Liquid Skin, Gas Phase, and Interface

Bond contraction also happens in liquid skins and gaseous molecules. For instances, bonds at methanol skin contracts by 4.5 % but the hydrogen bond expands by 5.9 % at water surface at room temperature [49]. The spacing between the first and the second atomic layers of liquid Sn, Hg, Ga, and In contracts by 10 % [50]. Phase change occurs at the droplet smface of n-alkane indicates the formation of a monolayer thick solid solution of the oil phase [51]. Bond contraction at liquid skin results in the surface strain and smface stress that drives and sustains liquid drop and gas-bubble formation. X-ray crystallography revealed that a Cr-Cr bond length contracts by 31.5 % from the bulk value of 0.255 to 0.174 nm for a Cr2 anion protected by three bidentate amidinate ligands [52] and 0.175 nm [53] for a neutral Cr2 complex. 

Bond contraction also happens to the hetero-coordinated atoms. Extended X-ray absorption fine structure (EXAFS) and X-ray absorption near edge spectroscopy (XANES) measmements revealed a 8 % contraction of distance between the As impurity (acceptor dopant) and the Te sub-lattice in CdTe [54]. At A1 twin grain boundaries, homo-junction dislocations have either compact or dissociated interlayers. The minimum stress op) required for moving an edge dislocation is 

However, exceptional cases exist. Based on the fine structure of boron and carbon /T-edges in EELS and TEM, Lu et al. [56] found local C-C bond expansion at sites with local high B content in the diamond grain. 

A few layer NB atomic sheet shows asymmetric interlayer relaxation effect. The ip-orbit hybridization of N and B results in an empty orbit of B and a lone-pair-filled orbit of N. The interlayer relaxation proceeds with the lone pair and empty orbit. At the side of B outward, the interface contracts substantially while the other side of N outward show strong expansion [57]. 

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