Search

Jamming and percolation of three-dimensional (3D) $k \times k \times k $ cubic objects ($k^3$-mers) deposited on simple cubic lattices have been studied by numerical simulations complemented with finite-size scaling theory. The $k^3$-mers were irreversibly deposited into the lattice. Jamming coverage $\theta_{j,k}$ was determined for a wide range of $k$ ($2 \leq k \leq 40$). $\theta_{j,k}$ exhibits a decreasing behavior with increasing $k$, being $\theta_{j,k=\infty}=0.4204(9)$ the limit value for large $k^3$-mer sizes. In addition, a finite-size scaling analysis of the jamming transition was carried out, and the corresponding spatial correlation length critical exponent $\nu_j$ was measured, being $\nu_j \approx 3/2$. On the other hand, the obtained results for the percolation threshold $\theta_{p,k}$ showed that $\theta_{p,k}$ is an increasing function of $k$ in the range $2 \leq k \leq 16$. For $k \geq 17$, all jammed configurations are non-percolating states, and consequently, the percolation phase transition disappears. The interplay between the percolation and the jamming effects is responsible for the existence of a maximum value of $k$ (in this case, $k = 16$) from which the percolation phase transition no longer occurs. Finally, a complete analysis of critical exponents and universality has been done, showing that the percolation phase transition involved in the system has the same universality class as the 3D random percolation, regardless of the size $k$ considered., Comment: 26 pages, 9 figures. arXiv admin note: text overlap with arXiv:1905.11438

O titanato de estrôncio (SrTiO3) possui estrutura cristalina do tipo perovsquita. Materiais com este tipo de estrutura são utilizados para diversas aplicações, tais como, sensores, atuadores, em células a combustível de óxido sólido, entre outros. Devido as suas interessantes propriedades físicas, o SrTiO3 vem sendo intensamente estudado, em especial com a introdução de dopantes. Portanto, neste trabalho foi investigada a influência de diferentes teores de Ca (1; 2,5 e 5% mol) e Pr (0,025; 0,050; 0,075 e 1% mol) na microestrutura e propriedades elétricas e dielétricas do SrTiO3, assim como o material sem aditivos (puro). Os resultados mostram que após a sinterização do SrTiO3 puro, a microestrutura consiste de grãos poligonais com tamanho médio micrométrico, além de texturas lisas e rugosas. A condutividade elétrica das amostras sintetizadas sinterizadas a 1450 e 1500ºC é máxima para 2 horas de patamar. Apenas as amostras de SrTiO3 contendo 1% em mol de Ca apresentam fase única. O tamanho médio de grãos das amostras contendo 1% em mol de Ca é 10,65 ± 0,28 µm e para teores acima deste valor ocorre crescimento significativo dos grãos. As medidas de condutividade elétrica mostraram que as amostras contendo a adição de 1% em mol de Ca possuem maior condutividade dos grãos em relação ao material puro. Para as amostras contendo teores de até 0,075% mol de Pr, pode-se observar alguns grãos lisos e outros rugosos e não há variação considerável do tamanho médio de grãos. As amostras contendo menor teor de Pr (0,025% mol) apresentam maior condutividade dos grãos e contornos de grãos. As amostras de SrTiO3 sintetizado sinterizadas a 1450ºC/10 h apresentaram permissividade elétrica colossal em temperatura ambiente em altas frequências. Strontium titanate (SrTiO3) exhibits cubic perovskite type crystalline structure at room temperature. Polycrystalline ceramics with this structure are potential candidates for a number of applications including sensors, actuators and in solid oxide fuel cells. Several properties of SrTiO3 are strongly dependent on addition of both donors and acceptors additives. Then, there is a growing interest for studying its properties as a function of type and concentration of additives. In this study, the effects of Ca (1, 2.5 and 5 mol%) and Pr (0.025 to 1 mol%) additions on microstructure and electric and dielectric properties of SrTiO3 were investigated. The microstructure of pure SrTiO3 consists of polygonal grains with average grain size in the micrometer range, and the electric conductivity is maximized after sintering for 2 h at 1450 and 1500ºC. Specimens containing 1 mol% Ca are single phase and the average grain size is 10.65 ± 0.28 µm, but for higher additive contents grain growth is observed. The electric conductivity of SrTiO3 with 1 mol% Ca is higher than that of the pure ceramic. Specimens containing Pr do not show significant grain growth, and the higher conductivity of grains and grain boundaries was achieved with 0.025 mol% Pr. Pure SrTiO3 sintered at 1450ºC for 10 h shows colossal dielectric permittivity (> 1.000) at room temperature, in contrast to specimens prepared with commercial powder (dielectric permittivity = 300), at high frequencies.

International audience; Crystal structure, magnetism and magnetocaloric properties of LaFe11.7Si1.3Ny (y = 0, 1.3) compounds have been studied by X-ray diffraction and magnetic measurements. The LaFe11.7Si1.3Ny compounds present a cubic NaZn13-type structure. Insertion of 1.3 nitrogen atoms per LaFe11.7Si1.3 formula increases the lattice parameter and Curie temperature from 11.467 to 11.733 A and from 190 to ∼230K, respectively. Besides, the absorption of nitrogen drives drastically the magnetic transition from first to second order and accordingly strongly decreases the magnetocaloric effect compared to the parent alloy. Under an external field change of 5T, the value of isothermal entropy change -AS is about 28 and 3.5J/kgK for LaFe11.7Si1.3 and LaFe11.7Si1.3N1.3, respectively, close to their Curie temperature. However, the relative cooling power RCP(S) of the nitride is about half that of the parent alloy.

The ground and excited state properties of the Cr3+ ion doped into the cubic host lattices Cs2NaYCl6 and Cs2NaYBr6 have been studied using density functional theory. A new symmetry based technique was employed to calculate the energies of the multiplets 4A2g, 4T2g, 2Eg, and 4T1g. The geometry of the CrX3 - 6 cluster was optimized in the ground and excited states. A Madelung correction was introduced to take account of the electrostatic effects of the lattice. The experimental Cr–X distance in the ground state can be reproduced to within 0.01 Å for both chloride and bromide systems. The calculated d–d excitation energies are typically 2000–3000 cm–1 too low. An energy lowering is obtained in the first 4T2g excited state when the octahedral symmetry of CrX3 - 6 is relaxed along the eg Jahn–Teller coordinate. The geometry corresponding to the energy minimum is in excellent agreement with the 4T2g geometry derived from high-resolution optical spectroscopy of Cs2NaYCl6:Cr3+. It corresponds to an axially compressed and equatorially elongated CrX3 - 6 octahedron.

Made available in DSpace on 2014-10-09T12:27:11Z (GMT). No. of bitstreams: 0 Made available in DSpace on 2014-10-09T13:57:37Z (GMT). No. of bitstreams: 0 ??xidos perovskita do tipo ABO3 t??m estrutura ideal c??bica, mas algumas distor????es nesse tipo de estrutura podem produzir mudan??a da simetria para ortorr??mbica ou rombo??drica. Na estrutura c??bica, o c??tion A (de maior raio at??mico) ocupa o centro de cada cubo, os c??tions B ocupam os 8 v??rtices e os ??nions situamse na metade de cada uma das 12 arestas de cada cubo. Distor????es ocorridas nessas estruturas produzem o surgimento de novas propriedades el??tricas, estruturais e magn??ticas, que instigam grande interesse ci??ntifico e tecnologico. Neste trabalho, foram estudados os compostos RCrO3 (R = Gd, Tb, Dy), conhecidas como ortocromitas. As amostras foram confeccionadas atrav??s do processo qu??mico Sol Gel. Ap??s a confec????o as amostras foram analisadas pela difra????o de raios-x na qual foi concluida que as amostras tiveram uma ??nica estrutura pertencente ao grupo espacial Pbnm. Logo foram realizadas medidas de correla????o angular gama-gama perturbada utilizando para isso os nucleos de prova 181Hf(181Ta) e 111In(111Cd) que substituam os ??tomos A ou B. Os n??cleos de prova foram introduzidos durante a fase de preparo das amostras. Um dos objetivos desse trabalho foi o estudo sobre varia????o do campo magn??tico hiperfino nos s??tios A e B como fun????o da temperatura, estrutura cristalogr??fica e a transi????o de fase antiferromagn??tica. As medidas foram feitas nas a temperaturas entre 20 e 300K para R = Gd, Tb e 20 a 800 K para R = Dy. Tamb??m foi medido o Gradiente de campo el??trico em fun????o da temperatura. Foi possivel observar a transi????o e comprovar o alinhamento dos Spins do Cr, conforme consta na literatura. As temperaturas de N??el (TN ) para as amostras s??o ~170 K, ~164 K, e ~148 K, respectivamente para os compostos GdCrO3, TbCrO3 e DyCrO3. Dissertacao (Mestrado) IPEN/D Instituto de Pesquisas Energeticas e Nucleares - IPEN-CNEN/SP

The electronic ground state of isolated Cr3+ introduced into the title compounds has been investigated with electron spin resonance and electron-nuclear double resonance spectroscopy. Simultaneously a multiple scattering (MS) Xalpha study of the (CrCl6)3− cluster has been performed. The experimental results agree with a cubic Cr site. They further show evidence for strong quadrupole interaction at the anion neighbor nuclei and for observably different covalency in the two hosts. Rather good agreement is found between the predictions of the MS Xalpha model and the experimental superhyperfine interaction constants but not with the Cr-hyperfine structure constant. It is suspected that the second neighboring Cs play a non-negligible role in the electronic structure of the cluster.

Les cristaux cubiques ferro ou ferrimagnétiques, présentant une anisotropie uniaxe, induite par traitement thermomagnétique, peuvent, à l'état vierge, être divisés en régions magnétiquement uniaxes. L'ignorance de cette division entraîne des erreurs sur la détermination de K1 qui peuvent aller jusqu'au changement apparent du signe de K1.