73Ge Solid-State NMR of Germanium Oxide Materials: Experimental and Theoretical Studies

A comprehensive series of crystalline germanates has been studied by ultrahigh-field Ge-73 NMR and quantum chemical calculations. Despite its low gyromagnetic ratio, low natural abundance and large quadrupole moment, interpretable spectra were obtained in almost all cases, demonstrating that Ge-73 is an accessible NMR nucleus. The spectra yield a wide range of quadrupole coupling constants (C-Q = 9 to 35 MHz), with calculations indicating a range twice that, which are rationalized principally in terms of the variation in Ge-O bond lengths. The isotropic chemical shifts appear to fall into distinct regions for four-, five-, and six-coordinate Ge, with increasing coordination number corresponding to lower frequencies. Both CASTEP and WIEN2k consistently underestimate the C(Q)s, suggesting that the exchange-correlation functional is poorly optimized for these systems. Ge-73 NMR spectra of alkali germanate glasses are broad and featureless, rendering them difficult to interpret in terms of specific structural elements, even with the well understood NMR parameters from the crystalline systems. This study represents the first systematic Ge-73 NMR investigation of solids, and shows that valuable structural information can be obtained in favorable cases.