Polarons, free charge localisation and effective dielectric permittivity in oxides

This review will deal with several types of free charge localisation in oxides and their consequences on the effective dielectric spectra of such materials. The first one is the polaronic localisation at the unit cell scale on residual impurities in ferroelectric networks. The second one is the collective localisation of free charge at macroscopic interfaces like surfaces, electrodes and grain boundaries in ceramics. Polarons have been observed in many oxide perovskites mostly when cations having several stable electronic configurations are present. In manganites, the density of such polarons is so high as to drive a net lattice of interacting polarons. On the other hand, in ferroelectric materials like BaTiO3 and LiNbO3, the density of polarons is usually very small but they can influence strongly the macroscopic conductivity. The contribution of such polarons to the dielectric spectra of ferroelectric materials is described. Even residual impurities as for example Iron can induce well defined anomalies at very low temperatures. This is mostly resulting from the interaction between localised polarons and the highly polarisable ferroelectric network in which they are embedded. The case of such residual polarons in SrTiO3 will be described in more details, emphasizing the quantum polaron state at liquid helium temperatures. Recently, several non-ferroelectric oxides have been shown to display giant effective dielectric permittivity. It is first shown that the frequency/temperature behaviour of such parameters is very similar in very different compounds (donor doped BaTiO3, CaCu3Ti4O12, LuFe2O4,Li doped NiO,...). This similarity calls for a common origin of the giant dielectric permittivity in these compounds. A space charge localisation at macroscopic interfaces can be the key for such extremely high dielectric permittivity.Comment: 17 pages, 11 figure

Dielectric and polarization experiments in high loss dielectrics: a word of caution

The recent quest for improved functional materials like high permittivity dielectrics and/or multiferroics has triggered an intense wave of research. Many materials have been checked for their dielectric permittivity or their polarization state. In this report, we call for caution when samples are simultaneously displaying insulating behavior and defect-related conductivity. Many oxides containing mixed valent cations or oxygen vacancies fall in this category. In such cases, most of standard experiments may result in effective high dielectric permittivity which cannot be related to ferroelectric polarization. Here we list few examples of possible discrepancies between measured parameters and their expected microscopic origin

Controlling internal barrier in low loss BaTiO3 supercapacitors

Supercapacitor behavior has been reported in a number of oxides including reduced BaTiO3 ferroelectric ceramics. These so-called giant properties are however not easily controlled. We show here that the continuous coating of individual BaTiO3 grains by a silica shell in combination with spark plasma sintering is a way to process bulk composites having supercapacitor features with low dielectric losses and temperature stability. The silica shell acts both as an oxidation barrier during the processing and as a dielectric barrier in the final composite

From bound states to resonances: analytic continuation of the wave function

Single-particle resonance parameters and wave functions in spherical and deformed nuclei are determined through analytic continuation in the potential strength. In this method, the analyticity of the eigenvalues and eigenfunctions of the Schroedinger equation with respect to the coupling strength is exploited to analytically continue the bound-state solutions into the positive-energy region by means of Pade' approximants of the second kind. The method is here applied to single-particle wave functions of the $^{154}Sm$ and $^{131}Eu$ nuclei. A comparison of the results with the direct solution of the Schroedinger equation shows that the method can be confidently applied also in coupled-channel situations requiring high numerical accuracy.Comment: 13 pages, 3 figure

Magnetic field tuning of polaron losses in Fe doped BaTiO3 single crystals

Artificial tuning of dielectric parameters can result from interface conductivity in polycrystalline materials. In ferroelectric single crystals, it was already shown that ferroelectric domain walls can be the source of such artificial coupling. We show here that low temperature dielectric losses can be tuned by a dc magnetic field. Since such losses were previously ascribed to polaron relaxation we suggest this results from the interaction of hopping polarons with the magnetic field. The fact that this losses alteration has no counterpart on the real part of the dielectric permittivity confirms that no interface is to be involved in this purely dynamical effect. The contribution of mobile charges hopping among Fe related centers was confirmed by ESR spectroscopy showing maximum intensity at ca T\sim40 K.Comment: Submitte

La magadiite, silicate sodique de néoformation des faciès évaporitiques du Kanem (littoral Nord-Est du lac Tchad)

Untersuchungen im Gelände und im Labor erbrachten den Beweis, dass Magadiit (NaSi₇ O₁₃ (OH)₃, 3H₂O) in den Tonablagerungen der Niederungen zwischen den Dünen des Kanem (nördliches Ufergebiet des Tchad-Sees) vorhanden ist. Dieses sodisches Silikat wurde nachgewiesen in den Niederungen, in denen ein eingeschränktes, sodisch-karbonatisches Milieu durch kapillaren Aufstieg aus in geringer Tiefe liegendem Grundwasser zustande kommt (bzw. zustande kommen könnte). Es tritt in zwei verschiedenen Erscheinungsformen auf : als Zwischenlagerschichten und als vereinzelte Knollen inmitten einer tonigen Grundmasse. Drei Entstehungsmechanismen werden nacheinander erörtert. 1. Chemische Ausfällung durch Eindampfen der alkalinischen Salzlösungen in den zurückbleibenden Zwischendünenseen. 2. Chemische Ausfällung nach Hydrolyse und Remobilisierung der Kieselsäure aus den Diatomeen-führenden Schichten des ursprünglichen Sedimentes. 3. Chemische Ausfällung der im Grundwasser gelösten Kieselsäure durch kapillare Verdampfung. Der letztgenannte Mechanismus, der für die Entstehung der Magadiit-Konkretionen verantwortlich ist, scheint auch heute noch weiterzuwirken in den Niederungen, die als Salinen in Betrieb sind und in denen das Grundwasser Anreicherungen an löslicher Kieselsäure bis zu 2 000 mg/1 aufweist.Натриевый силикат магалиит, новообразование озерной фации района Канем (северо-восточное побережие озера Чад) Полевые наблюдения и лабораторные исследования обнаружили присутствие ма-гадиита (NaSi₇ O₁₃ (OH)₃, 3H₂O) в глинистых отложениях междюнных западин Канема (северный край озера Чад). Этот натриевый силикат был обнаружен в депрессиях где развивается (или могла развиваться) замкнутая углекисло-натриевая среда благодаря капиллярному подьему от неглубоко залегающей поверхности грунтовой воды. Он представляется в виде переслаивающихся прослоев или в виде рассеянных в глинистой среде желваков. Поочередно рассматриваются три генетических механизма образования : 1) Химическое осаждение при испарении щелочных рассолов в остаточных междюнных озерах ; 2) Химическое осаждение после гидролиза и повторной мобилизации кремнезема диатомовых прослоек в первичных отложениях ; 3) Химическое отложение кремнезема растворенного в грунтовой воде при капиллярном испарении. Последний механизм, ответственный за образование желваков магадиита, продолжает по-видимому свое действие еще в настоящее время в депрессиях разрабатываемых как сильные месторождения и в грунтовой воде которых концентрация раст¬ воренного кремнезема достигает 2 000мг/л.Magadiite (NaSi₇ O₁₃ (OH)₃, 3H₂O) has been identified by field and laboratory studies in the clayey sediments of interdunal depressions in the Kanem (northern edge of Chad lake). This sodic silicate is present in all the depressions where a confined sodium carbonate environment is (or was) formed by capillary evaporation from shallow ground water. Inter-stratified beds up to 15 cm thickness and disseminated nodules of magadiite in the clays were observed. Three hypothesis for the magadiite's formation are proposed : 1. Chemical precipitation by evaporation from alkaline brines of interdunal lakes isolated by a lowering of the level of Lake Chad. 2. Chemical precipitation by interaction of alkaline brines with preexisting beds of diatoms. 3. Chemical precipitation by capillary evaporation of the interstitial alkaline brines. This last process is responsible for the formation of the magadiite concretions and occurs in the depressions where the carbonate salts (mainly trona) are quarried, and where the ground water contains as much as 2.000 mg/1 dissolved silica.Des études de terrain et de laboratoire ont montré la présence de magadiite (NaSi₇ O₁₃ (OH)₃, 3H₂O) dans les sédiments argileux des dépressions interdunaires du Kanem (bordure septentrionale du lac Tchad). Ce silicate sodique a été mis en évidence dans les dépressions où se développe (ou a pu se développer) un milieu confiné carbonaté sodique par remontées capillaires à partir d'une nappe phréatique peu profonde. II se présente sous deux faciès, en lits interstratifiés et en nodules épars au sein d'une trame argileuse. Trois mécanismes génétiques sont tour à tour envisagés. — 1. Précipitation chimique par évaporation des saumures alcalines des lacs interdunaires résiduels. 2. Précipitation chimique, après hydrolyse et remobilisation de la silice des lits diatomitiques du sédiment originel. — 3. Précipitation chimique de la silice dissoute dans la nappe par évaporation capillaire. Ce dernier mécanisme, responsable de la mise en place des concrétions de magadiite, semble se poursuivre actuellement dans les dépressions exploitées en salines, dans lesquelles la nappe présente des concentrations en silice soluble allant jusqu'à 2 000 mg/1.Maglione Gilbert. La magadiite, silicate sodique de néoformation des faciès évaporitiques du Kanem (littoral nord-est du lac Tchad). In: Bulletin du Service de la carte géologique d'Alsace et de Lorraine, tome 23, n°3-4, 1970. Sédimentologie et géochimie de la surface. pp. 177-190