Crystal growth and properties of novel multiferroic oxide compounds
Zusammenfassung der Projektergebnisse
For the first time polycrystalline and single crystals of RMn2-xFe(Co)xO5 for the wide range of x=[0...1] have been synthesized. Special synthesis conditions are required for single phase formation of the compound considered. Only modified sol-gel method under high oxygen ambient pressures up to 200 bar at 1000°C instead of conventional solid state powder synthesis can be applied for formation of RMn2-xFe(Co)x05 phases. Such complex synthesis conditions can explain the fact of some confused and contradictory results of some researchers on these or similar compounds but applied conventional synthesis technologies. Temperature stability range of RMn2-xFe(Co)x05 at 1 bar oxygen were studied. Those revealed that these phases decomposed in the range 1050°C - 1300°C depending on the nature of rare earth atom and doping element concentration. The higher is Fe or Co content in the compound the lower is the decomposition temperature. The difference between undoped and highly doped materials is around 250°C. Increasing the ambient oxygen pressure drastically increases the decomposition temperature. But even 100 bar oxygen is not sufficient to suppress the decomposition before their melting and corresponding crystallization from their own melts. Therefore a flux method was optimized for crystal growth of RMn2-xFe(Co)x05 compounds. Single crystals of them with high physical and chemical perfection were obtained with the size up to 20 mm. That it is more attractive for neutron scattering studies in comparison with 5 mm size crystals reported up to now. The following crystal growth parameters were optimized within the framework of this project: solvent compositions (including absolute solubility values), effect of excess or deficiency of the composing elements in the melt on crystallization of the compounds, effective segregation coefficients, ambient growth atmosphere and effect of evaporation the elements from the melt. On the basis of these data for the first time a preudobinary phase diagram corresponding to RMn2-xFe(Co)xO5 crystallization from flux have been constructed. That is an important step in understanding of crystallization features of the whole class of similar compound. The optimal growth parameters are assembled and presented those can by applied by other researchers for growth of such materials. X-ray diffraction studies of RMn2-xFexO5 showed that Fe3+ does indeed go randomly on the Mn3* site. On the basis of magnetic and neutron scattering studies it would suggest that ferroelectricity is not appearing only over a narrow temperature window as was considered. The following sequence of transitions on cooling was detected: ICM paraelectric to CM ferroelectric to ICM paraelectric (where ICM=incommensurate, CM= commensurate structure). Measurements of Raman scattering from the RMn2.xFexO5 crystals indicates a strong magnetoelastic coupling in the materials considered. Within the framework of the present project a "new" multiferroic materials was synthesized and partially investigated. That is Pb3Mn6O13. Exact oxygen concentration was defined and corresponding formula Pb3Mn6O13 instead of PbMn2O4 was confirmed. Single crystals up to 20 mm were obtained (crystal size <5 mm has been reported in the literature up to now). Studies of dielectric constant anomalies together with magnetic measurements allows us to conclude that all the attributes of a charge ferroelectric are presented in Pb3Mn6013. The results obtained within the framework of the project help significantly in understanding of the principal features of crystallization of the considered complex materials. Moreover, these can be easy applied to similar multiferroic crystals like RMn2-xTx05±5 with other R - rare earth and T - transition metal elements due to the similar crystallization nature of these compounds. That is important for activity of other researchers, whose interest is in the field of low temperature physics but not in crystal growth. Therefore applying of the ready recites of production of samples for further physical investigations will help to invest more time for their own research fields. The found unexpected interesting results for "newly" multiferroic Pb3Mn6O13 crystals can open a new wave of development of multiferroic materials. Up to now the intensive studies of physical phenomena in the single crystals grown within the framework of the project are conducting in the leading research centers and institutes (IFW Dresden, HMI Berlin, TU Dresden and MPI Stuttgart). From the first sight it is difficult to explain all the results observed due to the chemical and physical complexity of the systems considered and the lack of any reference information, because the materials have been obtained and investigated for the first time. But the already obtained results give us to conclude the basic magnetic and electrical properties of the ones. The principal results are preparing for publications and further collaborations.
