Glassy GaS: transparent and unusually rigid thin films for visible to mid-IR memory applications.

Phase-change materials based on tellurides are widely used for optical storage (DVD and Blu-ray disks), non-volatile random access memories and for development of neuromorphic computing. Narrow-gap tellurides are intrinsically limited in the telecom spectral window, where materials having a wider gap are needed. Here we show that gallium sulfide GaS thin films prepared by pulsed laser deposition reveal good transparency from the visible to the mid-IR spectral range with optical gap Eg = 2.34 eV, high refractive index nR = 2.50 over the 0.8 ≤ λ ≤ 2.5 µm range and, unlike canonical chalcogenide glasses, the absence of photo-structural transformations with a laser-induced peak power density damage threshold above 1.4 TW cm-2 at 780 nm. The origin of the excellent damage threshold under a high-power laser and UV light irradiation resides in the rigid tetrahedral structure of vitreous GaS studied by high-energy X-ray diffraction and Raman spectroscopy and supported by first-principles simulations. The average local coordination number appears to be m = 3.44, well above the optimal connectivity, 2.4 ≤ m ≤ 2.7, and the total volume of microscopic voids and cavities is 34.4%, that is, lower than for the vast majority of binary sulfide glasses. The glass-crystal phase transition in gallium sulfide thin films may be accompanied by a drastic change in the nonlinear optical properties, opening up a new dimension for memory applications in the visible to mid-IR spectral ranges.

Solvation Structure of Li+ in Methanol and 2-Propanol Solutions Studied by ATR-IR and Neutron Diffraction with 6Li/7Li Isotopic Substitution Methods.

Neutron diffraction measurements have been carried out on 10 mol % LiTFSA (TFSA: bis(trifluoromethylsulfonil)amide) solutions in methanol- d4 and 2-propanol- d8 to obtain information on the solvation structure of Li+. The detailed coordination structure of solvent molecules within the first solvation shell of Li+ was determined through the least-squares fitting analysis of the difference function between normalized scattering cross sections observed for 6Li/7Li isotopically substituted sample solutions. The nearest-neighbor Li+···O distance and coordination number determined for the 10 mol % LiTFSA-methanol- d4 solution are rLiO = 1.98 ± 0.02 Å and nLiO = 3.8 ± 0.6, respectively. In the 2-propanol- d8 solution, it has been revealed that 2-propanol- d8 molecules within the first solvation shell of Li+ take at least two different coordination geometries with the intermolecular nearest-neighbor Li+···O distance of rLiO = 1.93 ± 0.04 Å. The Li+···O coordination number, nLiO = 3.3 ± 0.3, is determined. Ion-pair formation in the LiTFSA-methanol and LiTFSA-2-propanol solutions has been investigated by the attenuated total reflection infrared spectroscopic method. Mole fractions of free, Li+-bound, and aggregated TFSA- are derived from the peak deconvolution analysis of vibrational bands observed for TFSA-.

Local Structure of Li+ in Concentrated Ethylene Carbonate Solutions Studied by Low-Frequency Raman Scattering and Neutron Diffraction with 6Li/7Li Isotopic Substitution Methods.

Isotropic Raman scattering and time-of-flight neutron diffraction measurements were carried out for concentrated LiTFSA-EC solutions to obtain structural insight on solvated Li+ as well as the structure of contact ion pair, Li+···TFSA-, formed in highly concentrated EC solutions. Symmetrical stretching vibrational mode of solvated Li+ and solvated Li+···TFSA- ion pair were observed at ν = 168-177 and 202-224 cm-1, respectively. Detailed structural properties of solvated Li+ and Li+···TFSA- contact ion pair were derived from the least-squares fitting analysis of first-order difference function, ΔLi(Q), between neutron scattering cross sections observed for 6Li/7Li isotopically substituted 10 and 25 mol % *LiTFSA-ECd4 solutions. It has been revealed that Li+ in the 10 mol % LiTFSA solution is fully solvated by ca. 4 EC molecules. The nearest neighbor Li+···O(EC) distance and Li+···O(EC)═C(EC) bond angle are determined to be 1.90 ± 0.01 Å and 141 ± 1°, respectively. In highly concentrated 25 mol % LiTFSA-EC solution, the average solvation number of Li+ decreases to ca. 3 and ca. 1.5. TFSA- are directly contacted to Li+. These results agree well with the results of band decomposition analyses of isotropic Raman spectra for intramolecular vibrational modes of both EC and TFSA-.