TY - JOUR

T1 - Bulk Glassy GeTe2: A Missing Member of the Tetrahedral GeX2Family and a Precursor for the Next Generation of Phase-Change Materials

AU - Tverjanovich, Andrey

AU - Khomenko, Maxim

AU - Benmore, Chris J.

AU - Bokova, Maria

AU - Sokolov, Anton

AU - Fontanari, Daniele

AU - Kassem, Mohammad

AU - Usuki, Takeshi

AU - Bychkov, Eugene

N1 - Publisher Copyright: © 2021 American Chemical Society. Copyright: Copyright 2021 Elsevier B.V., All rights reserved.

PY - 2021/2/9

Y1 - 2021/2/9

N2 - Vitreous germanium disulfide GeS2 and diselenide GeSe2 belong to canonical chalcogenide glasses extensively studied over the past half century. Their high-temperature orthorhombic polymorphs are congruently melting compounds, and the tetrahedral crystal and glass structure is largely preserved in the melt. In contrast, the ditelluride counterpart is absent in the Ge-Te phase diagram, which shows only a single compound, monotelluride GeTe. Phase-change materials based on GeTe have become a technologically important class of solids, and their structure and properties are also widely studied. Surprisingly, very scarce information is available for alloys having GeTe2 stoichiometry. Using a fast quenching procedure in silica capillaries, high-energy X-ray diffraction, and Raman spectroscopy supported by first-principles simulations, we show that bulk glassy GeTe2 differs substantially from the lighter GeX2 members, revealing 46% of trigonal germanium, 31% of three-fold coordinated tellurium, and only 20% of edge-sharing tetrahedra or pyramids. The fraction of homopolar Ge-Ge bonds is low; however, the population of dominant Te-Te dimers and Ten oligomers, n ≤ 10, appears to be significant. The complex structural and chemical topology of g-GeTe2 is directly related to the thermodynamic metastability of germanium ditelluride, schematically represented by the following reaction: GeTe2 ⇄ GeTe + Te. Disproportionation is complete above liquidus in the temperature range of semiconductor-metal transition, and the dense metallic GeTe2 liquid, mostly consisting of five-fold coordinated Ge species, exhibits high fluidity, strong fragility (m = 99 ± 5), and presumably a fast structural transformation rate combined with low atomic mobility in the vicinity of the glass transition temperature, favorable for reliable long-term data retention in nonvolatile memories. The observed and predicted characteristic features make GeTe2 a promising precursor for the next generation of phase-change materials, especially coupled with additional metal doping, depolymerizing the tetrahedral interconnected glass network and accelerating (sub)nanosecond crystallization.

AB - Vitreous germanium disulfide GeS2 and diselenide GeSe2 belong to canonical chalcogenide glasses extensively studied over the past half century. Their high-temperature orthorhombic polymorphs are congruently melting compounds, and the tetrahedral crystal and glass structure is largely preserved in the melt. In contrast, the ditelluride counterpart is absent in the Ge-Te phase diagram, which shows only a single compound, monotelluride GeTe. Phase-change materials based on GeTe have become a technologically important class of solids, and their structure and properties are also widely studied. Surprisingly, very scarce information is available for alloys having GeTe2 stoichiometry. Using a fast quenching procedure in silica capillaries, high-energy X-ray diffraction, and Raman spectroscopy supported by first-principles simulations, we show that bulk glassy GeTe2 differs substantially from the lighter GeX2 members, revealing 46% of trigonal germanium, 31% of three-fold coordinated tellurium, and only 20% of edge-sharing tetrahedra or pyramids. The fraction of homopolar Ge-Ge bonds is low; however, the population of dominant Te-Te dimers and Ten oligomers, n ≤ 10, appears to be significant. The complex structural and chemical topology of g-GeTe2 is directly related to the thermodynamic metastability of germanium ditelluride, schematically represented by the following reaction: GeTe2 ⇄ GeTe + Te. Disproportionation is complete above liquidus in the temperature range of semiconductor-metal transition, and the dense metallic GeTe2 liquid, mostly consisting of five-fold coordinated Ge species, exhibits high fluidity, strong fragility (m = 99 ± 5), and presumably a fast structural transformation rate combined with low atomic mobility in the vicinity of the glass transition temperature, favorable for reliable long-term data retention in nonvolatile memories. The observed and predicted characteristic features make GeTe2 a promising precursor for the next generation of phase-change materials, especially coupled with additional metal doping, depolymerizing the tetrahedral interconnected glass network and accelerating (sub)nanosecond crystallization.

UR - http://www.scopus.com/inward/record.url?scp=85099908876&partnerID=8YFLogxK

U2 - 10.1021/acs.chemmater.0c04409

DO - 10.1021/acs.chemmater.0c04409

M3 - Article

AN - SCOPUS:85099908876

VL - 33

SP - 1031

EP - 1045

JO - Chemistry of Materials

JF - Chemistry of Materials

SN - 0897-4756

IS - 3

ER -