Synthesis,structure And Properties Of All-inorganic Cesium-containing Lead-free Halide Double Perovskite

Since the stability of Bismush-based halide perovskites needs to be further studied,few species of these compounds have been reported and new photoelectric properties can be further explored.In this thesis,we have explored several novel all-inorganic cesium-containing lead-free halide double perovskites,concerning the synthesis,structure,and properties.The doping and modification of previously reported double perovskite halides were also investigated.We have found the following results in Cs₂B’BiX₆（B’=Li,Na,Ag,and K;X=Cl and Br）systems.1.For the Cs₂LiBiX₆（X=Cl and Br）system,we have successfully synthesized a new Bi-based halide double perovskite Cs₂LiBiCl₆ and Mn²⁺doped solid solution by a solid-state method.Using Rietveld structure refinement of X-ray powder diffraction（XRD）data and complementary ⁷Li-SSNMR techniques,and it is confirmed that the structure is a cubic double perovskite with highly ordered Li⁺and Bi³⁺.Compared with Na analogs,Cs₂LiBiCl₆has higher humidity sensitivity,but the indirect bandgap is relatively smaller,which is 3.15（2）e V.In the samples of Cs₂Li_1-xBi_1-xMn_2xCl₆（x=00002,0.004,0.008,and 0.024）,the luminescence spectrum of x=0.008 shows that the luminescence phase is red-shifted compared with that of Na,the central wavelength is about 612 nm,and the luminescence intensity is stronger.Based on the calculation of the tolerance factor（τ）and the density functional theory（DFT）,the stability mechanism of Cs₂LiBiCl₆ is discussed.Because of the smaller Li⁺ions,the tolerance factor（τ）of Cs₂LiBiCl₆ is larger,which indicates that it is very close to the boundary of stable perovskite and nonperovskite compounds,corresponding to the small decomposition energy in the theoretical calculation,so it is easy to coexist with other ternary phases.Moreover,the humidity sensitivity of Cs₂LiBiCl₆ is related to the tetrahedral coordination preference of Li⁺in the Cl^-sublattice.These mechanisms combining geometric factors and decomposition energy provide new insight into the stability of halide double perovskite.At the same time,the stability of Br-based halide double perovskite Cs₂LiBiBr₆ is explored:on the one hand,it can not be successfully synthesized in the experiment;on the other hand,it is difficult to be stable from the perspective of tolerance factor（τ）and decomposition enthalpy calculated by VASP.2.For Cs₂NaBiX₆（X=Cl and Br）system,we successfully synthesized Bi-based halide double perovskite Cs₂NaBiCl₆ and its Sn²⁺and Eu³⁺doped compounds by liquid phase method,and studied their luminescent properties.Cs₂NaBiCl₆ has high humidity stability with an indirect bandgap of 3.25（2）e V.Electrochemical tests show that Cs₂NaBiCl₆ is a good dielectric material,its relative permittivity increases with the increase of temperature,and the permittivity can still be maintained above 10 at 10⁴ Hz.The sample of Cs₂Na_1-xBi_1-xSn_2xCl₆（x=0.1 and 0.2）emits red light,which is a good red fluorescent material.The morphology of the sample prepared by liquid phase is more uniform and the size is smaller.However,there are no characteristic emission peaks of Eu²⁺and Eu³⁺in the fluorescence emission spectrum of Cs₂NaBiCl₆:Eu³⁺（doping content 0.1）sample,which indicates that Eu element doping does not enter into the Cs₂NaBiCl₆ lattice.Based on the calculation of tolerance factor（τ）,and density functional theory（DFT）,we discussed the stability mechanism of Cs₂NaBiCl₆ and Cs₂NaBiBr₆.3.For the Cs₂ABiX₆（A=Ag or K;X=ClorBr）system,we have studied the dielectric properties of Cs₂AgBiBr₆for the first time and found that it has a large value for relative dielectric constant（ε_r）>60 at room temperature and～100（10⁵ Hz）at 250℃.The Cs₂AgBiBr₆ partially decomposed at high pressure（10 GPa,250℃）.We have synthesized a new compound Cs₂KBi Cl₆.It is found that this compound is not suitable for describing the average cubic structure with a highly ordered arrangement of K and Bi.It has a large degree of local structure distortion and needs further local structure analysis（PDF）.We tried to synthesize Cs₂KBi Br₆ by various methods but failed.The structural instability is explained based on the tolerance factor（τ）.