CN111233778A - A high-temperature and high-pressure preparation and atmospheric-pressure interception method of confined high-density anhydrous alkali metal polymeric nitrogen NaN5 - Google Patents

Abstract

The invention relates to the technical field of high-energy density material preparation, and provides a high-temperature and high-pressure preparation and atmospheric-pressure interception method of confined high-density anhydrous alkali metal polymeric nitrogen _NaN5 . In the present invention, the sodium azide confined in the boron nitride nanotube is used as the starting material, and the high-pressure anhydrous alkali metal polymeric nitrogen Cm-NaN ₅ is obtained stably existing under high pressure through high pressure treatment; and laser heating treatment to obtain confined high-density anhydrous alkali metal polymeric nitrogen Pmn2 ₁ ‑NaN ₅ stably existing under high pressure; pressure relief after high pressure and laser heating treatment to obtain confined high-density stably existing under normal pressure Anhydrous alkali metal polymeric nitrogen P2/c-NaN ₅ . The method of the invention is simple and easy to operate, realizes the high-temperature and high-pressure preparation and atmospheric trapping of confined high-density anhydrous alkali metal polymeric nitrogen NaN ₅ for the first time, and provides an effective technical approach for the experimental preparation of novel anhydrous alkali metal polymeric nitrogen.

Description

High-temperature and high-pressure preparation and normalization of a confined high-density anhydrous alkali metal polymeric nitrogen NaN5 pressure interception method

technical field

The invention relates to the technical field of high-energy density material preparation, in particular to a high-temperature and high-pressure preparation and atmospheric-pressure interception method of confined high-density anhydrous alkali metal polymeric nitrogen _NaN5 .

Background technique

Polymeric nitrogen is a typical high energy density material (HEDM), in which nitrogen atoms are connected by NN bonds or N=N bonds, due to the NN bond energy (160KJ/mol)/N=N bond energy (418KJ/mol) Much lower than the N≡N bond energy in nitrogen (954KJ/mol), huge energy will be released when its depolymerization is restored to _N2 molecules. Pentazole compounds are a class of typical polymeric nitrogen materials. The nitrogen atoms in the nitrogen pentacycle (N ₅ ^- ) are on the same plane, and the nitrogen-nitrogen bond length is between the nitrogen-nitrogen single bond (NN) and the double bond (N =N) between.

In recent years, many pentazolium salts that can exist stably under ambient conditions have been obtained by chemical synthesis methods, including sodium pentazolium salts [Na ₈ (N ₅ ) ₈ (H ₂ O) ₃ ] _n and [Na(N ₅ )(H ₂ O)]·2H ₂ O. In these two sodium-based pentazole skeleton structures, sodium ion, bound water and free water play an important role in stabilizing the nitrogen pentacycle (N ₅ ^- ) therein. It is worth noting that there are a lot of water molecules in these two sodium-based pentazole skeleton structures, and the sodium-based pentazole structure cannot be separated from water molecules and exists stably under environmental conditions, and the two sodium-based pentazole skeletons are stable. The cage-like structure also leads to a great reduction in the density of the sodium-based pentazole structure.

So far, the sodium-based pentazole structure containing only metal sodium ion coordination has not been reported, and the higher-density anhydrous alkali metal polymeric nitrogen structure _NaN5 has not been reported either.

SUMMARY OF THE INVENTION

In view of this, the present invention provides a high-temperature and high-pressure preparation and normal-pressure interception method of confined high-density anhydrous alkali metal polymeric nitrogen NaN ₅ . The invention obtains the confined high-density anhydrous alkali metal polymer nitrogen Cm-NaN ₅ and Pmn2 ₁ -NaN ₅ which can exist stably under high pressure for the first time, and realizes the interception of the confined high-density anhydrous alkali metal polymerization under normal pressure. Nitrogen P2/c- _NaN5 .

In order to achieve the above-mentioned purpose of the invention, the present invention provides the following technical solutions:

A high-pressure preparation method of confined high-density anhydrous alkali metal polymeric nitrogen Cm-NaN ₅ , comprising the following steps:

The sodium azide confined in boron nitride nanotubes was encapsulated in a high-pressure cavity of a diamond anvil, and then pressurized to above 35 GPa to obtain a confined high-density anhydrous alkali metal polymer nitrogen Cm-NaN stably existing under high pressure ₅ .

A high-temperature and high-pressure preparation method of confined high-density anhydrous alkali metal polymeric nitrogen Pmn2 ₁ -NaN ₅ , comprising the following steps:

The sodium azide confined in boron nitride nanotubes was encapsulated in a high-pressure cavity of a diamond anvil, pressurized to more than 50GPa, and then subjected to 2000-2300K laser heat treatment to obtain a confined high-density nanotube that stably exists under high pressure. Water alkali metal polynitrogen Pmn2 ₁ -NaN ₅ .

A method for trapping at atmospheric pressure of confined high-density anhydrous alkali metal polymeric nitrogen P2/c-NaN ₅ , comprising the following steps:

The sodium azide confined in boron nitride nanotubes was encapsulated in a high-pressure cavity of diamond anvil, pressurized to more than 50GPa, then subjected to 2000-2300K laser heating treatment, and then decompressed to normal pressure to obtain Confinement high-density anhydrous alkali metal polymeric nitrogen P2/c-NaN ₅ that exists stably under the condition.

Preferably, the pressurized pressure transmission medium is liquid argon or liquid neon.

Preferably, the method for preparing the diamond-to-anvil high-pressure cavity is as follows: using rhenium foil as a gasket material, using diamond to pre-press the rhenium foil on the anvil to form an indentation; A hole is formed in the center as a high pressure cavity.

Preferably, the thickness of the pre-pressed rhenium foil is 40-60 μm.

The present invention provides the confined high-density anhydrous alkali metal polymeric nitrogen Cm-NaN ₅ obtained by the method described in the above scheme.

The present invention provides the confined high-density anhydrous alkali metal polymeric nitrogen Pmn2 ₁ -NaN ₅ obtained by the method described in the above scheme.

The present invention provides confined high-density anhydrous alkali metal polymeric nitrogen P2/c-NaN ₅ obtained by the method described in the above scheme.

The invention provides a high-pressure preparation method of confined high-density anhydrous alkali metal polymeric nitrogen Cm- _NaN5 , comprising the following steps: encapsulating azide confined in boron nitride nanotubes in a high-pressure cavity of a diamond anvil and then pressurized to above 35GPa to obtain confined high-density anhydrous alkali metal polymeric nitrogen Cm-NaN ₅ stably existing under high pressure. The present invention utilizes high pressure conditions to make NaN ₃ undergo structural phase transition, wherein azide N ₃ ^- dissociates and polymerizes to form N ₅ ^-ring , thereby obtaining confined high-density anhydrous alkali metal polymeric nitrogen that can exist stably under high pressure Cm- _NaN5 .

The invention provides a high-temperature and high-pressure preparation method of confined high-density anhydrous alkali metal polymeric nitrogen Pmn2 ₁ -NaN ₅ . Sodium azide, pressurized to more than 50GPa, and then subjected to 2000-2300K laser heating treatment to obtain confined high-density anhydrous alkali metal polymer nitrogen Pmn2 ₁ -NaN ₅ stably existing under high pressure. The present invention utilizes high temperature to promote the complete transformation of NaN ₃ to NaN ₅ structure across a higher potential barrier, and makes the crystallinity of the sodium nitrogen pentastructure better.

The invention also provides a method for trapping at atmospheric pressure of confined high-density anhydrous alkali metal polymeric nitrogen P2/ _c -NaN5, comprising the following steps: encapsulating the confined boron nitride nanotubes in a high-pressure cavity of a diamond counter-anvil Pressurize the sodium azide above 50GPa, then carry out 2000-2300K laser heating treatment, and then release the pressure to obtain a confined high-density anhydrous alkali metal polymer nitrogen P2/c-NaN that exists stably at room temperature and pressure ₅ . The invention utilizes high temperature to promote the complete transformation of NaN ₃ to NaN ₅ structure, and makes the crystallinity of the sodium nitrogen pentastructure better, and at the same time, under the confinement of the boron nitride tube, the normal pressure interception of NaN ₅ is realized.

In addition, the high temperature and high pressure preparation method and the atmospheric pressure interception method provided by the present invention do not require harsh experimental conditions, and the method is simple and easy to operate.

Description of drawings

Fig. 1 is the high-pressure in-situ Raman spectrum of Cm-NaN ₅ @BNNTs prepared in Example 1 at a pressure of 35 GPa;

Fig. 2 is the high-pressure in-situ synchrotron radiation angle-dispersion XRD spectrum of Cm-NaN ₅ @BNNTs prepared in Example 1 at 35 GPa;

Figure 3 shows the 3D crystal structures of Cm-NaN ₅ , Pmn2 ₁ -NaN ₅ and P2/c-NaN ₅ , wherein (a) is the 3D crystal structure of Cm-NaN ₅ and (b) is Pmn2 ₁ -NaN ₅ The 3D crystal structure of , (c) is the 3D crystal structure of P2/c-NaN ₅ ;

4 is a high-pressure in-situ Raman spectrum of Cm-NaN ₅ @BNNTs prepared in Example 2 under a pressure of 43 GPa;

Fig. 5 is the high-pressure in-situ synchrotron radiation angle-dispersion XRD spectrum of Cm-NaN ₅ @BNNTs prepared in Example 2 at 43 GPa;

6 is a high-pressure in-situ Raman spectrum of Cm-NaN ₅ @BNNTs prepared in Example 3 at a pressure of 115 GPa;

Fig. 7 is the high-pressure in-situ synchrotron radiation angle-dispersion XRD spectrum of Cm-NaN ₅ @BNNTs prepared in Example 3 at 115 GPa;

Fig. 8 is the high-pressure in-situ Raman spectrum of Pmn2 ₁ -NaN ₅ @BNNTs prepared in Example 4 at a pressure of 50 GPa;

Fig. 9 is the high-pressure in-situ synchrotron radiation angle-dispersion XRD spectrum of Pmn2 ₁ -NaN ₅ @BNNTs prepared in Example 4 at 50 GPa;

Figure 10 is the Raman spectrum of P2/c-NaN ₅ @BNNTs prepared in Example 5 under normal temperature and normal pressure conditions;

FIG. 11 is a synchrotron radiation XRD pattern of P2/c-NaN ₅ @BNNTs prepared in Example 5 under normal temperature and normal pressure conditions.

Detailed ways

The invention provides a high-pressure preparation method of confined high-density anhydrous alkali metal polymeric nitrogen Cm-NaN ₅ , comprising the following steps:

The sodium azide confined in boron nitride nanotubes was encapsulated in a high-pressure cavity of a diamond anvil, and then pressurized to above 35 GPa to obtain a confined high-density anhydrous alkali metal polymer nitrogen Cm-NaN stably existing under high pressure ₅ .

In the present invention, the method for preparing the high-pressure cavity of the diamond-to-anvil is preferably as follows: using rhenium foil as a gasket material, using diamond to pre-press the rhenium foil on the anvil, and using a laser drilling machine to form a rhenium foil in the center of the indentation Holes, as high pressure chambers. In the present invention, the thickness of the pre-pressed rhenium foil is preferably 40-60 μm; the diameter of the hole is preferably 1/3 of the diameter of the anvil surface of the diamond anvil. When the diameter of the anvil surface is 200 μm, the diameter of the hole is preferably 60-70 μm; the pressurized pressure transmission medium is preferably liquid argon or liquid neon, more preferably liquid argon, in the specific embodiment of the present invention Among them, it is preferable to use ruby microspheres below 10 μm as the pressure standard material to calibrate the pressure in the high pressure chamber.

In the present invention, the sodium azide confined in boron nitride nanotubes is specifically a NaN ₃ @BNNTs confinement nanocomposite material. By confining sodium azide in boron nitride nanotubes Obtained; the present invention has no special requirements for the sodium azide confined in boron nitride nanotubes, and can be prepared or purchased using methods well known to those skilled in the art.

The present invention encapsulates sodium azide confined in boron nitride nanotubes in a high-pressure cavity of a diamond counter-anvil, and then pressurizes it to more than 35GPa, specifically 35GPa, 43GPa or 115GPa. Under the action of pressure, NaN ₃ A structural phase transition occurs, wherein the azide N ₃ ^- dissociates and polymerizes to form an N ₅ ^-ring , thereby obtaining NaN ₅ . The high-density anhydrous alkali metal polymeric nitrogen NaN ₅ in boron nitride nanotubes has a space group of Cm and is denoted as Cm-NaN ₅ @BNNTs (where BNNTs represent boron nitride nanotubes).

The invention also provides a high-temperature and high-pressure preparation method of confined high-density anhydrous alkali metal polymeric nitrogen Pmn2 ₁ -NaN ₅ , comprising the following steps:

The sodium azide confined in boron nitride nanotubes was encapsulated in a high-pressure cavity of diamond anvil, pressurized to more than 50GPa, and then subjected to 2000-2300K laser heating treatment to obtain confinement high-density stably existing under high pressure Anhydrous alkali metal polymeric nitrogen Pmn2 ₁ -NaN ₅ .

In the present invention, the preparation method and pressurized medium of the diamond-to-anvil high-pressure cavity are the same as those in the above scheme, and will not be repeated here; the sodium azide and The above solutions are the same, and are not repeated here.

After the packaging is completed, the present invention is pressurized to more than 50GPa, specifically 50GPa or 60GPa, and then subjected to 2000-2300K laser heating treatment, preferably 2100-2200K laser heating treatment. In the present invention, a fiber laser with a wavelength of 1064 nm is preferably used for laser heating treatment. The present invention does not have special requirements on the specific conditions of the laser heating treatment, as long as the required temperature can be reached. The present invention utilizes laser to carry out heating treatment, and high temperature can promote the complete transformation of NaN ₃ to NaN ₅ structure across a higher potential barrier, and make the crystallinity of the sodium-nitrogen pentastructure better; Specifically, it is a high-density anhydrous alkali metal polymeric nitrogen NaN ₅ , which is stably existing under high pressure and confined in boron nitride nanotubes, and its space group is Pmn2 ₁ , denoted as Pmn2 ₁ -NaN ₅ @BNNTs ( where BNNTs represent boron nitride nanotubes).

The present invention also provides a method for trapping at atmospheric pressure of confined high-density anhydrous alkali metal polymeric nitrogen P2/ _c -NaN5, comprising the following steps:

The sodium azide confined in boron nitride nanotubes was encapsulated in a high-pressure cavity of diamond anvil, pressurized to more than 50GPa, then subjected to 2000-2300K laser heating treatment, and then decompressed to normal pressure to obtain Confinement high-density anhydrous alkali metal polymeric nitrogen P2/c-NaN ₅ that exists stably under the condition.

In the present invention, the preparation method and pressurized medium of the diamond-to-anvil high-pressure cavity are the same as those in the above scheme, and will not be repeated here; the sodium azide and The above solutions are the same, and are not repeated here.

After the packaging is completed, the present invention is pressurized to more than 50GPa, specifically 50GPa, 53GPa or 58GPa, and then subjected to 2000-2300K laser heating treatment, preferably 2100-2200K laser heating treatment, and then decompressed. In the present invention, a fiber laser with a wavelength of 1064 nm is preferably used for laser heating treatment. The present invention does not have special requirements on the specific conditions of the laser heating treatment, as long as the required temperature can be reached. In the present invention, the laser is used for heating treatment, and the high temperature can promote the complete transformation of NaN ₃ over a higher potential barrier to the NaN ₅ structure, and make the crystallinity of the sodium-nitrogen pentastructure better. The normal pressure interception of NaN ₅ is realized; what the present invention obtains under normal pressure is a high-density anhydrous alkali metal polymerized nitrogen NaN ₅ that is stably existing at normal temperature and normal pressure and confined in boron nitride nanotubes , whose space group is P2/c, denoted as P2/c-NaN ₅ @BNNTs (where BNNTs represent boron nitride nanotubes).

The present invention also provides the confined high-density anhydrous alkali metal polymeric nitrogen Cm-NaN ₅ , Pmn2 ₁ -NaN ₅ and P2/c-NaN ₅ obtained by the method described in the above scheme. For the first time, the present invention obtains the confined high-density anhydrous alkali metal polymeric nitrogen Cm-NaN ₅ and Pmn2 ₁ -NaN ₅ stably existing under high pressure for the first time; Domain high density anhydrous alkali metal polymeric nitrogen P2/c-NaN ₅ ; Cm-NaN ₅ , Pmn2 ₁ -NaN ₅ and P2/c-NaN ₅ provided by the present invention are sodium pentazoles only coordinated by metal sodium ions Structural compound, there is no water molecule in the skeleton structure of sodium-based pentazole, and has high energy density. In the present invention, based on the calculation of the decomposition of NaN ₅ structure into NaN ₃ and N ₂ under the corresponding pressure, Cm-NaN ₅ under 35GPa The theoretical energy density of Pmn21-NaN5 is 103.2kJ/mol at _50GPa , the theoretical energy density of Pmn21 _- NaN5 is 114.7kJ/mol at 0GPa, and the theoretical energy density of P2/c-NaN5 is _81.5kJ /mol at 0GPa.

The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention.

Example 1

The diamond with an anvil surface of 200 microns is selected to generate high pressure on the top anvil, and the metal rhenium foil with a purity of 99.97% is used as the gasket material, and the top anvil is pre-pressed with diamond to form an indentation, and the hole is drilled by laser. The machine forms a circular hole with a diameter of 70 μm in the center of the indentation as a sample cavity for encapsulating the NaN ₃ @BNNTs confined nanocomposite; liquid argon is sealed as the pressure transmission medium, and then ruby microspheres below 10 μm are loaded as the indenter. Standard substance used to calibrate the pressure in the sample chamber. Rotate the diamond anvil pressure nut, and carry out pressure loading under normal temperature conditions. When the pressure was increased to 35GPa, confined high-density anhydrous alkali metal polynitrogen Cm-NaN ₅ could be obtained, denoted as Cm-NaN ₅ @BNNTs.

Figures 1 and 2 are the high-pressure in situ Raman spectra of Cm-NaN ₅ @BNNTs at 35GPa pressure (black arrows in the figure represent the characteristic vibration of the N ₅ -ring) and high-pressure in situ synchrotron radiation angle-dispersion XRD spectra, respectively. (The * in the figure is marked as the diffraction peak of Cm-NaN ₅ phase), and (a) in Figure 3 is the 3D crystal structure of Cm-NaN ₅ . In the high-pressure in-situ Raman spectrum, the three characteristic peaks located at 200-400cm ^-1 are the lattice vibration modes of NaN ₃ high-pressure phase γ-NaN ₃ (space group is I4/mcm), and the characteristic peak located at 650cm ^-1 is The bending vibration mode of N ₃ ^- in γ-NaN ₃ , the characteristic peak located at 1490cm ^-1 is the symmetrical stretching vibration mode of N ₃ ^- in γ-NaN ₃ . The black arrows in the figure are marked as three new broad peaks, which are in good agreement with the theoretically calculated Raman peak positions of the Cm-NaN ₅ structure in the theoretical prediction, in which the Raman vibration peak located at ~280cm ^-1 is N ₅ ^- The lattice vibration of the ring, the Raman vibration peak at 800 cm ^-1 is N ₅ ^-ring bending vibration, and the Raman vibration peak at ~1100 cm ^-1 is assigned to N ₅ ^-ring asymmetric breathing and angular deformation vibration. In the high-pressure in situ synchrotron radiation angle-dispersion XRD pattern, the new diffraction peak at 35GPa is in good agreement with the theoretically predicted pattern of the Cm-NaN ₅ structure. In addition, the phase transition from the I4/mcm-NaN ₃ structure to the Cm-NaN ₅ structure was accompanied by the formation of the Cmmm-NaN ₂ structure.

Example 2

The press, sample chamber and pressure transfer medium were the same as those in Example 1. An appropriate amount of confined nanocomposite NaN ₃ @BNNTs was filled into the sample cavity, and then ruby microspheres were added as a pressure marker (to detect the pressure in the pressure cavity), and liquid argon was sealed as a pressure transmission medium for pressurization. When the pressure was increased to 43 GPa, confined high-density anhydrous alkali metal polymeric nitrogen Cm-NaN ₅ @BNNTs could be obtained. Figures 4 and 5 are the high-pressure in-situ Raman spectrum of the sample at 43GPa pressure (the black arrows in the figure represent the characteristic vibration of the N ₅ ^-ring .) and the high-pressure in-situ synchrotron radiation angle-dispersion XRD spectrum (* in the figure), respectively. It is marked as Cm-NaN ₅ phase diffraction peak), according to FIG. 4 and FIG. 5 , it can be seen that NaN ₅ is successfully obtained under high pressure in this example.

Example 3

The press, sample chamber and pressure transfer medium were the same as those in Example 1. An appropriate amount of confined nanocomposite NaN ₃ @BNNTs was filled into the sample cavity, and then ruby microspheres were added as a pressure marker (to detect the pressure in the pressure cavity), and liquid argon was sealed as a pressure transmission medium for pressurization. When the pressure is increased to 115GPa, the confined high-density anhydrous alkali metal polynitrogen NaN ₅ can be obtained, denoted as Cm-NaN ₅ @BNNTs.

The samples in the sample cavity were characterized by high-pressure in-situ Raman spectroscopy and high-pressure in-situ synchrotron radiation angle-dispersion XRD. The results are shown in Figures 6 and 7. Under the condition of 115GPa, the Raman spectrum shows the Raman characteristic vibration of NaN ₅ : the Raman vibration peak at 300-500cm ^-1 is assigned to the lattice vibration of _N5 ^- ring, and the Raman vibration peak at 830cm ^-1 The Raman vibration peaks at 1160 cm ^-1 are assigned to the bending vibrations of the _N5 ^- ring, and the asymmetric breathing and angular deformation vibrations of the _N5 ^- ring are assigned. The characteristic peak of N ₃ ^-bending vibration located in the range of 400-750 cm ^-1 and the characteristic peak of N ₃ ^-symmetric stretching vibration located at 1560 cm ^-1 disappeared completely, marking the complete transformation of γ-NaN ₃ to NaN ₅ . The high-pressure in situ synchrotron radiation XRD patterns are consistent with the Cm-NaN ₅ results.

Example 4

The press, sample chamber and pressure transfer medium were the same as those in Example 1. An appropriate amount of confined nanocomposite NaN ₃ @BNNTs was filled into the sample cavity, and then ruby microspheres were added as a pressure marker (to detect the pressure in the pressure cavity), and liquid argon was sealed as a pressure transmission medium for pressurization. When the pressure was raised to 50GPa, the sample in the sample cavity was heated to 2000K by high-pressure in-situ laser, and the confined high-density anhydrous alkali metal polynitrogen Pmn2 ₁ -NaN ₅ @BNNTs stably existed under high pressure was obtained.

Figures 8 and 9 are the Raman spectrum and synchrotron radiation angle dispersion XRD spectrum of the sample under the condition of 50GPa, respectively, and Figure 3(b) is the 3D crystal structure of Pmn2 ₁ -NaN ₅ . The N ₅ -characteristic vibrations appear in the Raman spectrum after laser heating ^: the two Raman bands at 150-540 cm ^-1 are assigned to the lattice vibration of the N ₅ ^-ring , and the Raman vibration peak at 800 cm ^-1 is assigned to The bending vibrations of the N ₅ ^-ring , the Raman vibration peaks at 1036, 1170 cm ^-1 are assigned to the asymmetric breathing and angular deformation vibrations of the N ₅ ^-ring . This is in good agreement with the theoretically calculated Raman peak positions of the Pmn2 ₁ -NaN ₅ structure in the theoretical prediction. The high-pressure in situ synchrotron radiation angle-dispersed XRD pattern is consistent with the theoretical calculation pattern of the Pmn2 ₁ -NaN ₅ structure.

Example 5

The press, sample chamber and pressure transfer medium were the same as those in Example 1. An appropriate amount of confined nanocomposite NaN ₃ @BNNTs was filled into the sample cavity, and then ruby microspheres were added as a pressure marker (to detect the pressure in the pressure cavity), and liquid argon was sealed as a pressure transmission medium for pressurization. When the pressure was raised to 50GPa, the sample in the sample cavity was heated to 2000K by high-pressure in-situ laser, and then the sample was depressurized to normal pressure to obtain the confined high-density anhydrous alkali metal polymer nitrogen P2/ c- _NaN5 @BNNTs.

Figure 10 and Figure 11 are the Raman spectrum and synchrotron radiation angle dispersion XRD spectrum of the sample under normal temperature and normal pressure conditions, respectively, and Figure 3(c) is the 3D crystal structure of P2/c-NaN ₅ . ^The N ₅ -characteristic vibrational peaks appear in the Raman spectrum at room temperature and pressure at 119cm ^-1 , 831cm ^-1 , 998cm ^-1 , 1115cm ^-1 and 1180cm ^-1 , which is consistent with the theoretical prediction of the P2/c-NaN ₅ structure. The calculated Raman peak positions are in good agreement. The XRD pattern of synchrotron radiation at room temperature and pressure is consistent with the theoretical calculation pattern of P2/c-NaN ₅ structure.

The above are only the preferred embodiments of the present invention. It should be pointed out that for those skilled in the art, without departing from the principles of the present invention, several improvements and modifications can be made. It should be regarded as the protection scope of the present invention.

Claims (9)

1. a high-pressure preparation method of confined high-density anhydrous alkali metal polynitrogen Cm-NaN ₅ , is characterized in that, comprises the following steps: The sodium azide confined in boron nitride nanotubes was encapsulated in a high-pressure cavity of a diamond anvil, and then pressurized to above 35 GPa to obtain a confined high-density anhydrous alkali metal polymer nitrogen Cm-NaN stably existing under high pressure ₅ . 2. A high-temperature and high-pressure preparation method of confined high-density anhydrous alkali metal polymeric nitrogen Pmn ₁ -NaN ₅ , characterized in that, comprising the following steps: The sodium azide confined in boron nitride nanotubes was encapsulated in a high-pressure cavity of a diamond anvil, pressurized to more than 50GPa, and then subjected to 2000-2300K laser heat treatment to obtain a confined high-density nanotube that stably exists under high pressure. Water alkali metal polynitrogen Pmn2 ₁ -NaN ₅ . 3. a method for trapping at atmospheric pressure of confined high-density anhydrous alkali metal polymeric nitrogen P2/c-NaN ₅ , characterized in that, comprising the following steps: The sodium azide confined in boron nitride nanotubes was encapsulated in a high-pressure cavity of diamond anvil, pressurized to more than 50GPa, then subjected to 2000-2300K laser heating treatment, and then decompressed to normal pressure to obtain Confinement high-density anhydrous alkali metal polymeric nitrogen P2/c-NaN ₅ that exists stably under the condition. 4. The method according to claim 1, 2 or 3, wherein the pressurized pressure transmission medium is liquid argon or liquid neon. 5. The method according to claim 1, 2 or 3, wherein the method for preparing the high-pressure chamber of the diamond anvil is as follows: using a rhenium foil as a gasket material, and using a diamond to pre-treat the anvil to the rhenium foil. Press to form an indentation; use a laser drilling machine to form a hole in the center of the indentation as a high pressure cavity. 6 . The method according to claim 5 , wherein the thickness of the pre-pressed rhenium foil is 40-60 μm. 7 . 7. The confined high-density anhydrous alkali metal polymeric nitrogen Cm-NaN ₅ obtained by the method of any one of claims 1, 4-6. 8. The confined high-density anhydrous alkali metal polymeric nitrogen Pmn2 ₁ -NaN ₅ obtained by the method according to any one of claims 2, 4 to 6. 9 . The confined high-density anhydrous alkali metal polymeric nitrogen P2/c-NaN ₅ obtained by the method according to any one of claims 3, 4 to 6. 10 .

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