New Li-Mg phosphates with a 3D framework: experimental and ab initio calculations

Area of Science:

• Solid-state chemistry
• Materials science
• Crystallography

Background:

• Lithium-magnesium phosphates are explored for energy storage and optical applications.
• Understanding structure-property relationships is crucial for developing new materials.

Purpose of the Study:

• To synthesize and characterize novel lithium-magnesium phosphate compounds.
• To investigate their crystal structure, ionic conductivity, and electronic properties.
• To evaluate the effect of scandium doping on material stability and optical characteristics.

Main Methods:

• Solid-phase synthesis.
• High-resolution time-of-flight neutron powder diffraction (TOF NPD) and X-ray powder diffraction (XRPD) for structural analysis.
• AC impedance spectroscopy for ionic conductivity measurements.
• Nuclear Magnetic Resonance Magic Angle Spinning (NMR MAS) studies.
• First-principles calculations for electronic structure and stability.
• UV-Vis spectroscopy for optical properties.

Main Results:

• Two new phosphates, LiMg₆(PO₄)₃(P₂O₇) and Li(Mg_5.62Sc_0.19Li_0.19)(PO₄)₃(P₂O₇), were synthesized with Pnma orthorhombic structures.
• Both materials exhibit significant lithium ion conductivity, with LiMg₆(PO₄)₃(P₂O₇) showing 3.6 × 10^-4 S cm^-1 at 950 °C.
• First-principles calculations indicate Sc stabilizes LiMg₆(PO₄)₃(P₂O₇) but destabilizes olivine LiMgPO₄.
• LiMg₆(PO₄)₃(P₂O₇) is a dielectric with a band gap of 5.35 eV, decreasing to 4.85 eV upon Sc doping.

Conclusions:

• The synthesized lithium-magnesium phosphates possess unique crystal structures conducive to lithium diffusion.
• The materials demonstrate potential for applications requiring ionic conductivity and as optical matrices.
• Scandium doping has a significant impact on the stability and electronic properties of these phosphate systems.