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Long-range Ordering in Pr(2)NiO(4+δ) Single Crystals studied by Resonant Soft X-Ray Scattering

Lanthanide oxides with the general formula of Ln2MO4+δ (where Ln = La, Pr, Nd and M = Cu, Ni, Co) are a class of perovskite oxides being part of the K2NiF4 family. As strongly correlated materials, their properties vary drastically on temperature and doping; the latter can extend over a wide range of concentration, modifying (anisotropic) oxygen mobility and valence states of the ions.
The chosen material to be investigated is Pr2NiO4+δ. Below certain critical temperatures, besides the antiferromagnetism, three- and/or two-dimensional long-range ordered superstructures can arise, creating electronically correlated states that only oxygen intercalation can induce. Those modulations are commonly referred to as static and dynamic orbital-, spin- and charge-stripe phases, i.e. wide local AFM regions divided by narrow anti-phase boundaries. They can be observed as commensurate and/or incommensurate diffrac- tion peaks by using high-brilliance X-ray sources, like synchrotron. However, their interpretation is challenging and their origin can be often confusing, due to their low intensity with respect to the structural Bragg peaks by a factor of 10−4 and the surface and bulk defects of the material, e.g. NiO segregation or Pr6O11 hydrolization. These elements provided the motivation to grow large single crystals
of Pr2NiO4+δ, in order to carry out analyses by exploiting resonant soft x-ray scattering (RSXS), a powerful technique which enhance the intensity of satellite reflections arising from competing states. By scattering with photon energies on resonance of the element’s elec- tronic transitions, RSXS is an element-specific, sensitive tool providing a combination of spectroscopic and spatial information. The thesis fo- cuses on the energy-, temperature- and polarization-dependent analysis of the ordered states measured.

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1 THE MATERIAL: STRUCTURE AND PROPERTIES OF PRASEODYMIUM NICKELATE 1.1 introduction The instability of a metal’s Fermi surface at low temperatures gives arise to the formation of spin or charge density waves, superconductivity, or other collective electronic ordering phenomena [1][2][3][4][5] . In such materials, known as strongly correlated materials, the mobility of the valence-state electrons is suppressed due to the interactions be- tween their mutual repulsion. The 3d-transition metal oxides (TMOs) represent an important category of the strongly correlated materials. In this thesis, the focus will be addressed to praseodymium nickelate, a transition metal oxide where the reduction of the valence shell leads to the switch of its behaviour from metallic to insulating and localization of spins [6][7][8] . The Fermi surface is modified depending on the repulsive interactions between valence-state electrons, leading to the formation of ordered states, which can be studied by using scattering techniques. The complexity of these structures requires high-quality and pure sam- ples and only single crystals can satisfy these conditions. The prepara- tion can be challenging but thanks to the development of techniques as the floating zone method (FZM), nowadays the growth of large single crystals is achievable in a reasonable time. 1.2 the crystal structure 1.2.1 The Perovskites and its derivatives The basic building block of many TMOs is the perovskite unit cell, named after the mineral CaTiO 3 , with general formula ABO [9] 3 (Fig- ure 1.1). It can be described by a three-dimensional (3D) cubic network ofcorner-sharingoctahedrawiththelargerAalkaline-earthorrare-earth metal cations occupying every site created by 8 BO 6 octahedra, where B is a transition metal, and the symmetry space group is Pm ¯ 3m [10] . The perovskite phases are very reactive and flexible in terms of oxygen stoichiometry and substitution of cations A and B with other cations of different sizes. In order to accommodate the electrostatic potential of doping ions, the octahedra rotate or distort their shape by tilting or elongate/shrink along favourable crystalline directions. These displace- ments within the unit cell lead to a lower symmetry and the size and the nature of A and B cations play a key role. Indeed, an important parameter to take into account the variation of the equilibrium positions of ions in the octahedra is the Goldschmidt tolerance factor t (Equation 1.1), which measures the degree of distor- tion as a function of the interatomic distances between cations and oxygen ions (d A−O and d B−O ) [11] . t = d A−O √ 2d B−O (1.1) 17

Tesi di Laurea Magistrale

Facoltà: Scienze Matematiche, Fisiche e Naturali

Autore: Francesco D'Acierno Contatta »

Composta da 86 pagine.

 

Questa tesi ha raggiunto 49 click dal 30/06/2016.

Disponibile in PDF, la consultazione è esclusivamente in formato digitale.