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Structural investigation in ferroelectric, magnetoresistive and multiferroïc Perovskite materials.

Updated on October 25, 2011
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Structural investigation in ferroelectric, magnetoresistive and multiferroïc Perovskite materials. Student : C. Lhaulé, .J. Dhahri, N. Ihzaz, S. Trujillo, A.Gasmi, M. Baazaoui Objectives The general goal of this subject is to understand the link between structural and physical properties in perovskites oxides with outstanding properties : relaxor ferroelectrics, colossal magnetoresistance and multiferroïc materials.  Main results
  • Ferroelectric Relaxors exhibit a broad and frequencydependent maximum of the dielectric permittivity as a function of temperature, instead of the sharp and frequency-independent anomaly found in classical ferroelectrics. The relaxor behaviour is usually related to the dynamics of randomly orientated polar nanoregions inside which cation displacements are correlated. However both the origin and nature of these nanoregions are highly debated in the literature. Getting insight in their structural nature requires accurate structural studies on the local and average scale. In the framework of the Ph-D thesis of C. Laulhé, we have studied the local structure in BaTi1-xZrxO3 relaxors (0.25 < x < 0.50) by associating X-ray absorption spectroscopy (EXAFS and XANES) at the Zr and Ti Kedges and neutron pair distribution function determinations. Although, according to X-ray diffraction, the average crystallographic structure is cubic, Ti atoms are found displaced from the centre of their octahedra by about 0.18 Å, like in the rhombohedral phase of the ferroelectric BaTiO3. Besides, Zr displacements remain small (less than 0.05 Å). These conclusions apply at all temperature in the range 10 - 300 K, i.e. below and above the maximum of the dielectric permittivity. Hence Ti displacements are responsible for the local polarization in BaTi1-xZrxO3 relaxors. The shape and volume of TiO6 and ZrO6 octahedra are quite different (Zr-O distance 2.10 Å and average Ti-O distance 2.02 Å), thus a further understanding of their connection is required to clarify the correlations between Ti displacements. A first answer has been obtained from the comparison between experiments and ab-initio calculations. The displacement direction of a given Ti atom is influenced by the nature of the atoms (Zr or Ti) in the neighbouring octahedra. This interesting and original result explains the limited range of ferroelectric correlations in BaTi1-xZrxO3 relaxors and underlines the key role of the Zr/Ti substitution in breaking the long range ferroelectric order present in BaTiO3.
Collaborations : ICMCB Bordeaux), InstitutNéel & ESRF /CRG FAME, Grenoble ; ILL Grenoble) ;
LPMMC & SIMAP Grenoble.
Alternative investigations on the local structure of potential relaxors systems was carried out by X-ray diffraction diffuse scattering measurements (synchrotron and standard CCD diffractometer mapping) on single crystal on the lead-free piezoelectric perovskites Na0.5Bi0.5TiO3 (NBT) and (Na0.5Bi0.5)0.89Ba0.11TiO3
(NBTBa0.11) (Ph. D. thesis S. Trujillo). Large amount of additional diffuse scattering for NBT is a clear evidence of a local structure different from the rhombohedral average symmetry (space group R3c). The additional scattering shows pronounced anisotropy and eventually produces satellite peaks along the pseudo-cubic <100>. By contrast the diffuse scattering observed for tetragonal NBT-Ba0.11 (space group P4mm) is directed along the pseudo-cubic <110> and thus very similar to previous studied relaxor systems (PMN, PZN). The different shape of diffuse scattering in NBT as compared to NBT-Ba0.11 lends support to the idea that the local structure in NBT is not related to the standard scenario of polar nanoregions (PNR ) used for describing traditional relaxor systems.
  • Colossal magnetoresistance (CMR) materials. Different polycrystalline manganites samples related to CMR systems (Lacunar controlled Nd0.93MnO2.96 (Ph. D. N. Ihzaz) and La0.67Ba0.33Mn1-xTixO3 (Ph. D. A. Gasmi ) have been investigated paying special attention to inhomogeneities at different scales (e.g. phase separation, magnetic cluster, polarons) Nd0.93MnO2.96 magnetization curves reveal an original behaviour. Magnetization changes sign twice on ZFC curve and strong negative magnetization is observed below 19 K on FC curve. On the basis of neutron diffraction, TEM microscopy and synchrotron radiation diffraction data this behaviour is interpreted by considering the coOngoingexistence of two canted antiferromagnetic phases having close chemical compositions and magnetic ordering temperatures. Ferromagnetic components in the clusters of both phases are antiferromagnetically coupled in the ra6nge 80 - 19 K. Ferromagnetic ordering at 19 K of Nd atoms is accompanied by a rearrangement of the Mn ferromagnetic components which become parallel in all the clusters. For La0.67Ba0.33Mn1-xTixO3 system the more salient structural feature is an increase of cell parameter and inter-ionic distances with increasing Ti rate which is ascribed to Ti4+/Mn4+ substitution. Compounds with x " 0.1 are ferromagnetic with a Curie temperature decreasing with increasing x. This can be related to the reduction of double exchange mechanism due to the elongation of the inter ionic distance. For larger Ti content a magnetic glass behaviour is observed with formation of relatively large magnetic cluster and disruption of the long range magnetic ordering. Semi-conductor to metallic transition is observed for x = 0 and x = 0.05 samples that is coupled with the ferromagnetic transition. For x # 0.1 a semiconductor behaviour with an increase of the resistivity by several order of magnitudes as compared with x = 0 and x = 0.05 samples is observed.
  • Multiferroïc materials. The influence of the potassium (K+) doping on the structure of multiferroic BiFeO3 and its relation with ferroelectric and magnetic properties was investigated for perovskites with composition Bi1-xKxFeO3 in the range 0<x<0.07 (Ph. D. Ja. Dhahri). The structure determination on single crystals obtained by the flux method shows that as the K+ content increases, the average cations displacements decrease reducing the polar character of doped samples with respect to pure BiFeO3 and leading to a change from rhombohedral to a pseudocubic symmetry. A structural disorder is related to the substitution of K+, which results in strong diffuse scattering (DS) located at the bottom of the Bragg peaks. Magnetic measurements reveal that all the compounds remain antiferromagnetic at room temperature (RT) with almost no change in the transition temperature (Néel temperature TN).
Collaborations : Dept.of Physics U.of Warwick, United Kingdom, Faculté des Sciences de Monastir. Tunisie, Institut Néel, ILL Grenoble, SIMAP, LEMA Tours.

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Date of update October 25, 2011


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