RT info:eu-repo/semantics/bachelorThesis
T1 Reacción de estado sólido en compuestos polimorfos tipo RE2(MoO4)3 monitorizados por termodifractrometría en un sincrotrón
A1 Ramirez Rodriguez, Nivaria Rut
AB This work is focuses on the study of the solid-state synthesis and phase transitions of rare-earthmolybdates with formula 𝑅𝐸2(𝑀𝑜𝑂4)3 by X-Ray thermodiffraction, with powder samples. Thisfamily of compounds is interesting because it consists of up to 10 different polymorphs. Thus, theirstructural variety gives them important physical properties with interesting applications.The stoichiometric mixture of the oxides 𝑀𝑜𝑂3 and 𝑅𝐸2𝑂3 where 𝑅𝐸 ≡ 𝑁𝑑, 𝑆𝑚, 𝐸𝑢 𝑦 𝐺𝑑 wereused as initial samples. Since this work is not completely experimental, we decided to introduce thisfamily of compounds, in detail, by plotting and describing the different crystal structures of the twomain polytypes: modulated scheelites, and ferroic phases. For this purpose, we made a bibliographictour from the fundamental to modern crystallography, defining concepts such as polytypes, crystallinesymmetry, space and superspace groups, modulated structures, among others.The experiment was performed at the ESRF synchrotron (Grenoble, France); specifically, with theSpanish beamline BM25-A, six year ago. The collected data have never been fully analysed, so, it hasbeen necessary to retrieve and review the experimental conditions and explain them in detail. Whilewe are reviewing the experimental data, we have decided to explain how a synchrotron works and itsadvantages over a conventional X-ray tube. In addition, we also review some basic concepts ofdiffraction for describing the diffraction by crystalline powder. Regarding the experimental conditions,we distinguish between the first heating, in which the compounds with stoichiometry RE2Mo3O12 wereformed (from room temperature up to 900ºC), and the other cooling and heating cycles, to study thephase transitions. The schedule followed for the Nd and Sm samples was similar and it consisted ofmore cycles than for the Eu and Gd samples, as time in these experiments is limited. The heating andcooling cycles were carefully plotted for each sample.There were more than 100 diffractograms, so my role was to help identify and refine some of thesediffractograms, in particular the pure phases and the last cycles of the refinements. Before that, I hadto explain and distinguish between the phase identification and Le Bail refinement. To achieve thephase identification, we had to plot most of the diffractograms and compare them with the simulateddiffractogram for each phase, whose crystal structure was obtained with the help of the ICSD database.In addition, we obtained more quantitative results, such as the lattice parameters, with Le Bailrefinements of some selected phases identified at different temperatures. The most difficult work wasthe identification and refinement of, we believe, all the non-stoichiometric crystalline phases (includingstarting oxides) before the formation of the α-phase. Afterwards, it was easier to observe the α ⇔ βtransition at high temperature.As we progressed in this work, we completed a phase diagram within this family of rare-earthmolybdates, studying the sequence and reversibility of the phase transitions. To do so, we have takeninto account the temperatures of each cycle and the ionic radii of the rare earths. Some of the phasesand transitions found had not been studied before, for example, the non-reversible transitions from theβ’ phase, obtained at room temperature by quenching (very fast cooling to freeze the crystallinestructure at ambient conditions), to the α-phase or the 𝐿𝑎2(𝑀𝑜𝑂4)3 phase, normally obtained bycooling. Along the way we have found a possible phase mixture or an incommensurable phase for the𝑁𝑑2(𝑀𝑜𝑂4)3, during the heating cycle, also from the β’ phase. In contrast, we have not studied thebetter known β’ ⇔ β’ (ferroelectric-paraelectric) phase transition.From the conclusions obtained we can carry out further refinements and evaluate the thermaldependence of the lattice parameters, as well as publish a scientific paper based on this work.The work has been divided into three chapters:The first chapter was entitled: Introduction to molybdates with 𝑅𝐸2(𝑀𝑜𝑂4)3 stoichiometry, crystalstructures, polymorphisms and phase transitions. It reviews the state of the art, motivations, aims andobjectives of the work and explains the organisation of the work. The second section was devoted tobasic explanations of symmetry and direct lattice, crystal systems and crystal classes, space groups andthe reciprocal lattice. In the third section we described the different crystals with formula𝑅𝑒2(𝑀𝑜𝑂4)3 divided into modulated scheelites (including the 𝐿𝑎2(𝑀𝑜𝑂4)3- and the α-phase) andthe ferroelectric-ferroelastic phase and paraelectric-paraelastic phase (i.e. the ferroic phases β and β’).Finally, we described other possible molybdates with different RE/Mo ratios.In the second chapter entitled: Diffraction techniques and experimental conditions. We focused onsynchrotron radiation: storage rings and synchrotron radiation sources, the properties of synchrotronradiation and, in particular, the BM25 - X-Ray 'Beamline' from the Spanish CRG. Followingsections were dedicated to the X-ray diffraction and polycrystalline samples. From a schematicdiagram of a powder diffractometer, we give the experimental conditions of thermodiffractionincluding heating-cooling schedules.The third chapter is devoted to the analysis of the results and discussion. First, we explain how thedifferent phases can be identified from the experimental diffractograms. For this purpose, we modelledthe complete profile with the structural data obtained from the ICSD database and compare them withexperimental ones. Second, we explain the Le Bail least-squares refinement and the particularstrategies. We presented and discussed the results of the first heating cycle (𝑅𝑒2(𝑀𝑜𝑂4)3-phasesformation) and the subsequent cooling and heating cycles (thermal evolution and phase transitions).We end this chapter with conclusions and possible future work.Due to the very large and varied literature reviewed (more than fifty articles) and in order not to losethe work done, we added an important part in the supplementary material. Here we include the mostcomplex descriptions, mathematical developments and some very specific definitions.
YR 2021
FD 2021
LK http://riull.ull.es/xmlui/handle/915/22351
UL http://riull.ull.es/xmlui/handle/915/22351
LA es
DS Repositorio institucional de la Universidad de La Laguna
RD 23-abr-2024