Penta-MN8 family: First realization of type-5 pentagonal tessellation in 2D hexagonal crystals with intriguing properties

Tessellation of the Euclidean plane by congruent pentagons has long been of great interest. However, periodic pentagonal patterns realized in crystallography reported so far are limited to type-2 or type-4 tiling in typical square lattices. Based on the recently synthesized N18 macrocycles together with Hückel aromaticity and 18-electron rules, we design a family of pentagonal metal polynitrides, penta-MN8 (M = Mg, Ca, Sr, Ba; Sn, Pb; Cd; Mo, W), the first materials realization of type-5 mathematical pattern with novel pentagonal tiles in 2D hexagonal lattices. The penta-MN8 family exhibits rich stable phases (γ, α/α′, and β) and diverse Poisson's ratios ranging from 0.299 to -0.488. Among these nitrogen-richest monolayer nitrides hitherto, penta-MgN8 possesses firm stability up to 700 K and under low pressure originating from unique aromaticity over the polymeric nitrogen network, which could be synthesized under ∼50 GPa as confirmed by global structure search. Moreover, penta-MgN8 exhibits a fully flat band near Fermi level over the whole 2D Brillouin zone, deriving from the combination of pentagonal pattern and hexagonal lattice as verified with tight-binding model analysis. Such electronic structures fill in the blank of clear kagome flat bands in known kagome nitrides and pentagon-based materials, resulting in very heavy fermions with huge effective mass (55.7 m0) that are favorable to achieve quantum zero-field Wigner crystals. Our work presents a new pentagonal-materials class with unprecedented coordination, lattice symmetry, and physical properties compared to existing pentagon-based systems, which makes a breakthrough in exploring the materials realization of the mathematical models for pentagonal tessellation.