Controllable Preparation And Thermochromic Properties Of Perovskite Rare Earth Chromates

In recent years,the rare-earth orthochromite perovskites chromate has shown great application potential in energy materials,environmental catalysis,gas sensing,intelligent manufacturing and other fields due to its rich structural regulatory properties,and has been widely concerned by physics,chemistry,materials science and other disciplines.Especially the optical properties and ion conduction properties of the materials have been widely used in harmful gas purification reaction and solid oxide fuel cells and other emerging technology industries.However,the traditional method to prepare the materials relies on high temperature sintering technology,and the product components have poor controllability.For the stability required by the application of high-end materials,it is still necessary to expand the high-quality manufacturing technology to achieve the controllable preparation of the chemical composition,crystal structure and crystal size of the materials.High quality powder single crystal with uniform size and controllable composition can be prepared by hydrothermal synthesis.In particular,mild hydrothermal synthesis technology assisted by mineralizer can effectively reduce the synthesis temperature and pressure of traditional hydrothermal reaction through mineralizer assisted dehydration,saving energy required for production,but also effectively avoid production risks brought by high temperature and high pressure.Our research group has been committed to the development of minerality-assisted hydrothermal preparation technology of perovskite structure chromates for a long time,especially our self-developed programmed decrystallization technology has realized the controllable synthesis of more than ten kinds of perovskite structure rare earth chromates.In this paper,we will further explore the synthesis of rare-earth chromates with variable valence state and the irreversible thermochromism of the products produced in the process of heating,and find that this group of materials has the potential to be used in thermal memory pigments.Specific research contents are as follows:1.Low temperature hydrothermal synthesis a series rare-earth orthochromite perovskites RCrO₃（R=Eu,Tb,Y）single crystals has been achieved for the first time,and it is found that they show continuous irreversible thermochromism with increasing temperature.The effects of hydrothermal mineralization temperature and concentration of mineralizer on the crystallization behavior of perovskite chromate with different rare earth ions at A-site were systematically studied.By means of powder X-ray diffraction,scanning electron microscopy,Raman spectroscopy,high resolution transmission electron microscopy,infrared absorption spectroscopy and other means,the structure,crystal growth habit and characteristic functional groups of the materials were systematically studied.The magnetic properties and thermochromic properties of the three samples were tested.The results show that the three A-site Eu,Tb and Y perovskite rare earth chromates prepared in this chapter belong to the orthogonal structure of Pbnm space group,all show high structural distortion,and can be modulated by A-site ion size.Magnetic tests show that all the synthesized samples exhibit paramagnetic antiferromagnetic transition,and Eu CrO₃also exhibits morphology-dependent magnetic behavior.Thermochromic test showed that the color of the three samples changed significantly with the increase of temperature.When the temperature returned to room temperature,the color of the samples could not be recovered,indicating that the series of samples had thermal memory effect.In addition,with the change of the A-site rare-earth ion radius,the temperature point at which the samples show obvious thermochromism and the mechanism of the material change accordingly.The thermochromic properties of perovskite rare earth chromate were controlled by the synthesis of the A site of perovskite,and they are expected to provide new members for temperature indication materials.2.In view of the influence of the distortion degree of CrO₆octahedron in RCrO₃on the material properties,we further select Lu³⁺with the smallest lanthanide ion radius to doped into the A site of La CrO₃to adjust its structure.A series of perovskite La_1-xLux CrO₃（x=0,0.1,0.2）single crystals with high degree of crystallization and high purity were successfully synthesized by self-developed mild hydrothermal method for the first time.According to the X-ray powder diffraction test and Rietveld refinement,it is confirmed that the sample belongs to the orthogonal structure of perovskite with space group Pbnm（No.62）.Moreover,the distortion degree of the sample structure increases gradually with the increase of the doping amount of A site Lu³⁺.Infrared test results show.With the increase of Lu³⁺doping,the deformation vibration of O-Cr-O and the tensile vibration of Cr-O move towards the direction of high wave number.La_1-xLu_xCrO₃（x=0,0.1,0.2）series samples showed irreversible thermochromic behavior,and with the increase of Lu³⁺doping,the color transition temperature point decreased significantly,through the A-site ion size mismatch can effectively adjust the color change temperature zone of the series materials.XPS studies on La CrO₃samples before and after sintering show that the irreversible thermochromic mechanism of the series materials can be attributed to the oxidation of some Cr³⁺in the materials under temperature field.In conclusion,a series of new perovskite structure rare earth chromate single crystal powder materials have been synthesized in this paper by using the low temperature hydrothermal synthesis programmed decrystallization technology developed by our research group,which provides technical reserve and material basis for its large-scale production and application in the field of high-end intelligent materials.