Vacancy distribution in maghemite

Abstract

x F 2+x [5] with the CaF 2 structure. Non-matrix reflections correspond to these objects in local electron diffraction patterns, and they have been described as defect volumes (DV) of unknown structure [5]. We try to identify DV in this work. Ion radii difference of Ba 2+ and La 3+ is maximal among M 1-x La x F 2+x compounds, that simplifies an identification of reflections belonging to volumes of different compositions, but their atomic scattering amplitudes are practically equal, that excludes an observation of composition variations with the help of the z-contrast. In Ca 1-x La x F 2+x differences of scattering amplitudes of Ca and La allow to use the z-contrast, and Ca 1-x La x F 2+x crystals form another group of studied objects. Ba 0.69 La 0.31 F 2.31 , Ba 0.75 La 0.25 F 2.25 , B a 0.35 La 0.65 F 2.35 , C a 0.85 La 0.15 F 2.15 , Ca 0.95 La 0.05 F 2.05 have been grown by the crystallization from a melt. Ba 1-x La x F 2+x were studied in as-grown and annealed (at 1173 K and 1132 K) states. Ca 0.35 La 0.65 F 2.35 was studied in as-grown and annealed (at 1280 K) states. Other Ca 1-x La x F 2+x were studied after annealing at 1280 K. Visible density of DV in Ba 1-x La x F 2+x is two orders more after annealing than before it. Although a few anomalies are observed in electron diffraction patterns[6], DV have been identified as twins [5]. This DV nature is confirmed by: i. appearance of non-matrix reflections when an orientation of the object deviates from the exact [111] projection (as a result of the capture of reflections of the twin Laue zones), and their disappearance in the exact [111] projection. ii. variations of non-matrix reflection nets when projections are changed, iii. positions of non-matrix reflections in [111], [011], [255], [114] projections, etc. Increasing twin density during annealing means their deformation origin as a result of relaxation of misfit strains between volumes of different compositions and generation of the 1/6<211> dislocations, but signs of composition stratification in Ba 1-x La x F 2+x have no been revealed. However these signs are observed in all Ca 1-x La x F 2+x , wherein twin density grows also during annealing, although less intensive than in Ba 1-x La x F 2+x , and wherein nano-twins adjoin with inclusions of a phase with a lattice derivative from the tisonite lattice. Existence of nano-dimensional deformation twins can be considered as the manifestation of the nano-structuredness.

Highly water-soluble nanocrystal powders of magnetite and maghemite coated with gluconic acid: Preparation, structure characterization, and surface coordination

“…These materials are difficult to be identified because of their similarities in structure and properties, especially in their nano state. Some identification methods have been reported, such as X-ray diffraction (XRD) [30][31][32], X-ray absorption spectroscopy (XAS) [33], X-ray photoelectron spectroscopy (XPS) [34,35], Mössbauer spectra [36,37], and Fourier transform infrared (FTIR) spectra [30]. Each of these methods has its advantages and disadvantages.…”