ScRu₂B₃ and Sc₂RuB₆: Borides Featuring a 2D Infinite Boron Clustering

Area of Science:

• Solid State Chemistry
• Materials Science
• Crystallography

Background:

• The Sc-Ru-B system is explored to understand ternary boride formation.
• Previous reports of 'ScRuB₄' require verification.
• Ternary borides often exhibit complex and novel crystal structures.

Purpose of the Study:

• To synthesize and characterize new scandium ruthenium borides.
• To determine the crystal structures of ScRu₂B₃ and Sc₂RuB₆.
• To investigate the phase diagram of the Sc-Ru-B system at 800 °C.

Main Methods:

• Argon-arc melting followed by annealing at 800 °C.
• Single crystal X-ray diffraction for structure determination.
• Electron Probe Microanalysis (EPMA) and powder X-ray diffraction for phase analysis.
• Ab initio electronic structure calculations (generalized gradient approximation).
• Electrical resistivity measurements.

Main Results:

• Two new borides, ScRu₂B₃ (new structure type, Cmcm) and Sc₂RuB₆ (Y₂ReB₆-type, Pbam), were successfully synthesized and their structures refined.
• ScRu₂B₃ features an intergrowth of CeCo₃B₂- and AlB₂-related slabs with puckered boron hexagons.
• Sc₂RuB₆ exhibits 2D nets of condensed boron polygons (pentagons, hexagons, heptagons) between metal layers.
• The Sc-Ru-B system at 800 °C was mapped, confirming Sc₂Ru₅B₄ and ruling out 'ScRuB₄'.
• ScRu₄B₄ (LuRu₄B₄-type, I4₁/acd) was also identified in cast alloys.
• Electronic structure calculations predict a moderate density of states at the Fermi level.
• Electrical resistivity measurements confirm ScRu₂B₃ and Sc₂RuB₆ are metals.

Conclusions:

• Novel ternary borides ScRu₂B₃ and Sc₂RuB₆ with unique structural motifs have been discovered.
• The study clarifies the phase relationships in the Sc-Ru-B system at 800 °C.
• Both synthesized borides exhibit metallic behavior consistent with electronic structure calculations.