Switching to Hidden Metallic Crystal Phase in Phase-Change Materials by Photoenhanced Metavalent Bonding

Abstract

Metavalent bonding is crucial for the determination of phase transition and improvement of device performance in phase-change materials, which are attracting interest for use in memory devices. Although monitoring dielectric and phononic parameters provides a direct measure of the metavalent bonding, the control of phase-change phenomena and metavalent bonding in the dynamical regime has yet to be demonstrated. This study reports the photoenhanced metavalent bonding and resulting hidden metallic crystalline state of Ti-doped Sb2Te3, a representative phase-change material with ultralong sustainability. Using ultrafast terahertz spectroscopy, Ti0.4Sb2Te3 was discovered to possess ultralong pump–probe dynamics, which is retained over hundreds of picoseconds, unlike the short-lived state of undoped Sb2Te3. Moreover, for Ti0.4Sb2Te3 during the long-lived transmission change, the infrared-active phonon is highly softened, even more than the amount of a thermal phonon shift, indicating the photoenhancement of lattice anharmonicity. Such a long-lived relaxation implies photoinduced transition into a crystalline state of ultrastrong metavalent bonding in Ti0.4Sb2Te3, on the basis of comparisons of the dynamical dielectric constant and temporal phonon shift. Our results show the realization of photoengineering of phase-change materials by tuning electron sharing or transferring.

Revisiting the Nature of Chemical Bonding in Chalcogenides to Explain and Design their Properties

“…[121] In addition to that, several phenomena have been attributed to metavalent bonding, such as the formation of dynamic dipoles in PbTe due to the softening of the TO branch, [122,123] the absence of translational shear fault defects in some layered chalcogenides, [124] and the switching mechanism in amorphous ovonic threshold switching materials under an electric field. [125] So-called photoenhanced metavalent bonding has been observed in pump-probe experiments by Yang et al [126] Under high pressures, metavalent bonding emerges in several compounds. For example, in orpiment, where it is accompanied by the softening of several vibrational modes and the decrease of the phonon bandgap, [127] in p-type layered SnSe [128] and in the compressed rhombohedral phase of SnSb 2 Te 4 .…”

“…[ 125 ] So‐called photoenhanced metavalent bonding has been observed in pump‐probe experiments by Yang et al. [ 126 ] Under high pressures, metavalent bonding emerges in several compounds. For example, in orpiment, where it is accompanied by the softening of several vibrational modes and the decrease of the phonon bandgap, [ 127 ] in p‐type layered SnSe [ 128 ] and in the compressed rhombohedral phase of SnSb 2 Te 4 .…”

Conversion between Metavalent and Covalent Bond in Metastable Superlattices Composed of 2D and 3D Sublayers

“…Inherent vacancies change their atomic sites based on the competition between enthalpic and entropic gain that minimizes the electrostatic repulsion at their sites and maximizes randomness in atomic configurations, respectively. The direct observation of the evolution in chemical states of inherent vacancies would extend the experimental limits by combining with the state-of-art techniques on fragility, time-resolved ultrafast transition, Peierls distortion, and coherent atomic displacements. ,,− Those interdisciplinary convergences would extend the understanding on the phase-change operations such as kinetic physical factors of inherent vacancies for phase-transition speed and activation energy. As the materials discussed are also of considerable interest in topological insulators and Weyl semimetals, understanding the kinetics of their inherent vacancies would inspire studies on the change in spin textures .…”