Scandium-45 NMR of pyrope-grossular garnets: Resolution of multiple scandium sites and comparison with X-ray diffraction and X-ray absorption spectroscopy

Abstract

Here we present 45Sc and 27Al NMR results on Sc-doped pyrope (Mg3Al2Si3O12), grossular (Ca3Al2Si3O12), and an 80% grossular-20% pyrope garnet (grs80) that have recently been well-studied by X-ray diffraction and X-ray spectroscopies. Clearly distinct NMR peaks are observed for Sc in the eight-coordinated X site (pyrope and grs80) and in the six-coordinated Y site (grossular and grs80). X-ray and NMR data agree that only eight-coordinated Sc is present in pyrope and that six-coordinated Sc is predominant in grossular; however, the XRD results also indicated significant X and Z site (four-coordinated) Sc in the Ca-rich garnet. Possible reasons for this apparent discrepancy are discussed. We demonstrate that 45Sc NMR is potentially a useful new method for studies of the site occupancies of Sc3+ in oxides and silicates, at least in experimental systems where its concentration is a few percent or greater.