Pressure-Driven Energy Band Gap Narrowing of λ-N_(2)

Probing the energy band gap of solid nitrogen at high pressures is of importance for understanding pressuredriven changes in electronic structures and insulator-to-metal transitions under high pressure.The λ-N_(2) formed by cold compression is known to be the most stable one in all solid nitrogen phases observed so far.By optimizing the optical system,we successfully measured the high-pressure absorption spectra of λ-N_(2) covering the polymericnitrogen synthetic pressures(124 GPa-165 GPa).The measured optical band gap decreases with increasing pressure,from 2.23 eV at 124 GPa to 1.55 eV at 165 GPa,with a negative pressure coefficient of-18.4 meV/GPa,which is consistent with the result from our ab initio total-energy calculations(-22.6 meV/GPa).The extrapolative metallization pressure for theλ-N_(2) is around 288(18)GPa,which is close to the metallization pressure(280 GPa)for the η-N_(2) expected by previous absorption edge and direct electrical measurements.Our results provide a direct spectroscopic evidence for the pressure-driven band gap narrowing of solid nitrogen.

Regulation of Ionic Bond in GroupⅡB Transition Metal Iodides

Using a swarm intelligence structure search method combining with first-principles calculations,three new structures of Zn-I and Hg-I compounds are discovered and pressure-composition phase diagrams are determined.An interesting phenomenon is found,that is,the compounds that are stable at 0 GPa in both systems will decompose into their constituent elements under certain pressure,which is contrary to the general intuition that pressure always makes materials more stability and density.A detailed analysis of the decomposition mechanism reveals the increase of formation enthalpy with the increase of pressure due to contributions from bothΔU andΔ[P V].Pressure-dependent studies of theΔV demonstrate that denser materials tend to be stabilized at higher pressures.Additionally,charge transfer calculations show that external pressure is more effective in regulating the ionic bond of Hg-I,resulting in a lower decomposition pressure for HgI_(2)than for ZnI_(2).These findings have important implications for designs and syntheses of new materials,as they challenge the conventional understanding on how pressure affects stability.

Downshift of d-states and the decomposition of silver halides

The ionicity of ionic solids is typically characterized by the electronegativity of the constituent ions.Electronegativity measures the ability of electron transfer between atoms and is commonly considered under ambient conditions.Howeve r,external stresses profoundly change the ionicity,and compressed ionic compounds may behave differently.Here,we focus on silver halides,with constituent ions from one of the most electropositive metals and some of the most electronegative nonme tals.Using first-principles calculations,we find that the strengths of the ionic bonds in these compounds change greatly under pressure owing to downshifting of the Ag 4d-orbital.The center of this orbital is lowered to fill the antibonding state below the Fermi level,leading to chemical decomposition.Our results suggest that under pressure,the orbital energies and correspondingly the electronegativities still tune the ionicity and control the electron transfer,ionicity,and reactivity of both the metal and the nonmetal elements.However,the effects of orbital energies start to become dominant under pressure,causing substantial changes to the chemistry of ionic compounds and leading to an unusual phenomenon in which elements with substantial electronegativity differences,such as Ag and Br,do not necessarily form ionic compounds,but remain in their elemental forms.