Determining a Structural Distortion and Anion Ordering in La2Si4N6C via Computation and Experiment

Area of Science:

• Materials Science
• Solid-State Chemistry
• Crystallography

Background:

• La2Si4N6C was initially reported in the Pnma space group.
• Ab initio calculations are crucial for predicting material properties and stability.

Purpose of the Study:

• To investigate the dynamic stability of La2Si4N6C.
• To determine the precise crystal structure and anion ordering of La2Si4N6C.
• To validate computational predictions through experimental synthesis and characterization.

Main Methods:

• Ab initio phonon dispersion curve calculations.
• High-temperature synthesis route.
• High-resolution synchrotron X-ray and neutron powder diffraction.
• Total scattering analysis using neutron pair distribution function (PDF).

Main Results:

• Ab initio calculations predicted a structural instability in orthorhombic La2Si4N6C, indicating a dynamic instability and a monoclinic distortion pathway to space group P21/c.
• Experimental synthesis and co-refinement of diffraction data confirmed the lower symmetry crystal structure.
• Neutron diffraction and PDF analysis revealed full anion ordering, with carbon exclusively located in C(SiN3)4 units.

Conclusions:

• The study unequivocally solves the crystal structure of La2Si4N6C, demonstrating its dynamic instability and lower symmetry.
• Combining ab initio computation with advanced experimental techniques like synchrotron X-ray and neutron diffraction is a powerful approach for accurate crystal structure determination from polycrystalline samples.