Competition between Ferrimagnetism and Magnetic Frustration in Zinc Substituted YBaFe4O7

The substitution of zinc for iron in YBaFe4O7 has allowed the oxide series YBaFe4-xZnxO7, with 0.40 < x < 1.50, belonging to the "114" structural family to be synthesized. These oxides crystallize in the hexagonal symmetry (P63mc), as opposed to the cubic symmetry (F-43m) of YBaFe4O7. Importantly, the d.c. magnetization shows that the zinc substitution induces ferrimagnetism, in contrast to the spin glass behaviour of YBaFe4O7. Moreover, a.c. susceptibility measurements demonstrate that concomitantly these oxides exhibit a spin glass or a cluster glass behaviour, which increases at the expense of ferrimagnetism, as the zinc content is increased. This competition between ferrimagnetism and magnetic frustration is interpreted in terms of lifting of the geometric frustration, inducing the magnetic ordering, and of cationic disordering, which favours the glassy state.Comment: 24 pages, 10 figure

Sintering mechanism and grain growth in CaCu 3 Ti 4 O 12 ceramics

In this work, a study of the densification of CaCu 3 Ti 4 O 12 (CCTO) ceramics during conventional sintering process is presented. Dilatometric study was conducted in detail to find out which diffusion mechanism is responsible for densification. Based on the shrinkage curves, it was shown that densification is mostly controlled by grain boundary diffusion. The microstructure evolution of CCTO during conventional sintering was studied through the sintering trajectory, which corresponds to a unique relationship between grain size and density. According to SEM analysis, it was shown that abnormal grain growth takes place for samples sintered at high temperature stage with density andgt;80% of the theoretical density. Finally, the Master Sintering Curve (MSC) method was also used to characterize the densification process of CCTO and estimate the apparent sintering activation energy (Q = 730 kJ mol -1 ). © 2019 Elsevier Ltd and Techna Group S.r.l

Sintering mechanism and grain growth in CaCu 3 Ti 4 O 12 ceramics

International audienceIn this work, a study of the densification of CaCu 3 Ti 4 O 12 (CCTO) ceramics during conventional sintering process is presented. Dilatometric study was conducted in detail to find out which diffusion mechanism is responsible for densification. Based on the shrinkage curves, it was shown that densification is mostly controlled by grain boundary diffusion. The microstructure evolution of CCTO during conventional sintering was studied through the sintering trajectory, which corresponds to a unique relationship between grain size and density. According to SEM analysis, it was shown that abnormal grain growth takes place for samples sintered at high temperature stage with density andgt;80% of the theoretical density. Finally, the Master Sintering Curve (MSC) method was also used to characterize the densification process of CCTO and estimate the apparent sintering activation energy (Q = 730 kJ mol -1 ). © 2019 Elsevier Ltd and Techna Group S.r.l

Valence and spin-state transition in cobaltates revisited by x-ray magnetic circular dichroism

International audienceThe compounds (Pr1−ySmy)0.7Ca0.3CoO3 belong to a class of Pr-based cobaltates presenting a unique case of simultaneous valence (charge transfer between Pr and Co ions) and spin-state transition (of the Co3+ ions), hereafter referred to as VSST. The present study sheds light on the debated issues of the Co4+ and Co3+ spin states, by combining x-ray absorption spectroscopy (XAS) at Co and Pr edges and x-ray magnetic circular dichroism (XMCD) at Co L2,3 edges. XAS experiments at both L3 and M4,5 Pr edges attest to the appearance of Pr4+ below the VSST at T∗=106K, and allow a precise characterization of the evolution of the Co4+ content as a function of the temperature. XMCD at the Co L2,3 edges at 5 K, and in magnetic field up to 9 T, directly tackles the issue of the Co4+ spin state. It is found that the Co4+ ions are most likely in a low spin state, and experience ferromagnetic interactions at T≪T∗. On the basis of temperature dependent XMCD at 9 T, the fingerprint of the VSST on the Co moments is isolated, and found to be consistent with bulk magnetization data when accounting for the rare-earth contributions derived from reference samples. These temperature dependent XMCD data are used to characterize the evolutions of the various valence/spin state of the Co species involved in the VSST. It appears that the Co3+ moments above T∗ are not consistent with a pure intermediate spin state, whereas they can be well reproduced by considering a low/high spin mixture. Finally, these XMCD results are compared to those derived from fitting of the XAS spectra recorded in zero field at various temperatures

Silver intercalation in SPS dense TiS<inf>2</inf> Staging and thermoelectric properties

Polycrystalline samples in the series AgxTiS2 with x varying from 0 to 0.2 were prepared using solid-liquid-vapor reaction and spark plasma sintering. Depending on the x content, it is found that different stages can occur with intercalation, from the so-called 1T-TiS2 (stage 1) to ordered Ag1/6TiS2 (stage 2). Randomly intercalated Ag cations in the van der Waals gap of stage 1 and stage 2 based TiS2 structures induce a strong decrease of lattice thermal conductivity through structural disorder. A decrease in electrical resistivity and the absolute value of the Seebeck coefficient with increasing Ag content supports also the charge transfer to the Ti 3d conduction band, enhancing the power factor in the specific temperature range. Thus, the combined effects of Ag intercalation are beneficial to the improvement in ZT, reaching around 0.45 at 700 K in Ag intercalated compounds. © The Royal Society of Chemistry 2015