The structure analyses of NaGaSi3O8, NaAlGe3O8 and NaGaGe3O8 glasses by X-ray diffraction and EXAFS measurements

The short-range structures of NaGaSi3O8, NaAlGe3O8 and NaGaGe3O8 glasses, which have feldspar structures in crystalline phases, have been determined by X-ray diffraction and EXAFS spectroscopy techniques. The X-ray radial distribution functions showed coordination numbers of T atoms (T=Si,Al,Ge,Ga) in the glasses a little larger than four. Correspondingly, the T–O distances in NaAlGe3O8 and NaGaGe3O8 glasses obtained by X-ray radial distribution analyses are a little longer than those of NaAlGe3O8 and NaGaGe3O8 feldspars. The Ge–O and Ga–O distances of NaGaSi3O8, NaAlGe3O8 and NaGaGe3O8 glasses obtained by EXAFS spectroscopy are also a little longer than those in low-quartz type GeO2 and in GaO4 tetrahedra of β-Ga2O3, respectively. If the Si–O and Al–O distances are 1.61 and 1.75 Å respectively, the T–O distances obtained from EXAFS analyses are considered to agree with the values determined by X-ray radial distribution analyses. These results suggest that the structures of the glasses are essentially based on GeO4 and GaO4 tetrahedra and contain a small amount of GeO6 and GaO6 octahedra. The radial distribution functions of these glasses are well interpreted by model structures with cristobalite type linkages of GeO4 and GaO4 tetrahedra. The long range order diminishes from NaGaSi3O8, NaAlGe3O8 to NaGaGe3O8 glasses in this order. The densities of the glasses approach to those of the corresponding feldspar crystals in the same order.