Synthesis And Characterization Of The Perovskite Chromate

Scientific development depends largely on the new materials，which is also ahot topic in scientific research. Functional materials are those can perform optical,electrical, magnetic, acoustic, and thermal qualities under certain conditions.Functional materials contain different types of things, therein to, perovskite oxidehas a potential application in magnetic electrical, optical field due to the doubleexchange theory and the John-Teller effect can dramatically change the interaction ofcharge, spin, orbital, and lattice in the compounds, as well as its high degree ofstability. Nowadays, there are kinds of ways to synthesize new materials. Solid statereaction method is widely used for its readily available raw materials and relativelysimple operation. Hydrothermal method is another widely used synthetic way. Thereaction temperature of the hydrothermal technique is usually between130℃and250℃, the time to form perovskite phase is significantly lower than for conventionalprocessing techniques. Autogenous pressure can be measured in the process of thereaction and it could accelerate the dialysis of the inorganic or organic matter withwater. The hydrothermal processing avoid the step of calcinations which happens inthe general liquid phase synthesis, in this way, we can reduce agglomerationphenomenon of the matters so that we could control the particle size and the purityof the resultant.LaCrO₃belongs to the perovskite oxides；It has been widely used in theelectrode material of the MHD generator, heat-sensitive materials, electrode andinterconnector of the solid oxide fuel cell for its excellent properties in the magnetic,electrical and so on. But on the other hand, the pure phase LaCrO₃also has somedrawbacks, such as the high electrical resistivity at room temperature, poor thermalshock resistance and mechanical properties. Thus, metal elements（especiallyalkaline-earth elements） doping LaCrO₃or adding other substances to LaCrO₃hasbeen widely used to improve the electrical conductivity, thermal stability, thermalexpansion properties of the LaCrO₃materials, so that it could be used in more fields. In this paper, we study the properties of the LaCrO₃, and it can be divided into twoparts:1, It is widely accepted that the doped LaCrO₃is a P-type electronic conductor inoxidizing conditions, and the conduction mechanism is small polaron theory whichconsiders the variable of the electrical conductivity is related to the valence state ofthe Cr. In this paper, solid-phase method is used to synthesize doping LaCrO₃,La_0.8Ca_0.2CrO₃and La_0.7Ca0.2Sr_0.1CrO₃. We study its high thermal stability,relationship between conductivity and the temperature, dependence of ACconductivity with frequency, and the valence state of the Cr. By comparison, thesame compounds were synthesized by solid state reaction under nitrogen conditions.Valence state of the Cr was also studied by X-ray photoelectron spectrometer.Through the experiments, we came to know the properties of the compoundsLa_0.8Ca_0.2CrO₃and La_0.7Ca_0.2Sr_0.1CrO₃synthesized by solid state reaction. Therelationship of AC conductivity with frequency is in line with Jonscher power-law,La_0.7Ca_0.2Sr_0.1CrO₃compound has a super thermal stability and electricalconductivity under the same temperatures. The valence states of the Cr are+3and+6through X-ray photoelectron spectrometer. The same result is also found in thecompounds La_0.8Ca_0.2CrO₃and La_0.7Ca_0.2Sr_0.1CrO₃synthesized by solid state reactionunder nitrogen conditions.2Under hydrothermal processing, the reaction temperature is low; the reactants arein a subcritical or supercritical state which could increase the reaction speed.Therefore, we could get some differences on the structure and nature of the sameproduct through different synthesize method. We explored a new hydrothermalsynthesis of LaCrO₃, the differences are the relative low temperature, no need for thereaction precursor during the reaction in comparison to the method mentioned in theliterature.In this paper, we use KOH as the mineralizer and adopt a relative lowtemperature to synthesize LaCrO₃. The impacts of the different reaction temperatures,different reaction time, and different concentration of alkali on the sample morphology and yield were studied during the reaction process. In the end, we findthat the reaction temperature is240℃，reaction time is6days：as well as thetemperature is260℃，reaction time is4days. Properties of the LaCrO₃weredetermined through X-ray diffractometer, differential thermal analysis and X-rayphotoelectron spectrometer. An exothermic peak in the408℃could be observed inDTA picture, the morphology of the LaCrO₃synthesized by hydrothermal method iscubic, and the valence state of Cr is+3.