⁷¹Ga-⁷⁷Se connectivities and proximities in gallium selenide crystal and glass probed by solid-state NMR

We introduce two-dimensional (2D) ⁷¹Ga-⁷⁷Se through-bond and through-space correlation experiments. Such correlations are achieved using (i) the J-mediated Refocused Insensitive Nuclei Enhanced by Polarization Transfer (J-RINEPT) method with ⁷¹Ga excitation and ⁷⁷Se Carr-Purcell-Meiboon-Gill (CPMG) detection, as well as (ii) the J- or dipolar-mediated Hetero-nuclear Multiple-Quantum Correlation (J- or D-HMQC) schemes with ⁷¹Ga excitation and quadrupolar CPMG (QCPMG) detection. These methods are applied to the crystalline β-Ga₂Se₃ and the 0.2Ga₂Se₃-0.8GeSe₂ glass. Such glass leads to a homogeneous and reproducible glass-ceramic, which is a good alternative to single-crystalline Ge and polycrystalline ZnSe materials for making lenses transparent in the IR range for thermal imaging applications. We show that 2D ⁷¹Ga-⁷⁷Se correlation experiments allow resolving the ⁷⁷Se signals of molecular units, which are not resolved in the 1D ⁷⁷Se CPMG spectrum. Additionally, the build-up curves of the J-RINEPT and the J-HMQC experiments allow the estimate of the ⁷¹Ga-⁷⁷Se J-couplings via one and three-bonds in the three-dimensional network of β-Ga₂Se₃. Furthermore, these build-up curves show that the one-bond ¹J_71Ga-77Se couplings in the 0.2Ga₂Se₃-0.8GeSe₂ glass are similar to those measured for β-Ga₂Se₃. We also report 2D ⁷¹Ga Satellite Transition Magic-Angle Spinning (STMAS) spectrum of β-Ga₂Se₃ using QCPMG detection at high magnetic field and high Magic-Angle Spinning frequency using large radio frequency field. Such spectrum allows separating the signal of β-Ga₂Se₃ and that of an impurity.