Bulk Glassy GeTe₂: A Missing Member of the Tetrahedral GeX₂ Family and a Precursor for the Next Generation of Phase-Change Materials

Abstract

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 Te n 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.

Polymorphism of CdTe in the Few‐Monolayer Limit

“…One is that the presence of heavy chalcogen atoms favors the formation of vdW solids. We note here that vdW blocks terminated by chalcogene planes were also proposed for thin slabs of such materials as SiTe 2 and GeTe 2 , [10,11] which possess a different structure in the bulk phase. A related result was reported for quasi 2D GeTe, viz., the Te termination of the surface was found to be energetically more favourable than Ge termination.…”

Unraveling the Atomic Structure of Bulk Binary Ga–Te Glasses with Surprising Nanotectonic Features for Phase-Change Memory Applications

“…The local gallium coordination remains intact (Table ) as well as the shape and position of the first neighbor peak for g GaTe ( r ) partials (Figure ). In contrast, the Ge–Te NN distances in GeTe 2 become markedly longer in metallic liquid, and the g GeTe ( r ) partial function exhibits an asymmetric shape with a long- r tail without a clear minimum …”

“…The 156 cm −1 mode is similar to that in a-Te and Te−Te stretching modes in glassy binary tellurides: Ge− Te, As−Te, and Se−Te. 31,73,74 The fractional amplitude of this mode A 156 (x) decreases with x, approaching zero at x = 0.4, lim x→0.4 A 156 (x) = 0 (Figure 6c). In contrast, the position of a lower-frequency counterpart at ≈128 cm −1 is similar to that in cubic Ga 2 Te 3 , and the fractional amplitude A 128 (x) increases with x and lim x→0.4 A 128 (x) = 1.…”

“…The neutron diffraction data for metallic Ga 0.2 Te 0.8 were available only in a limited Q range from 1.7 to 9 Å –1 and represent a linear interpolation between Ga 0.14 Te 0.86 and Ga 0.28 Te 0.72 melts . As previously reported, ,, the used hybrid GGA/PBE0 functional has a clear advantage compared to GGA/PBE or GGA/PBEsol both for glasses and liquids, except associated heavy computational costs. Figure illustrates a distinct difference in the glass and liquid interference Q [ S X ( Q ) – 1] and pair-distribution g X ( r ) functions in direct comparison with experimental data.…”

“…A broad asymmetric P ( q ) function in metallic liquid is centered at q ≈ 0.9 and exhibits a long tail up to q = 0.4 (Figure i). The low- q part, 0.4 ≲ q ≲ 0.8, is typical for distorted defect octahedra . A regular defect octahedron GaTe 4 , with and , has q = 5/8, that is, just in the middle of this q range.…”