Evidencias de descomposición en multiferróicos RE2(MoO4)3 antes de PIA

Abstract

In this work we present the study of rare earth molybdates RE=(Eu, Tb and Ho) with formula RE2(MoO4)3. Under ambient conditions, Eu- and Tb-containing compounds can be found in the phases α and β0 , while holmium-containing can be found in the phase γ and β0 under these conditions. In the study of the β0 -Tb2(MoO4)3 compression by the CCDD group, the hypothesis of a transition to a new phase, called the phase δ, was considered. Subsequently, in the study of Y2(MoO4)3 synthesised unconventional conditions, non-stoichiometric oxide and molybdate phases with dif fractograms very similar to those of the phase δ were obtained. This opened the door to another possible hypothesis about the β0 → δ transition, that it was a decomposition induced by high pressure. For its verification, the synt hesis of the named compounds was carried out, modifying the solid state synthesis, applying a pressure of 0.66 GPa for compacting powder samples and increasing or decreasing the synthesis temperature, which was different for each case. We carry out a study of the crystalline structures of the most relevant phases involved: chelite-α, β-β0 and γ. Additionally, symmetry relationships provide clarity in understanding the phase transitions. A routine diffractogram was performed for each compound synthesised by using the X-ray diffractometer available at SIDIX (X-ray Facilitiy of the La Laguna Univer sity). Diffraction data collected under pressure were provided by the CCDD (group with which the supervisors of this work are researching). Note that, in addition to the β0 -Tb2(MoO4)3 data, β0 -phase of Ho and Eu molybdates data were also available. Using the ICSD database we simulated the different phases that would be expec ted to be found, and with which a visual identification of the phases was performed. Applying the Le Bail refinement method, the intensities of the full profile were refined as a verification of the existence of the expected phases. The synthesis of europium molybdate was carried out at 500oC, 550oC and 600oC. After the analysis and refinements, different mixtures of phases with struc tural types Sm2O3, MoO3, Eu4Mo7O27, Eu2Mo4O15 and the phase α-Eu2(MoO4)3 were detected. Furthermore, by analysing the pure β0 -Eu2(MoO4)3 phase under pres sure, it was observed that around 2.23 GPa a phase transition occurs, interpreted as a decomposition into the β0 , Eu2O3 and Eu2Mo4O15 phases, while at 5 GPa an amorphisation undergoes. Holmium oxide, β0 -Ho2(MoO4)3, nonstoichiometric Y2Mo4O15 phases were iden tified for the holmium molybdated synthetised at 600oC. Under pressure, as in the case of the europium molybdate, a phase transition occurs around 2.3 GPa in which the β0 -phase, the europium oxide and the Y2Mo4O15 phase are involved and the non-reversible amorphous phase starts at around 5 GPa. In these cases as well as the one studied by the CCDD group on Tb molybdate, when the decompression is carried out, the initial β0 -RE2(MoO4)3 phase is not com pletely recovered, so the phase transition is not reversible, which leads us to think that it is a decomposition induced by pressure. In addition to the experimental work and the analysis and discussion of the results, we would like to highlight the literature review carried out, which is a very important part of this dissertation. On the one hand, the research was contextualised, its interest was explained and an exhaustive description of the crystal structures of the materials studied was made. It was also necessary to: 1) review and introduce some crystallographic terms that were later used in the description of these structures. 2) Describe the experimental techniques used, reviewing their physical foundations. 3) Explain the tools used in the analysis of the data. Therefore, the work was structured as follows: Chapter 1. Introduction. This is divided into three sections: the state of the art, the motivations and how the work was organised. Chapter 2. Crystallographic basis for the description of polymorphs with for mula RE2(MoO4)3. In the first section we explain concepts about crystal structures and symmetry relations: direct lattice and symmetry, point and space groups and group-subgroup relations. In the second section we describe the crystal structures of the RE2(MoO4)3 family of compounds: the α, β0, γ and other non-stoichiometric phases. Chapter 3. Experimental preparation and diffraction. Here we explain how com pounds are prepared by solid state synthesis, giving details of the material, the equip ment, the stoichiometric calculations and the solid state reaction. The second section is devoted to X-ray diffraction by powder samples. Diffraction concepts are introdu ced, the operation of powder diffractometers is explained and measurement conditions at the SIDIX and DIAMOND synchrotron are given. Chapter 4. Analysis of results and conclusions. The first section explains the procedure followed for the identification of the phases, using databases, CIF files and simulating the possible phases. The second section explains the refinement by the Le Bail method. In the third section, the diffractograms measured in SIDIX (pha se identification and refinement) are analysed and discussed. In the fourth section, the diffractograms measured in DIAMOND (phase identification and refinement) are analysed and discussed. In the fifth section the conclusions are developed and in the sixth section the possible continuation of this TFG is proposed.