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Low Temperature Single-Crystal X-ray Diffraction on A(1-x)A'xMnO3 (A=La,Eu and A'=Ba,Sr)
(2006)
- The present thesis is devoted to A(1-x)A'xMnO3 (A=La and Eu, A'=Ba and Sr) compounds with distorted perovkite-type structure commonly denoted as manganites. The interest toward these compounds manifested when colossal magnetoresistance (CMR) was discovered in manganites. We have selected three particular compositions in order to study three different aspects of their structures, that are: the analysis of the structural behavior (and its correlation to magnetic and electronic properties) of rhombohedral La0.815Ba0.185MnO3 and orthorhombic La0.89Sr0.11MnO3 in dependence on temperature, the studies of the monoclinic symmetry of La0.815Ba0.185MnO3 at T=160K and the evidences of the splitting of the A-site in Eu0.60Sr0.40MnO3. We investigated these materials by single-crystal X-ray diffraction. For La0.815Ba0.185MnO3 the structure was analyzed in the temperature range of 188.9-295K. The temperature of 188.9K has been selected as minimum since the crystal undergoes a first-order structural phase transition from rhombohedral R-3c to monoclinic I2/c at TS=187.1K. We found a change in the thermal expansion coefficient at the transition temperature of the paramagnetic (PI) to ferromagnetic (FM) phase transition (TC=251K), with a larger expansion of the FM phase. Progressive charge localization due to strong electron-phonon interactions in the PI phase might be responsible for a smaller thermal expansion coefficient in the PI phase. Precursor effects near the phase transition at TS towards the monoclinic phase occur for Mn-O distances, Mn-O-Mn angles and the volume of the MnO6 polyhedron, providing a picture of how the structure reacts approaching the structure phase transition. Furthermore, our results allow to conclude that the apparent driving force for the structural phase transition is the replacement of shear-type distortions by Jahn-Teller-type distortions of the MnO6 octahedra, together with a stabilization of the La environment. The same crystal as for studies of the rhombohedral phase was used for analysing the monoclinic phase, but what was a single-crystal in the rhombohedral phase becomes twinned in the monoclinic phase. With the aid of omega-theta mapping of reflection profiles we found that this material displays monoclinic symmetry I2/c despite many reports in the literature of an orthorhombic symmetry Pbnm for this compound as well as for related materials A(1-x)A'xMnO3 with x about 0.2. Another member of the family of perovkite-type materials is represented by La0.89Sr0.11MnO3, with orthorhombic symmetry with space group Pbnm at ambient conditions. The crystal structure was investigated in dependence on temperature, with the aim of analyzing structural changes correlated with the magnetic transitions. A complex evolution of the magnetization measurements against temperature was found. The crystal structure remains orthorhombic over the investigated temperature range 110-240K but several extremes are seen for the lattice parameters as well as for structural and thermal parameters at temperatures where also changes in the magnetic state are observed. The lattice parameters have extreme values at the Curie Temperature of TC=150K. The present results show small anomalies in the temperature dependencies of bond lengths and bond angles, that occur at T=170K. This temperature coincides with the onset of magnetic ordering rather than with TC. Less pronounced maxima at 170K are found for the temperature parameters of oxygen, while the shortest La-O bond is virtually independent on temperature. On the basis of these observations we propose that the tilts and deformations of the MnO6 octahedra are determined by the shortest La-O distance as a kind of boundary condition. The thermal expansion is then accounted for by structural rearrangements while keeping the shortest La-O distance constant at the value corresponding to optimal chemical bonding. The coincidence of structural anomalies with the onset of magnetic order strongly suggests that magnetic ordering occurs at temperatures where the crystal structure allows favorable magnetic interactions. The final part of this work concerns the structural investigation by single-crystal X-ray diffraction at room temperature of Eu0.60Sr0.40MnO3. This composition was never studied before and we found that the crystal structure is orthorhombic with space group Pbnm. Initially, the same positional and displacement parameters were refined for both Eu and Sr atoms, but the difference Fourier maps showed that they do not occupy the same position. The refinement of separate positions reduced the R value, and features in the difference Fourier map were considerably reduced, leading to the first observation of a splitting of the A-A' crystallographic site of A(1-x)A'xMnO3 compounds.
