Peculiar bond characters of fivefold coordinated octet compound crystals.

An-An Sun,Shang-Peng Gao,Gong Gu

doi:10.1039/d0sc00292e

An-An Sun, Shang-Peng Gao + Show 1 more

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https://doi.org/10.1039/d0sc00292e

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Abstract

The present work exemplifies complementary perspectives offered by the band and bond pictures of solids, with an emphasis on the chemical intuition pertaining to the latter, especially in the presence of interfaces. The modern computational method of constructing a unique set of maximally localized Wannier functions from delocalized band states imparts new interpretations to the familiar concept of chemical bonds in the context of crystalline solids. By bridging the band and bond pictures using advanced computational tools, we reveal for the first time the unusual bond characters of a long-predicted fivefold coordinated structure of binary octet compounds ANB8−N consisting of AA′ stacked planar AB honeycombs. While the isolated monolayer retains the familiar pz–π bonding in a honeycomb framework as in graphene and hexagonal boron nitride, the bulk foregoes in-plane π bonding and embraces out-of-plane ⋯A–B–A–B⋯ chain bonding via overlapping pz orbitals. Not only does the chemical intuition gained by invoking the bond picture clarify the chemical nature of the fivefold coordination, but it also facilely explains a salient discrepancy in theoretical predictions in otherwise sound ample experimental evidence in the form of epitaxial thin films, paving the way towards rational synthesis of such thin films for optoelectronic applications. On the other hand, we show that the conduction band minimum, important in determining the electrical and optical properties, is a distinctly extended state that can only be properly described within the band picture.

Highlights

The band and bond pictures of solids offer complementary perspectives, with the latter conducive to chemical intuition especially in the presence of interfaces and defects.[1,2] Chemical bond characters of a material, not apparent from the extended energy and momentum eigenstates in the band picture, can provide insight into the structure, properties, and synthesis of the material, as we shall show here for a vefold coordinated phase of binary octet compounds ANB8ÀN consisting of AA0 stacked planar AB honeycombs[3,4,5] (Fig. 1a and b).Under ambient conditions, group IV–IV, III–V, and many II– VI binary octets exhibit fourfold coordinated polymorphs including zinc blende and wurtzite, as well as numerous stacking variations,[6,7,8] whereas higher ionicity compounds are stable in structures of higher coordination numbers (e.g. MgO (z-BN) is the thermodynamically stable phase until z-BN was generally accepted as such.[9]The vefold coordinated structure is an intermediate predicted for some ANB8ÀN between phases characterized by coordination numbers Nc 1⁄4 4 and 6, along the paths of structural transitions exhibiting a general trend of increasing Nc with increasing ionicity upon successive compression.[4,5,10,11,12,13,14,15,16,17] This structure has the same symmetry (P63/mmc) as h-BN and is referred to as the h-MgO structure (just as NaCl for the rocksalt structure, the compound not necessarily MgO) since it was rst predicted for MgO4 or as the 5-5 structure for the mutual vefold coordination,[5,16,17,18,19] or as HX standing for hexagonal.[11]
By bridging the band and bond pictures using advanced computational tools, we reveal for the first time the unusual bond characters of a long-predicted fivefold coordinated structure of binary octet compounds ANB8ÀN consisting of AA0 stacked planar AB honeycombs
Chemical bond characters of a material, not apparent from the extended energy and momentum eigenstates in the band picture, can provide insight into the structure, properties, and synthesis of the material, as we shall show here for a vefold coordinated phase of binary octet compounds ANB8ÀN consisting of AA0 stacked planar AB honeycombs[3,4,5] (Fig. 1a and b)

Summary

Introduction

The band and bond pictures of solids offer complementary perspectives, with the latter conducive to chemical intuition especially in the presence of interfaces and defects.[1,2] Chemical bond characters of a material, not apparent from the extended energy and momentum eigenstates in the band picture, can provide insight into the structure, properties, and synthesis of the material, as we shall show here for a vefold coordinated phase of binary octet compounds ANB8ÀN consisting of AA0 stacked planar AB honeycombs[3,4,5] (Fig. 1a and b).Under ambient conditions, group IV–IV, III–V, and many II– VI binary octets exhibit fourfold coordinated polymorphs including zinc blende and wurtzite, as well as numerous stacking variations,[6,7,8] whereas higher ionicity compounds are stable in structures of higher coordination numbers (e.g. MgO (z-BN) is the thermodynamically stable phase until z-BN was generally accepted as such.[9]The vefold coordinated structure is an intermediate predicted for some ANB8ÀN between phases characterized by coordination numbers Nc 1⁄4 4 and 6, along the paths of structural transitions exhibiting a general trend of increasing Nc with increasing ionicity upon successive compression.[4,5,10,11,12,13,14,15,16,17] This structure has the same symmetry (P63/mmc) as h-BN and is referred to as the h-MgO structure (just as NaCl for the rocksalt structure, the compound not necessarily MgO) since it was rst predicted for MgO4 or as the 5-5 structure for the mutual vefold coordination,[5,16,17,18,19] or as HX standing for hexagonal.[11]. Both bulk h-AlN and h-MgO exhibit wider valence band energy spans along each of these lines than their respective wurtzite counterparts, indicating a stronger interlayer interaction for the vefold coordinated polymorph than for the fourfold.

Results

Conclusion