CN114438592B - A kind of monoclinic crystal form and preparation method thereof - Google Patents

Abstract

α＝90°，β＝119.192±0.006°，γ＝90°，

所述晶型空间群为C2/m，所述晶系属于单斜晶系。本发明利用高温高压条件，可以克服红磷蒸气压高容易引起石英管爆管的限制，从而制备出底心单斜结构的MoP₂单晶，该晶体结晶质量好，经单晶X射线衍射证实为单相，单晶X射线衍射的结果表明单晶内无杂质及栾晶。The invention relates to the technical field of preparation of new materials, in particular to a monoclinic crystal form and a preparation method thereof. The molecular formula of the monoclinic crystal form is MoP ₂ ; the monoclinic crystal form includes the following unit cell parameters

α=90°, β=119.192±0.006°, γ=90°,

The crystal form space group is C2/m, and the crystal system belongs to the monoclinic crystal system. The invention utilizes high temperature and high pressure conditions to overcome the limitation that the high vapor pressure of red phosphorus can easily cause the quartz tube to burst, thereby preparing a MoP ₂ single crystal with a monoclinic structure at the bottom. The crystal quality of the crystal is good, and it is confirmed by single crystal X-ray diffraction It is a single phase, and the results of single crystal X-ray diffraction show that there is no impurity and luanite in the single crystal.

Description

A kind of monoclinic crystal form and preparation method thereof

technical field

The invention relates to the technical field of preparation of new materials, in particular to a monoclinic crystal form and a preparation method thereof.

Background technique

In recent years, due to the rich crystal structure types and novel physical and chemical properties of the metal phosphorus-rich compound TPn (T is a metal, and the value of n is 2, 2.5, 3 and 4, etc.), it has aroused extensive research interests of researchers. TP ₂ is an important system among metal phosphorus-rich compounds. In the periodic table of chemical elements, Cr, Mo and W belong to the same main group, and these three metal elements can chemically react with red phosphorus to form CrP ₂ , MoP ₂ and WP ₂ . Currently, the crystal structures of these three compounds reported in the literature are: CrP ₂ has a bottom-centered monoclinic structure (space group: C2/m), and MoP ₂ has a bottom-centered orthorhombic structure (space group: Cmc2 ₁ ). However, WP ₂ has two different crystal structures (bottom-centered orthorhombic structure (space group: Cmc2 ₁ ) and bottom-centered monoclinic structure (space group: C2/m)) due to different preparation temperatures. Cr, Mo and W belong to the same main group and have very similar outer electronic structures. WP ₂ has crystal polymorphism (that is, the same chemical composition has different crystal structures). So does the isoelectronic compound MoP ₂ with the same main group as WP ₂ also have crystal polymorphism? At present, the above-mentioned CrP ₂ , MoP ₂ and WP ₂ crystals are all grown in a high-temperature furnace (well type furnace, box type furnace, etc.) by flux method, and the pressure of sample synthesis is normal pressure. In the Mo-P binary phase diagram that has been reported in the literature, MoP ₂ with a stoichiometric ratio of Mo to P of 1:2 has only a bottom-centered orthorhombic structure, while MoP ₂ with a bottom-centered monoclinic structure has not been reported. The problem that cannot be overcome at present is that the common high-temperature furnace in the laboratory cannot provide a high-pressure (for example, 5GPa, about 50,000 atmospheres) environment for the sample reactants, and only the bottom of MoP ₂ can be prepared by the normal-pressure high-temperature preparation method. The heart is orthogonal.

In addition, since the melting point of red phosphorus is very low (590 degrees Celsius), the melting points of Cr, Mo and W are very high, which are 1907 degrees Celsius, 2623 degrees Celsius and 3422 degrees Celsius, respectively. When synthesizing metal phosphorus-rich compounds, there is a large temperature difference between the melting points of red phosphorus and metal elements. Therefore, it is necessary to slowly raise the temperature under the premise of adding a large amount of flux (such as Sn) to prevent the quartz tube from bursting, resulting in normal pressure. The sample preparation speed of the solid-phase method is very slow (more than half a month). Another point, also the most important point, is that the vapor pressure of red phosphorus is very high (4357 kPa at 590 degrees Celsius, about 41 atmospheres). risks of.

Contents of the invention

In view of the above-mentioned shortcomings of the prior art, the object of the present invention is to provide a monoclinic crystal form and a preparation method thereof.

α＝90°，β＝119.192±0.006°，γ＝90°，

所述晶型空间群为C2/m，所述晶系属于单斜晶系。In order to achieve the above object and other related purposes, one aspect of the present invention provides a monoclinic crystal form, the molecular formula of the monoclinic crystal form is MoP ₂ ; the monoclinic crystal form comprises The following unit cell parameters

α=90°, β=119.192±0.006°, γ=90°,

The crystal form space group is C2/m, and the crystal system belongs to the monoclinic crystal system.

In some embodiments of the present invention, in the monoclinic crystal form, Mo is in an eight-coordination field, and each Mo atom coordinates with eight P atoms respectively.

In some embodiments of the present invention, the monoclinic crystal form is a black crystal with metallic luster.

Another aspect of the present invention provides a method for preparing the monoclinic crystal form of the present invention, the method comprising the following steps:

1) Compressing elemental molybdenum and red phosphorus into tablets and wrapping hexagonal boron nitride to prepare inclusions;

2) reacting the inclusions described in step 1) at 900-1100° C. under a pressure of 4-6 GPa to obtain a monoclinic crystal form.

In some embodiments of the present invention, in the step 1), the molar ratio of elemental molybdenum to red phosphorus is 1:2.5-3.5.

In some embodiments of the present invention, in the step 1), the elemental molybdenum and red phosphorus are ground first, and then pressed into tablets to form a cylindrical shape.

In some embodiments of the present invention, the diameter of the cylinder is 3.5-5.0 mm, and the height is 3.2-6.0 mm.

In some embodiments of the present invention, in the step 1), the hexagonal boron nitride is selected from a hexagonal boron nitride sheet or a hexagonal boron nitride tube; the thickness of the hexagonal boron nitride sheet wrapped and pressed sheet is 0.5-0.8 mm; the wall thickness of the hexagonal boron nitride tube is 0.3-0.5 mm.

In some embodiments of the present invention, in the step 2), after the reaction, the temperature is lowered at a rate of 50-100° C. per hour, and the temperature of the sample is slowly lowered to 800-900° C.; then quenched to room temperature, and finally Slowly release the pressure to normal pressure.

In some embodiments of the present invention, the step 2) is to carry out the reaction in a large cavity press.

In some embodiments of the present invention, the step 2) is to calibrate the pressure and temperature before the reaction; the pressure is calibrated using the resistance-pressure curve of metal Bi; the temperature is calibrated by the crystal structure of silicon dioxide Phase change calibration.

Description of drawings

Fig. 1 is a schematic diagram of the device for synthesizing samples at high temperature and high pressure according to the present invention.

Fig. 2 is the optical photo of the MoP ₂ single crystal sample and geometrical dimensions of the present invention.

Fig. 3 shows the orientation and high symmetry diffraction crystal plane of the MoP ₂ single crystal sample of the present invention.

Fig. 4 is the single crystal diffraction pattern of the MoP ₂ single crystal sample of the present invention along three different directions ((a) (hk0), (b) (h0l), (c) (0kl)).

Fig. 5 is a schematic diagram of the crystal structure of bottom-core monoclinic MoP ₂ of the present invention.

Fig. 6 is a micro-region image of the sample of the present invention under an electron microscope.

Fig. 7 is an analysis diagram of chemical element types and chemical components of samples of the present invention.

Fig. 8 is a simulation diagram of the energy band structure of the bottom core monoclinic MoP ₂ crystal of the present invention and the orthorhombic MoP ₂ crystal of the prior art.

Fig. 9 is a simulation diagram of the density of states of the bottom-core monoclinic MoP ₂ crystal of the present invention and the orthorhombic MoP ₂ crystal of the prior art.

Fig. 10 is a comparison diagram of the resistance test of the bottom-core monoclinic MoP ₂ crystal of the present invention and the orthorhombic MoP ₂ crystal of the prior art.

Detailed ways

The inventors of the present invention have found through a large number of experiments that the difficulties in the prior art can be overcome by using high temperature and high pressure synthesis methods. The large chamber press has the advantages of physical mechanical pressurization, magnesium oxide eight-sided wrapping and pyrophyllite sealing, which can provide a sealed and high-strength sample chamber for the sample. The physical pressure provided by the large chamber press is much greater than the vapor pressure of the sample at high temperature. Therefore, the large-cavity press is very suitable for sealing chemical reactants with high vapor pressure, plays an extremely important role in the research and exploration of new materials, and is an important means of preparing metal phosphorus-rich compounds. The present invention synthesizes a MoP ₂ single crystal sample with a monoclinic structure for the first time by adopting a high temperature and high pressure method on a 2000-ton Kawai type large cavity press. On this basis, the present invention has been accomplished.

α＝90°，β＝119.192±0.006°，γ＝90°，

所述晶型空间群为C2/m，所述晶系属于单斜晶系。The first aspect of the present invention provides a monoclinic crystal form, the structural formula of the monoclinic crystal form is MoP ₂ ; the monoclinic crystal form includes the following unit cell parameters

α=90°, β=119.192±0.006°, γ=90°,

The crystal form space group is C2/m, and the crystal system belongs to the monoclinic crystal system.

采集数据时单晶衍射仪管压：50kV，管流30mA。采用ShelXT 2018/2(Sheldrick,2018)解析晶体结构，获得全部3个非氢原子位置，使用最小二乘法修正结构参数和判别原子种类，使用几何计算法和差值Fourier法获得全部原子位置，最终可靠因子R₁＝0.0557,wR₂＝0.1653，优度因子(Goodness～of～fit on F²)＝1.177。最终确定样品化学计量式为MoP₂，计算分子量为157.88，材料的晶体密度为5.726g/cm^～3。In the monoclinic crystal form provided by the present invention, the determination of single crystal data is not particularly limited, and single crystal testing instruments known to those skilled in the art can be used. In a specific embodiment, the single crystal data of the monoclinic crystal form is collected at 150K on a Bruker (Bruker) D8 Venture double-target small molecule X-ray single crystal diffractometer The single crystal X-ray diffraction data of the Mo Kα radiation

Tube pressure of single crystal diffractometer when collecting data: 50kV, tube current 30mA. Use ShelXT 2018/2 (Sheldrick, 2018) to analyze the crystal structure, obtain all three non-hydrogen atom positions, use the least square method to correct the structural parameters and distinguish the atomic types, use the geometric calculation method and the difference Fourier method to obtain all atomic positions, and finally Reliability factor R ₁ =0.0557, wR ₂ =0.1653, goodness factor (Goodness~of~fit on F ² )=1.177. Finally, the stoichiometric formula of the sample was determined to be MoP ₂ , the calculated molecular weight was 157.88, and the crystal density of the material was 5.726 g/cm ³ .

In the monoclinic crystal form provided by the present invention, in the monoclinic crystal form, Mo is in an eight-coordination field, and each Mo atom coordinates with 8 P atoms respectively.

The molecular formula of the crystal form described in the present invention is further determined by energy dispersive spectrum analysis, and the molecular formula is MoP ₂ . The present invention uses energy dispersive spectroscopy (energy dispersive spectroscopy, Desktop Scanning Electron Microscope ~ Phenom ProX) to analyze the chemical composition of the sample.

The second aspect of the present invention provides a method for preparing the monoclinic crystal form described in the first aspect of the present invention, the method comprising the following steps:

1) Compressing elemental molybdenum and red phosphorus into tablets and wrapping hexagonal boron nitride to prepare inclusions;

2) reacting the inclusions described in step 1) at 900-1100° C. under a pressure of 4-6 GPa to obtain a monoclinic crystal form.

In the method for synthesizing monoclinic crystals provided by the present invention, the step 1) is to compress elemental molybdenum and red phosphorus into tablets and then wrap hexagonal boron nitride to prepare inclusions. Among them, elemental molybdenum and red phosphorus need to be limited in a suitable ratio range. The molar ratio of elemental molybdenum to red phosphorus is 1:2.5~3.5. In one embodiment, the molar ratio of the elemental molybdenum to red phosphorus is 1:2.5-3; or 1:3-3.5. Usually, elemental molybdenum and red phosphorus can be mixed in a glove box, for example, they can be mixed uniformly in an argon atmosphere, mortared in an agate mortar for half an hour, and pressed into a diameter by a powder tablet press (pressure: 5 MPa). It is cylindrical with a height of 3.5 to 5.0 mm and a height of 3.2 to 6.0 mm. In some embodiments, the diameter of the cylinder is 3.5-4.0 mm, 4.0-4.5 mm, or 4.5-5.0 mm, etc. The height of the cylinder is 3.2-4.0 mm, 4.0-5.0 mm, 5.0-6.0 mm, 3.5-5.5 mm, etc.

Hexagonal boron nitride is a material that is resistant to high temperature and high pressure and has stable chemical properties. In the present invention, the hexagonal boron nitride can be a hexagonal boron nitride sheet or a hexagonal boron nitride tube. When a hexagonal boron nitride tube is used, the thickness of the tube wall can be, for example, 0.5-0.8 mm, 0.5-0.6 mm, 0.6-0.7 mm, or 0.7-0.8 mm, or 0.55-0.75 mm, 0.5-0.7 mm, 0.6-0.8 mm etc. The hexagonal boron nitride sheet is wrapped and pressed with a thickness of 0.3-0.5 mm, 0.3-0.4 mm, or 0.4-0.5 mm. Because red phosphorus reacts with noble metal Pt under high temperature and high pressure, different from the sample wrapped with noble metal in the existing high temperature and high pressure experiment, the sample reactant in the present invention is wrapped with high temperature and high pressure resistant and chemically stable hexagonal boron nitride, Reduce the chance of generating impurity.

In the synthesis method of the monoclinic crystal form provided by the present invention, the step 2) reacts the inclusions described in step 1) at 900-1100° C. under a pressure of 4-6 GPa to obtain the monoclinic crystal form things. Wherein, in some embodiments, the aforementioned reaction temperature may also be 900-1000°C, or 1000-1100°C, 950-1050°C, etc. The reaction pressure is 4-5Gpa, or 5-6Gpa, or 4.5-5.5Gpa. In the step 2), after the reaction, the temperature is lowered at a rate of 50-100° C. per hour, and the temperature of the sample is slowly lowered to 800-900° C.; then it is quenched to room temperature, and finally the pressure is slowly released to normal pressure. In some embodiments, in the aforementioned steps, the temperature may also be lowered at a rate of 50-80° C., or 80-100° C., 60-90° C., or 70-80° C. per hour. Slowly lower the temperature of the sample to 800-850°C, or 850-900°C, etc. Then it was quenched to room temperature, and finally the pressure was slowly released to normal pressure.

Usually, any reactor capable of providing the above-mentioned high temperature and high pressure will do. In some embodiments, for example, a large-cavity press can be used for the reaction, more for example, a Kawai-type large-cavity press, and a 2,000-ton Kawai-type large-cavity press can be selected. For specific use, as shown in Figure 1, the inclusions are first put into the tantalum heating furnace, and the surroundings of the inclusions can be sealed with zirconia heat-insulating plugs, and the carbon heating furnace is placed in the silicon dioxide heat-insulating tube, and finally put The samples were synthesized on a 2,000-ton Kawai-type large-cavity press in a pressure-transmitting medium in magnesium oxide octahedra doped with 5% cobalt oxide. And after controlling the aforementioned high-pressure and high-temperature conditions, cooling and pressure relief, the sample was taken out from the magnesium oxide octahedron, and black crystals with metallic luster were observed under a microscope.

It should be further explained that, in the step 2) before the reaction, the pressure and temperature are first calibrated; the pressure is calibrated using the resistance-pressure curve of metal Bi; the temperature is calibrated by the crystal structure phase transition of silicon dioxide . The heating temperature is controlled by controlling the heating power, and the temperature is measured by a tungsten-rhenium thermocouple.

The beneficial effects of the present invention are as follows:

Utilize the high temperature and high pressure conditions of the present invention, can overcome the limitation that the high vapor pressure of red phosphorus easily causes the quartz tube to explode, thereby prepare the MoP ₂ single crystal of the monoclinic structure of the bottom core, the crystal quality of this crystal is good, through single crystal X-ray diffraction It is confirmed to be a single phase, and the result of single crystal X-ray diffraction shows that there is no impurity and luanite in the single crystal.

Orthorhombic phase and monoclinic phase MoP ₂ are both compensated semi-metallic materials, which are expected to be applied to the fields of magnetic memory, magnetic sensor or magnetic switch. The orthorhombic phase MoP ₂ is a three-dimensional stacked crystal structure (MoP ₇ polyhedra are facets (consisting of three phosphorus atoms) links along the a axis, points (phosphorus atoms) links on the b axis, and compound links (point links and edge link)), the chemical bond is stronger. It is difficult to thin the sample, and it is not easy to make a device. The monoclinic phase belongs to layered materials, and the layers are combined by van der Waals bonds, the chemical bonds are relatively weak, and it is easy to make thin films. Another point, and the most important point, from a thermodynamic point of view, the orthorhombic phase belongs to the low-temperature phase, while the monoclinic phase belongs to the high-temperature phase. When the above materials are applied to devices, heat is generated during use. Therefore, high-temperature monoclinic phases are more stable in performance than low-temperature orthorhombic phases.

Below in conjunction with embodiment further illustrate the beneficial effect of the present invention.

In order to make the object, technical solution and beneficial technical effect of the present invention clearer, the present invention will be further described in detail below in conjunction with examples. However, it should be understood that the embodiments of the present invention are only for explaining the present invention, not for limiting the present invention, and the embodiments of the present invention are not limited to the embodiments given in the description. The specific experimental conditions or operating conditions that are not indicated in the examples were made under conventional conditions, or made under the conditions recommended by the material supplier.

In addition, it should be understood that one or more method steps mentioned in the present invention do not exclude that there may be other method steps before and after the combined steps or other method steps may be inserted between these explicitly mentioned steps, unless otherwise There are descriptions; it should also be understood that the combined connection relationship between one or more devices/devices mentioned in the present invention does not exclude that there may be other devices/devices before and after the combined devices/devices or those explicitly mentioned Other devices/apparatus can also be interposed between the two devices/apparatus, unless otherwise stated. Moreover, unless otherwise stated, the numbering of each method step is only a convenient tool for identifying each method step, and is not intended to limit the sequence of each method step or limit the scope of the present invention. The change or adjustment of its relative relationship is in In the case of no substantive change in the technical content, it shall also be regarded as the applicable scope of the present invention.

In the following examples, the reagents, materials and instruments used are commercially available unless otherwise specified.

Example 1

1) Preparation of cylindrical reactants of molybdenum and red phosphorus under normal pressure

Mix chemically pure elemental molybdenum and red phosphorus in a molar ratio of 1:2.5 in a glove box, and mortar them in an agate mortar for half an hour. The reactant powder was compressed into a cylinder with a diameter of 3.5 mm and a height of 3.2 mm by a powder tablet press (pressure: 5 MPa). Because red phosphorus reacts with noble metal Pt under high temperature and high pressure, different from the sample wrapped with noble metal in the existing high temperature and high pressure experiment, the sample reactant in the present invention is wrapped with high temperature and high pressure resistant and chemically stable hexagonal boron nitride, Reduce the chance of generating impurity.

2) Put the above sample and the hexagonal boron nitride protective sheath together into a tantalum heating furnace. Then put the sample into a high-pressure assembly for high-pressure high-temperature synthesis (see Figure 1 for the assembly drawing). Sample synthesis was performed on a 2000-ton Kawai-type large-chamber press. First slowly increase the pressure to 5GPa at room temperature, then start the heating program to heat to 1100 degrees Celsius, keep it warm for 2 hours under high temperature and high pressure conditions, and then cool down at a rate of 100 degrees Celsius per hour, slowly cooling the temperature of the sample to 900 degrees Celsius, and then Quenching to room temperature, and finally depressurizing slowly to normal pressure. Take out the sample from the magnesium oxide octahedron, and it can be seen under the microscope that the sample is a black crystal with metallic luster (see Figure 2).

β＝119.192度。基于解析的晶体学信息文件，发明人绘制了底心单斜晶系MoP₂的晶体结构示意图(图5)，每个Mo原子周围有8个P原子。Single crystal X-ray diffraction analysis is performed on the sample obtained after quenching, and Fig. 3 shows the crystal orientation and high symmetry diffraction crystal plane of the MoP ₂ single crystal sample of the present invention. It can be seen from Fig. 4 that the single crystal diffraction spot of the sample is smaller and brighter, which proves that the MoP ₂ single crystal prepared by the present invention has no impurities and crystal phenomenon, and the crystal quality is good. The crystallographic information file (Crystallographic Information File) of the sample was obtained through the single crystal analysis software. The lattice parameters are: space group: C2/m, and the unit cell parameters are:

β = 119.192 degrees. Based on the analyzed crystallographic information files, the inventors drew a schematic diagram of the crystal structure of bottom-centered monoclinic MoP ₂ ( FIG. 5 ), with 8 P atoms surrounding each Mo atom.

Example 2

1) Preparation of cylindrical reactants of molybdenum and red phosphorus under normal pressure

Chemically pure elemental molybdenum and red phosphorus were mixed uniformly in a glove box at a molar ratio of 1:3, and ground in an agate mortar for half an hour. The molybdenum phosphorus powder was compressed into a cylinder with a diameter of 3.5 mm and a height of 3.2 mm with a powder tablet press. The initial reactant of the cylindrical sample is protected by a hexagonal boron nitride tube and a disc to reduce the probability of generating impurity phases.

2) Put the above sample and the hexagonal boron nitride protective sheath together into a tantalum heating furnace. Then put the sample into a high-pressure assembly for high-pressure high-temperature synthesis. The sample synthesis is carried out on a 2,000-ton Kawai-type large-cavity press. First slowly increase the pressure to 6GPa at room temperature, then start the heating program to heat up to 1200 degrees Celsius, keep warm for 3 hours under high temperature and high pressure conditions, and then cool down at a rate of 100 degrees Celsius per hour to slowly cool the temperature of the sample to 1000 degrees Celsius, and then Quenching to room temperature, and finally depressurizing slowly to normal pressure. Excess phosphorus was used as a self-melting agent, which was removed with dilute nitric acid after the sample was taken out.

Same as in Example 1, the sample obtained after quenching also has a single crystal with metallic luster. It was studied by X-ray diffraction of the single crystal, and it was found that the sample had a monoclinic crystal system, and the crystal quality was good without the presence of lucite and impurities.

Example 3

1) Preparation of cylindrical reactants of molybdenum and red phosphorus under normal pressure

Chemically pure elemental molybdenum and red phosphorus were mixed uniformly in a glove box at a molar ratio of 1:3.5, and ground in an agate mortar for half an hour. The molybdenum phosphorus powder was compressed into a cylinder with a diameter of 3.5 mm and a height of 3.2 mm with a powder tablet press. The initial reactant of the cylindrical sample is protected by a hexagonal boron nitride tube and a disc to reduce the probability of generating impurity phases.

2) Put the above sample and the hexagonal boron nitride protective sheath together into a tantalum heating furnace. Then put the sample into a high-pressure assembly for high-pressure high-temperature synthesis. The sample synthesis is carried out on a 2,000-ton Kawai-type large-cavity press. First slowly increase the pressure to 4GPa at room temperature, then start the heating program to heat up to 1000 degrees Celsius, keep warm for 3 hours under high temperature and high pressure conditions, and then cool down at a rate of 100 degrees Celsius per hour, slowly cooling the temperature of the sample to 900 degrees Celsius, and then Quenching to room temperature, and finally depressurizing slowly to normal pressure.

Same as in Example 1, the sample obtained after quenching also has a single crystal with metallic luster. It was studied by X-ray diffraction of the single crystal, and it was found that the sample had a monoclinic crystal system, and the crystal quality was good without the presence of lucite and impurities.

The crystal that embodiment 1 obtains carries out single crystal X-ray diffraction test as follows:

采集数据时单晶衍射仪管压：50kV，管流30mA。采用ShelXT 2018/2(Sheldrick,2018)解析晶体结构，获得全部3个非氢原子位置，使用最小二乘法修正结构参数和判别原子种类，使用几何计算法和差值Fourier法获得全部原子位置，最终可靠因子R₁＝0.0557,wR₂＝0.1653，优度因子(Goodness～of～fit onF²)＝1.177。最终的结构优化是使用SHELXL程序完成的，使用全矩阵技术最小化F²的平方和偏差。结果详见表1。The single crystal X-ray diffraction data of MoP ₂ were collected on the Bruker (Bruker) D8 Venture dual-target small molecule X-ray single crystal diffractometer at 150K, where the molybdenum target irradiated

Tube pressure of single crystal diffractometer when collecting data: 50kV, tube current 30mA. Use ShelXT 2018/2 (Sheldrick, 2018) to analyze the crystal structure, obtain all three non-hydrogen atom positions, use the least square method to correct the structural parameters and distinguish the atomic types, use the geometric calculation method and the difference Fourier method to obtain all atomic positions, and finally Reliability factor R ₁ =0.0557, wR ₂ =0.1653, goodness factor (Goodness~of~fit onF ² )=1.177. The final structural optimization was done using the SHELXL program, using the full matrix technique to minimize the sum-of-squares deviation of ^F2 . The results are detailed in Table 1.

Table 1. Crystal data and structure refinement of MoP ₂ (150K)

The crystal obtained in embodiment 1 carries out the energy dispersive x-ray energy spectrum test as follows:

The present invention analyzes the chemical composition of the sample through energy dispersive X-ray energy spectroscopy (test conditions: area size: 112 microns, voltage, 15KV, point detector (BSD Full)). Figure 6 is the micro-area image of the sample under the electron microscope (FOV: 112μm, Mode: 15kV-Point, Detector: BSD Full, Time: OCT 27 2020 15:09). Figure 7 shows the chemical element types and chemical composition analysis of the samples. Experimental results show that the sample contains only two elements, molybdenum and phosphorus. As shown in Table 2, the proportion of molybdenum is 33.51%, the proportion of phosphorus is 66.49%, and the stoichiometric ratio of molybdenum and phosphorus is 1:2. The energy dispersive X-ray energy spectrum analysis results are consistent with the single crystal structure analysis results of the present invention. The chemical formula of the sample was certified as MoP ₂ .

Table 2 Analysis of chemical elements contained in samples

Example 4

Simulation of the energy band structure and density of states of the monoclinic _MoP2 of the present invention and the orthorhombic _MoP2 of the prior art

Simulation software name: Cambridge Serial Total Energy Package (CASTEP)

1) Simulation parameters: Exchange correlation energy: Generalized gradient approximation (GGA));

2) Cut-off energy: 290 electron volts (monoclinic phase);

3) Inverted space grid size: 8*8*4;

4) Energy error: 1*10 ^-6 eV/atom.

The test results are shown in Figures 8 and 9.

Both the orthorhombic MoP ₂ and the monoclinic MoP ₂ are compensatory semi-metallic materials. As _shown in Fig _. Overlapping, it is expected to be applied to fields such as magnetic memory, magnetic sensor or magnetic switch. The orthorhombic phase MoP ₂ is a three-dimensional stacked crystal structure (MoP ₇ polyhedra are facets (consisting of three phosphorus atoms) links along the a axis, points (phosphorus atoms) links on the b axis, and compound links (point links and edge link)), the chemical bond is stronger. Therefore, it is more difficult to thin the sample, and it is not easy to make a device. The monoclinic MoP ₂ is a layered material, and the layers are combined by van der Waals bonds, the chemical bonds are relatively weak, and it is easy to make a thin film. Figure 8 shows the energy band structures of orthorhombic phase (black dotted line) and monoclinic phase (red solid line) _MoP2 . The monoclinic phase (space group number: No. 12) is high, but by comparing the energy band structures of the orthorhombic phase MoP ₂ and the monoclinic phase MoP ₂ , the applicant found that the energy band structure of the orthorhombic phase is more complicated than that of the monoclinic phase, It is difficult to analyze the electronic structure of the sample, which is not conducive to future device planning and design. Figure 9 shows the comparison results of the density of states of the orthorhombic phase (black dotted line) and the monoclinic phase (red solid line). Near the Fermi surface, the density of states of the monoclinic phase MoP ₂ is 2 electrons/electron volts, while that of the orthorhombic phase The density of states is 1 electron/electron volt. The above results indicate that the monoclinic phase _MoP2 is more advantageous than the orthorhombic phase in terms of future device development.

Example 5

The resistance test experiment of monoclinic phase MoP ₂ of the present invention and existing orthorhombic phase MoP ₂

Instrument used for testing: Physical Property Measurement System (PPMS)

Test method: Calibrate the four-electrode method, that is, two electrodes pass current, and the other two electrodes test voltage.

Link wire: gold wire, pasted on the sample with conductive silver paste.

Temperature range: 2K-300K

Current size: 50mA

Test results: As shown in Figure 10, the orthorhombic phase MoP ₂ and the monoclinic phase MoP ₂ both exhibit metallic behavior in the temperature range from room temperature to 2 Kelvin, and the resistance value is between 0.09 ohm and 0.18 ohm, which is comparable to that of common metals The values are comparable. The resistance value of the orthorhombic phase MoP ₂ is higher than that of the monoclinic phase MoP ₂ in the whole temperature test range, and the experimental result is consistent with the theoretical simulation result of the present invention. The resistance value of the monoclinic phase is low, and when it is made into a device in the future, the power consumption is small and the thermal effect is low, which is conducive to the safe operation of the device.

The MoP ₂ monoclinic system of the present invention belongs to the compensation type semi-metallic material, and is expected to be applied in fields such as magnetic memory, magnetic sensor or magnetic switch.

The above-mentioned embodiments are only illustrative to illustrate the principles and effects of the present application, but are not intended to limit the present application. Any person familiar with the technology can modify or change the above-mentioned embodiments without departing from the spirit and scope of the present application. Therefore, all equivalent modifications or changes made by those skilled in the art without departing from the spirit and technical ideas disclosed in the application shall still be covered by the claims of the application.

Claims (9)

1. A monoclinic crystal substance, the molecular formula of which is MoP ₂ ；

The monoclinic crystal type substance comprises the following unit cell parameters

α＝90°，β＝119.192±0.006°，γ＝90°，

The crystal form space group is C2/m, and the crystal system belongs to a monoclinic system.

2. The monoclinic crystal of claim 1, wherein Mo is in an octadentate field and each Mo atom is coordinated with 8P atoms;

and/or the monoclinic crystal type substance is a black crystal with metallic luster.

3. A method of preparing a monoclinic crystal according to any one of claims 1 to 2, comprising the steps of:

1) Tabletting elementary molybdenum and red phosphorus and then coating hexagonal boron nitride to prepare a coating body; the molar ratio of the simple substance molybdenum to the red phosphorus is 1:2.5 to 3.5;

2) Reacting the inclusion in the step 1) at 900-1100 ℃ under the pressure of 4-6 GPa to obtain the monoclinic crystal type substance.

4. The method according to claim 3, wherein in the step 1), the single molybdenum and red phosphorus are ground and then pressed into a cylindrical shape.

5. The method of claim 4, wherein the diameter of the cylinder is 3.5-5.0 mm and the height is 3.2-6.0 mm.

6. The method according to claim 3, wherein in step 1), the hexagonal boron nitride is selected from hexagonal boron nitride flakes or hexagonal boron nitride tubes; the thickness of the hexagonal boron nitride sheet wrapping tablet is 0.5-0.8 mm; the thickness of the tube wall of the hexagonal boron nitride tube is 0.3-0.5 mm.

7. The method for preparing a monoclinic crystal form according to claim 3, wherein in the step 2), after the reaction is finished, the temperature is reduced at a rate of 50-100 ℃ per hour, and the temperature of the sample is slowly reduced to 800-900 ℃; then quenching to room temperature, and finally slowly releasing pressure to normal pressure.

8. The method according to claim 3, wherein the step 2) is carried out in a large chamber press.

9. The method according to claim 3, wherein the step 2) comprises calibrating the pressure and temperature before the reaction; the pressure is calibrated by using a resistance-pressure curve of metal Bi; the temperature is calibrated by phase transition of the crystal structure of the silicon dioxide.

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