China
Africa
Explore chapters and articles related to this topic
Eu3+-Based Orange-Red-Emitting Inorganic Color Convertors: An Overview
Published in Sanjay J. Dhoble, B. Deva Prasad Raju, Vijay Singh, Phosphors Synthesis and Applications, 2018
Pyrochlore oxides, expressed by the general formula A23+B24+O7, have a structure derived from the fluorite. Two kinds of cations are ordered on the A- and B-sites, and one-eighth of the anions are missing with ordered vacancies. Disordering of the cation sites coupled with oxygen disorder on the anion vacancies within the pyrochlore structure results in a defective fluorite structure [182]. The crystal structure of RE2Ti2O7 is shown in Fig. 8.25. Cation A can be a trivalent RE, a divalent alkaline earth, or a monovalent alkali ion. Cation B will be a 3d, 4d, or 5d transition metal ion of an appropriate oxidation state for charge neutrality. The A-site (16d) cations are eight- coordinated and are located within scalenohedra (distorted cubes) that contain six equal oxygens (O) and two additional axial oxygens (O’) at a slightly shorter distance from the central cations, whereas the smaller B-site (16c) cation resides in sixfold coordination, forming (BO6) oxygen octahedra. The coordination geometry of A ions in the pyrochlore structure shows. As the fractional coordinate x of O’ oxygen decreases, the (AO8)n- scalenohedra become highly depressed in shape. A number of Eu3+-doped pyrochlores, A2B2O7 (A = Y, La, Gd; B = Ti, Zr, Hf, Sn), have been reported as promising candidates for phosphors.
Leaching with Acids
Published in C. K. Gupta, T. K. Mukherjee, Hydrometallurgy in Extraction Processes, 2019
In naturally available forms, niobium and tantalum occur in combination with oxygen and one or more other metals as niobates and tantalates in various types of deposits. Of these, the important ones are niobite-tantalite as pegmatites, pyrochlore as carbonatites, and niobium-tantalum-bearing cassiterite as placer deposits. Niobite-tantalite represents an iso-morphous series of niobates and tantalates of iron and manganese. Chemically, the mineral is essentially a ferrous manganese tantalate-niobate, (Fe,Mn)(Ta,Nb)2O6, the mineral being known as niobite when Nb2O3 predominates, tantalite when Ta2O5 predominates, and tan-taloniobite when both oxides are present in significant amounts. Niobite serves principally as a source for niobium, tantalite for tantalum, and tantaloniobite, of course, for both. Pyrochlore is a complex niobate of calcium and sodium containing variable amounts of iron, titanium, rare earths, fluorine, and sometimes thorium or uranium. The mineral is represented chemically as NaCaNb2O6F, and some sources contain small amounts of Ta2O5. Pyrochlores are today the most significant source of niobium. Niobium-tantalum-bearing cassiterite deposits, when processed through mining and beneficiation, yield a niobite-tantalite concentrate with most of the tantalum in the cassiterite concentrate. The smelting operations carried out with cassiterite concentrate yield tantalum-rich slags which form an important source mainly of tantalum. Table 2 presents the analyses of typical columbite, tantalite, and pyrochlore ores.
Investigation of structural and dielectric properties of subsolidus bismuth iron niobate pyrochlores
Published in Journal of Asian Ceramic Societies, 2020
F.A. Jusoh, K.B. Tan, Z. Zainal, S.K. Chen, C.C. Khaw, O.J. Lee
The X-ray diffraction (XRD) patterns of Bi3.36Fe2.08+xNb2.56−xO14.56−x (−0.32 ≤ x ≤ 0.56) pyrochlores are shown in Figure 1. The solid solution limit is confirmed to be −0.24 to 0.48 due to the absence of other secondary phases. Trace amounts of BiNbO4 and Fe2O3 are denoted by their minute peaks at 2θ: 30.44 °, 31.42 ° (BiNbO4, ICDD: 01–082-0348); 33.14 °, 35.54 ° (Fe2O3, ICDD: 01–089-0597), respectively. These phase-pure pyrochlores crystallized in a cubic structure with a space group of Fdm, No. 227, which is similar to the reported BFN pyrochlore (ICDD, 00–052-1774). In general, the ideal pyrochlore structure consisted of two types of cation coordination polyhedron (A- and B-sites) with the A cations (usually ~1 Å ionic radius) were eight-fold coordinated while the smaller B cations (~0.6 Å ionic radius) were six-fold coordinated, respectively [13]. In the BFN system, the Bi3+ (1.17 Å) and Nb5+ (0.64 Å) cations are expected to occupy the A- and B-sites, respectively. Meanwhile, the Fe3+ cation is expected to occupy both A- and B-sites similar to other Bi-based pyrochlores [9–11]. Assuming both sites are fully occupied, the general formula of BFN system could be represented as (Bi3.36Fe0.64)(Fe1.44+xNb2.56-x)O14.56-x. It is worthwhile highlighting that a full occupancy at the A-site by Bi3+ cation should be avoided as the coupling among the 6s2 lone pairs may result in unit cell distortion. In this case, the partial occupancy of Fe3+ cation at the A-site is expected to reduce the coupling effect [14]. However, the Fe3+ cation (0.78 Å) is considered too small to fit into the eight-coordinated A-site. Hence, high bismuth content is required as to stabilize the pyrochlore structure [15,16].