High-pressure synthesis of PbN2, the missing group 14 AN2-type compound
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2025-01-01 |
| Journal | Journal of Materials Chemistry A |
| Authors | Ken Niwa, Hirokazu Ogasawara, Takuya Sasaki, Shunsuke Nomura, Gendai Azuma |
| Institutions | Université de Poitiers, Nagoya University |
| Analysis | Full AI Review Included |
Executive Summary
Section titled âExecutive Summaryâ- Novel Compound Synthesis: Successfully synthesized Lead Pernitride (PbN2), the missing Group 14 AN2-type compound, via direct reaction of Pb and N2 using a Laser-Heated Diamond Anvil Cell (LHDAC) above 50 GPa.
- Unique Crystal Structure: PbN2 crystallizes in a tetragonal system (CuAl2-type, I4/mcm), featuring Pb atoms coordinated to eight N atoms (PbN8 units) and encapsulating anionic nitrogen dimers (N2)2-.
- Structural Divergence: This CuAl2-type structure is distinct from the pyrite-type (FeS2) observed in previously synthesized Group 14 pernitrides (SiN2, GeN2, SnN2).
- Stability and Decomposition: PbN2 maintains structural integrity up to approximately 90 GPa during compression but decomposes irreversibly into Pb and N2 upon decompression below 15 GPa. It is not recoverable at ambient pressure.
- Mechanical Properties: Preliminary analysis yielded a zero-pressure bulk modulus (K0) of 65(3) GPa, indicating weaker Pb-N interactions compared to other Group 14 pernitrides.
- Bonding Insight: Experimental and DFT evidence suggests the encapsulated nitrogen dumbbells act as anionic units (N2)2-, confirming charge transfer from the lead lattice.
Technical Specifications
Section titled âTechnical Specificationsâ| Parameter | Value | Unit | Context |
|---|---|---|---|
| Synthesis Pressure (Minimum) | >50 | GPa | Required for direct reaction of Pb + N2. |
| Synthesis Temperature (Estimate) | >2000 | K | Based on intense thermal radiation observed during laser heating. |
| Crystal System | Tetragonal | - | CuAl2-type structure (Space Group I4/mcm, No. 140). |
| Lattice Parameter (a) @ 55.1 GPa | 3.297(1) | A | Initial indexing result (P lattice equivalent). |
| Lattice Parameter (c) @ 55.1 GPa | 3.089(2) | A | Initial indexing result (P lattice equivalent). |
| Lattice Parameter (a) @ 47 GPa | 4.6266(1) | A | Le Bail refinement (I4/mcm setting). |
| Lattice Parameter (c) @ 47 GPa | 6.1278(2) | A | Le Bail refinement (I4/mcm setting). |
| Zero-Pressure Bulk Modulus (K0) | 65(3) | GPa | Determined by 2nd-order Birch-Murnaghan EOS fit. |
| Decomposition Pressure | ~15 | GPa | Observed during room temperature decompression. |
| High-Pressure Stability Limit | ~90 | GPa | Retained structure during room temperature compression. |
| Volume Reduction (Pb + N2 to PbN2) | ~18% | - | Volumetric comparison at synthesis pressure. |
| Calculated N-N Bond Length (0 GPa) | 1.21 | A | DFT calculation (increases slightly to 1.27 A at 100 GPa). |
| Low Wavenumber Raman Mode (Eg) | 130 (at 50 GPa) to 49 (at 0 GPa) | cm-1 | Calculated mode, shows inverse pressure response. |
| Experimental Raman Peak (Unexplained) | 2000-2200 | cm-1 | Observed high wavenumber peak, possibly activated N2 species. |
| DFT Cutoff Energy | 570 | eV | Used for ultrasoft pseudopotentials in CASTEP calculations. |
Key Methodologies
Section titled âKey Methodologiesâ- High-Pressure Synthesis: Utilized a Laser-Heated Diamond Anvil Cell (LHDAC) with 350 ”m culet anvils. Lead foil (99.99%) was loaded into a sample chamber drilled into stainless steel or rhenium gaskets, followed by filling with liquid nitrogen (N2) as the pressure medium.
- Reaction Conditions: Samples were compressed to >50 GPa and heated using dual-sided infrared laser irradiation (λ = 1090 nm), achieving estimated temperatures exceeding 2000 K.
- In Situ Structural Analysis (XRD): High-pressure synchrotron X-ray powder diffraction (XRD) was performed at BL2S1 and BL10XU (SPring-8). Data analysis included phase identification, indexing (using PDIndexers and DICVOL06), and Le Bail refinement (RIETAN-FP) to confirm the tetragonal I4/mcm structure.
- In Situ Vibrational Analysis (Raman): Raman spectroscopy was conducted using Argon ion (488 nm) or diode-pumped (473 nm) lasers to monitor the emergence and shift of vibrational modes, confirming the presence of N-N dimers.
- Pressure Calibration: Pressure was determined using the ruby fluorescence peak method (at lower pressures) and the equation of state (EOS) of the rhenium gasket or solid nitrogen (at higher pressures).
- First-Principles Modeling (DFT): Density Functional Theory (DFT) calculations (CASTEP, VASP) using GGA-PBE were employed for geometry optimization, total electronic energy calculations, charge density mapping, and phonon calculations (PHONOPY) to validate the CuAl2-type structure and predict Raman spectra.
Commercial Applications
Section titled âCommercial Applicationsâ- High-Energy Density Materials (HEDMs): Pernitrides are a primary focus for HEDMs due to the high energy stored in N-N bonds. This research provides fundamental structural data necessary for computational screening and synthesis of novel, potentially recoverable, high-nitrogen compounds.
- Advanced Functional Ceramics: The study contributes to the knowledge base of Group 14 nitrides, which are critical components in high-performance ceramics (e.g., Si3N4). Understanding high-pressure phase transitions (pyrite-type to CuAl2-type) guides the development of new, high-density ceramic phases.
- High-Pressure Synthesis Technology: The successful methodologyâcombining LHDAC, synchrotron characterization, and DFTâis directly applicable to synthesizing other novel binary or ternary compounds involving volatile elements (e.g., Pn-N, transition metal-N systems).
- Crystal Structure Prediction (CSP): The experimental validation of the CuAl2-type structure for PbN2 serves as a crucial benchmark for evolutionary algorithms and DFT models used in predicting material structures under extreme conditions.
View Original Abstract
The direct reaction of Pb with molecular nitrogen was investigated using a laser-heated diamond anvil cell combined with high-pressure in situ synchrotron X-ray powder diffraction measurements and Raman spectroscopy.