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Abstract: A phase of trigonal structure has been reported in previous articles to be piezoelectric with a very high longitudinal d 33 coefficient of several thousand pm/V (J. Solid State Chem. 184 (2011) 2023, 2033). It was observed in Ta2O5 thin films and assumed to be the result of a reversible monoclinic–trigonal phase transition. However, new investigations are reported because the cell parameters of this phase are actually very close to those of the natrotantite mineral Na2Ta4O11. From analyses by X-ray energy dispersive spectroscopy (XEDS), the chemical composition of this trigonal phase corresponds to ( with . The origin of sodium, potassium, iron and oxygen atoms is found to be due to a pollution coming from alumina crucibles used for thermal treatments. Knowing both atomic structures Ta2O5 and Na2Ta4O11 and their structural relationships, observed by high resolution transmission electron microscopy (HRTEM), the mechanism of the reaction of transformation is re-examined and discussed. This mechanism implies that Ta5+ vacancies likely exist in monoclinic structures of tantalum oxide and that electrical neutrality could be due to incorporated proton H+ instead of O2− vacancies or Ta5+ interstitials. [Copyright &y& Elsevier]

Abstract: Birefringent and piezoelectric properties of Ta2O5 ceramic thin films of monoclinic and trigonal structures were analyzed. The birefringence, observed by reflected polarized light microscopy, yields information on thin film microstructures, crystal shapes and sizes and on crystallographic orientations of grains of trigonal structure. Such an information was considered for investigating piezoelectric properties by laser Doppler vibrometry and by piezoresponse force microscopy. The vibration velocity was measured by applying an oscillating electric field between electrodes on both sides of a Ta2O5 film deposited on a Si substrate which was pasted on an isolating mica sheet. In this case, it is shown that the vibration velocity results were not only from a converse piezoelectric effect, proportional to the voltage, but also from the Coulomb force, proportional to the square of the voltage. A huge piezoelectric strain effect, up to 7.6%, is found in the case of Ta2O5 of trigonal structure. From an estimation of the electrical field through the Ta2O5 thin film, this strain likely corresponds to a very high longitudinal coefficient d 33 of several thousand picometers. Results obtained by piezoresponse force microscopy show that trigonal grains exhibit a polarization at zero field, which is probably due to stress caused expansion in the transition monoclinic–trigonal, presented in a previous article (part I). [Copyright &y& Elsevier]

Abstract: A new structural state 25L-Ta2O5, obtained from sintering and annealing treatments of a Ta2O5 powder, is identified both by electron diffraction and high resolution imaging on a transmission electron microscope (TEM). According to general rules for the different L-Ta2O5 structures proposed by Grey et al. (J. Solid State Chem. 178 (2005) 3308), a structural model is derived from their crystallographic data on 19L-Ta2O5. This model yields simulated images in agreement with high resolution TEM observations of the structure oriented along its [001] zone axis, but only for a very thin crystal thickness of less than 1.2nm. Such a limitation is shown to be due to a modulation of the structure along its [001] axis. Actually, from an analysis of a diffuse scattering and of its evolution into satellites reflections as a function of the cooling rate, a second order incommensurate phase transition can be assumed to occur in this compound. The property of single phase samples observed by TEM is also verified by X-ray powder diffraction. In a discussion about studies performed by different authors on incommensurate structures in the system Ta2O5–WO3, it is noticed that TEM results, similar to ours, indicate that phase transitions could be expected in these structures. [ABSTRACT FROM AUTHOR]

Structural and magnetic properties were studied on La1−xMnO3±δ nanocrystalline powders exhibiting different La/Mn ratios. These compounds were prepared using a gel combustion method based on a cation solution soaking by acrylamide polymerization. Structural properties were studied both by transmission electron microscopy and X-ray diffraction (XRD). Complete chemical composition analyses were performed by induced coupled plasma spectroscopy and by iodometric titration. Proportions of parasitic phases in samples, as La2O3 or Mn3O4, and actual compositions of La1−xMnO3±δ phases were then determined from refinements of XRD data and sample chemical compositions. As a result, perovskite structure is not any more stable for La/Mn<0.9 as it decomposes into a mixing of La0.9MnO3 and of Mn3O4 phases, in agreement with results on thermodynamic equilibrium in the La–Mn–O phase diagram. For La/Mn>0.9, a high oxygen excess is observed and leads to consider the creation of vacancies on both lanthanum and manganese sites, whose concentrations are evaluated. Magnetic properties agree well with the proposed structures and sample compositions since for La/Mn<0.9, for which a La0.9MnO3 phase is always found, the Curie temperature remains constant and equal to 295 K (the highest temperature never observed before in such series of compositions), while for La/Mn>0.9, there is a formation of Mn vacancies giving rise to a lowering of Curie temperature resulting of a frustration of ferromagnetic interactions. [Copyright &y& Elsevier]