Preferred selenium incorporation and unexpected interlayer bonding in the layered structure of Sb₂Te_{3− x} Se _x

Abstract

We have performed a detailed structural analysis of several crystals from the Sb2Te3−xSex solid solution (with x = 0–1.55) using a combination of single-crystal and powder X-ray diffraction and high-resolution scanning transmission electron microscope imaging combined with energy-dispersive X-ray spectroscopic mapping. The experimental study was supported by quantum-chemical calculations. All compounds crystallize in the rhombohedral tetradymite structure type in which the atoms occupy three symmetrically independent crystallographic sites; the two chalcogenide sites A1 and A2, and the antimony site Sb. They form quintuple A2–Sb–A1–Sb–A2 layers perpendicular to the [001] direction, held together via A2–A2 chalcogenide-chalcogenide interactions. The results of our experimental study show unambiguously, and in agreement with quantum-chemical calculations, a preferred incorporation of selenium into the A2 position inside the layers. An unexpected increase of the van-der-Waals inter-layer distances is observed for low Se contents (x < 1) and discussed with the help of DFT calculations.

The Orbital Origins of Chemical Bonding in Ge−Sb−Te Phase‐Change Materials**

“…The most obvious and indisputable structural property is given by the too small van-der-Waals gaps in layered PCM systems which refuses a convincing explanation until today. [9] The aforementioned property set has proven to be an apt identifier for phase-change behavior although its origins have so far eluded an in-depth bonding analysis based on orbitals. Nonetheless, it has been possible to empirically map [10] various materials including PCM based on an electron-density partitioning scheme and thus derived descriptors for ionicity (electrons transferred, ET) and covalency (electrons shared, ES), thereby identifying prominent PCM as being positioned between the covalent and metallic bonding regimes.…”

“…The most obvious and indisputable structural property is given by the too small van‐der‐Waals gaps in layered PCM systems which refuses a convincing explanation until today. [9] …”

Orbitale als Ausgangspunkt der Chemischen Bindung in Ge−Sb−Te‐Phasenwechselmaterialien**

“…Die offensichtlichste und unbestreitbarste Struktureigenschaft sind die zu kleinen van-der-Waals-Lücken in schichtartigen PCM-Systemen, die sich bis heute nicht überzeugend erklären lassen. [9] Das zuvor genannte Eigenschaftsportfolio hat sich als geradezu typisch für das Phasenwechselverhalten erwiesen, obwohl sich ihre Ursache bisher einer eingehenden Bindungsanalyse (auf Orbitalbetrachtungen fußend) entzogen hat. Nichtsdestoweniger war es möglich, verschiedene Mate-rialien, darunter auch PCM, auf der Grundlage einer Elektronendichteaufteilung empirisch abzubilden [10] und daraus Deskriptoren für Ionizität (übertragene Elektronen, ET, von engl.…”

The Orbital Origins of Chemical Bonding in Phase-Change Materials

“…The most obvious and indisputable structural property is given by the too small van-der-Waals gaps in layered PCM systems which refuses a convincing explanation until today. [8] The aforementioned property set has proven to be an apt identifier for phase-change behavior although its origins have so far eluded an in-depth bonding analysis based on orbitals. Nonetheless, it has been possible to empirically map [9] various materials including PCM based on an electron-density partitioning scheme and thus derived descriptors for ionicity (electrons transferred, ET) and covalency (electrons shared, ES), thereby identifying prominent PCM as being positioned between the covalent and metallic bonding regimes.…”

“…[18] The somewhat mysterious structural gap between two layers (that is, between the terminal Te atoms) has often been referred to as a van-der-Waals (vdW) gap, for obvious reasons, but a closer look reveals that this gap is significantly (12%) smaller than twice the sum of the vdW radius. [8] Because there is no external pressure in the GPa range, there must be more than simple vdW forces; indeed, previous projected COHP analysis has already detected small but significant covalency across the gap. [11] The latter stems from multicenter bonding, as elaborated below, present in the entire set of phase-change materials studied here.…”