Low Temperature Controllable Hydrothermal Syntheses, Magnetic Properties And Thermochromism Of Perovskite Structure Rare-Earth Chromites

Perovskite structure rare earth chromates have been widely studied due to its excellent and versatile physical and chemical properties.Crystal surface engineering is an important means for fine tunable material physical and chemical properties in many fields,which can promote further understanding of the intrinsic relationship between structure and properties,and provide technical support for improving the existing properties of materials and developing new properties.Samples with good crystallinity,purity and uniform grain size can be obtained by hydrothermal reaction at low temperature,and many single crystals of perovskite materials can be synthesized by hydrothermal method at low temperature,which provides a feasible way to further study the intrinsic properties of the corresponding crystals.In this dissertation,the hydrothermal growth,controllable doping and facet-controllable crystal growth of perovskite-type rare earth chromate single crystals are realized by means of low-temperature hydrothermal preparation,which provides an effective strategy for crystal growth of perovskite-type rare earth chromate.The main research contents of this paper has been listed as below:1.We developed a low temperature hydrothermal preparation method of two-step crystalization.Under the low temperature hydrothermal conditions,four perovskite-type RECrO₃?RE=Er,Tm,Yb,Lu?micron crystals were successfully prepared and characterized.We find that the single crystals obtained by low temperature hydrothermal method have higher octahedral distortion angle than that reported in the literature.Moreover,the distortion degree of CrO₆ increases gradually with the decrease of the ionic radii of A-site heavy rare earth,which is in agreement with Raman and infrared spectroscpy.Compared with that of literature,the samples of heavy rare earth chromite microcrystals by hydrothermal method have a lower Néel transition temperature,and weak ferromagnetic phase under the low temperature condition comes from the antisymmetric exchange between the corner-shared CrO₆ octahedron distortion.The process of dehydration and hydrothermal crystallization developed in this work is also applicable for the synthesis of other amphoteric metal oxides.2.Shape-tuneable synthesis of perovskite structured rare-earth chromites RECrO₃?RE=La,Pr,Nd?via a mild hydrothermal method.We successfully prepared RECrO₃series micron sized single crystal samples with high energy crystal surface index using NH₄⁺as the crystal facet tailoring agent which is decomposed from urea in mild hydrothermal condition.We hypothesized that NH₄⁺adsorbed on the surface of CrO₆octahedron via a weak interaction,which inhibited the growth rate of the corresponding crystal surface,so that it could be retained.The crystal morphology and the formation of crystal facets are in agreement with BFDH theory.This work provides a feasible and effective strategy for the facet-controllable growth of rare earth chromate crystals.3.Low temperature controlled synthesis,characterization and magnetic behavior of B-site orderedly doped LaCr_1-x-x Mn_xO₃?x=0.1,0.2 and 0.3?single crystal.B-site Mn-doped LaCrO₃ series micro single crystals were successfully prepared by low temperature hydrothermal method.XPS results revealed that the valence state of Cr and Mn in the sample are both+3.The sample was canted-antiferromagnetism to paramagnetism at near room temperature.With the increase of Mn doping,the remanent susceptibility of the sample increased exponentially.We calculated the magnetic entropy of the series samples and found that when the temperature was lower than 40 K,the spin glass behavior appeared.This work proves that the controlled doping of B-site Mn can be realized under hydrothermal conditions,and the obtained doping samples have higher magnetic ordering degree compared with other synthesis methods.4.Mild hydrothermal synthesis and thermochromic properties of LaCr_1-xAl_x O₃?x=0.05,0.08,0.12,0.15?series single crystal samples.We have successfully prepared LaCr_1-xAl_xO₃ series micron-sized cubic-shape single crystals in mild hydrothermal conditions.Crystal structures were determined via Rietveld analysis of powder x-ray diffraction results in detail.The samples exhibited composition and temperature dependent thermochromic behavior in the range of room temperature to 600 ^oC,which gradually changeed from green to black.There are reversible and irreversible thermochromic regions in the range of temperature.Reversible thermochromism originates from the heat-induced lattice expansion,while irreversible thermochromism originates from the internal production of part of Cr⁶⁺of the sample at high temperature.To conclude,we have finished the study of synthetic method design,controllable doping,and crystal facet controllable growth of perovskite-structure rare-earth chromites in this dissertation.Structure-properties relationship of these chromites have been systematically measured and analyzed.We believe that this dissertation will be benefiting the related studies on chromites and orther amphoteric oxide materials.