Papers by Keyword: Modulated Structure

Abstract: The effect of La substitution on the structure and its modulation in samples with a nominal composition of Bi₂Sr_2-xLa_xCaCu₂O_8+d where 0≤x≤0.3 superconducting phase was investigated. The X-ray powder diffraction (XRPD) results showed an almost pure Bi-2212 phase, confirmed by the estimated amount of element concentration using dispersive energy X-ray spectroscopy (EDX). Scanning electron microscopy (SEM) micrographs showed a decrease of both crystallite size and connectivity when Lanthanum concentration increases. Le Bail refinement using the Jana2006 program gives a very good value of goodness of fit (GOF) factor. Also, the refinement reveals that for x=0.3 the orthorhombic cell transforms to the tetragonal one and the modulation vector q increases when increasing La concentration.

Abstract: The microstructural evolution in Cu-1.5 wt % Ti alloy aged at 400 °C was investigated by high resolution electron microscopy (HREM). The hardness and electrical conductivity of this alloy have been also characterized. The electron metallographic results showed that the sequence of the decomposition in the studied Cu-1.5 wt % Ti alloy can be summarized as follows: a modulated structure resulting from spinodal clustering → formation of clusters and then ordered fcc phase → formation of LRO β’-Cu₄Ti which distributed periodically along the <100>_Cu directions. The ordered fcc phase showed a cube-on-cube OR with matrix, while the LRO β’-Cu₄Ti showed an orientation relationship of [001]_Cu//[001]_β’ and (310)_Cu//(100)_β’. After aging for 24 h, the hardness and electrical conductivity of this alloy reached 175 HV and 25.3 % IACS, respectively. The spinodal clustering is responsible for the hardening of the alloy during the initial 30 min aging. The ordered fcc phase and β’-Cu₄Ti phase makes a significant contribution to the strengthening of the alloy during the advanced stage of aging.

Abstract: The simple-perovskite system Bi_1-xLa_xFeO₃ (BLFO) is one of mixed-oxide systems having the multiferroic BiFeO₃ as an end material, which shows the ferroelectric and antiferromagnetic orders in its ground state. Because of the paraelectric nature of LaFeO₃ as another end material, the ferroelectric-to-paraelectric state change can be expected to occur in the mixed-oxide system BLFO. The interesting feature of BLFO is that there are both the PbZrO₃-type and incommensurately modulated (IM) states in the intermediate La-content range between the ferroelectric-R3c and paraelectric-Pnma states. Although the detailed features of these two states have not been understood sufficiently, in this study, we focus on the IM state around x = 0.20, and have investigated the crystallographic features of prepared BLFO samples with 0.15 ≤ x ≤ 0.35, mainly by transmission electron microscopy. It was found that six kinds of superlattice reflections were, for instance, present in the reciprocal lattice of the state at 300 K for x = 0.20, in addition to fundamental reflections due to the cubic simple-perovskite structure. To understand the appearance of these superlattice reflections, we regarded the state as a modulated-structure state in this study. Concretely, the modulated structure was assumed to be characterized by the appearance of both the incommensurate wave with q_I = <1/(4+δ) 1/(4+δ) 0>_c and the commensurate wave with q_II = <1/4 0 0>_c in the normal-Imma structure. In addition, the appearance of the two modulation waves could also produce the superlattice reflections at the <1/4 1/4 1/4>_c-, <1/2 0 0>_c-, and <1/2 1/2 0>_c-type positions in the reciprocal lattice. From the comparison with the experimental data obtained in this study, our modulated-structure model seems to be appropriate for the IM state in the vicinity of the PbZrO₃-type/IM state boundary.

Abstract: The modulated structure characterized by periodic concentration fluctuations is caused by the up-hill diffusion of solute atoms due to the spinodal decomposition, when the Gibbs free energy surface of the solid solution has a convex curve. This convex shape in the free energy surface will appear in multi-component systems, even though the energy in each binary system does not show the convex curve, when a binary system exhibits a strong attractive interaction, i.e., the Gibbs free energy shows a deep concave curve. This may be the case that steel containing an element having a strong tendency of carbide forming. The Fe-Ni-V-C system was selected in this study. V is a strong carbide forming element and forms VC carbide based on f.c.c. lattice. A series of experiments was carried out using Fe-25Ni-3V-3C (mol%) alloy after aging at elevated temperatures. A modulated structure was observed in specimens aged at temperature range of 550 °C to 650 °C, and the maximum temperature occurring the spinodal decomposition was estimated to be 715 °C in Fe-25Ni-3V-3C alloy. Furthermore, the coefficient of the gradient energy caused by composition fluctuations was estimated to be 2.8 x 10-15 J.m2/mol.

Abstract: The results for 3D anisotropic Ising model with competing interactions (ANNNI) investigated by the Monte Carlo methods are presented. The temperature dependence of thermal parameters is calculated. The character of all possible phase transitions in the model is analyzed.

Abstract: We have investigated the modulated structure of the misfit-layered crystal Bi1.8Sr2.0Rh1.6Ox by means of electron diffraction and high-resolution electron microscopy. This compound consists of two interpenetrating subsystems of a hexagonal RhO2 sheet and a distorted four-layered rock-salt-type (Bi,Sr)O block. Both subsystems have common a-, c-axes and β-angles with a = 5.28 Å, c = 29.77 Å and β = 93.7º. On the other hand, the crystal structure is incommensurated parallel to the b-axes, among which b1 = 3.07 Å for the RhO2 sheet and b2 = 4.88 Å for the (Bi,Sr)O block. The misfit ratio, b1/b2 ~ 0.63, characterizes the structural analogue as [Bi1.79Sr1.98Oy]0.63[RhO2]. This compound has two modulation vectors, i.e., q1 = – a* + 0.63b1* and q2 = 0.17b1* + c*, and the superspace group is assigned as the Cc(1β0, 0μ1)-type from the electron diffraction patterns. High-resolution images taken with the incident electron beam parallel to the a- and c-axes clearly show displacive as well as compositional modulations.