Dynamical Bond Formation in KNi₂Se₂

Abstract

Emphanisis, or the appearance out of nothing, has been used to describe the phenomena of spontaneous atom off-centering and dipole formation at elevated temperatures in lead chalcogenides. Here, we provide spectroscopic evidence of spontaneous formation of metal-metal bonds above T ~ 50 K in the layered metal KNi2Se2. These bonds form zig-zag chains that lower the local symmetry from tetragonal to monoclinic. Energy-resolved pair distribution function measurements exclude a pure phonon origin of our observations, and instead imply the existence of extra, slowly fluctuating, Ni-Ni bonds above T = 50 K. Density functional theory calculations support this lower symmetry configuration as an instability of tetragonal KNi2Se2. We thus demonstrate that the phenomena of emphansis is not limited to local electric dipole formation, but can also be driven by the formation of metal-metal bonds.

When IV−VI Meets I−V−VI2: A Reinvigorating Thermoelectric Strategy for Tin Monochalcogenides

“…The simplest example for illustration would be the sequenced solid−liquid−gas state transition of an ordinary substance that undergoes melting and then evaporation with increasing temperature. However, a very rare contrary phenomenon where the crystal structure shows local symmetry breaking arising from an undistorted high‐symmetry phase upon warming was discovered in cubic PbTe and PbS in 2010, and subsequently in several other cubic compounds such as SnTe, PbSe, CsSnBr 3 , (SnSe) 0.5 (AgSbSe 2 ) 0.5 , etc ., as well as very recently in less‐high‐symmetry tetragonal AgGaTe 2 and KNi 2 Se 2 compounds [71–77] . In these compounds, a local “off‐centering” breaks the symmetry upon heating, which was termed “ emphanisis ”, meaning “appearing out of nothing” in Greek [78] .…”

“…However, a very rare contrary phenomenon where the crystal structure shows local symmetry breaking arising from an undistorted high-symmetry phase upon warming was discovered in cubic PbTe and PbS in 2010, and subsequently in several other cubic compounds such as SnTe, PbSe, CsSnBr 3 , (SnSe) 0.5 (AgSbSe 2 ) 0.5 , etc., as well as very recently in less-highsymmetry tetragonal AgGaTe 2 and KNi 2 Se 2 compounds. [71][72][73][74][75][76][77] In these compounds, a local "off-centering" breaks the symmetry upon heating, which was termed "emphanisis", meaning "appearing out of nothing" in Greek. [78] Although some possible explanations have been hypothesized, the exact origin of such emphanisis remains a mystery.…”