Correlations of structural, magnetic, and dielectric properties of undoped and doped CaCu3Ti4O12

The present work reports synthesis, as well as a detailed and careful characterization of structural, magnetic, and dielectric properties of differently tempered undoped and doped CaCu3Ti4O12 (CCTO) ceramics. For this purpose, neutron and x-ray powder diffraction, SQUID measurements, and dielectric spectroscopy have been performed. Mn-, Fe-, and Ni-doped CCTO ceramics were investigated in great detail to document the influence of low-level doping with 3d metals on the antiferromagnetic structure and dielectric properties. In the light of possible magnetoelectric coupling in these doped ceramics, the dielectric measurements were also carried out in external magnetic fields up to 7 T, showing a minor but significant dependence of the dielectric constant on the applied magnetic field. Undoped CCTO is well-known for its colossal dielectric constant in a broad frequency and temperature range. With the present extended characterization of doped as well as undoped CCTO, we want to address the question why doping with only 1% Mn or 0.5% Fe decreases the room-temperature dielectric constant of CCTO by a factor of ~100 with a concomitant reduction of the conductivity, whereas 0.5% Ni doping changes the dielectric properties only slightly. In addition, diffraction experiments and magnetic investigations were undertaken to check for possible correlations of the magnitude of the colossal dielectric constants with structural details or with magnetic properties like the magnetic ordering, the Curie-Weiss temperatures, or the paramagnetic moment. It is revealed, that while the magnetic ordering temperature and the effective moment of all investigated CCTO ceramics are rather similar, there is a dramatic influence of doping and tempering time on the Curie-Weiss constant.Comment: 10 pages, 11 figure

Local structure study about Co in YBa2_2(Cu1−x_{1-x}Cox_x)3_3O7−δ_{7-\delta} thin films using polarized XAFS

We have studied the local structure around Co in YBa$_2$(Cu$_{1-x}$Co$_x$)$_3$O$_{7-\delta}$ thin films with three different concentrations: x=0.07, 0.10, 0.17, and in a PrBa$_2$(Cu$_{1-x}$Co$_x$)$_3$O$_{7-\delta}$ thin film of concentration x=0.05 using the X-ray Absorption Fine Structure (XAFS) technique. Data were collected at the Co $K$-edge with polarizations both parallel and perpendicular to the film surface. We find that the oxygen neighbors are well ordered and shortened in comparison with YBCO Cu-O values to 1.80 \AA{} and 1.87 \AA{} in the $c$-axis and $ab$-plane, respectively. A comparison of further neighbors in the thin film and powder data show that these peaks in the film are suppressed in amplitude relative to the powder samples, which suggests there is more disorder and/or distortions of the Co environment present in the thin films.Comment: 14 pages; To be submitted to Phys. Rev.

The Crystal Structure of Ba (Bi0.977K0.023)O3 by Single-Crystal X-Ray Diffraction

We determined by X-ray diffraction the structure of a slightly K-doped BaBiO3 single-crystal. The best refinements were obtained by describing the structure in the monoclinic I2/m space group similarly to the structure of the undoped compound. In this description the Bi cations occupy two crystallographically independent sites. We found that at this low K concentration, the K atoms are not, as expected, substituting for the Ba cations but are located on one of the two Bi sites. The two octahedral Bi sites are alternatively compressed and elongated along the c-axis. The formal valences of the two Bi cations deduced from the Brown and Altermatt formula are 3.79 and 4.44, respectively. This indicates that these cations are disproportionated into Bi3+ and Bi5+; however, they are only partially ordered

Two-phase structural refinement of La2CuO4.032 at 15 K

A two-phase refinement of a crystal for the compound La2CuO4+δ (δ=0.032), based on the neutron diffraction data collected at 15 K with the D9 diffractometer at ILL, using {double barred pipe}=0.48 A ̊, has been carried out. One phase (30%) consists of stoichiometric La2CuO4 domains, while the other (70%) of oxygen-rich La2CuO4.048 domains. The percentages of each phase, which have been refined together with the other parameters (scale factor, positional parameters, thermal factors and occupancy factors of the oxygen atoms), agree very well with the value determined from χAC measurements. The La2CuO4.048 structure is essentially the same as the average structure reported in ref. [1], the only difference being the oxygen content which is found to be δ= δ 0.70. The extra oxygen, O(4), is found to be in between two LaO layers in a similar position as the oxygen atoms located between two Nd layers in the N2CuO4 structure. The insertion of extra oxygen causes the displacement of some of the oxygen O(1) towards the O(3) positions. Different models are proposed for the distortion induced by this insertion according to the experimental value, 3.3 [6], found for the ratio of the amount of O(3) to that of O(4). If this ratio is assumed to be 3 and the O(3) atoms are localized about the insertion, then the formation of a short O(4)-O(3) bond would occur. The models not requiring the formation of the short bond correspond to ratios of 2 or 4. In the latter case there would be four displaced O(1), but due to the rigidity of the oxygen octahedra only two O(3) would be bonded to O(4), the other two being the apically opposite oxygen atoms of the same octahedra. © 1990

Structural aspects of the phase separation in La2CuO4.032

The average structure of superconducting La CuO has been determined by single-crystal neutron diffraction data. The excess oxygen is located between two adjacent LaO layers. Its presence distorts the apical-oxygen sublattice in such a way that a short OO bond is formed (1.64Å). By scanning several hkl reflections, we have confirmed that a phase separation occurs below room temperature. The peaks of one phase are in agreement with the space group Cmca and the unit cell proposed by Jorgensen et al. and Cox et al.. However, a monoclinic unit cell is needed to index the second phase reflections. © 1989. 2 4.03

MAGNETIC BEHAVIOUR AND PHASE TRANSITION IN [ThCu3] (Mn4)O12

Le composé [ThCu3] (Mn4)o12 appartient à la série des oxydes [AC3] (B4)O12 de structure pérovskite dans lesquels les sites [A] icosaédriques sont occupés par les cations Th4+, les sites [C] plans-carrés par les cations Cu2+ et les sites (B) octaédriques par les cations Mn3+ et Mn4+ en proportions égales. Diverses mesures indiquent un comportement ferrimagnétique avec une température d'ordre TN = 430 K, une aimantation spontanée à 4,2 K de 9 µB/unité de formule et une transition à Tt = 180 K. La structure ferrimagnétique, déduite des expériences de diffraction neutronique est en accord avec les valeurs des interactions magnétiques tirées des mesures d'aimantation et de susceptibilité. La transition Tt a été étudiée par des mesures d'aimantation et de diffraction des rayons X. Elle est caractérisée par une variation thermique anormale du paramètre de maille et du moment magnétique, sans changement de la symétrie cristallographique et magnétique.The compound [ThCu3] (Mn4)O12 belongs to the series of [AC3] (B4) O12 perovskite-like oxides in which the icosahedral sites [A] are occupied by the Th4+ cations, the square planar sites [C] by the Cu2+ cations, and the octahedral sites (B) by the Mn3+ and Mn4+ cations in equal proportion. Previous measurements indicated a ferrimagnetic behavior with an ordering temperature TN= 430 K, a spontaneous magnetization of 9µB/formula unit at 4.2 K and a magnetic transition at Tt = 180 K. The ferrimagnetic structure, as determined from powder neutron diffraction data, is consistent with the values of magnetic interactions that can be estimated from magnetization and susceptibility measurements. The low temperature transition has been studied by magnetization measurements and X-Rays techniques. It seems to be of second order with anomalies in the cubic cell parameter and of the magnetic moments. No changes in cristallographic symmetry and magnetic structure are observed

Structural aspects of the phase separation in La2CuO4.032

The average structure of superconducting La2CuO4.032 has been determined by single-crystal neutron diffraction data. The excess oxygen is located between two adjacent LaO layers. Its presence distorts the apical-oxygen sublattice in such a way that a short OO bond is formed (1.64Å). By scanning several hkl reflections, we have confirmed that a phase separation occurs below room temperature. The peaks of one phase are in agreement with the space group Cmca and the unit cell proposed by Jorgensen et al. and Cox et al.. However, a monoclinic unit cell is needed to index the second phase reflections. © 1989

STRUCTURAL CHARACTERIZATION, PREPARATION AND MAGNETIC PROPERTIES OF SEVERAL ROOM-TEMPERATURE MAGNETIC OXIDES WITH THE PEROVSKITE-LIKE STRUCTURE

Nous avons préparé par synthèse hydrothermale classique ou sous haute pression une nouvelle série d'oxydes magnétiques à la température ambiante dont la structure dérive de la pérovskite ABO3. La formule générale de ces composés est [AC3] (B4)O12. Leur structure est cubique ; le paramètre de maille est double de celui de la pérovskite idéale. Les cations B occupent le centre d'octaèdres d'oxygène mettant en commun un atome. Ils forment des chaînes se propageant en zig-zag dans les trois directions [100]. Il existe un ordre 1/3 entre les cations A et C. Les cations A sont entourés par 12 voisins oxygène équidistants alors que les cations C sont entourés par 4 voisins oxygène équidistants formant un carré et par 8 autres atomes d'oxygène beaucoup plus éloignés. Un tel arrangement avec 4 oxygènes très proches explique pourquoi seuls des cations Jahn-Teller tels que Cu2+ ou Mn3+ peuvent occuper les sites C. Ces cations sont d'ailleurs probablement nécessaires à la stabilité de la structure. Deux sous-séries ont été préparées : [ACu2+3] (B4)O12 et [AMn3+3 (B4) O12. Lorsque les cations B sont Mn4+ ou Mn3+ ou les deux, les composés de la sous-série du cuivre sont fortement ferro- ou ferrimagnétiques au-dessus de la température ambiante alors que les composés de la sous-série du manganèse sont antiferromagnétiques. Nous avons étudié les propriétés magnétiques de plusieurs composés et déterminé leur structure magnétique par diffraction neutronique sur des échantillons de poudre.A new series of room temperature perovskite-like magnetic oxides has been synthesized by using either the high pressure or the hydrothermal technique. The general formula of these compounds is [AC3] (B4)O12. Their cubic structure is derived by doubling the lattice parameter of the undistorted perovskite structure. The oxygen octahedra which are occupied by the B cations form zig-zag chains along the [100] directions. The A sites are still surrounded by 12 equidistant oxygen atoms while the C sites are surrounded by four close ones arranged as a small square and by eight oxygen atoms further away. The square arrangement of the first neighbours explains why only Jahn-Teller cations, such as CU2+ and Mn3+, have been observed to occupy these sites. These cations are probably necessary to insure the stability of the structure. Two subseries have been prepared so far : [ACu2+3] (B4)O12 and [AMn3+3 (B4) O12. When the B cations are Mn4+ ou Mn3+ or both, the compounds of the copper subseries are strongly ferro- or ferrimagnetic above room temperature, whereas the compounds of the manganese subseries are antiferromagnetic. Magnetic properties of several compounds have been investigated and magnetic structures have been determined from neutron diffraction powder data

High pressure synthesis and structure of the superconducting mercury cuprates (Hg1-xMx)Ba2Can-1CunO2+2n+delta with M = C, S.

In mercury superconducting cuprates synthesized at high pressure, partial carbon substitution on the mercury site occurs when the precursors are not absolutely carbon free. Hg-12(n-1)n samples made from carbonate-containing precursors show lower Tcs and smaller c parameter. A model of the structural arrangement of the CO3 -2 groups is proposed from neutron diffraction data and crystal-chemistry considerations. The third oxygen of the oxyanion is situated in the (Hg,C)O∂ layer and hinders the incorporation of extra-oxygen atoms. The (Hg1-xCx)Ba2CuO4+∂ phase diagram shows three different phases, namely HgBa2CuO4+∂, Hg0.5C0.5Ba2CuO4+∂ and CBa2CuO4+∂. The latter two phases are not superconducting due to the insufficient hole concentration on the superconducting layer. Sulphur can also partially replace mercury in Hg1-xSxBa2CuO4+∂ and forms a solid solution up to x=0.15. Neutron diffraction shows that the sulphur atoms arrange as S04 -2 oxyanions. The larger S-O distances induce steric limitations to the presence of other sulphate anions in the same mesh