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Papers by Cristian Perca
The fruitful contribution of Broadband Dielectric Spectroscopy (BDS) to study hierarchical materi... more The fruitful contribution of Broadband Dielectric Spectroscopy (BDS) to study hierarchical materials applied to batteries electrodes has been previously shown [1-5]. The results demonstrate that the broadband dielectric spectroscopy technique is very sensitive to the different scales of the electrode architecture involved in the electronic transport, from interatomic distances to macroscopic sizes, as well as to the morphology at these scales, coarse or fine distribution of the constituents. When the frequency increases, different kinds of polarizations appear from interatomic distances to macroscopic sizes (Fig. 1) and give rise to dielectric relaxations in the following order: (a) space-charge polarization (low-frequency range) due to the interface sample/current collector; (b) polarization of clusters of particles (micronic scale) and (c) polarization of particles due to the existence of resistive junctions between them; d) electron transfers (nanometric or interatomic scale).The...
Cette these porte sur la synthese et l'etude des proprietes structurales et magnetiques de pe... more Cette these porte sur la synthese et l'etude des proprietes structurales et magnetiques de perovskites RBaMn2O6-d (ou d = 0 et 0,5). En ce qui concerne le compose YBaMn2O6, les mesures par DSC indiquent que celui-ci subit trois transitions : deux autour de 500K et la troisieme a 750K. L'etude par diffraction des rayons X et de neutrons a montre que la phase de plus haute temperature cristallisait dans le groupe P4/nbm. Les diffractogrammes de rayons X et de neutrons ne presentent qu'un seul changement alors que deux transitions sont observees par DSC au voisinage de 500K. La phase intermediaire a ete affinee dans le groupe d'espace C2/m. La phase d'ordre de charges (en dessous de 500K) etant de basse symetrie, nous avons realise un affinement combine rayons X-neutrons. Les resultats obtenus indiquent un ecart de seulement 15% par rapport a un ordre de charges ideal Mn3+/Mn4+. L'affinement de la structure magnetique et les mesures de susceptibilite magnetique ...
The determination of the magnetic structures of manganites has always been at the root of their f... more The determination of the magnetic structures of manganites has always been at the root of their fundamental understanding [1]. We studied the magnetic structures of half-doped charge ordered manganites that are either show the prototype [1] CE-type magnetic structure (Pr1/2Ca1/2MnO3), or variants of this order (YBaMn2O6 [2] and Pr0.6Ca0.4MnO3) with neutron diffraction. The study of Pr1/2Ca1/2MnO3 (ILL, France) is the first ever done on a single crystal and it essentially confirms the pioneering picture [1], whereas the NPD studies of YBaMn2O6 [2] (PSI, Switzerland) and Pr0.6Ca0.4MnO3 [3] (ISIS, UK), give two unprecedented results. The YBaMn2O6 magnetic structure corroborates the hotly debated ordering of Zener Polarons [4], and high resolution NPD data evidence a new spin reorientation transition around T˜20K far below its TN˜170K in Pr0.6Ca0.4MnO3 [3] that has so far only been vaguely observed. We will discuss the consequences that these results have on the still hotly debated understanding of the connection between charge/orbital and spin orderings in the manganites. [1] Wollan, E.O. and Koehler, W.C. Rev. 100, 545 (1955) [2] A. Daoud-Aladine et al., Phys. Rev. Lett.: 101 166404 (2008) [3] A. Daoud-Aladine et al., unpublished [4] M. Coey Nature 430, 155-157 (8 July 2004)
The fruitful contribution of Broadband Dielectric Spectroscopy (BDS) to study hierarchical materi... more The fruitful contribution of Broadband Dielectric Spectroscopy (BDS) to study hierarchical materials applied to batteries electrodes has been previously shown [1-5]. The results demonstrate that the broadband dielectric spectroscopy technique is very sensitive to the different scales of the electrode architecture involved in the electronic transport, from interatomic distances to macroscopic sizes, as well as to the morphology at these scales, coarse or fine distribution of the constituents. When the frequency increases, different kinds of polarizations appear from interatomic distances to macroscopic sizes (Fig. 1) and give rise to dielectric relaxations in the following order: (a) space-charge polarization (low-frequency range) due to the interface sample/current collector; (b) polarization of clusters of particles (micronic scale) and (c) polarization of particles due to the existence of resistive junctions between them; d) electron transfers (nanometric or interatomic scale).The...
Cette these porte sur la synthese et l'etude des proprietes structurales et magnetiques de pe... more Cette these porte sur la synthese et l'etude des proprietes structurales et magnetiques de perovskites RBaMn2O6-d (ou d = 0 et 0,5). En ce qui concerne le compose YBaMn2O6, les mesures par DSC indiquent que celui-ci subit trois transitions : deux autour de 500K et la troisieme a 750K. L'etude par diffraction des rayons X et de neutrons a montre que la phase de plus haute temperature cristallisait dans le groupe P4/nbm. Les diffractogrammes de rayons X et de neutrons ne presentent qu'un seul changement alors que deux transitions sont observees par DSC au voisinage de 500K. La phase intermediaire a ete affinee dans le groupe d'espace C2/m. La phase d'ordre de charges (en dessous de 500K) etant de basse symetrie, nous avons realise un affinement combine rayons X-neutrons. Les resultats obtenus indiquent un ecart de seulement 15% par rapport a un ordre de charges ideal Mn3+/Mn4+. L'affinement de la structure magnetique et les mesures de susceptibilite magnetique ...
The determination of the magnetic structures of manganites has always been at the root of their f... more The determination of the magnetic structures of manganites has always been at the root of their fundamental understanding [1]. We studied the magnetic structures of half-doped charge ordered manganites that are either show the prototype [1] CE-type magnetic structure (Pr1/2Ca1/2MnO3), or variants of this order (YBaMn2O6 [2] and Pr0.6Ca0.4MnO3) with neutron diffraction. The study of Pr1/2Ca1/2MnO3 (ILL, France) is the first ever done on a single crystal and it essentially confirms the pioneering picture [1], whereas the NPD studies of YBaMn2O6 [2] (PSI, Switzerland) and Pr0.6Ca0.4MnO3 [3] (ISIS, UK), give two unprecedented results. The YBaMn2O6 magnetic structure corroborates the hotly debated ordering of Zener Polarons [4], and high resolution NPD data evidence a new spin reorientation transition around T˜20K far below its TN˜170K in Pr0.6Ca0.4MnO3 [3] that has so far only been vaguely observed. We will discuss the consequences that these results have on the still hotly debated understanding of the connection between charge/orbital and spin orderings in the manganites. [1] Wollan, E.O. and Koehler, W.C. Rev. 100, 545 (1955) [2] A. Daoud-Aladine et al., Phys. Rev. Lett.: 101 166404 (2008) [3] A. Daoud-Aladine et al., unpublished [4] M. Coey Nature 430, 155-157 (8 July 2004)