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

We introduce two-dimensional (2D) Ga-Se 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 Ga excitation and Se 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 Ga excitation and quadrupolar CPMG (QCPMG) detection. These methods are applied to the crystalline β-GaSe and the 0.2GaSe-0.8GeSe 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 Ga-Se correlation experiments allow resolving the Se signals of molecular units, which are not resolved in the 1D Se CPMG spectrum. Additionally, the build-up curves of the J-RINEPT and the J-HMQC experiments allow the estimate of the Ga-Se J-couplings via one and three-bonds in the three-dimensional network of β-GaSe. Furthermore, these build-up curves show that the one-bond J couplings in the 0.2GaSe-0.8GeSe glass are similar to those measured for β-GaSe. We also report 2D Ga Satellite Transition Magic-Angle Spinning (STMAS) spectrum of β-GaSe 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-GaSe and that of an impurity.