The atomic structure of niobium and tantalum containing borophosphate glasses

A complete structural study has been carried out on sodium borophosphate glass containing increasing amounts of either niobium or tantalum. A combination of high energy x-ray diffraction, neutron diffraction, extended x-ray absorption fine structure, nuclear magnetic resonance, and infrared and Raman spectroscopy has been used to discern the local atomic structure of each component and the changes with M content, where M is either niobium or tantalum. The glasses are found to consist of tetrahedral borate and phosphate with octahedral MO6. As expected, B and P play the roles of tetrahedral network formers. At low M content there are isolated MO6 units with PcM and BcM linkages that contribute to the glass network. As the M content increases, the number of McM links increases, and at the highest M content each MO6 unit is connected to several others. The octahedra become significantly distorted as the niobium content increases, an effect that is not seen for tantalum.