Tuning The Optical Properties And Research On High-Frequency Magnetoelectric Properties Of （CH₃NH₃）₂MCl₄（M=Co,Cu）

The framework structure of hybrid organic-inorganic perovskite（HOIPs）materials is synthesized by the coordination of metal ions and organic functional groups,which have excellent optoelectronic and magnetoelectric properties.It has broad application prospects in photovoltaic,sensing,information storage and other fields.At present,the research on HOIPs materials mostly focuses on three-dimensional perovskite crystals,with the structural formula AMX₃,where A and X are organic ions,and M is a metal ion.Compared with three-dimensional perovskites,two-dimensional perovskites have attracted much attention due to their higher stability and structural tunability.The perovskite monolithic framework is composed of metal ions and inorganic ions to form polyhedral planes,with organic ions interspersed between the planes.These organic cations act as insulating barriers,confining charge carriers in two dimensions,leading to decrease in quantum fluorescence yields and limiting the development of photovoltaic devices.Therefore,the research on the relationship between crystal structure and optical properties and how to control the optical properties have become problems to be solved.The research object of this paper is a two-dimensional layered organic-inorganic hybrid perovskite,its molecular formula is A₂MX₄,where A corresponds to methylammonium organic cation（CH₃NH₃⁺）,M is a metal cation(Co²⁺or Cu²⁺)and X is a halide anion（Cl^-）.High-quality single crystals were prepared based on the slow evaporation method.While exploring the structural characteristics of single crystals,thermal and magnetic properties of single crystals were also explored,and optical properties under pressure were further studied.In addition to applications in optoelectronics and other fields,We have also applied perovskite to a new field—the field of microwave absorption,which serves as a scientific reference for promoting the development of the field of microwave absorption.（1）In this paper,high-quality（CH₃NH₃）₂CoCl₄（MA₂CoCl₄）single crystal was prepared by slow evaporation method.The MA₂CoCl₄ single crystal was characterized and tested by XRD,Raman and VSM.As the temperature decreases,the MA₂CoCl₄ single crystal undergoes a structural phase transition.when the temperature is further reduced,magnetic order states appear.The relationship between the crystal structure and optical properties under pressure is further explored.The MA₂CoCl₄ single crystal appears blue under normal pressure.The color changed to dark green at 25 GPa.At the same time,the band gap also showed an overall increasing trend with the increase of pressure.That is,the change of the band gap can be regulated by pressure,and its optical properties can be further regulated.（2）A high-quality（CH₃NH₃）₂CuCl₄（MA₂CuCl₄）single crystal was prepared by the slow evaporation method.Low-temperature magnetic tests show that when the temperature is lowered to around 11 K,the magnetic moment suddenly increases,and we believed that the MA₂CuCl₄ single crystal changed from paramagnetic to ferromagnetic.The pressure test of MA₂CuCl₄ single crystal showed that as the pressure increases,the band gap gradually decreases,that is,the goal of band gap regulation can be achieved by the pressure method.（3）The problem of electromagnetic wave interference has become one of the problems that people need to solve urgently.We prepared（CH₃NH₃）₂CoCl₄ and（CH₃NH₃）₂CuCl₄polycrystals by anti-solvent method,then blended them with polypyrrole（PPy）and polyaniline（PANI）conductive polymers in certain proportions,and then tested the microwave properties of these two composites.The results show that when the ratio of（CH₃NH₃）₂CoCl₄/PPy is 1:3,the best microwave absorption ability is exhibited.With a thickness of 1.76 mm,the best reflection loss（RL=-46.3 dB）is achieved at 14.8 GHz;its effective absorption bandwidth is 5.5 GHz（RL≤-10 dB）.When the ratio of（CH₃NH₃）₂CuCl₄/PANI is 1:1,the best microwave absorption ability is exhibited.That is,with a thickness of 2.68 mm,the best reflection loss（RL=-49.7 dB）is achieved at 8.9 GHz;its effective absorption bandwidth is 3.1 GHz（RL≤-10 dB）.