Lattice relaxation in solid solutions: long-range vs. short-range structure around Cr3+ and Co2+ in oxides and silicates

This dissertation reports the results derived from the 3-year doctoral thesis project aimed at exploring some oxide and silicate structures as promising ceramic pigments with enhanced colorimetric properties with respect to the traditional colorants. Solid solutions of perovskite, alumoniobite, and melilite compounds were obtained by doping octahedral and tetrahedral coordination sites with transition metal ions (e.g. Cr3+, Co2+, and Zn2) through a solid-state synthesis performed by means of an industrial-like process. The analytical techniques adopted to investigate the synthesized compounds allowed the determination of the "averaged" crystal structure, or the so termed long-range properties, and the short-range properties (i.e. the local structure around the substituting ions) through X-ray powder diffraction and electron absorption spectroscopy (EAS), respectively. As stated by Geiger (2001) "an understanding of the microscopic, mesoscopic and macroscopic properties and of the behaviour of solid solutions under different conditions is a challenge for all disciplines concerned with the solid state". As a matter of fact, the precise determination of a structure around impurities results fundamental to provide detailed information on their incorporation and on physical properties. For instance, in the case of the solid solutions here reported, the lattice incorporation of transition metal ions as impurities is the cause of their gradual coloration. Most of the times, such a coloration is more intense as greater is the impurity amount. The final goal of this work, was attained by calculating the structural relaxation coefficient for each studied solid solution by combining the mean with the local bond distances achieved by XRPD and EAS, respectively

Co-doped willemite ceramic pigments: technological behavior, crystal structure and optical properties

Cobalt-doped willemite is a promising blue ceramic pigment, but some important aspects concerning crystal structure, optical properties and technological behavior are still undisclosed. In order to get new insight on these features, willemite pigments (Zn2-xCoxSiO4, 0<x<0.3) were synthesized by the ceramic route and characterized from the structural (XRPD with Rietveld refinement), optical (DRS and colorimetry), microstructural (SEM, STEM, TEM, EDX, EELS) and technological (simulation of the ceramic process) viewpoints. The incorporation of cobalt in the willemite lattice, taking preferentially place in the Zn1 tetrahedral site, induces an increase of unit cell parameters, metal-oxygen distances, and inter-tetrahedral tilting. It causes shifting and enhanced splitting of spin-allowed bands of Co2+ in tetrahedral coordination, implying slight changes of crystal field strength Dq and Racah B parameter, but increasing spin-orbit coupling parameter l. Willemite pigments impart deep blue hue to ceramic glazes and glassy coatings with a coloring performance better than commercial Co-bearing colorants in the 800-1200?C range. Detailed SEM-TEM investigation and microanalysis proved that no diffusion phenomena occur at the pigment-glassy coating interface and that willemite pigments are chemically inert during firing at 1050?

Fabrication of a Highly NO2-Sensitive Gas Sensor Based on a Defective ZnO Nanofilm and Using Electron Beam Lithography

Hazardous substances produced by anthropic activities threaten human health and the green environment. Gas sensors, especially those based on metal oxides, are widely used to monitor toxic gases with low cost and efficient performance. In this study, electron beam lithography with two-step exposure was used to minimize the geometries of the gas sensor hotplate to a submicron size in order to reduce the power consumption, reaching 100 °C with 0.09 W. The sensing capabilities of the ZnO nanofilm against NO2 were optimized by introducing an enrichment of oxygen vacancies through N2 calcination at 650 °C. The presence of oxygen vacancies was proven using EDX and XPS. It was found that oxygen vacancies did not significantly change the crystallographic structure of ZnO, but they significantly improved the electrical conductivity and sensing behaviors of ZnO film toward 5 ppm of dry air

Malayaite Ceramic Pigments: a Combined Optical Spectroscopy and Neutron/X-ray Diffraction Study

Ceramic pigments based on the Cr-doped malayaite structure were synthesized by solid state reaction and characterized by optical spectroscopy and combined X-ray and neutron powder diffraction in order to elucidate the still unclear chromium substitution mechanisms. The results show that coloration is actually due to simultaneous occurrence of Cr4+ and Cr3+ ions in the crystal lattice. Spectroscopy data confirm that Cr4+ is replacing Sn4+ in the octahedral site and, in minor amount, Si4+ in the tetrahedral site. In addition, neutron powder diffraction data suggest that Cr3+ substitution for octahedral Sn4+ is charge balanced by formation of oxygen vacancies with no preference over the different oxygen sites. Upon incorporation of Cr ion, the SnO6 octahedra exhibit an off-centre displacement of central cation which in turn induces a rearrangement of both the octahedral and tetrahedral coordination shells

Compressibility of orthorhombic perovskites. The effect of transition metal ions (TMI)

Interest in perovskites evenly spans Materials Science and Geophysics. Due to their inimitably lattice flexibility enabling small as well as large ions to be accommodated, perovskites have become a base structure for new technological applications. Understanding the mechanisms governing their evolution at non-ambient conditions (such as high-pressure and high-temperature) is fundamentally important both for devising functional materials and in order to provide the most reliable possible deep-Earth model. With particular attention being paid to the chemical nature of the constituent ions, a suite of orthorhombic perovskites has been selected and contrasted using several parameterizations and models. A new perspective on the pressure-induced distortion of orthorhombic perovskite structures has enabled their compressional behaviour to be redefined

La collezione Gasser nel Museo di Mineralogia dell'Università di Padova

Nel capitolo viene descritta la collezione mineralogica Gasser presente nel Museo di Mineralogia dell'Università di Padova, e comprende: - le vicende storiche legate all'acquisto della collezione; - il lavoro di riordino e di inventariazione della collezione; - i campioni rappresentativi della collezione; - i falsi mineralogici

Predicting viscosity and surface tension at high temperature of porcelain stoneware bodies: A methodological approach

The shear viscosity and the glass-vapor surface tension at high temperature are crucial to understand the viscous flow sintering kinetics of porcelain stoneware. Moreover, the pyroplastic deformation depends on the viscosity of the whole body, which is made up of a suspension of crystals dispersed in the melt. The existing fundamental theoretical background, along with semi-empirical constitutive laws for viscous flow sintering and glass densification, can be exploited through different approaches to estimate the physical properties at high temperatures starting from amount and chemical composition of the melt. In this work, a comprehensive attempt to predict the properties of the liquid phase is proposed by means of a detailed overview of existing models for viscosity and surface tension of glasses and melts at high temperature. The chemical composition of the vitreous phase and its physical properties at high temperature are estimated through an experimental approach based on the qualitative and quantitative chemical and phase analyses (by Rietveld refinement of X-ray powder diffraction patterns) of different porcelain-like materials. Repercussions on the firing behavior of ceramic bodies, are discussed. Comparative examples are provided for porcelain stoneware tiles, vitreous china and porcelain bodies, disclosing differences in composition and properties but a common sintering mechanism

Vanadium-induced color in calcium aluminates: grossite (CaAl4O7) and hibonite (CaAl12O19)

The occurrence of samples from an exotic mineralization from Sierra de Comechingones, San Luis, Argentina, opens a vivid scientific debate on its possible genesis. In addition to dellagiustaite (a new mineral of the spinel supergroup, ideally Al2V2+O4 [1]), hibonite (ideal formula CaAl12O19) and grossite (ideal formula CaAl4O7) are main components. Hibonite is purple and occurs as centimetric euhedral phenocrystals, while grossite occurs as interstitial light violet crystals up to a few millimeters. Furthermore, crystals of both minerals frequently have tubular inclusions of a V-rich phase isostructural with a non-stoichiometric vanadium oxide (approximately V2O) indicating very low oxygen fugacity. Sierra de Comechingones is a 100 km long formation composed of Neoproterozoic metamorphic rocks, mainly high grade migmatites, as well as Paleozoic granitoids [ref. in 1], but the metamorphic grade so far recorded in bedrock exposures is too low for the formation of the peculiar mineral assemblage described above. Comparable rocks have been described from Mt. Carmel (northern Israel), where similar super-reduced mineral assemblages are found to have crystallized from high-T melts trapped in xenoliths within picritic-tholeiitic lavas ejected from Cretaceous volcanoes [2]. Besides its possible origin, the mineralogical assemblage from Sierra de Comechingones offers a unique case-study on the coloration of its constituent minerals. Indeed, high concentrations of vanadium cause very unusual coloration in both hibonite and grossite. In the hibonite structure vanadium ions, in various valence states (divalent, trivalent, and tetravalent), may be distributed over five different polyhedra, namely, three unequal octahedra [M1 (D3d), M4 (C3v) and M5 (Cs)], the M3 tetrahedron (C3v), and the unusual 5-fold coordinated trigonal bipyramid M2 (D3h) [e.g. 3]. The possible location of vanadium ion in grossite is limited to two tetrahedrally coordinated sites. The combination of single crystal X-ray diffraction and absorption spectroscopy techniques aided by chemical analyses has provided details regarding the nature of the vanadium-induced colour in both hibonite and grossite crystals. In hibonite, both M4 octahedral and M2 trigonal bipyramid sites of the hibonite R-block are partially occupied by V3+. Due to the strong polarization of its spectral bands, V2+ is also located at the M4 octahedral site of the hibonite R-block. Chemical analyses coupled with the refinement of the electron density of V ions suggested that the vanadium ions occupy about the 6% and 2% of the M4 and M2 sites, respectively. V3+-occupancy at the M2 site (characterized by three short bond distances in the ab-plane and two long bond distances along the c-axis) results in a very high crystal field parameter, Dq, of 2100 cm-1. Polarized optical absorption spectra of grossite reveal no indications of V2+. All observed absorption bands can be assigned to V3+ in tetrahedral coordination. Lack of information on the geometric relations between crystallographic and optical main axes makes the determination from recorded optical spectra of the V3+ distribution among the two available tetrahedrally coordinated sites inconclusive. Yet, due to longer bond distances and a higher degree of polyhedral distortion V3+ is likely to be located at the T2 site. [1] F. Cámara 1, L. Bindi, A. Pagano, R. Pagano, S.E.M. Gain, W.L. Griffin Minerals 2019, 9, 4. [2] W.L. Griffin, S.E.M. Gain, J.-X. Huang, M. Saunders, J. Shaw, V. Toledo, S.Y. O'Reilly Am. Mineral. 2019, 104, 207. [3] M. Nagashima, T. Armbruster, T. Hainschwang Mineral. Mag. 2010, 74, 871