Cr_5.7Si_2.3P₈N₂₄-A Chromium(+IV) Nitridosilicate Phosphate with Amphibole-Type Structure.

Area of Science:

• Materials Science
• Inorganic Chemistry
• Crystallography

Background:

• Amphibole minerals are a significant class of rock-forming silicates.
• The synthesis of nitridic analogs of silicate structures is challenging.
• Chromium-nitrogen (Cr-N) chemistry is an emerging field with potential applications.

Purpose of the Study:

• To synthesize and characterize the first nitridic analog of an amphibole mineral.
• To explore the crystal structure and properties of the novel compound Cr₅.₇Si₂.₃P₈N₂₄.
• To expand the understanding of chromium-nitrogen chemistry.

Main Methods:

• High-pressure, high-temperature synthesis (1400°C, 12 GPa) using binary nitrides and nitrogen sources.
• Single-crystal X-ray diffraction with synchrotron radiation for crystal structure elucidation.
• Energy dispersive X-ray spectroscopy (EDS) for elemental analysis.
• Atomic resolution scanning transmission electron microscopy (STEM).
• Optical spectroscopy for band gap determination.
• Magnetic susceptibility measurements and Electron Paramagnetic Resonance (EPR) for oxidation state analysis.

Main Results:

• Successful synthesis of the quaternary nitridosilicate phosphate Cr₅.₇Si₂.₃P₈N₂₄.
• Determination of the crystal structure (space group C2/m) featuring PN₄ tetrahedra in zweier double chains and (Si,Cr)-centered octahedra in ribbons.
• Identification of ordered and disordered Si/Cr sites via STEM.
• Measurement of a band gap of 2.1 eV and confirmation of paramagnetic behavior.
• Assignment of the oxidation state as Cr⁺IV through magnetic and EPR studies.

Conclusions:

• The study reports the first nitridic amphibole analog, Cr₅.₇Si₂.₃P₈N₂₄, synthesized under extreme conditions.
• The detailed structural and chemical analysis provides significant insights into Cr-N chemistry.
• This work opens avenues for designing novel nitride materials with structures analogous to prevalent silicate frameworks.