阅读说明：本技术 一种理论与实验结合的钙钛矿材料晶体结构优选方法 (Theory and experiment combined perovskite material crystal structure optimization method ) 是由 孔祥刚 虞游 沈艳红 于 2021-09-24 设计创作，主要内容包括：本发明公开了一种理论与实验结合的钙钛矿材料晶体结构优选方法,该方法为：首先,参照MAPbBr-(3)结构,采用CALYPSO方法寻找到MAPbBr-(3-x)(BF-(4))-(x)稳定的晶体结构,然后,按照寻找到的MAPbBr-(3-x)(BF-(4))-(x)稳定晶体结构,确定PbBr-(2)和MABF-(4)的比例,将PbBr-(2)粉末和MABF-(4)粉末按照不同比例掺杂混合,制备不同结构MAPbBr-(3-x)(BF-(4))-(x)屏蔽材料,最后,对得到的不同结构屏蔽材料进行性能评价,找到最优性能的MAPbBr-(3-x)(BF-(4))-(x)晶体结构及屏蔽材料,此方法通过引入空间群对结构产生的限制,有效减少搜索空间自由度,增加结构种群的多样性；引入成键特征矩阵,实现对结构的指纹表征,排除相似结构,引入基于粒子群优化算法的结构演化方法高效探索势能面,大大提高了最优性能的晶体结构的优化难度,节约工作量,工作准确度大大提升。(The invention discloses a theory and experiment combined perovskite material crystal structure optimization method, which comprises the following steps: first, refer to MAPbBr 3 The structure adopts CALYPSO method to find MAPbBr 3‑x (BF 4 ) x Stable crystal structure, then, according to the found MAPbBr 3‑x (BF 4 ) x Stabilizing the crystal structure and determining PbBr 2 And MABF 4 In a ratio of PbBr 2 Powder and MABF 4 The powder is mixed according to different proportions to prepare MAPbBr with different structures 3‑x (BF 4 ) x Shielding material, finally, evaluating the performance of the obtained shielding material with different structures to find MAPbBr with optimal performance 3‑x (BF 4 ) x The method effectively reduces the freedom degree of search space and increases the diversity of structure population by introducing the limitation of space population on the structure; the bonding characteristic matrix is introduced, the fingerprint representation of the structure is realized, the similar structure is eliminated, the structure evolution method based on the particle swarm optimization algorithm is introduced to efficiently explore the potential energy surface, the optimization difficulty of the crystal structure with the optimal performance is greatly improved, the workload is saved, and the working accuracy is greatly improved.)

1. A method for crystal structure optimization of a perovskite material by combining theory and experiment, characterized in that the method for crystal structure optimization comprises the following steps:

step S1: reference MAPbBr₃The structure adopts CALYPSO method to find MAPbBr_3-x(BF₄)_xA stable crystal structure;

step S2: according to MAPbBr obtained in step S1_3-x(BF₄)_xStable crystal structure, determination of PbBr₂And MABF₄Preparing MAPbBr with different structures_3-x(BF₄)_xThe shielding material is subjected to performance evaluation on the obtained shielding materials with different structures, and MAPbBr with optimal performance is found_3-x(BF₄)_xCrystal structure and shielding material.

2. The method of claim 1, wherein the CALYPSO method of step S1 is specifically:

by introducing the limitation of the space group on the structure, the freedom degree of the search space is effectively reduced, and the diversity of the structure group is increased; and introducing a bonding characteristic matrix, realizing fingerprint representation of the structure, eliminating similar structures, and introducing a structure evolution method based on a particle swarm optimization algorithm to efficiently explore a potential energy surface.

3. The crystal structure optimization method of perovskite material combined with theory and experiment as claimed in claim 1, wherein the step S2 is to prepare different structure MAPbBr_3-x(BF₄)_xThe specific method of the shielding material comprises the following steps:

reacting PbBr₂Powder and MABF₄Mixing the powder according to different proportions, and grinding until the powder is uniformly mixed;

rotating the uniformly mixed powder mixture by using a ball mill, wherein the color of the mixture finally changes from light yellow to brown to gray black;

the grayish black powder was compressed by a tablet press to obtain round flakes.

4. A method of optimizing the crystal structure of a perovskite material combined with theory and experiment as claimed in claim 3, wherein the temperature of grinding to mix uniformly is room temperature.

5. A method of optimising the crystal structure of a perovskite material in combination of theory and experiment as claimed in claim 3 wherein the ball mill is a planetary ball mill.

6. A method of optimising the crystal structure of a perovskite material combined theoretically and experimentally according to claim 3, wherein the ball mill is rotated at 300rpm for 5min at intervals of 10 min.

7. A method of optimizing the crystal structure of a perovskite material combined with theory and experiment as claimed in claim 3, wherein the tablet press presses for 5min with a pressure of 200 MPa.

8. A method of optimising the crystal structure of a perovskite material combined with theory and experiment as claimed in claim 3, wherein the circular flakes have a diameter of 20 cm.

Technical Field

The invention relates to the technical field of material development of gamma ray and neutron comprehensive shielding performance, in particular to a method for optimizing a crystal structure of a perovskite material by combining theory and experiment.

Background

With the continuous development of atomic energy science, the application of nuclear energy and radioactive nuclides is becoming more and more extensive, and high-energy ray particles generated in the nuclear reaction process also attract more and more intense attention of people. Among these high-energy radiation particles, gamma rays and neutrons have the greatest influence on humans and are also most widely used. Effective shielding has been achieved with a single ray, whether gamma or neutron. In a complex mixed environment of gamma rays and neutron symbiosis, a single ray shielding material cannot realize the comprehensive shielding of neutrons and gamma rays. Therefore, it is a hot point of research to design and develop materials having excellent comprehensive shielding properties for gamma rays and neutrons.

The material for comprehensively shielding gamma rays and neutrons is rich in elements with high atomic number as gamma ray absorbers on the one hand and low atomic number elements with high neutron absorption sections on the other hand. Moreover, the elements with high atomic number and the elements with low atomic number are also required to be uniformly distributed, so that the uniformity and the shielding stability of the shielding material are ensured. The organic-inorganic hybrid perovskite single crystal material is rapidly developed in the photoelectric field, and meanwhile, the research in the radiation shielding field is also developed. On the basis that the perovskite material is rich in high atomic number element Pb, in order to prepare a material for comprehensively shielding gamma rays and neutrons, low atomic number element doping modification is carried out on the perovskite material so as to ensure that the modified material has a sufficiently high neutron absorption section. B atoms have a high neutron absorption cross section and a broad capture spectrum, and thus are widely used to improve the neutron absorption performance of materials. Due to fluoroborate (BF)₄ ^-,0.218nm) and I^-(0.220nm) approach, and researchers have utilized BF on the basis of already obtained perovskite single crystals and thin film materials₄ ^-To I^-Is substituted to synthesize (C)₄H₉NH₃₎₂Pb(BF₄)₄The surface density of the B element is obviously improved, and the possibility is provided for developing a high-performance neutron absorption material.

By virtue of the advantages of stable crystal structure, easy doping modification, good chemical uniformity, irradiation stability and the like, the perovskite structure is an excellent carrier for designing novel wide-energy-region neutron and gamma-ray absorber materials. The perovskite structure is used as a framework, the high-Z-value elements and the low-Z-value elements are uniformly and stably fixed in the perovskite structure, and the absorber material with wide energy spectrum absorption and excellent comprehensive absorption performance of neutrons and gamma rays can be obtained by utilizing the characteristic that the high-Z-value elements absorb gamma rays and the low-Z-value elements absorb neutrons. How to regulate and control the radiation absorption effect of high and low Z value elements, and designing and synthesizing a stable perovskite structure material with a uniform and controllable structure becomes the key for developing a material for comprehensively absorbing neutrons and gamma rays in a wide energy region.

To deeply understand MAPbBr_3-x(BF₄)_xThe neutron and gamma ray irradiation damage mechanism of the material fully exerts the advantages of the high/low Z value doping modified all-inorganic perovskite material, and the invention designs the all-inorganic perovskite material MAPbBr_3-x(BF₄)_x. Firstly, a method of simulating, establishing and quantifying a neutron and gamma-ray irradiation damage physical model by adopting a first principle and combining experimental verification is adopted, the perovskite structure, the high-Z Pb and low-Z B radiation absorption effect are systematically researched, and the irradiation damage defect formation rule is obtained. Then, in experiments, halogen in the traditional organic-inorganic hybrid perovskite material is replaced by fluoroborate ions, the dual characteristics of good gamma ray absorption performance of high-Z-value elements and high boron atom neutron absorption cross section are fully exerted, the stability is improved, and the fully-inorganic perovskite material MAPbBr with excellent comprehensive absorption performance on neutrons and gamma rays in a wide energy region is obtained_3-x(BF₄)_xIn order to develop neutron and gamma-ray detector absorbing materials with excellent performanceAnd laying experimental and theoretical foundations.

Disclosure of Invention

The invention aims to overcome the defects of the prior art and provides a method for optimizing the crystal structure of a perovskite material, which combines theory and experiment, and comprises the following steps: first, refer to MAPbBr₃The structure adopts CALYPSO method to find MAPbBr_3-x(BF₄)_xStable crystal structure, then, according to the found MAPbBr_3-x(BF₄)_xStabilizing the crystal structure and determining PbBr₂And MABF₄In a ratio of PbBr₂Powder and MABF₄The powder is mixed according to different proportions to prepare MAPbBr with different structures_3-x(BF₄)_xShielding material, finally, evaluating the performance of the obtained shielding material with different structures to find MAPbBr with optimal performance_3-x(BF₄)_xThe method effectively reduces the freedom degree of search space and increases the diversity of structure population by introducing the limitation of space population on the structure; the bonding characteristic matrix is introduced, the fingerprint representation of the structure is realized, the similar structure is eliminated, the structure evolution method based on the particle swarm optimization algorithm is introduced to efficiently explore the potential energy surface, the optimization difficulty of the crystal structure with the optimal performance is greatly improved, the workload is saved, and the working accuracy is greatly improved.

In order to realize the technical purpose, the following technical scheme is adopted:

a method for optimizing the crystal structure of a perovskite material by combining theory and experiment comprises the following steps:

step S1: reference MAPbBr₃The structure adopts CALYPSO method to find MAPbBr_3-x(BF₄)_xA stable crystal structure;

step S2: according to MAPbBr obtained in step S1_3-x(BF₄)_xStable crystal structure, determination of PbBr₂And MABF₄Preparing MAPbBr with different structures_3-x(BF₄)_xThe shielding material is subjected to performance evaluation on the obtained shielding materials with different structures, and MAPbBr with optimal performance is found_3-x(BF₄)_xCrystal structure and shielding material.

Further, the calipaso method in step S1 specifically includes:

by introducing the limitation of the space group on the structure, the freedom degree of the search space is effectively reduced, and the diversity of the structure group is increased; and introducing a bonding characteristic matrix, realizing fingerprint representation of the structure, eliminating similar structures, and introducing a structure evolution method based on a particle swarm optimization algorithm to efficiently explore a potential energy surface.

Further, in the step S2, different structures MAPbBr are prepared_3-x(BF₄)_xThe specific method of the shielding material comprises the following steps:

reacting PbBr₂Powder and MABF₄Mixing the powder according to different proportions, and grinding until the powder is uniformly mixed;

rotating the uniformly mixed powder mixture by using a ball mill, wherein the color of the mixture finally changes from light yellow to brown to gray black;

the grayish black powder was compressed by a tablet press to obtain round flakes.

Further, the temperature for grinding to be uniformly mixed is room temperature.

Further, the ball mill is a planetary ball mill.

Further, the ball mill was rotated at 300rpm for 5min at intervals of 10 min.

Further, the tablet press presses for 5min at a pressure of 200 MPa.

Further, the diameter of the circular sheet is 20 cm.

The invention has the beneficial effects that:

the method for optimizing the crystal structure of the perovskite material by combining theory and experiment is provided, and comprises the following steps: first, refer to MAPbBr₃The structure adopts CALYPSO method to find MAPbBr_3-x(BF₄)_xStable crystal structure, then, according to the sought MAPbBr_3-x(BF₄)_xStabilizing the crystal structure and determining PbBr₂And MABF₄In a ratio of PbBr₂Powder and MABF₄The powder is mixed according to different proportions to prepare MAPbBr with different structures_3-x(BF₄)_xShielding material, finally, evaluating the performance of the obtained shielding material with different structures to find MAPbBr with optimal performance_3-x(BF₄)_xThe method effectively reduces the freedom degree of search space and increases the diversity of structure population by introducing the limitation of space population on the structure; the bonding characteristic matrix is introduced, the fingerprint representation of the structure is realized, the similar structure is eliminated, the structure evolution method based on the particle swarm optimization algorithm is introduced to efficiently explore the potential energy surface, the optimization difficulty of the crystal structure with the optimal performance is greatly improved, the workload is saved, and the working accuracy is greatly improved.

Drawings

FIG. 1 shows MAPbBr in the example of the present application₃、MAPbBr₂BF₄And MAPbBr (BF)₄)₂The structure of (1);

FIG. 2 shows MAPbBr in the example of the present application₃、MAPbBr₂BF₄And MAPbBr (BF)₄)₂The theoretical XRD pattern spectrum of (1);

FIG. 3 shows MAPbBr in the example of the present application_3-x(BF₄)_xA thermodynamic property diagram of;

FIG. 4 shows MAPbBr in the example of the present application_3-x(BF₄)_xThe absorption spectrum of (a);

FIG. 5 shows MAPbBr in the example of the present application_3-x(BF₄)_xAttenuation coefficient contrast maps for absorbing gamma rays of different energies;

FIG. 6 shows MAPbBr in the example of the present application_3-x(BF₄)_xA comparison graph of the absorption effect of B content in the material on thermal neutrons;

Detailed Description

The invention will be further described with reference to the accompanying drawings, without limiting the scope of the invention to the following:

in order to better understand the technical scheme of the invention, the operation process is described as follows:

example 1

The first principle calculations of this example based on the density functional theory were all done using the VASP software package, all taking into account the van der waals correction (Vdw). All the cross-correlation functionals used for the calculations are the pbe (per Burke ernzerhof) functionals in the Generalized Gradient Approximation (GGA), the electron-ion interactions being described by the PAW-plus-plane-wave method. For MAPbBr₃The structure is subjected to convergence test, and 500eV is taken as the plane wave truncation energy. In addition, a k-point network of 7 × 9 × 7 size is automatically generated using the Monkhorst-Pack method. When energy self-consistency and structure optimization calculation are carried out, the convergence standards of energy and force are respectively 1.0 multiplied by 10^-5eV anddue to BF₄ ^-The position of the group structure coordinate is unknown, firstly, the CALYPSO method is adopted to determine MAPbBr_3-x(BF₄)_xThen, the electronic structure, thermodynamics and other related properties of the structure are studied in detail.

Reacting PbBr₂Powder and MABF₄Mixing the powder according to different proportions, and grinding the powder by using a mortar at room temperature until the powder is uniformly mixed; the uniformly mixed powder mixture was rotated for 5min at 300rpm intervals of 10min by a planetary ball mill, and the color of the mixture finally changed from light yellow to brown to gray black. The grayish black powder was then compressed for 5min at a pressure of 200MPa using a tablet press to obtain round flakes having a diameter of 20 cm. The sheet can be used for testing ray shielding. Regarding gamma ray shielding performance, attenuation coefficients under high energy spectrum are calculated by adopting WinXcom software. The neutron and gamma ray shielding performance of the structure is studied experimentally. The neutron shielding performance of the sample is measured by using a 252Cf neutron source and a lithium glass detector, and the gamma ray shielding performance of the sample is measured by using a 133Ba//60Co gamma radioactive source and a Clover high-purity germanium detector.

1. Determination of crystal structure

Reference MAPbBr₃The structure adopts CALYPSO method to find MAPbBr_3-x(BF₄)_xThe most stable crystal structure, and optimization,as shown in fig. 1, the corresponding theoretical XRD results are shown in fig. 2, with BF₄ ^-When the radical is doped, MAPbBr₂BF₄Middle BF₄ ^-Replace MAPbBr₃Br at the middle 4c position^-Ion, MAPbBr (BF)₄)₂Middle BF₄ ^-Replace MAPbBr₃Br at the middle 8d position^-Ions. From the optimization results, MAPbBr₂BF₄The lattice constant is slightly reduced, causing the crystal to shrink slightly, while MAPbBr (BF)₄)₂The lattice constant of (a) is slightly increased, causing a slight expansion of the crystal, and the atomic coordinate occupancy of the lattice constant is shown in table 1. Relative to MAPbBr (BF) on an XRD diffraction pattern₄)₂Structure, MAPbBr₂BF₄The distortion is more pronounced.

TABLE 1 MAPbBr optimized₃、MAPbBr₂BF₄And MAPbBr (BF)₄)₂Lattice constant and atomic coordinates and occupancy of the system

2. Preparation of MAPbBr of different structures_3-x(BF₄)_xShielding material

Reacting PbBr₂Powder and MABF₄Mixing the powder according to different proportions, and grinding the powder by using a mortar at room temperature until the powder is uniformly mixed; the uniformly mixed powder mixture was rotated for 5min at 300rpm intervals of 10min by a planetary ball mill, and the color of the mixture finally changed from light yellow to brown to gray black. Pressing the gray black powder with a tablet press at 200MPa for 5min to obtain round sheet with diameter of 20cm to obtain MAPbBr₂BF₄And MAPbBr (BF)₄)₂A material. The sheet can be used for testing ray shielding.

3、MAPbBr_3-x(BF₄)_xThermodynamic properties

From MAPbBr_3-x(BF₄)_xThe BF is researched in four aspects of free energy, entropy, equal capacitance heat capacity and total energy₄ ^-The influence of substitutional doping of the group. The corresponding properties vary linearly with increasing doping concentration, as shown in fig. 3. From the free energy point of view, with BF₄ ^-The increase of the doping amount and the lowest free energy at high temperature means that the structure is more stable, and the equivalent bulk heat capacity is gradually increased along with the increase of the temperature and is also along with BF₄ ^-The doping amount increases.

4、MAPbBr_3-x(BF₄)_xOptical Properties of the System

From MAPbBr_3-x(BF₄)_xAbsorption coefficient of BF₄ ^-The influence of substitutional doping of the group. The light absorption spectra of the doped material in three directions were calculated using the first principle method, as shown in fig. 4. The comparison shows that in the x and z directions, along with BF₄The increase of the doping concentration causes the blue shift phenomenon of different degrees. The situation is different in the y-direction, MAPbBr (BF)₄)₂A red shift occurs, which may be related to the lattice distortion caused by doping, resulting in electrons at shallow levels in the valence band, which readily absorb the photons for energy level transitions.

5、MAPbBr_3-x(BF₄)_xRadiation resistance of the system

As shielding material for neutron and gamma ray, perovskite material MAPbBr_3-x(BF₄)_xAre necessarily subject to bombardment by energetic particles, producing various defects, which are studied here from the standpoint of vacancy generation. To analyze Br atom being BF₄ ^-Influence of group substitution on vacancy formation all non-equivalent vacancy formation energies in the system were calculated as shown in table 2. The comparison analysis shows that the vacancy forming energy of most atoms is increased along with the increase of the doping concentration, particularly the B atom vacancy forming energy with the function of absorbing neutrons is the highest, which indicates that the material has higher radiation resistanceStrong and has better performance as a shielding material.

TABLE 2MAPbBr₃、MAPbBr₂BF₄And MAPbBr (BF)₄)₂Energy of formation of individual vacancies in the system

In addition, it is theoretically calculated that gamma rays of different energies enter MAPbBr_3-x(BF₄)_xThe attenuation coefficient after the system is shown in FIG. 5. The larger attenuation coefficient in the low and high energy ranges means MAPbBr (BF)₄)₂The ability to shield gamma rays is greater.

MAPbBr measurement by cadmium difference method_3-x(BF₄)_xThe absorption efficiency of the material for thermal neutrons (fig. 6). The results show that the thermal neutron absorption rate of the material is linearly related to the mass fraction of B atoms. This also indicates that the B atoms are chemically doped and uniformly distributed in the perovskite structure. With MAPbI_3-x(BF₄)_xCompared with the materials, the absorption effects of the two materials on thermal neutrons are basically consistent and do not change significantly, mainly the structures and the B element contents of the two materials are relatively close, so the improvement of the shielding performance is mainly in the aspect of gamma rays.

On the basis of keeping the synthesis method of the original lead-based perovskite material, boron fluoride ions are doped into the lead-based perovskite material by a thin film deposition method, the doping proportion of the boron fluoride ions is changed, and various CH are prepared₃NH₃PbBr_(3-x)BF_4xResearch on the sample lining finds that boron fluoride ions can be well combined in the lead-based perovskite material to form a lead-based perovskite structure crystalline phase.

MAPbBr₂BF₄The absorption of the material for 81keV gamma-rays reaches 53.2%. CH (CH)₃NH₃PbBr_(3-x)BF_4xThe thermal neutron absorption rate of the material is in linear relation with the mass fraction of B atoms, and the comprehensive shielding performance exceeds 50%. Theoretical research proves that BF is doped₄The radical can improve the radiation-resistant stability of the material and is beneficial to further maintaining the improved shielding performance. This indicates a BF-based basis₄ ^-The substituted perovskites have broad prospects.

In summary, the invention discloses a method for optimizing the crystal structure of a perovskite material by combining theory and experiment, which comprises the following steps: first, refer to MAPbBr₃The structure adopts CALYPSO method to find MAPbBr_3-x(BF₄)_xStable crystal structure, then, according to the found MAPbBr_3-x(BF₄)_xStabilizing the crystal structure and determining PbBr₂And MABF₄In a ratio of PbBr₂Powder and MABF₄The powder is mixed according to different proportions to prepare MAPbBr with different structures_3-x(BF₄)_xShielding material, finally, evaluating the performance of the obtained shielding material with different structures to find MAPbBr with optimal performance_3-x(BF₄)_xThe method effectively reduces the freedom degree of searching space and increases the diversity of structural population by introducing the limitation of the space population on the structure; the key forming characteristic matrix is introduced, the fingerprint representation of the structure is realized, the similar structure is eliminated, the potential energy surface is efficiently explored by introducing a structure evolution method based on a particle group optimization algorithm, the optimization difficulty of the crystal structure with the optimal performance is greatly improved, the workload is saved, and the working accuracy is greatly improved.

Thus, it will be appreciated by those skilled in the art that while embodiments of the invention have been illustrated and described in detail herein, many other variations or modifications can be made, which are consistent with the principles of the invention, and which are directly determined or derived from the disclosure herein, without departing from the spirit and scope of the invention. Accordingly, the scope of the invention should be understood and interpreted to cover all such other variations or modifications.