Simple Molecules under High‐Pressure and High‐Temperature Conditions - Synthesis and Characterization of α‐ and β‐C(NH)2 with Fully sp3‐Hybridized Carbon
At a Glance
Section titled “At a Glance”| Metadata | Details |
|---|---|
| Publication Date | 2023-12-15 |
| Journal | Angewandte Chemie |
| Authors | Thaddäus J. Koller, Siyu Jin, Viktoria Krol, Sebastian J. Ambach, Umbertoluca Ranieri |
| Institutions | University of Edinburgh, China University of Geosciences |
| Analysis | Full AI Review Included |
Executive Summary
Section titled “Executive Summary”This research successfully synthesized and characterized two novel carbon diimide phases, α-C(NH)2 and β-C(NH)2, under extreme high-pressure and high-temperature (HPHT) conditions relevant to planetary science.
- Novel Materials Synthesis: Two previously unknown compounds, α-C(NH)2 and β-C(NH)2, were synthesized from simple C/N/H precursors (malononitrile, DCDA, melamine) using Laser-Heated Diamond Anvil Cells (LHDAC).
- Structural Confirmation: Both phases feature fully sp3-hybridized carbon atoms, forming extended covalent networks, similar to diamond or high-pressure SiO2 polymorphs.
- α-C(NH)2 Structure: Crystallizes in a distorted β-cristobalite structure (space group Fdd2). It exhibits a high bulk modulus (K0 ≈ 148 GPa) but is not recoverable at ambient pressure.
- β-C(NH)2 Structure & Recoverability: Built from complex imide-bridged 2,4,6,8,9,10-hexaazaadamantane (HAA) units, forming two interpenetrating diamond-like networks. It is fully recoverable to ambient conditions and stable in air (K0 ≈ 93 GPa).
- Thermodynamic Mapping: Density Functional Theory (DFT) calculations mapped the relative thermodynamic stabilities, explaining why different precursors (DCDA vs. melamine) yield different products under similar HPHT conditions.
- Relevance: These materials contribute to the understanding of exotic chemistry occurring within the interiors of ice giants like Uranus and Neptune, which are rich in C, N, and H.
Technical Specifications
Section titled “Technical Specifications”| Parameter | Value | Unit | Context |
|---|---|---|---|
| Synthesis Pressure (DAC-1) | 37 | GPa | Malononitrile precursor |
| Synthesis Temperature (DAC-1) | 2400 | K | Malononitrile precursor |
| Synthesis Pressure (DAC-3) | 36 | GPa | Melamine precursor |
| Synthesis Temperature (DAC-3) | 1400 | K | Melamine precursor |
| α-C(NH)2 Bulk Modulus (K0) | 148(2) | GPa | Experimental (3rd order BMEOS) |
| α-C(NH)2 Bulk Modulus (K0) | 145.4(4) | GPa | Theoretical (DFT) |
| β-C(NH)2 Bulk Modulus (K0) | 93(4) | GPa | Experimental (2nd order BMEOS) |
| β-C(NH)2 Bulk Modulus (K0) | 89.8(10) | GPa | Theoretical (DFT) |
| α-C(NH)2 Lattice (37 GPa) | a=5.36, b=5.63, c=6.13 | A | Distorted β-cristobalite structure |
| β-C(NH)2 Lattice (36 GPa) | a=10.85, b=11.39, c=12.40 | A | Hexaazaadamantane network |
| C-N Bond Lengths (α-C(NH)2) | 1.421(3) and 1.435(3) | A | sp3-hybridized C-N single bonds |
| N-C-N Bond Angles (α-C(NH)2) | 103.65 to 118.8 | ° | Deviation from ideal tetrahedral (109.5°) |
| β-C(NH)2 Recoverability | Full | N/A | Stable in air at ambient conditions |
| α-C(NH)2 Stability | Not recoverable | N/A | Decomposes upon decompression |
Key Methodologies
Section titled “Key Methodologies”The synthesis and characterization relied on a combination of high-pressure techniques and advanced synchrotron analysis:
- Precursor Loading: Simple C/N/H molecular precursors (Malononitrile, DCDA, or Melamine) were loaded into BX90-type Diamond Anvil Cells (DACs).
- High-Pressure Compression: Samples were compressed to target pressures ranging from 36 GPa to 45 GPa.
- Laser Heating (LHDAC): Samples were heated using a laser, with power increased stepwise until an intense flash of light was observed (T > 1000 K, sustained for approximately five seconds).
- In Situ X-Ray Diffraction (XRD): Products were analyzed using Synchrotron Single-Crystal X-Ray Diffraction (SCXRD) at the Extreme Conditions Beamline P02.2 (DESY) and the High Pressure Beamline ID27 (ESRF).
- Equation of State Determination: Samples were stepwise decompressed, and unit cell volumes were measured to calculate the Bulk Modulus (K0) using the 2nd or 3rd order Birch-Murnaghan Equation of State (BMEOS).
- Density Functional Theory (DFT) Modeling: Calculations were performed to verify experimental structures, determine precise hydrogen atom positions, confirm dynamic stability (phonon frequencies), and calculate relative enthalpies and Gibbs free energies of formation.
Commercial Applications
Section titled “Commercial Applications”The synthesis of novel, highly condensed carbon nitrides with sp3 bonding networks and high bulk moduli opens pathways for applications in extreme environments and advanced materials engineering.
| Application Area | Relevance to C(NH)2 Phases |
|---|---|
| Superhard Materials & Coatings | High bulk moduli (K0 up to 148 GPa) and extended sp3 covalent networks suggest high hardness, suitable for abrasive tools, protective coatings, and high-wear components. |
| High-Performance Ceramics | Carbon nitrides are predicted to have high thermal conductivity, making them candidates for heat sinks and thermal management components in electronics and aerospace. |
| Proton Exchange Membranes (PEMs) | The presence of imide (NH) groups allows for the formation of hydrogen bonds, potentially enabling high proton conductivity, crucial for next-generation fuel cells and electrochemical devices. |
| High-Energy Density Materials | The highly condensed cage structure of β-C(NH)2 (hexaazaadamantane units) and its high nitrogen content are characteristics sought after in high-energy density materials (HEDMs). |
| Extreme Environment Engineering | The stability of β-C(NH)2 under high pressure and its recoverability to ambient conditions make it valuable for components used in deep-sea, geological, or aerospace applications. |
View Original Abstract
Abstract The elements hydrogen, carbon, and nitrogen are among the most abundant in the solar system. Still, little is known about the ternary compounds these elements can form under the high‐pressure and high‐temperature conditions found in the outer planets’ interiors. These materials are also of significant research interest since they are predicted to feature many desirable properties such as high thermal conductivity and hardness due to strong covalent bonding networks. In this study, the high‐pressure high‐temperature reaction behavior of malononitrile H 2 C(CN) 2 , dicyandiamide (H 2 N) 2 C=NCN, and melamine (C 3 N 3 )(NH 2 ) 3 was investigated in laser‐heated diamond anvil cells. Two previously unknown compounds, namely α‐C(NH) 2 and β‐C(NH) 2 , have been synthesized and found to have fully sp 3 ‐hybridized carbon atoms. α‐C(NH) 2 crystallizes in a distorted β‐cristobalite structure, while β‐C(NH) 2 is built from previously unknown imide‐bridged 2,4,6,8,9,10‐hexaazaadamantane units, which form two independent interpenetrating diamond‐like networks. Their stability domains and compressibility were studied, for which supporting density functional theory calculations were performed.