High-Pressure Investigation of the Phase Diagram of the C-N-Alkali Metal Systems
Qian Zhang^1,*, Zara Kucernak¹, Akun Liang¹, Umbertoluca Ranieri¹, James Spender¹, Sarah Bolton¹, Elissaios Stavrou^2,3,4, Dominique Laniel<sup>1,*   

1</sup> Centre for Science at Extreme Conditions and School of Physics and Astronomy, University of Edinburgh, Edinburgh EH9 3FD, UK
² Materials Science and Engineering Program, Guangdong Technion-Israel Institute of Technology, Shantou, Guangdong 515063, China
³ Department of Materials Science and Engineering, Technion-Israel Institute of Technology, Haifa 32000, Israel
⁴ Guangdong Provincial Key Laboratory of Materials and Technologies for Energy Conversion, Guangdong Technion-Israel Institute of Technology, Shantou 515063, China

Q.Zhang-127@sms.ed.ac.uk, Dominique.Laniel@ed.ac.uk

Binary carbon nitrides have attracted significant research interest due to their remarkable physical properties, such as superhardness, ultra-incompressibility, and electrochemical performance.^[1–3] At high pressure conditions, the interest in carbon nitrides is shifting toward ternary systems, which offer many exciting perspectives. First, introducing an element that favors electronic charge transfer enables the formation of novel carbon-nitrogen species^[4] at low synthesis pressures; of great interest for fundamental chemistry.^[5] Secondly, besides a few lanthanides^[4] and hydrogen systems,^[5] the phase diagrams of ternary C-N systems at extreme conditions remain largely unexplored. Thirdly, the CN₄ tetrahedra present in C₃N₄ polymorphs,^[1,2] C(NH₂) solids^[5] and lanthanide carbonitrides,^[4] form robust frameworks that can be functionalized by the third element; appealing for technological applications.^[6,7] In particular, a CN₄-based porous framework would be ideal for battery anodes, permitting improved charge storage, fast ion transport and low volume expansion.^[8] With this in mind, carbon-nitrogen-alkali metal systems are particularly intriguing, as alkali cations could facilitate of CN₄ network formation at low pressures through a charge transfer, and later be removed at ambient conditions—similarly to the Na₄Si₂₄ allotrope^[7].

In this work, we investigated the Na- and K-C-N systems at pressures below 50 GPa using laser-heated diamond anvil cells (LHDACs). The sodium and potassium carbonitrides NaCN, NaN(CN)₂ and KCN were employed as precursors and laser-heated at various pressures. Raman spectroscopy as well as synchrotron single-crystal X-ray diffraction was performed in order to assess if a chemical reaction had taken place and, if so, determine the nature of the reaction products. Remarkably, four hitherto unknown compounds were synthesized, namely Na₅(N₂)(CN₂)₂, the α-Na₂CN₂ and β-Na₂CN₂ polymorphs, and K₂CN₂ (see Figure 1). All four solids are comprised of the cyanamide anion ([N=C=N]^2-).

Figure 1. Crystal structure of the novel compounds (a) α-Na₂CN₂ (b) β-Na₂CN₂, (c) Na₅(N₂)(CN₂)₂ and (d) K₂CN₂. Grey, brown, yellow and purple spheres represent nitrogen, carbon, sodium and potassium atoms, respectively

In this talk, we will discuss the crystal chemistry of these novel compounds, their implications for the ternary phase diagram of these systems and draw parallels between the arrangement adopted by cyanamides with the isostructural and isoelectronic azide (N₃^-) anion in alkali azides. The (meta)stability range of these compounds will also be addressed, and in particular whether or not they are expected to be recoverable to ambient conditions. Additionally, we will present preliminary experiments up to 120 GPa, where the cyanamide anions are expected to concatenate, forming polymeric, CN₄-based, frameworks. Such frameworks could be of interest for energy storage applications, particularly as potential anode materials in high-performance batteries.


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high pressure,diamond anvil cell,single crystal XRD,laser heating,battery materials