Single Structure Widely Distributed in a GeTe−Sb₂Te₃ Pseudobinary System:  A Rock Salt Structure is Retained by Intrinsically Containing an Enormous Number of Vacancies within its Crystal

Abstract

GeTe(1-x)-Sb2Te3(x) sputtered amorphous film was crystallized into a simple NaCl-type structure through instantaneous laser irradiation over a wide composition range from x = 0 to at least 2/3. When the ratio of Sb2Te3 increases, a vacancy is generated at every Na site for two Sb atoms. The fraction of vacancies, v(x), changes according to x/(1 + 2x), and the cubic root unit cell volume varies with a strong correlation to v(x). Through these created vacancies, valence electrons provided by adjacent Ge/Sb and Te atoms remain constant regardless of the composition, ensuring that these electrons occupy predominantly the bonding molecular orbitals. This results in crystal chemical stability, with the closed shell p-p bondings in the valence electrons arranging the crystal's atomic configuration into an NaCl-type structure.

Impact of deposition conditions on the crystallization kinetics of amorphous GeTe films

“…2b. Regardless of the deposition temperatures, the GeTe films were composed of the same phase, with the expected trigonal symmetry [18]. Weak Te peaks were also observed due to the Te-rich composition of the films.…”

Structure of amorphousGe8Sb2Te11:<

“…4,5 At room temperature, samples with 0 Յ x Յ 1 3 crystallize into a deformed NaCl ͑rhombohedral͒ structure and the remainder into an NaCl ͑cubic͒ phase. The NaCl structures are stabilized by vacancies that increase in number as x increases.…”

“…The semiconducting nature of c-GST-8,2,11 is closely related to the presence of vacancies on Na sites, which help stabilize the rocksalt structure ͓the p 3 -bonded structure must include 6N͑Te͒ p electrons͔. 5,15 Deviations from the pseudobinary tie line shift the band gap and change conductivity and optical reflectivity. The overall shapes of the DOS profiles display a threeband-structure characteristic of materials with average valence near 5 ͑5.14 for GST-8,2,11͒.…”

“…4 Vacancies play an essential role in stabilizing the NaCl structure over a wide range of x. 5 Density-functional calculations have been performed for crystalline structures with compositions ͑GeTe͒ 1−x ͑Sb 2 Te 3 ͒ x and nearby stoichiometries. 6 2 3 , respectively͔͒ suggest that the stability of these compounds is enhanced by maximizing the number of Te atoms surrounded by three Ge and three Sb atoms.…”

Structural phase transitions on the nanoscale: The crucial pattern in the phase-change materialsGe2Sb2Te5and GeTe

“…Extended x-ray absorption fine structure ͑EXAFS͒ measurements on GST have found significant concentrations of Ge-Ge bonds and indications that overcoordinated Te atoms play a role, 12 and there has been renewed focus on the role of vacancies. 13,14 Density functional ͑DF͒ calculations suggest that the metastable structure is consistent with rocksalt symmetry but comprises two units ordered along the ͓111͔ direction. Higher temperatures lead to two-dimensional linear or tangled clusters that are ordered in the perpendicular direction.…”