Transport and Magnetic Properties of R1-xAxCoO3 (R=La, Pr and Nd; A=Ba, Sr and Ca)

Transport and magnetic measurements have been carried out on perovskite Co-oxides R1-xAxCoO3 (R=La, Pr, and Nd; A=Ba, Sr and Ca; 0<x<0.5: All sets of the R and A species except Nd1-xBaxCoO3 have been studied.). With increasing the Sr- or Ba-concentration x, the system becomes metallic ferromagnet with rather large magnetic moments. For R=Pr and Nd and A=Ca, the system approaches the metal- insulator phase boundary but does not become metallic. The magnetic moments of the Ca-doped systems measured with the magnetic field H=0.1 T are much smaller than those of the Ba- and Sr-doped systems. The thermoelectric powers of the Ba- and Sr-doped systems decrease from large positive values of lightly doped samples to negative ones with increasing doping level, while those of Ca-doped systems remain positive. These results can be understood by considering the relationship between the average ionic radius of R1-xAx and the energy difference between the low spin and intermediate spin states. We have found the resistivity-anomaly in the measurements of Pr1-xCaxCoO3 under pressure in the wide region of x, which indicates the existence of a phase transition different from the one reported in the very restricted region of x~0.5 at ambient pressure [Tsubouchi et al. Phys. Rev. B 66 (2002) 052418.]. No indication of this kind of transition has been observed in other species of R.Comment: 9 pages, 8 figures. J. Phys. Soc. Jpn. 72 (2003) No.

Charge ordering and dielectric properties in the near half-doped Pr0.79Na0.21MnO3 perovskite

[Abstract] The Pr0.79Na0.21MnO3 perovskite has been prepared in polycrystalline form by a ceramic method. We have carried out its structural characterization by synchrotron X-ray powder diffraction (SXRPD) at room temperature and at 150 K (T<TCO). In the charge ordering (CO) state, the SXRPD data can be refined both on the basis of the site-centred model and the bond-centred model proposed for half-doped manganites, without a clear advantage of one model over the other. From the dielectric point of view, at the CO temperature this manganite shows a maximum in the dielectric constant whose origin is intrinsic, and cannot be attributed to the presence of extrinsic factors. We relate this dielectric behaviour to the formation of polar entities at the temperature of charge condensation, due to an asymmetric charge distribution intermediate between site-centred and bond-centred type.Xunta de Galicia; PGIDIT06PXB103298PRMinisterio de Educación y Ciencia; Project FEDER MAT 2004-0513

Studies on the power factor of (Ba,Sr)Co2+xRu4−xO11 compounds

[Abstract] We have prepared polycrystalline single-phase ACo2+xRu4−xO11 (A = Sr, Ba; 0 ≤ x ≤ 0.5) using the ceramic method and we have studied their structure, electrical resistivity and Seebeck coefficient, in order to estimate their power factor (P.F.). These layered compounds show values of electrical resistivity of the order of 10−5 Ωm and their Seebeck coefficients are positive and range from 1 μV K−1 (T = 100 K) to 20 μV K−1 (T = 450 K). The maximum power factor at room temperature is displayed by BaCo2Ru4O11 (P.F.: 0.20 μW K−2 cm−1), value that is comparable to that shown by compounds such as SrRuO3 and Sr6Co5O15.Ministerio de Ciencia e Innovación; Project FEDER MAT 2007-6669

Influence of aluminum doping on the properties of LiCoO2 and LiNi0.5Co0.5O2 oxides

We have prepared LiCo1−yAlyO2 and LiNi0.5−yAlyCo0.5O2 (0≤y≤0.3) powder samples by a low temperature sol–gel method using succinic acid as chelating agent. We have studied the details of their crystallographic and local structure by X-ray diffraction (XRD) and FTIR spectroscopy, respectively; we have analyzed their chemical composition by ICP and obtained information about the morphology of the polycrystalline particles by SEM. Also, we have studied the electrochemical performance of the as-prepared materials in the LiLiNi0.5−yAlyCo0.5O2 cells cycled in the potential range 2.5–4.2 V finding that the overall capacity of the oxides has been reduced due to the metal substitution. For example, at 4.2 V cut-off, the charge capacity of the LiLiNi0.35Al0.15Co0.5O2 cell is ca. 115 mA h/g. However, more stable charge–discharge cycling performances have been obtained as compared to those displayed by the native oxides. Finally, we have characterized the kinetics of Li-diffusion by the galvanostatic intermittent titration technique and, according to our results, Al substitution provides an increase in the chemical diffusion coefficients of Li ions in the LiNi0.5−yAlyCo0.5O2 matrix.Spanish and French Foreing Office; PAI Picasso 00717TCSpanish and French Foreing Office; HF 1999-010

Field-induced magnetic anisotropy in La0.7Sr0.3CoO3

Magnetic anisotropy has been measured for the ferromagnetic La0.7Sr0.3CoO3 perovskite from an analysis of the high-field part of the magnetization vs. field curves, i.e., the magnetic saturation regime. These measurements give a magnetic anistropy one order of magnitude higher than that of reference manganites. Surprisingly, the values of the magnetic anisotropy calculated in this way do not coincide with those estimated from measurements of coercive fields which are one order of magnitude smaller. It is proposed that the reason of this anomalous behaviour is a transition of the trivalent Co ions under the external magnetic field from a low-spin to an intermediate-spin state. Such a transition converts the Co3+ ions into Jahn-Teller ions having an only partially quenched orbital angular momentum, which enhances the intra-atomic spin-orbit coupling and magnetic anisotropy.Comment: Accepted of publication in Europhysics Letters, 11 pages, 5 figure

IR characterization of Ln2−xSrxCoO4 (x≥1; Ln=La, Nd) oxides

[Abstract] We have recorded the FTIR spectra of powder samples of Ln2−xSrxCoO4 (Ln=La, Nd) at room temperature. We have identified the infrared active modes (3A2u+4Eu), and analyzed how they change as a function of Ln and the Sr doping. We correlate the obtained results with structural data obtained from powder X-ray diffraction studies and with the electronic properties displayed by these samples

Multiferroic behavior in the new double-perovskite Lu2_2MnCoO6_6

We present a new member of the multiferroic oxides, Lu$_2$MnCoO$_6$, which we have investigated using X-ray diffraction, neutron diffraction, specific heat, magnetization, electric polarization, and dielectric constant measurements. This material possesses an electric polarization strongly coupled to a net magnetization below 35 K, despite the antiferromagnetic ordering of the $S = 3/2$ Mn$^{4+}$ and Co$^{2+}$ spins in an $\uparrow \uparrow \downarrow \downarrow$ configuration along the c-direction. We discuss the magnetic order in terms of a condensation of domain boundaries between $\uparrow \uparrow$ and $\downarrow \downarrow$ ferromagnetic domains, with each domain boundary producing a net electric polarization due to spatial inversion symmetry breaking. In an applied magnetic field the domain boundaries slide, controlling the size of the net magnetization, electric polarization, and magnetoelectric coupling

Magnetic Structures of High Temperature Phases of TbBaCo2O5.5

Neutron diffraction studies have been carried out on a single crystal of oxygen-deficient perovskite TbBaCo2O5.5 in the temperature range of 7-370 K. There have been observed several magnetic or structural transitions. Among these, the existence of the transitions to the insulating phase from the metallic one at ~340 K, to the one with the ferromagnetic moment at ~280 K and possibly to the antiferromagnetic one at ~260 K, with decreasing temperature T correspond to those reported in former works. We have studied the magnetic structures at 270 K and 250 K and found that all Co3+ ions of the CoO6 octahedra are in the low spin state and those of the CoO5 pyramids carry spins which are possibly in the intermediate spin state. Non-collinear magnetic structures are proposed at these temperatures. Two other transitions have also been observed at the temperatures, ~100 K and ~250 K.Comment: 9 pages, 2 tables, 10 figure

Narrowing the Tolerance Factor Limits for Hybrid Organic-Inorganic Dicyanamide-Perovskites

[Abstract] In this work we focus in setting the limits of the tolerance factor and the size of the A-cations that stabilize the perovskite structure in hybrid dicyanamide compounds [A][Mn(dca)3]. For this purpose, we propose an alternative, simple approach to calculate a more realistic effective ionic radius for the large and anisotropic A-cations often present in these type of compounds. We test the proposed procedure by analysing the crystal structures of [A][Mn(dca)3] dicyanamide hybrids reported in the literature and recalculating the tolerance factors of such compounds, as well as by preparing five new [A][Mn(dca)3] members, discussing also the influence of the A-cation shape in the stability limits of the perovskite structure. Interestingly, such methodology is not only useful to develop new compounds of the emerging family of (multi)functional multi(stimuli)-responsive dicyanamide materials but can also be applied to other hybrid organic-inorganic perovskites and related materials.As for financial support, the authors thank Ministerio de Economía y Competitividad MINECO and EU-FEDER (projects MAT2017-86453-R and PDC2021-121076-I00), and Xunta de Galicia for the collaboration agreement “Development of research strategic actions Universidade da Coruña I ​+ ​D ​+ ​i 2021–2022: CICA-Disrupting Projects 2021SEM-A3 (NanoCool). J.G.-B. and J.M.B.-G. acknowledge Xunta de Galicia for Predoctoral and Postdoctoral Fellowships, respectively. I.D.-F thanks Ministerio de Universidades for a FPU Predoctoral Fellowship. A. G.-F. and U.B.C thank the Carl Tryggers foundation and the Göran Gustafsson foundation for financial suppor

Anomalous and Colossal Thermal Expansion, Photoluminescence, and Dielectric Properties in Lead Halide-Layered Perovskites With Cyclohexylammonium and Cyclopentylammonium Cations

[Abstract] A detailed study of lead halide-layered perovskites with general formula A2PbX4 (where A is cyclohexylammonium (CHA) or cyclopentylammonium (CPA) cation and X is Cl− or Br− anion) is presented. Using variable temperature synchrotron X-ray powder diffraction, we observe that these compounds exhibit diverse crystal structures above room temperature. Very interestingly, we report some unconventional thermomechanical responses such as uniaxial negative thermal expansion and colossal positive thermal expansion in a perpendicular direction. For the polymorphs of (CHA)2PbBr4, the volumetric thermal expansion coefficient is among the highest reported for any extended inorganic crystalline solid, reaching 480 MK−1. The phase transitions are confirmed by calorimetry and dielectric measurements, where the dielectric versus temperature curves show anomalies related with the order-disorder phase transitions. In addition, these compounds exhibit a broad photoluminescence (PL) emission with a large Stokes shift, which is related with an exciton PL emission.This work was financially supported by the Ministerio de Ciencia e Innovación (MICINN) and EU-ERDF under the projects MAT2017-86453-R and PDC2021-121076-I00. We thank Alba synchrotron for the provision of beamtime at BL04-MSPD beamline (experiment 2019023280). We also thank prof. Dr. R. Artiaga and Dr. J.J. López-Beceiro (University of A Coruña) for assistance during DSC measurements. J.M.B.G. thanks Xunta de Galicia for a postdoctoral fellowshi