitution in highly symmetric environments, such as perovskite-structured oxynitrides, because of its high sensitivity to symmetry changes in the local electronic surroundings of nuclei.

Nanocrystalline BaTi0.95Sn0.05O3 (BTS-5) powder was synthesised from glycolate-precursors, and used to sinter fine-grain BTS-5 ceramics. We compare sintering behaviour, microstructure as well as dielectric and electromechanical properties of the advanced ceramics with ceramics sintered from classical mixed oxide powder.

The thermal expansion ST has been measured in the system BaTi1−xSnxO3, both for the pure compositions x=0 (BT) and x=1 (BS), and for solid solutions 0.025≤x≤0.2 (BTS). For all ceramics examined, a non-linear temperature dependence ST(T) has been observed at elevated temperatures T>400K. This is related to thermally generated impurities and, below the Burns-temperature Td of BT and BTS, to the non-linear strain contribution of polar nanoregions. With increasing Sn-content x, a steep increase of the Burns-temperature is found in BTS for compositions x≥0.025.

AbstractFor Abstract see ChemInform Abstract in Full Text.

A new procedure for the preparation of a core–shell-structured BaTiO3 precursor (core=TiO2; shell=BaCO3) will be described. The structure of this precursor is characterized by electron microscopy (environmental scanning electron microscopy; energy disperse X-ray spectroscopy), whereas the development of phases during thermal treatment is followed by X-ray powder diffraction.

Additives of SiO2 lower the sintering temperature of BaTiO3 ceramics and promote anomalous grain growth. However, the nature of the working liquid phase is not clear. A binary eutectic is out of the question because the system BaTiO3–SiO2 is not a binary one, as shown by Robbins [Robbins, C. R., Synthesis and growth of fresnoite (Ba2TiSi2O8) from a TiO2 flux and its relation to the system BaTiO3–SiO2. Journal of Research of the National Buereau of Standards — A. Physics and Chemistry, 1970, 74A, 229–232]. Guha and Kolar [Guha, J. P. and Kolar, D., Phase equilibria, sintering characteristics and dielectric properties in the BaTiO3-rich portion of the system BaO–TiO2–SiO2. In 5th Conference on Ceramics for Electronics, Liblice, 1974, pp. 1–9] found a ternary eutectic with a composition of 33 mol% BaO, 54 mol% TiO2 and 13 mol% SiO2. We examined powder tips of this composition and ones of pure SiO2 in contact with BaTiO3 green bodies by heating microscopy. The results suggest that the ternary eutectic develops in sintering bodies with SiO2 addition. During further sintering the composition of the liquid phase is changed and this modified melt is able to trigger anomalous grain growth.

In order to investigate the influence of second phases of CaTiSiO5 (sphene) or Ba2TiSi2O8 (fresnoite) on the microstructure formation and electrical properties of BaTiO3 ceramics, we first studied the thermochemical behaviour of the very complex system BaTiO3-CaTiSiO5. On the basis of a mass balance we describe the solid-state reactions qualitatively.

In order to investigate the influence of various industrial barium carbonates on the thermal decomposition of CaCO3/BaCO3/TiO2 mixtures thermoanalytical measurements of mixtures of the composition xCaCO3/(1-x)BaCO3/TiO2 with 0.04 less-than-or-equal-to x less-than-or-equal-to 0.24 were carried out. The decomposition curve is clearly divided into two sections according to the different thermal stabilities of CaCO3 and BaCO3. The quantitative analysis shows for the CaCO3 region that up to 100% more CO2 has been evolved than calculated from theory. This will be discussed in relation to the different granulometric behaviour of the different barium carbonates used.

AbstractCeramic materials have been known for several thousands of years, but an intensive research in the last two decades was leading to the advanced materials which are becoming established in todays technology and which are expected to form the backbone of developments to the next century. The properties of the final ceramics are greatly predeterminated by the characteristics of the starting powder materials. This situation stimulated the increase of studies on preparation of taylor made powders with regard to purity, homogeneity, reactivity, grain size and grain size distribution. Starting with the classical solid state reaction a detailed review is given of wet chemical preparation techniques for perovskite type materials. Briefly some physical methods of powder preparation are mentioned. Furthermore important processes accompanying the preparation as drying, control of agglomeration behaviour and powder characterization are discussed, respectively.