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.

Binary Ge–Te and ternary Ge–Sb–Te systems belong to flagship phase-change materials (PCMs) and are used in nonvolatile memory applications and neuromorphic computing. The working temperatures of these PCMs are limited by low-T glass transition and crystallization phenomena. Promising high-T PCMs may include gallium tellurides; however, the atomic structure and transformation processes for amorphous Ga–Te binaries are simply missing. Using high-energy X-ray diffraction and Raman spectroscopy supported by first-principles simulations, we elucidate the short- and intermediate-range order in bulk glassy Ga x Te1–x , 0.17 ≤ x ≤ 0.25, following their thermal, electric, and optical properties, revealing a semiconductor–metal transition above melting. We also show that a phase change in binary Ga–Te is characterized by a very unusual nanotectonic compression with the high internal transition pressure reaching 4–8 GPa, which appears to be beneficial for PCM applications increasing optical and electrical contrast between the SET and RESET states and decreasing power consumption.

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Non-linear optical (NLO) crystals are widely used in advanced photonic technologies for second harmonic and difference frequency generation (SHG and DFG, respectively), producing coherent light at frequencies where existing lasers are unavailable. Isotropic glasses do not exhibit SHG or DFG, except temporary induced anisotropy under external stimuli. However, recent reports on glasses with chiral structural motifs show promising permanent NLO properties. We propose an alternative solution: hybrid molecular/network glasses with non-centrosymmetric HgI2 monomers. Mercury (II) iodide consists of linear HgI2 triatomic molecules in the vapor phase and in the yellow orthorhombic polymorph stable above 400 K. At lower temperatures, the tetragonal red form is composed of corner-sharing HgI4/2 tetrahedra forming a layered extended framework. There is a gap in the molecular evolution; direct structural measurements of the liquid HgI2 phase are missing. Using high-energy X-ray scattering, pulsed neutron diffraction and Raman spectroscopy supported by structural and vibrational modeling, we show that the mercury (II) iodide melt and HgI2-containing sulfide glasses are built-up by bent HgI2 monomers (the bond angle ∠I-Hg-I = 156±2° in the melt). The non-centrosymmetric entities imply intrinsic optical non-linearity of the second order, confirmed by a strong SHG response.

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